110 RG PNC 00000 000826 EOR Abbey Mills Route

7/27/2019 110 RG PNC 00000 000826 EOR Abbey Mills Route

http://slidepdf.com/reader/full/110-rg-pnc-00000-000826-eor-abbey-mills-route 1/79

Spring 2012

Engineering optionsreportAbbey Mills route



Please note:

Further details are provided in the Final Report on SiteSelection Process (doc ref: 7.05) that can be found onthe Thames Tideway Tunnel section of the PlanningInspectorate’s web site.



110-RG-PNC-00000-000826 | Spring 2012

Engineering options

reportAbbey Mills route





Engineering options report Abbey Mills route

i

Thames Tunnel

Engineering options report

Abbey Mills routeList of contents

Page number

1 Executive summary ......................................................................................... 1 2 Introduction ...................................................................................................... 3

2.1 Background ............................................................................................. 3 2.2 Purpose of this report .............................................................................. 4 2.3 Engineering design development ............................................................ 5

3 System design and engineering requirements .............................................. 7 3.1 System design and engineering assumptions ......................................... 7 3.2 Health and safety considerations ............................................................. 7 3.3 System requirements ............................................................................... 7 3.4 Engineering geology .............................................................................. 15 3.5 Tunnel engineering and construction requirements ............................... 20 3.6 CSO engineering and construction requirements .................................. 30

4 Main tunnel drive options .............................................................................. 35 4.1 Introduction ............................................................................................ 35 4.2 Main tunnel engineering: Options preparation ....................................... 35 4.3 Main tunnel engineering: Options assessment ...................................... 49

5 Connection tunnel drive options .................................................................. 57 5.1 CSO connection options ........................................................................ 57 5.2 Connection tunnel: Drive options ........................................................... 63

6 Conclusions and recommendations ............................................................ 69 7 Next steps ....................................................................................................... 71

The following can be found in the accompanying document Engineering optionsreport - Appendices - Abbey Mills route (110-RG-PNC-000000-000827):

Appendix A – Assumptions register

Appendix B – Drawings

Appendix C – Time chainage

Appendix D – Geology




ii

List of figures

Page number

Figure 3.1 Main tunnel routes considered................................................................. 12 Figure 3.2 Typical CSO interception arrangement .................................................... 13 Figure 4.1 Main tunnel site types .............................................................................. 36 Figure 4.2 Main tunnel site zones for all three routes ............................................... 37 Figure 4.3 Main tunnel site zones for the Abbey Mills route ..................................... 37 Figure 5.1 Type A CSO connection .......................................................................... 58 Figure 5.2 Type B CSO connection .......................................................................... 59 Figure 5.3 Type C CSO connection .......................................................................... 60 Figure 5.4 Type D CSO connection .......................................................................... 61 Figure 5.5 Type E CSO connection .......................................................................... 62

List of tables

Page number

Table 3.1 CSO control measures ............................................................................... 8 Table 3.2 Geology of the London Basin.................................................................... 16 Table 3.3 Chalk aquifer groundwater levels in 2008 and imposed pressure at tunnel

invert (east of Shad) ................................................................................ 20 Table 4.1 Grouping of shortlisted main tunnel sites for the Abbey Mills route post-

phase two consultation ............................................................................ 38 Table 4.2 Drive options: Consideration of practical drive lengths ............................. 43 Table 4.3 Initial provisional main tunnel drive options............................................... 44 Table 4.4 Interim main tunnel drive options .............................................................. 47 Table 4.5 Interim list of main tunnel drive options ..................................................... 48 Table 4.6 Programme assumptions for comparison of options ................................. 54 Table 4.7 Summary of construction durations for main tunnel drive options............. 55 Table 4.8 Final list of main tunnel drive options ........................................................ 56 Table 5.1 Frogmore connection tunnel: Drive options .............................................. 64 Table 5.2 Greenwich connection tunnel: Initial drive options .................................... 65 Table 5.3 Greenwich connection tunnel: Final drive options ..................................... 66 Table 5.4 North East Storm Relief Type A CSO connection tunnel drive options

matrix ....................................................................................................... 67




iii

List of abbreviations

AOD above Ordnance Datum

ATD above tunnel datumCSO combined sewer overflow

Defra Department of Environment Food and Rural Affairs

EA Environment Agency

EU European Union

EPB earth pressure balance (type of TBM)

GWT groundwater table

m/s metres per second

m3/s cubic metres per second

NESR North East Storm Relief

OD Ordnance Datum (mean sea level at Newlyn in Cornwall)

Ofwat Water Services Regulatory Authority

PLA Port of London Authority

PS pumping station

SMP System master plan

SR storm relief STW sewage treatment works

TBM tunnel boring machine




iv



1 Executive summary

Engineering options report Abbey Mills route 1

1 Executive summary

1.1.1 The need for an engineering options report is outlined in the Site selectionmethodology paper (Summer 2011) 1.

1.1.2 Following phase two consultation, this report was prepared as part of theprocess to support the proposed sites and proposed route for the ThamesTunnel project (the ‘project’) that will be publicised in accordance withSection 48 of the Planning Act 2008. It is specific to the Thames Tunnelproject, but also takes cognisance of the Lee Tunnel project, which isexpected to be completed before the Thames Tunnel project.

1.1.3 This report should be read as a technical document; therefore the contenthas been kept brief in the understanding that the reader is familiar with thetechnical subject matter.

1.1.4 The report begins by defining the overall engineering requirements to beconsidered as part of the development of engineering options for theproject. The requirements are broadly summarised without providing anyin-depth justification since the main aim of the report is to identify tunneldrive options.

1.1.5 Three main tunnel routes between west London and Beckton SewageTreatment Works (STW) were identified as part of design development,and the Abbey Mills route was chosen as the preferred option. The Report on phase one consultation concluded that, having considered thefeedback received, following phase one consultation the Abbey Mills routeremained the preferred route for phase two consultation. Followingconsideration of the second phase of consultation, only the Abbey Millsroute was taken forward for further evaluation in this report as theproposed route.

1.1.6 The report also presents our methodology for determining possible optionsfor constructing the main tunnel along the Abbey Mills route. Themethodology is based on engineering requirements and the shortlist of main tunnel sites provided by the ‘site selection process’, which identifiespotentially suitable sites for use as either main tunnel drive, intermediateor reception sites, in order to facilitate the construction of the main tunneland its subsequent operation. Drive options associated with the shortlisted

CSO sites for connection tunnels that link two or more CSOs are alsoconsidered in this report.

1.1.7 In order to build the project, it is necessary to ‘drive’ a series of tunnels toconnect a number of tunnel sites. Possible permutations of tunnel drivescenarios (‘drive options’) for the presented sites are establishedsystematically for evaluation.

1.1.8 There is an important relationship between tunnel drive optioneering andsite selection. The relative desirability of the feasible drive options is also

1

Terminology: prior to the two phases of consultation the project identified ‘preferred sites’. These sites and thedrive strategy were the subject to public consultation. Following phase two consultation, the project reviewed andidentified the ‘proposed sites’ for Section 48 publicity.



1 Executive summary


examined in terms of engineering factors. These and factors identified byother disciplines, such as planning, environment, community and property,will be addressed in workshops and used in conjunction with the sitesuitability reports to determine the proposed sites and the proposedscheme, which will be presented in the Section 48: Report on site

selection process.1.1.9 This report describes the appropriate engineering options that are

available to drive the tunnels. These options are presented as a scheduleof feasible tunnel drive options to be taken forward to subsequent stagesin the site selection process.

1.1.10 Finally, engineering factors that will be used to provide content todetermine the proposed sites and associated drive options for the tunnelsare presented. These factors will be used in the Section 48: Report on siteselection process to examine the advantages and disadvantages of thedrive options.



2 Introduction


2 Introduction

2.1 Background

2.1.1 The Site selection methodology paper and Site selection background technical paper are the main documents that guide our site selectionprocess.

2.1.2 In summary, the Site selection methodology paper comprises three mainstages. Stage 1 includes a site identification and filtering process, which iscarried out in three main parts:

a. 1A: Creation of a long list of potential sites, with an explanation of howinformation is to be verified and moved to part 1B

b. 1B: Creation of a shortlist of potential sites, with an explanation of howinformation is to be verified and moved to part 1C

c. 1C: Creation of a preferred list of sites, with an explanation of howinformation is to be verified and moved to Stage 2 (consultation).

2.1.3 Arriving at the preferred list of sites involves three steps: the first two stepstake place concurrently and the third step brings together the findings of the first two:

a. The suitability of all sites on the final shortlist is assessed in moredetail in site suitability reports.

b. An engineering options report sets out the tunnel drive options.

c. Optioneering workshops bring together the disciplines to discusskey factors from the site suitability reports and engineering optionsreport in order to determine the preferred drive options and associatedsites.

2.1.4 There is an important relationship between tunnel drive optioneering andsite selection. The role of the engineering options report in the process isto define the engineering requirements and to set out the drive options tobe taken forward for evaluation. It also explains how the possible optionsfor delivering the three main tunnel routes are determined. Possiblepermutations of tunnel drive scenarios are considered in order to identify

all the feasible drive options, based on the potential number of tunnelboring machines (TBMs) used and the shortlisted sites that they could bedriven from and received at.

2.1.5 An engineering options report was produced at the following stages of theproject to take account of information current at the time:

a. prior to phase one consultation

b. prior to phase two consultation

c. post phase two consultation (prior to Section 48 publicity)

2.1.6 Prior to phase one consultation, the work for Stage 1 of the site selection

process, from identification of the long list to the preferred list of sites for phase one consultation, was carried out in 2009 and 2010. As part of that



2 Introduction


process, the Engineering options report (Spring 2010) was prepared toconsider the drive options associated with the shortlisted sites for threedifferent tunnel routes: the River Thames route, the Rotherhithe route andthe Abbey Mills route.

2.1.7 At phase one consultation (September 2010 to January 2011) we

presented the preferred sites and preferred route along with the shortlistedsites and other routes that had been discounted. The phase oneconsultation feedback was collated into the Report on phase oneconsultation. Analysis of the consultation feedback received concludedthat the Abbey Mills route remained the preferred route.

2.1.8 Prior to phase two consultation, consideration was given to phase oneconsultation feedback and a number of emerging factors that triggered aseries of site selection ‘back-checks’. The back-checks led to a number of site changes and new drive options. As part of this process, theEngineering options report – Abbey Mills route (Summer 2011) was

prepared to consider the drive options associated with the revisedshortlisted sites for the Abbey Mills route. This process and the changeswere presented in the Phase two scheme development report (Winter 2011).

2.1.9 At phase two consultation the revised preferred sites along the Abbey Millsroute were presented together with the shortlisted sites that had beendiscounted. Some of the preferred sites presented at phase twoconsultation were different to those presented at phase one consultation,including changes to the associated drive strategy.

2.1.10 As a consequence of phase two consultation feedback and other

emerging information, a review of sites and drive options was undertaken. As part of that process, this Engineering options report – Abbey Mills route (Spring 2012) has been prepared to consider the drive options associatedwith the shortlisted sites for the Abbey Mills route as part of the post phasetwo consultation review of sites.

2.2 Purpose of this report

2.2.1 This report has been prepared as part of the process to support theproposed sites and proposed route for the project that will be publicised inaccordance with Section 48 of the Planning Act 2008. It is specific to the

project but also takes cognisance of the Lee Tunnel project, which isexpected to be completed before the Thames Tunnel project.

2.2.2 The Site selection methodology paper states that the engineering optionsreport should consider:

a. how sites work in combination as well as options for the main tunnelalignment and combined sewer overflow (CSO) connections

b. how options for the tunnel alignment and CSO connection points canbe refined, having regard to the availability and spacing of suitablemain tunnel sites, in addition to the potential for combined use of sites.Cost considerations associated with engineering options, transportand energy should be compared and discussed.



2 Introduction


2.2.3 This report identifies and refines possible main tunnel drive options for the Abbey Mills route and considers the overall location and grouping of theshortlisted sites. The information in this report will be used in subsequentworkshops to identify the proposed sites and drive strategy.

2.2.4 The findings of this Engineering options report – Abbey Mills route (Spring

2012) will help to inform subsequent stages of the selection process for the proposed scheme that will be presented in the Section 48: Report onsite selection process.

2.2.5 This Engineering options report – Abbey Mills route (Spring 2012) covers:

a. Section 3: System design and engineering requirements. This sectionsets out a high-level description of the system, geological, tunnellingand CSO engineering requirements to be considered in thedevelopment of engineering options, and the subsequent selection of a proposed drive strategy and the associated list of proposed maintunnel sites for the Abbey Mills route. This section states and broadlysummarises the requirements without providing an in-depth justification for the system and engineering requirements.

b. Section 4: Main tunnel drive options and Section 5: Connection tunneldrive options: These sections summarise the tunnel optionsconsidered and the analysis and refinement of these options. Theanalysis also considers the relationship of the tunnel options to theavailable groups of shortlisted sites.

2.2.6 This report only considers the development of options from an engineeringperspective. The suitability of each site is not discussed here and ispresented in the site suitability reports.

2.2.7 In considering the tunnel drive options this report does not identifyproposed tunnel routes or sites. The selection of the proposed tunnelalignment, CSO sites and main tunnel sites will be assessed at later stages in the process (selection of the proposed sites and proposedproject). These stages will be carried out by a multidisciplinary team andreported in the Section 48: Report on site selection process. Theconsiderations in this Engineering options report – Abbey Mills route(Spring 2012), along with site suitability reports, will feed into and informthese stages.

2.3 Engineering design development2.3.1 The project comprises a main tunnel that would run from west to east

London and be integrated into the existing sewerage system viaconnection tunnels and drop shafts in order to control 34 ‘unsatisfactory’ CSOs. These tunnels would store and transfer the intercepted flows toBeckton STW via the Lee Tunnel.

2.3.2 The Lee Tunnel project, which comprises a tunnel to connect Abbey MillsPumping Station to Beckton STW and works to control the Abbey MillsPumping Station CSO, has been consented and construction started in2010.



2 Introduction


2.3.3 The Thames Tunnel project’s site selection process recognises that theengineering design will need to proceed parallel to the site selectionprocess and that there is an iterative relationship between the two.

2.3.4 Design development activities have included:

a. engineering designs and studies of various components of the projectand identification of potentially feasible tunnel routes

b. ‘system master planning’ to define the sewage system operationchanges and facilities that would be needed to control and limitoverflows

c. logistics studies for construction, transportation and river navigation

d. field investigations, including ground investigations and surveys.

2.3.5 This Engineering options report – Abbey Mills route (Spring 2012) drawson the relevant aspects of these studies and investigations, as well as the

results of the site selection shortlisting process.



3 System design and engineering requirements



3.1 System design and engineering assumptions

3.1.1 The assumptions on which this report was based are identified and listedin an assumptions register in Appendix A, which can be found in theaccompanying document, Engineering options report – Abbey Mills route – Appendices (Spring 2012). These assumptions and other requirementsare discussed in the sections below.

3.2 Health and safety considerations

3.2.1 Health and safety is of paramount importance to Thames Water and theproject team.

3.2.2 Through risk assessment and management, the project team is working in

accordance with industry codes and project standards, with the aim of achieving world-class health and safety objectives.

3.2.3 The project has a plan and policies in place to ensure compliance with theConstruction (Design and Management) Regulations 2007 . According tothese statutory requirements, so far as is reasonably practicable, everydesigner shall avoid foreseeable risks to the health and safety of anyperson carrying out construction work; the designer shall eliminatehazards which may give rise to risks; and reduce risks from any remaininghazards.

3.2.4 In addition, the Health and Safety at Work etc Act 1974 places generalduties on employers to conduct their operations in such a way as toensure, so far as is reasonably practicable, that others (including thegeneral public) are not exposed to risks to their health or safety.

3.2.5 In assessing health and safety risks there is a contrast in magnitudebetween above-ground work, where risk can generally be controlledthrough proven good management practices, and underground work,where the remaining risks are higher as they are dictated by confinedspace, restricted heavy-lifting facilities, a mechanically controlledatmosphere, risk of inundation, distance for escape or rescue, andvariable ground conditions.

3.3 System requirements

3.3.1 The need and overarching requirements for the project are described inthe Needs Report . The Needs Report provides the legal and regulatorycontext and the need for a solution to meet the regulatory drivers.

3.3.2 The concept of the project is to:

a. control discharges from 34 CSOs

b. store CSO discharges

c. transfer CSO discharges for treatment.





3.3.3 The engineering requirements to be taken forward in assessing theengineering tunnel route and alignment options are summarised andbriefly discussed in the following sections. Design development is ongoingand therefore the implications of any future changes will need to be further assessed and reviewed.

3.3.4 This section of the report focuses on system requirements relevant to theselection of sites and tunnel alignments.

Developments in design requirements

3.3.5 Design developments have updated the requirements of the project suchthat now only 18 of the 34 CSOs require direct interception. The remainingCSOs are to be controlled by other measures.

3.3.6 Table 3.1 lists the control measures proposed for the 34 CSOs.

Table 3.1 CSO control measures

CSOref

CSO Method of overflow control

CS01X Acton Storm Relief Interception

CS02X Stamford Brook Storm Relief Control measures at other CSOs indirectly

control this CSO

CS03X North West Storm Relief Interception and pumping station operation

changes at Hammersmith Pumping Station

indirectly control this CSO

CS04X Hammersmith PumpingStation

Interception and pumping station operationchanges

CS05X West Putney Storm Relief Interception

CS06X Putney Bridge Interception

CS07A

CS07B

Frogmore Storm Relief – Bell

Lane Creek

Frogmore Storm Relief –

Buckhold Road

Interception

CS08A

CS08B

Jews Row Wandle Valley

Storm Relief

Jews Row Falconbrook Storm

Relief

Modifications already in place so CSO is

indirectly controlled**

CS09X Falconbrook Pumping Station Interception

CS10X Lots Road Pumping Station Interception

CS11X Church Street Controlled indirectly by sewer relief works at

other CSOs

CS12X Queen Street Controlled indirectly by sewer relief works at





CSO

ref CSO Method of overflow control

other CSOs

CS13A

CS13B

Smith Street – Main Line

Smith Street – Storm Relief

Controlled indirectly by sewer relief works at

other CSOs

CS14X Ranelagh Interception and additional sewer

connection relief*

CS15X Western Pumping Station Controlled indirectly by sewer relief works at

other CSOs

CS16X Heathwall Pumping Station Interception

CS17X South West Storm Relief Interception

CS18X Kings Scholars Pond Controlled indirectly by sewer relief works atother CSOs

CS19X Clapham Storm Relief Interception

CS20X Brixton Storm Relief Interception

CS21X Grosvenor Ditch Controlled indirectly by sewer relief works at

other CSOs

CS22X Regent Street Interception via additional sewer connection

relief*

CS23X Northumberland Street Controlled indirectly by sewer relief works at

other CSOs

CS24X Savoy Street Controlled indirectly by sewer relief works at

other CSOs

CS25X Norfolk Street Controlled indirectly by sewer relief works at

other CSOs

CS26X Essex Street Controlled indirectly by sewer relief works at

other CSOs

CS27X Fleet Main Interception and additional sewer

connection relief*

CS28X Shad Thames Pumping

Station

Pumping station modifications***

CS29X North East Storm Relief Interception

CS30X Holloway Storm Relief Local modifications***





CSO

ref CSO Method of overflow control

CS31X Earl Pumping Station Interception

CS32X Deptford Storm Relief Interception

CS33X Greenwich Pumping Station Interception and pumping station operation

changes

CS34X Charlton Storm Relief Pumping station operation changes at

Greenwich Pumping Station and

improvements at Crossness STW

* The additional sewer connection relief at Ranelagh, Regent Street and Fleet Main CSOs

would be connections to the northern Low Level Sewer No.1.

** This CSO was planned to be controlled via interception at phase one consultation stage

and does not require a worksite.*** These CSOs were planned to be controlled via interception at phase one consultation

stage; they are now planned to be controlled by other methods and require a worksite.

3.3.7 Further elements that the project should provide as a minimum are listedbelow:

a. The westerly start point of the project should connect to the ActonStorm Relief CSO.

b. The easterly end point of the tunnel should connect to the Lee Tunnelat Abbey Mills Pumping Station (PS) (only for the Abbey Mills route).

c. Relieving flows in the northern Low Level Sewer No.1 at theRanelagh, Regent Street and Fleet Main CSO sites should givesufficient control to reduce local CSO spills so that direct interceptionis no longer required for the Northumberland Street, Church Street,Smith Street, Kings Scholars Pond, Grosvenor Ditch, Savoy Street,Norfolk Street and Essex Street sewers.

d. The system should ensure the health and safety of operatives, thepublic and third parties. This means providing, during both theconstruction and operational phases, a hydraulically safe and robustsystem with no risk of flooding or adverse transient conditions; secureand resilient facilities; appropriate levels of ventilation and air treatment; and safe methods and facilities for access and egress intoand out of the main and connection tunnels.

Main tunnel routes

3.3.8 Design development has identified three possible tunnel routes: the River Thames route, the Rotherhithe route and the Abbey Mills route.

3.3.9 The River Thames route largely follows the route of the River Thames, theRotherhithe route alignment cuts across the Rotherhithe Peninsula, andthe Abbey Mills route connects to the Lee Tunnel at Abbey Mills. The latter became feasible when the depth of the Lee Tunnel shaft at the AbbeyMills PS end was increased to avoid difficult geological conditions on the





Lee Tunnel route. This would enable a continuous gradient for the maintunnel and satisfy the design constraints for the overall vertical tunnelalignment and system hydraulic requirements.

3.3.10 The three routes were consulted on at phase one consultation and the Abbey Mills route was presented as the preferred route. Analysis of the

consultation feedback received concluded that the Abbey Mills routeremained the preferred route for phase two consultation. Followingconsideration of the consultation feedback, only the Abbey Mills route isevaluated further in this report as the ‘proposed route’.

3.3.11 The three routes are illustrated in Figure 3.1.





12

Figure 3.1 Main tunnel routes considered





13

Control and interception of CSO flows

3.3.12 The CSOs to be controlled by interception are listed in Table 3.1.

3.3.13 The interception and connection of CSO flows to the main tunnel typicallycomprises four major elements: a CSO interception chamber, a

connection culvert, a drop shaft, and a connection tunnel, as illustratedbelow in Figure 3.2. A description of the construction elements is providedin the Site selection background technical paper and discussed further inSection 3.6.

Figure 3.2 Typical CSO interception arrangement

Tunnel hydraulic requirements

3.3.14 The main purpose of the tunnel system is to store and transfer CSO flowsin order to reduce CSO discharges. The background for the size of thetunnels is provided in paragraph 3.5.3.

3.3.15 The tunnel system must be self-cleansing; therefore the velocity of flowmust be high enough to move detritus without requiring further flushing.It has been determined that a gradient in excess of approximately one in850 generates flow velocities that exceed 1m/s during event cycles andinternational experience has shown that this is sufficient to meet theproject’s objective.

3.3.16 The gradient of the connection tunnels is generally between one in 400

and one in 500 in order to achieve flow capacities while not exceedingmaximum peak velocities.





14

3.3.17 Large tunnel systems can be prone to hydraulic pressure effects due tothe generation of transient (temporary surge flow) conditions. Controlfeatures need to be incorporated into the tunnel design and mode of operation in order to manage these transient pressure effects.

System functional and operational requirements3.3.18 In order to ensure safe operation, access to, and inspection and

maintenance of the tunnel, design development is based on the followingcriteria and features:

a. The main tunnel and connection tunnels must be designed asgenerally ‘maintenance free’ and have a design life of 120 years.Tunnel entry for inspection and maintenance is only planned to takeplace approximately every ten years.

b. The ten-year inspection would be a major undertaking in its own right,which would involve extensive planning and provision of temporary

works to permit entry.

c. The designated access points to the tunnel system would be maintunnel shafts and CSO drop shafts that connect to the main tunneldirectly. During construction, extensive temporary facilities areprovided for safety purposes. Once after temporary facilities havebeen removed the long term maintenance requirements control thesafe spacing of the main tunnel shafts. These shafts would allow theinsertion and removal of specialist inspection and maintenancevehicles during the ten-yearly inspection of the tunnel. At this stage of design development, it is assumed that the spacing between the

permanent access points should not exceed 9km. Long, largediameter connection tunnels would have similar access provisions.

d. The main tunnel shafts and on-line CSO drop shafts would beprovided with large access openings to permit inspection plant to belowered into/removed from the tunnel and provide emergencyaccess/egress. CSO and main tunnel sites must be selected to ensuresufficient space for two mobile cranes to service the shafts.

e. Permanent air management facilities would be provided, includingventilation and monitoring of exhaust air quality, along with air treatment facilities (odour control).

f. Control gates would be provided to isolate the tunnel system andprevent flows from entering. These gates would be controlled from acentral control room to provide an overview of the system from asingle point. The gates would also be used to isolate the tunnel fromin-flows during maintenance inspections, which are currentlyenvisaged to take place every ten years.

g. The tunnel’s operating regime would be integrated with the operatingregimes at pumping stations, particularly Abbey Mills PS, GreenwichPS, Beckton STW, and Crossness STW.

h. Fixed ladders and access ways would not be provided to the bottom of shafts or the main tunnel due to the potential for corrosion and thelikelihood of damage during surge events, which has occurred on





15

similar projects. Specific arrangements would be developed for safeaccess to carry out inspection and maintenance of the CSO dropshafts, connection tunnels and the main tunnel. Fixed ladder accesswould be provided to subsurface MEICA equipment, odour controlequipment and other equipment that requires routine inspection and

maintenance.3.3.19 When considering the spacing of main tunnel shafts for the completed

system, and based on the experience of other major CSO systems, it isassumed that maintenance and inspection teams would travel through themain tunnel by means of an inspection vehicle supported by a back-upstandby vehicle. This would reduce the transit time and permit a wider range of equipment to be carried with relative ease. It would also facilitateaccess to the internal circumference of the tunnel for inspection purposes.Vehicular access is practicable in the tunnel system, given the diameter of the main tunnel and the fact that the system would be dry when

inspections are carried out as all penstocks that control the flow into thesystem would be securely locked off.

3.3.20 Access to the connection tunnels would also be required duringinspection. The length of the connection tunnels is highly variabledepending on location, and varies from 16m to 4,600m. Provision for emergency egress would be made at the drop shafts by means of suitableaccess openings and space for cranes to operate a man-rider. Theconnection tunnel to Greenwich PS would be inspected using a similar inspection vehicle to that used for the main tunnel.

3.4 Engineering geology

Route geology

3.4.1 The route geology has been established using the British GeologicalSurvey (BGS) ‘Lithoframe50’ model, from which geological long sectionshave been prepared. This has been supplemented by project-specific siteinvestigations, including a seismic refraction survey, ground and over-water boreholes, and field and laboratory testing, as well as the installationof piezometers to establish water levels.

3.4.2 The geological long section for the Abbey Mills main tunnel route, derivedfrom the Lithoframe50 model, is provided in Appendix D of the

Engineering options report –

Abbey Mills route –

Appendices (Spring2012).

3.4.3 The basic geological descriptions within the London Basin geologicalsequence are provided in Table 3.2.





16

Table 3.2 Geology of the London Basin

Era Group FormationBrief description of

formation

Approximaterange of

thickness

(m)Recent

AlluviumSoft clays, silts, sands andgravels. May contain peat.

0 – 5

FloodplainTerrace

Medium to dense sand, flintand chert gravel, occasionalcobbles and boulders.

0 – 10KemptonParkTerrace

Tertiary Thames

LondonClay

Very stiff, fissured, silty,

locally fine to medium sandyclay.

>100

Harwich

Swanscombe member:

Sandy clay to clayey sand(< 2m) with some fine tomedium black roundedgravel.

Blackheath member:

Dense to very dense flintgravel (with occasionalcobbles) in silty or clayey,glauconitic, fine to mediumsand matrix.

Oldhaven member:

Very dense clayey sand withgravel and shells – oftencemented as limestone.

0 – 10

LambethGroup

Woolwich

Stiff, dark grey to black claywith locally abundant shell

debris and strong limestonebeds (100 to 200mm thick).

10 – 20

Reading

Very stiff to hard, multi-coloured (light blue-greymottled red, orange, brownand purple), locally sandyclay.

Upnor Gravel, glauconitic andorganic sand, silt and clay.

5 – 7





17

Era Group FormationBrief description of

formation

Approximaterange of

thickness(m)

Thanet Sand Formation(including Bullhead Bedat base <~0.5m)

Very dense silty to very siltysand. The lowest ~0.5msometimes consists of fine,medium and coarse, angular flint gravel.

10 – 15

Cretaceous ChalkSeaford*

Homogeneous chalk withnodular flint horizons(>100mm thick).

circa 40

Lewes*Heterogeneous nodular chalk with nodular flint

horizons and marl seams.

circa 50

NB: *Limited to those formations of the ‘White Chalk’ subgroup expected within the ThamesTunnel project. (Upper and Middle Chalk are now known collectively as ‘White Chalk’.)

3.4.4 The distribution of strata along the route is largely controlled by theLondon Basin Syncline, which plunges gently eastwards. Thus, beneath acover of made ground and recent deposits, the succession of tertiarydeposits is gradually exposed west to east along the river until Chalkoccurs at an outcrop around Greenwich.

3.4.5 The anticipated geology at the proposed main tunnel invert is as follows:

a. London Clay Formation: from the western end of the tunnel to justwest of Albert Bridge (Harwich at the base approximately betweenCremorne Wharf and Albert Bridge).

b. Lambeth Group: starts to enter the tunnel invert just east of AlbertBridge, forming the lower third of the face by Chelsea Bridge, and thefull-face by Tideway Walk. The tunnel continues in full-face LambethGroup to just east of London Bridge.

c. Thanet Sand Formation: within the invert and the lower third of theface between Blackfriars Bridge and London Bridge, becoming full-face from just east of London Bridge to just west of Tower Bridge.

d. White Chalk subgroup: downstream from just east of Tower Bridge.

3.4.6 Faulting at London Bridge is expected to repeat the sequence, and mixed-face conditions in the Lambeth Group and Thanet Sand Formation areexpected from Chelsea Bridge through to Tower Bridge, with only a shortsection wholly in the Thanet Sand Formation close to Tower Bridge.

3.4.7 Various structural geological models provide different interpretations of thestructural setting across the London Basin, but they all generally indicateregular faulted block groundmass in Chalk and northwest by southeasttrending faults that cut the basic east –west main synclinal form.





18

3.4.8 The dominant structural geological features are:

a. the Hammersmith Reach Fault Zone, a series of north-northwest –south-southeast trending faults beneath and adjacent to the east sideof Hammersmith Bridge. A 5m displacement to the east is noted.

b. the Putney Bridge Fault, a series of southeast – northwest trendingfaults on the syncline with the axis to the west of Putney Bridge, withvertical displacement on top of Lambeth Group strata on the easternhanging wall of approximately 2m.

c. the Chelsea Embankment (Albert Bridge) Fault Zone, a series of north – south and south-southwest – north-northeast trending faultsbetween Battersea and Chelsea bridges, which would intersect thetunnel alignment at near to perpendicular. Up to 5m verticaldisplacement of strata has been noted over this zone, resulting in upliftof the top of Lambeth Group deposits on the east side of AlbertBridge.

d. the Lambeth Anticline, a north-northwest – south-southeast trendingfaulted anticline between Vauxhall and Lambeth Bridges thatintersects the tunnel alignment at an oblique angle with a difference instrata level of approximately 5m.

e. the London Bridge Fault Graben, a southeast – northwest trendinggraben-type feature arranged between Cannon Street and Tower Bridge, with known vertical displacement in excess of 10m.

f. the Greenwich Fault Zone, a southwest – northeast trending feature,which was investigated in detail during the Lee Tunnel project ground

investigation in 2008. Up to 20m downthrow is anticipated to thenorthwest in a series of stepped faults. The fault runs generally parallelto the main syncline, southwest – northeast from Greenwich toBeckton, crossing the River Thames downstream of the ThamesBarrier, and is in close proximity to Greenwich PS.

3.4.9 Other structural features include the North Greenwich Syncline (now moregenerally known as the Plaistow Graben), Millwall Anticline and Beckton Anticline, all of which trend northeast – southwest, contrary to the mainbasin axis.

3.4.10 There is a risk of scour hollows located on previous drainage channels

formed by the River Thames, which are often found at the confluence of the existing tributaries, eg, at the River Fleet, River Lee and River Wandle.The features usually contain a variety of granular deposits and/or disturbed natural materials and are localised and steep-sided.

3.4.11 The scour hollow in the vicinity of the Blackwall Tunnel is the only scour hollow known to penetrate into Chalk. Elsewhere, the hollows only affectthe tertiary deposits and, more particularly, London Clay. Basal depths arenormally 5m to 20m below ground level, with the exception of 33m atBattersea Power Station and Hungerford Bridge.

3.4.12 Of the known scour hollows, only the hollow at Hungerford Bridge is close

to the main tunnel. This feature attains a base level of 72mATD in LondonClay near the south bank, equivalent to only 10m above the tunnel crown.





19

Such features may, however, have greater implications for the shallower connection tunnels in other locations.

3.4.13 The presence of flints within the Chalk may cause severe wear to theTBM, which would require frequent and hazardous interventions for inspection and maintenance of the TBM cutterhead. Therefore, an

important part of the project’s ground investigations is to investigate theChalk structure, permeability and characteristics of flint-related features.

3.4.14 A number of flint bands are present within the Chalk. Within the SeafordChalk, two well-defined flint bands that are used as marker horizons (notnecessarily the thickest seams) are the Bedwells Columnar and SevenSisters. The Bedwells typically comprise a discontinuous layer of verylarge, irregular flints, up to approximately 500mm high by 300mm indiameter. Previous projects have found that they have a compressivestrength of 600mPa. The Seven Sisters is a continuous band, with flintsbetween 100mm and 150mm thick.

3.4.15 The selection of the appropriate TBM is important in this respect and aslurry TBM is preferred for the section of the route in Chalk. A slurry TBMwas used successfully on the Channel Tunnel Rail Link Thames crossingnext to the QE2 Bridge and, most recently, the same type was procured bythe contractor for the Lee Tunnel project. The advantages of this type of TBM include the ability to deal with water-bearing fissures in Chalk and toconvey flint pieces in a fluid slurry, as opposed to a potentially damagingabrasive paste from an earth pressure balance (EPB) TBM. The need for hazardous interventions would be reduced by selecting a slurry TBM for Chalk, however slurry TBMs are not appropriate for use in clay of the type

expected on other sections of the tunnel alignment (seeparagraph 3.5.20).

Hydrogeology

3.4.16 The major aquifer of the London Basin lies in the Chalk. It is whollyunconfined to the east but confined to the west below the tertiary strataand the London Clay Formation in particular. The Chalk aquifer isgenerally in hydraulic continuity with the overlying Thanet Sand Formationand sometimes the base of the Lambeth Group, particularly the gravel partof the Lower Mottled Beds and the Upnor Formation. The Environment Agency (EA) refers to this combined aquifer as the Chalk-Basal Sands

aquifer.

3.4.17 Local aquicludes can exist in the overlying Lambeth Group, in particular the Woolwich Formation Laminated Beds, leading to perched groundwater tables. Historical records of engineering schemes have stated that these‘perched’ features retain hydrostatic pressures of up to 40m, which mayresult in high inflows at tunnel level and particularly into shafts duringconstruction.

3.4.18 The Harwich Formation (Blackheath Member) is also known to containhigh groundwater levels in places, which cause problems during tunnelconstruction.





20

3.4.19 A minor regional aquifer lies within the floodplain and river terracedeposits; due to the connection to the River Thames, it is generally tidal,with an average level of 100mATD (0mAOD) +/- 2.5m.

3.4.20 Regional monitoring of the Chalk aquifer is reported by the EA and specificmonitoring data is available over the years 2000 to 2008. Data indicates a

depressed groundwater table in central London at 60mATD withgroundwater levels close to Blackfriars Bridge at 62mATD (refer to thegroundwater level contour plan of the London Basin in Appendix D).However, the latest ground investigations undertaken by the projectindicate that groundwater levels in the Chalk from Rotherhithe to Charltonare 10m higher than the reported EA levels.

3.4.21 Groundwater pressure in the Chalk would have an important bearing ontunnelling, especially the construction of junctions between the maintunnel and the connection tunnels. Table 3.3 shows the 2008 levels in theChalk aquifer eastwards from Tower Bridge, using the data obtained from

the EA.

Table 3.3 Chalk aquifer groundwater levels in 2008 and imposedpressure at tunnel invert (east of Shad)

Tunnel sectionTower Bridge

NESRAbbeyMills

Approx tunnel invert mATD 50 45 40

Approx GWT level 2008 mATD 72 78 92

Approx GWT pressure bar 2.5 3.5 4.0

NB: * Highest levels indicated in Lee Tunnel project and Thames Tunnel projectmonitoring holes.

3.4.22 Short-term effects of pumping can still have a demonstrable impact onregional equipotentials. For example, levels decreased significantly due toabstractions in supply wells at Battersea/Brixton that commenced in 2002.The groundwater level was drawn down approximately 18m local to thewells, 10m in central London near the River Fleet, and 6m in the vicinity of Tower Bridge and the Battersea Power Station area.

3.4.23 The EA reports that the groundwater that feeds the Chalk aquifer from thesoutheast interacts with the River Thames from Greenwich to Woolwich asit flows northwest to Stratford, then west to central London. In theGreenwich to Woolwich area, there is evidence of saline intrusion withinthe aquifer.

3.5 Tunnel engineering and construction requirements

Risk management considerations

3.5.1 In addition to the health and safety requirements described in Section 3.2, there is a requirement that, in order to make the project insurable, project

risk must be managed in accordance with the British Tunnelling Societyand the Association of British Insurers’ Joint Code of Practice for Risk





21

Management of Tunnel Works in the UK . The objective of the code is topromote and secure best practice to minimise and manage risksassociated with tunnelling works and to set out best practices to beadopted. At the core of the code is an obligation for owners, designers andcontractors to have processes in place to identify and manage risks

throughout the life of the project.3.5.2 The project has a risk management plan and procedures in place to

manage and control risks and comply with the requirements of the Joint Code of Practice for Risk Management of Tunnel Works in the UK . Refer also to Health and safety engineering risk considerations in Section 4.3.

General tunnel considerations

Tunnel diameters

3.5.3 Tunnels should be sized to suit the hydraulic performance of the systemand the required storage capacity. The majority of the main tunnel needs

to be 7.2m internal diameter, but the western most tunnel drive sectionmay be smaller depending on the drive length as follows:

a. between a site in the Acton area and a site in the Barn Elms area, theinternal diameter would be 6m

b. between a site in the Acton area and the Wandsworth Bridge area, theinternal diameter would be 6.5m.

3.5.4 CSOs would be connected to the main tunnel via interceptionchambers/drop shafts and connection tunnels. These tunnels should besized to carry the design flows from the CSOs at a gradient that would limit

maximum flow velocities to 5m/s in order to ensure hydraulic stability andlimit scour potential. The size of the connection tunnels would vary from2.2m to 5m internal diameter depending on the flow. The minimum tunnelsize for safe man access is assumed to be 2.2m internal diameter.

Vertical tunnel alignments

3.5.5 The overriding factors that control the tunnel slope and elevation (verticaltunnel alignment) are:

a. hydraulic functional performance

b. constraints imposed by existing and proposed third-party infrastructure

c. tie-in connection level to the Lee Tunnel at Abbey Mills PS in order tomaintain gravity flow throughout (for the Abbey Mills Route).

3.5.6 The main third-party constraints for the main tunnel include:

a. the Thames Water Ring Main Barnes to Barrow

b. the Thames Water Lee Valley Water Tunnel near HammersmithBridge

c. the proposed National Grid ‘Wimbledon to Kensal Green’ cable tunnel.

3.5.7 The vertical distance that separates the Lee Valley Raw Water Tunnel andthe main tunnel crossing above is approximately 3m. Other existing deep-level service tunnels, including National Grid’s Richmond to Fulham highpressure pipeline tunnel and a number of BT Openreach tunnels, also





22

present constraints on the alignment. In addition, the planned NationalGrid ‘Wimbledon to Kensal Green’ tunnel would require co-ordination toensure that the projects remain compatible. The distance between thetunnel and other existing third-party underground tunnels not listed aboveis less critical to the vertical tunnel alignment.

3.5.8 The potential connection tunnel to connect the Earl PS, Deptford StormRelief and Greenwich PS CSOs to the main tunnel would be restrictedvertically by the Jubilee Underground line (which crosses the RotherhithePeninsula); the proposed UKPN cable tunnel from New Cross to WellcloseSquare Scheme; and the proposed National Grid Hurst to New Crosscable tunnel.

3.5.9 The alignment would be designed to minimise the overall impact on third-party structures. A programme of work is underway to quantify the impactson third-party infrastructure including bridges, tunnels, buildings andutilities.

Horizontal tunnel alignments

3.5.10 Drive options between a number of geographic site zones are identifiedand compared in Section 4 of this report. Once the individual site optionshave been considered and assessed as part of the Final report on siteselection process, the detailed tunnel alignment will be decided. Thealignment must satisfy the hydraulic flow regime requirements as set out inparagraphs 3.3.14 to 3.3.17.

3.5.11 The minimum preferred horizontal radius for the main tunnel is 600m for practicable construction purposes; however this could be reduced to 500mwhen constrained. Smaller diameter, segmentally lined connection tunnelstypically have a minimum radius of 300m, although techniques can beemployed to achieve smaller radii.

3.5.12 In order to minimise the effect of tunnelling on third-party infrastructure,the tunnel should, as far as practicable:

a. pass under the centre of the mid-deck span of bridges to maximiseclearance to the bridge foundations

b. avoid interfaces with sensitive existing structures, such as the originalThames Tunnel (Brunel’s ‘Thames Tunnel’, which now carries theOverground railway line) and the Rotherhithe road tunnel

c. avoid passing beneath tall buildings on deep piles

d. maximise clearance to third-party infrastructure.

3.5.13 The alignment of the CSO connection tunnels would generally be basedon the location of the main tunnel and main tunnel shafts, along withhydraulic considerations. In many cases the risks of setting up TBMs for the very short connection tunnels would offset the benefits in terms of ground control. Where connection tunnels are unlikely to be machinedriven and ground conditions are expected to be poor, tunnel lengthshould be minimised as far as practicable.





23

Tunnel lining

3.5.14 The lining of the main tunnel is assumed to comprise a primary andsecondary lining system. The primary lining installed during the excavationcycle would consist of a ring of tapered, reinforced concrete segments,approximately 350mm thick. The secondary lining would be 300mm-thick,

cast-in-situ reinforced concrete in order to provide the required internaldiameter for the finished tunnel. For the purposes of this report, theconnection tunnels are also assumed to have a secondary lining.

Shaft sizes

3.5.15 The main tunnel drive shafts are anticipated to have an internal diameter between 25m and 30m, with depths ranging from approximately 30m inwest London to 65m in east London. Shafts of 25m diameter areconsidered the minimum size required to launch a TBM and accommodateall the equipment required for the safe construction of the tunnel. Shafts of 30m diameter may be required to accommodate multiple hydraulic dropstructures or for use as double drive shafts.

3.5.16 The intermediate shafts and reception shafts for the main tunnel areassumed to have an internal diameter of between 15m and 25m.

3.5.17 The internal diameter of CSO shafts ranges from 6m to 24m to suit thehydraulic requirements, although at some locations it may beadvantageous to connect the CSO connection culvert directly to a maintunnel shaft.

Tunnelling and shaft construction methods

Tunnelling construction methods3.5.18 In order to construct the project within the required timeframe (paragraph

4.3.33), sections of the tunnel would need to be built concurrently. Inaddition, management of construction risk and the suitability of TBM typesfor the varying ground conditions along the route would also affect thedetermination of the number of TBMs to be used.

3.5.19 The geology and hydrogeology along each tunnel alignment influence theselection of the TBM type. Full-face TBMs would be required to supportthe ground during tunnelling in order to prevent excess excavation andgroundwater inflow, and to minimise ground movement. Full-face TBMs

are designed to incorporate a pressure bulkhead that separates the face – that is the unexcavated ground – from the completed segmentally-linedtunnel.

3.5.20 Full-face TBMs can be either earth pressure balance machines (EPBTBM) or slurry TBMs. Convertible TBMs, which have been used in other projects, can operate as either EPB or slurry TBMs; however, this is anecessary compromise that results in the need for additional plant andequipment. Convertible TBMs introduce additional risks and impact on theprogramme because there is a need to allow for changes to theoperational method. They also perform sub-optimally in both modes. For the purposes of this report, it has been assumed that specific machineswould be used according to the predicted ground conditions. It is assumedthat EPB-type TBMs would be used for the tunnel drives through London





24

Clay and the Lambeth Group west of Tower Bridge and a slurry-type TBMfor the eastern drives through Chalk, as described in paragraph 3.4.15.

Shaft construction methods

3.5.21 The geology, hydrogeology, depth and size of shafts would influence the

method of construction. Various methods can be used, including:a. precast concrete segmental-lined caisson or underpinned construction

b. sprayed concrete lining

c. reinforced concrete sunk caisson

d. secant piled wall

e. diaphragm wall.

3.5.22 The construction of shafts in the London Clay is likely to be byconventional methods, with segmental lining either underpinned or sunk

as a caisson. Sprayed concrete linings may also be used in conjunctionwith sheet piles for support of any groundwater-bearing superficialdeposits.

3.5.23 Where the shafts are very deep and constructed through mixed groundconditions under high groundwater pressures, diaphragm wall constructionis the most likely method of construction due to greater vertical accuracy. A secant piled wall method could also be used. In general, the diaphragmwall type of shaft construction requires a larger working area than other methods. A diaphragm wall shaft is a reinforced concrete lined shaft thatcomprises individually installed, abutting vertical concrete wall panels,which are constructed in the ground using specialist plant prior to the

excavation of the ground within the centre of the shaft.

Ground treatment and control of groundwater

3.5.24 For all methods of shaft construction, groundwater would need to becontrolled to enable safe excavation and sinking of the shaft and toconstruct the base slab.

3.5.25 In some locations, ground treatment may be required to improve thenatural state of the ground in advance of shaft construction or tunnelling.The term ‘ground treatment’ covers a variety of techniques to strengthenor stabilise the ground, including:

a. injection of chemical or cementitious grouts (depending on the groundencountered) to form blocks that can be excavated without collapsing

b. ground freezing, where injection pipes circulate brine or liquid nitrogento freeze the groundwater and produce a stable block that can beexcavated, but this is costly and takes a long time to implement.

c. compressed air, where the air pressure at the face of the tunnel isincreased using air locks and compressors to resist the inflow of groundwater and maintain face stability. Over the years this techniquehas been replaced by closed-face tunnelling machines due to severehealth and safety implications such as bone necrosis. The 8.8m-highface of the main tunnel and the potentially high compressed air pressures required to resist groundwater pressures (5.5bar) make it





25

unlikely to be appropriate except for TBM face interventions. The 7.2minternal diameter main tunnel has an approximate external diameter of 8.8m, based on a 350mm primary lining thickness, a 300mmsecondary lining thickness, and an assumed 150mm overcut.

d. de-watering to control the inflow of groundwater into shafts and tunnel

excavations to ensure excavation stability. This can take the form of either regional (widespread) or localised de-watering methods,depending on the purpose and the extent of the pressure reductionrequired. These methods include deep borehole wells or localiseddrains, well points and injector wells.

Main tunnel site requirements

Main tunnel sites

3.5.26 Three types of main tunnel site may be needed to construct the maintunnel: drive sites, reception sites and intermediate sites.

3.5.27 The main tunnel would be driven from main tunnel drive shafts, whichwould be equipped to enable the efficient excavation and construction of the tunnel.

3.5.28 Reception shafts would be used to remove the TBM from the tunnel at theend of a drive. Where a site is a sufficient size, a shaft could be used for both drive and reception purposes.

3.5.29 Intermediate shafts could be used to gain access to the main tunnel boreduring construction, either to inspect and/or maintain the TBM or toprovide access for secondary lining construction (should a secondary

lining be required).Location of sites

3.5.30 The required number and spacing of sites for tunnel construction would besubject to the following considerations:

a. the project construction period

b. the TBM types, which must be appropriate to the predicted geologicalconditions

c. the risk of TBM breakdowns/servicing requirements and their severityand frequency, which increase with the length of the drive

d. the emergency egress for the construction workforce, which wouldbecome more difficult the longer the length of the drive.

3.5.31 The number of TBMs, and hence the number of associated drive sites,would depend on balancing the appropriate type of TBM for the groundconditions, the available main drive site locations , geology, programme,environment, amenity, health and safety, risk, cost, and procurementconsiderations.

3.5.32 Where possible, CSO connection tunnels would be constructed from maintunnel sites in order to reduce the space required at the CSO sites. Where

CSO connection tunnels are driven from main tunnel sites, the CSO sites





26

would comprise smaller reception sites. Excavated material from the CSOconnection tunnels could also be handled at the main tunnel sites.

Main tunnel site requirements

3.5.33 The Site selection background technical paper (Summer 2011) provides

information on construction activities at main tunnel sites and their sizerequirements. The sizes are summarised as follows:

a. main tunnel drive sites from which slurry TBMs would be drivenrequire approximately 20,000m2, whereas sites that drive EPB TBMswould require approximately 18,000m

2. If site space is constrained, it

may be possible for an EPB TBM to be driven from a 15,000m2 site;however, this may reduce the efficiency of tunnel operations andincrease the risk of delays.

b. main tunnel reception or intermediate sites would range from 5,000m2

for sites with shafts constructed in the London Clay to 7,500m2, if deep

diaphragm walling is proposed.3.5.34 The construction activities that follow tunnel excavation would be less

onerous with respect to site spatial requirements. Activities would includesecondary tunnel lining (if required), shaft lining, buildings and surfaceworks, and mechanical and electrical fit-out works.

Construction logistics

3.5.35 For the purposes of this Engineering options report – Abbey Mills route(Spring 2012), the following logistical needs have been considered:

a. the ability to provide efficient site layouts

b. logistics hubs

c. critical services (power)

d. transport of materials and equipment

e. main tunnel segment fabrication and supply.

Site layouts for logistics

3.5.36 The layout of individual sites for logistics purposes would depend on thespecific site use and local constraints. The Site selection background technical paper (Summer 2011) indicates illustrative layouts for different

types of site.

Logistics hubs

3.5.37 The supply and servicing of the smaller CSO sites could be carried out assatellites to the main tunnel sites. Main tunnel sites might therefore requirean allowance for a logistics hub area for facilities to service the satellitesites. This has not been taken into account at this stage of the project andwould likely be the contractor ’s responsibility.

Critical services: power

3.5.38 The temporary power supply requirements for construction sites typically

varies from 1.25MVA to 3.5MVA for the smaller CSO sites, and up to





27

12.5MVA to 17.5MVA for large main tunnel drive sites that serve a singleTBM and 25MVA for a double drive site.

3.5.39 The number and potential spacing of sites for main tunnel drives is suchthat, for the majority of areas, it is unlikely that current capacity would besufficient or available from existing UK Power Networks when construction

commences. Therefore, power supply improvement works would berequired at main tunnel drive sites and should be planned toaccommodate new substation installations.

3.5.40 Discussions with UK Power Networks have established that it would beprudent to plan for the early procurement of power supplies for maintunnel drive sites to ensure that sufficient supply would be available for theTBMs in order to meet the project programme.

Transport of materials and equipment

3.5.41 Construction of the shafts and tunnel works would require a wide variety of

materials and equipment to be transported to and from the working sites.3.5.42 Excavated material from the main tunnel would need to be taken away

from the drive sites and a variety of materials delivered, in particular theconcrete segments for the main tunnel lining. Other logistical activitieswould include workforce arrival/departure, equipment deliveries/return,consumables and, for drive sites, delivery of the large TBM components.

3.5.43 Due to the large volume of materials to be transported, the objective is touse the river to transport main tunnel excavated material by barge and toenable the contractor to move other materials by river where practicableand cost-effective.

3.5.44 The practicality of rail freight transportation depends on the proximity of main tunnel sites to suitable rail sidings and the local network’s capacityfor freight movements.

3.5.45 It is anticipated that some deliveries would also need to be transported byroad even where barge and/or rail transport facilities are available. Anynecessary highway routes would be identified as part of projectdevelopment. Major deliveries/removals would be subject to specificmovement restrictions and conditions imposed by police and trafficauthorities.

Main tunnel segment fabrication and supply

3.5.46 The supply of tunnel lining segments to individual drive site locationswould depend on the final site location and the location of the fabricationfacility or facilities. This has not been taken into account at this stage of the project and it would be the contractor ’s responsibility to fabricate andsupply segments.

Handling and disposal of excavated material

Material type and handling

3.5.47 The main types of excavated material would be London Clay, Lambeth

Group, Thanet Sand Formation and Chalk.





28

3.5.48 The type of material and choice of TBM would dictate the materialhandling and treatment requirements. The consistency of the excavatedmaterial would vary from relatively firm London Clay paste to Chalk slurry.

3.5.49 For site planning purposes, allowance has been made for onsite storageof excavated material for five days’ production. This would allow for

resolution of issues relating to maintenance, plant breakdown and bargeoperations on the River Thames. Where there are site space constraintsbut good transport links, it may be possible to reduce the allowance tothree days’ storage at the risk of delays should this prove insufficient.

Quantities and programme requirements

3.5.50 The total quantity of excavated material for all tunnels and shafts isanticipated to be in the region of 1.7million m3 (in situ quantity). This wouldvary, depending on the tunnel alignment and connections.

3.5.51 The in situ volume of (unbulked) excavated material arising per drive at

main tunnel drive sites would be approximately 300,000m3

to 500,000m3

,assuming a tunnel length of between 5km to 8km.

3.5.52 Where two drives are carried out from the same site location, the requiredstorage capacity would increase if the drives are to be carried outsimultaneously.

3.5.53 Tunnelling advance rates would dictate the requirements for materialremoval. For preliminary planning purposes, a rate of approximately1,000m3 to 2,000m3 is assumed per day from a site, depending on TBMtype and ground conditions.

Marine transport

3.5.54 The feasibility and use of marine transport for the removal of excavatedmaterial from potential main tunnel drive sites along the river is dependenton location.

3.5.55 Operations in the upper reaches of the River Thames beyondHammersmith Bridge are considered impractical due to restrictions of bridge height, tidal range, and the width of the navigable channel. Thesefactors would impose constraints on barge movements that wouldsubstantially reduce the quantity of material and rate of removal, makingthe viability of exclusive marine transport in these areas unacceptable.

3.5.56 Operations between Putney Bridge and Hammersmith Bridge areconsidered challenging, especially when servicing peak tunnelling rates.Sites along this length of the River Thames could be accessed andserviced, however it would require careful planning to mitigate theproblems associated with navigational constraints.

3.5.57 Downstream of Putney Bridge there are fewer navigational constraints andtherefore it would be possible to reduce the number of barge movementsby using larger barges on the lower reaches of the River Thames to theeast. Hence, only 350t barges could be used around Putney Bridge,1,000t barges in the vicinity of Battersea Power Station, and 1,500t barges

or larger downstream of Tower Bridge.





29

3.5.58 The Abbey Mills Pumping Station site is located on the River Lee, adjacentto the Three Mills Lock. At this location, the river is tidal and only navigablefor about four hours on each tide. Downstream of the site, the river isnarrow and constrained by physical features, including low bridges. Although not impossible, using the river to transport the materials required

to service a main tunnel drive would introduce cost and programme risksthat would need to be carefully investigated before taking the final decisionto use a site to drive the main tunnel. For the purposes of this optionsreport, a drive from Abbey Mills is included as a feasible option to beevaluated against other options during subsequent stages of siteselection.

In-river facilities

3.5.59 Jetty/wharf structures and their location with respect to the navigationalchannel, together with associated dredging of the river for accesspurposes, would be site specific. Each main tunnel drive site with no

substantial jetty or deep water wharf facilities would likely require abespoke solution with specific consents from the Port of London Authorityand the EA.

3.5.60 The above issues in respect of in-river facilities are more onerous on theupper reaches of the river. Consequently, upstream of HammersmithBridge – and to a lesser extent upstream of Putney Bridge – the scale of facilities required for barges would likely impinge on the river and river users, in such a way that would challenge feasibility and create risks toother river users.

3.5.61 Some risks to in-river facilities and barge movements relate to other river

users and depend on the need to obtain a marine risk assessment for operations. It is noted that in the upper reaches of the river beyond PutneyBridge the presence of recreational users, such as rowers and smallboats, presents a significant hazard and risk to be considered whenevaluating sites.

Disposal of material

3.5.62 The methods of treatment, transport and disposal of excavated materialare dependent on the nature and consistency of the material and therequirements for final disposal.

3.5.63 The overall policy is to favour marine transport for main tunnel excavatedmaterial along the River Thames, where practicable and cost-effective.

3.5.64 Details of potential disposal sites are not discussed or considered in thisreport. This will be covered by the project’s ‘waste management strategy’,as part of the Environmental Impact Assessment.

CSO connections to the main tunnel

3.5.65 Where the CSO connection tunnels would connect directly to the 7.2minternal diameter main tunnel, it is assumed that the internal diameter of the connection tunnel would be no greater than 4.5m. The junctions wouldbe axis-to-axis and have a horizontal angle to the main tunnel of approximately 70 degrees, where practical. Where the internal diameter of the main tunnel is smaller than 7.2m, the connection tunnel diameter





30

would need to be of an appropriate size. The limitation on connectiontunnel diameter is due to constructability and the design requirement toachieve a stable structural opening.

3.5.66 The CSO connections to the main tunnel are grouped into five genericoptions/types, which are outlined in greater detail in Section 5.1.

Connection to the Lee Tunnel

3.5.67 The main tunnel would connect to the Lee Tunnel at Abbey Mills PS. Theproposed arrangement is for the main tunnel to connect to the Lee Tunnel‘Shaft F’ (the proposed Lee Tunnel shaft located at Abbey Mills PS). Theconnection would need to provide a smooth hydraulic confluence tocombine the flows from both the Abbey Mills CSO and the main tunnel.The design of the connection would need to minimise disruption to theoperation of the Lee Tunnel.

Impact on third-party infrastructure

3.5.68 The nature of the operations involved in the construction of the maintunnel and associated shafts has the potential to cause groundmovements that could affect existing third-party infrastructure andbuildings. The horizontal and vertical alignment of the main tunnel shaftlocations and construction methodologies would be selected to avoid or minimise, as far as reasonably practical, the impact of ground movementon third-party infrastructure.

3.5.69 Searches of historical and other records have revealed groundwater abstraction wells located within the alignment corridor, some of which are

operational. The tunnel alignment would avoid any adverse effect on thesewells wherever reasonably possible.

3.5.70 In addition to road and underground rail transport tunnels, searches haverevealed a number of existing deep-level service tunnels, includingNational Grid’s Richmond to Fulham high pressure pipeline tunnel and anumber of BT Openreach tunnels. The planned National Grid ‘Wimbledonto Kensal Green’ tunnel is also noted, along with the proposed UK Power Networks New Cross to Wellclose Square cable tunnel. The alignment of the main tunnel and connection tunnels would avoid these assets withacceptable clearances.

3.5.71 Liaison with third parties has commenced with the objective of obtaining‘Approval in Principle’ agreements to cross major assets where possible.This includes an assessment of all significant assets, development of preliminary instrumentation and monitoring plans, and identification of mitigation works where necessary. The scope includes tunnels, bridges,river walls, utilities and existing buildings.

3.6 CSO engineering and construction requirements

General considerations

3.6.1 The design requirements for CSOs are outlined in Section 3.3 together

with a list of the controls required for all 34 CSOs. Current findings indicatethat 18 CSOs require interception, including three connections to the





31

existing northern Low Level Sewer No.1. The remaining 16 CSOs could becontrolled indirectly (see Table 3.1).

3.6.2 The CSO interceptions identified comprise a combination of direct gravityoverflows and pumping stations. In each case, the location of the CSOinterception works would be constrained by the layout of the existing

sewer system.

3.6.3 In general, interception of gravity CSOs would be downstream of the lastincoming connection to the overflow before the overflow sewer reachesthe river in order to ensure that the CSO interception would not bebypassed during a storm event.

3.6.4 In terms of intercepting flows from pumping stations, there are advantagesand disadvantages associated with both pre- and post-pumpinginterception. For example, intercepting flows pre-pumping allows for directgravity interception without reliance on the pumps, which would createenergy savings, whereas post-pumping interception allows the pumps tobe used regularly and reduces the need for special maintenance facilities.If the pumps are not used regularly, maintenance procedures are requiredperiodically to start pumps manually to ensure that they do not seize up. Inpractice, the criterion that governs whether pumping station flows areintercepted pre- or post-pumping is likely to be the availability of suitableCSO sites.

CSO interception: Design and construction

3.6.5 The CSO interceptions typically consist of the following elements:

a. interception chamber

b. connection culvert

c. drop shaft

d. connection tunnel.

3.6.6 Details of each of these elements are outlined below and illustrated inFigure 3.2.

CSO interception chambers

3.6.7 The CSO interception chamber would typically be a box-shaped structurepositioned on the line of the existing sewer pipe. The purpose of this

structure is to intercept the CSO flow and direct it into the connectionculvert that leads to the drop structure.

3.6.8 The size of the interception chamber would be determined to suit theexisting sewer and to accommodate the maximum flow requirements for interception. This would be achieved by means of a combination of calculations and physical modelling.

3.6.9 The depth of the interception chamber would be determined by the depthof the existing sewer.

3.6.10 It is envisaged that the interception chambers would be constructed as a

reinforced concrete structure. However, the construction methodology for the chamber would depend on depth, ground conditions and other site-





32

specific criteria. In general, sheet piling may be used to carry out theexcavation in order to construct the chamber. Where the depth of thechamber precludes the use of sheet piling, an alternative method such assecant piling may be required.

3.6.11 The CSO overflow facility would be retained permanently for use as an

overflow for the system. The overflow would also need to be maintainedduring the construction of the interception works in order to construct theinterception chamber and maintain the functionality of the existingsewerage system during storm events for the duration of the constructionperiod prior to commissioning the project.

3.6.12 The overflow to the river would be protected by double isolation in the formof two lines of flap gates. The flap gates would either utilise the existingflap gate arrangement (where acceptable) or, in some cases, a newstructure and flap gate arrangement.

3.6.13 The interception chamber would also be protected against reversesurcharge flows from the drop shaft by means of two lines of flap gateslocated on the line of the proposed connection culvert. An actuatedmotorised penstock would also be positioned within the interceptionchamber at the junction with the connection culvert. This penstock wouldremain open during normal operative procedures, but would be closed toprevent diversion of flows through the connection culvert during tunnelmaintenance activities.

3.6.14 A control kiosk would be required at each CSO interception site to operatethe motorised penstock. This kiosk might also be used to accommodateother control and monitoring equipment.

3.6.15 An opening would be required in the roof of the interception chamber tofacilitate maintenance access and allow for repair or replacement of theflap gates and penstock in the future. These openings would be fitted withsuitable lockable covers. It is envisaged that the roof of the chamber wouldbe at or below ground level. The covers to the openings would bepositioned at ground level.

CSO connection culverts

3.6.16 The CSO connection culvert would join the interception chamber to thedrop shaft. The intention is to minimise the length of the CSO connection

culvert by positioning the chamber and shaft as close together as possible,although this would depend on the individual constraints at each site.

3.6.17 The depth of the connection culvert would typically be determined by thedepth of the existing sewer, which in turn would set the depth of theinterception chamber. In some cases, it might be necessary to increasethe depth of the connection culvert to minimise the impact on third-partyassets, particularly where the culvert needs to pass under existingstructures or utilities.

3.6.18 The connection culvert would be sized to accommodate the requiredcontrolled or maximum design flow rate.

3.6.19 The means of construction of each CSO connection culvert would bedetermined by the constraints at each site. Typical means of construction





33

might include open cut supported by sheet piling or an open cut trenchsupport system, micro-tunnelling (utilising precast concrete pipe units), or sprayed concrete lining tunnelling connections. Therefore, the crosssection of the connection culvert could be either circular or box-shapedand could comprise precast concrete pipes, precast concrete culvert units,

or sprayed or in situ concrete.3.6.20 A series of access manholes might also be required along the length of

the culvert to facilitate the installation, removal, inspection andmaintenance of the flap gates and penstocks.

3.6.21 For foreshore interception of CSOs, the interception chamber might beaccommodated within the top of the drop shaft. No connection culvertwould be required.

CSO drop shafts

3.6.22 The purpose of the drop shaft is to drop the intercepted flows from the

CSO to the level of the main tunnel or, in some cases, to the level of theconnection tunnel, with an acceptable amount of air entrainment. Threeforms of mechanism have been considered to drop the flows within thedrop shaft, which are summarised as follows:

3.6.23 Straight drop: Due to energy dissipation, the use of a straight drop is onlyconsidered appropriate where the drop is less than 10m in height. Thedirect drop approach would maintain the flow within the pipe rather thanallowing it to become a ‘waterfall’. For the majority of CSOs, the drop isgreater than 10m and therefore a straight drop would not be used.

3.6.24 Cascade drop: Cascade platforms are used within shafts to dissipate

energy for drops greater than 10m. The cascade would typically includealternating platforms at intervals of approximately 3m to 6m over the totaldepth of the shaft, which dissipates the energy in stages as the flows dropto the required level. Due to the regular inspection and maintenanceregime required for cascade-type drops and the associated health andsafety issues, cascade type drop shafts are not preferred.

3.6.25 Vortex drop: Vortex drop tubes can be used for drops greater than 10m.In order to generate the vortex at the top of the drop tube, vortex tubes areenvisaged to be between 0.9m and 3m in diameter. A vortex drop is asystem that accelerates and spins the flow so that it adheres to the wall of

the tube, which is a proven and robust means of transferring flows from ashallow structure to a deep tunnel.

3.6.26 Drop shafts would be sized to accommodate maximum flows, havingregard to the mechanism used to drop the flow to tunnel level.

Connection tunnels

3.6.27 Connection tunnels would take flows either between two drop shafts or from one drop shaft to the main tunnel/main tunnel shaft.

3.6.28 Details of the types and methods of connecting CSOs to the main tunnelare outlined in Sections 4.3 and 5.1.





34

Air management

3.6.29 When the tunnel system fills with CSO discharges the air would bedisplaced and, when the flow is removed from the system, the air wouldneed to return. When the tunnel system is empty, the design includes ameans of refreshing the air within the system. Therefore, the interaction

between combined sewage inflow and management of air requirementsneeds to be considered and addressed.

3.6.30 The air management system would involve a combination of air extractionand intake structures as well as buildings to house air treatmentequipment. The size and configuration of the structures would dependprimarily on how air moves through the system and the amount of air to bemoved.

Construction sites and logistics

Site requirements

3.6.31 CSO site requirements would depend on the size of the connectiontunnels; diameter, depth, and type of drop shaft; space requirements for construction activities; access constraints; and whether the drop shaft is tobe used as a drive or reception shaft for a connection tunnel.

Considerations for in-river sites

3.6.32 In-river (foreshore) sites are under consideration at a number of locations.In general, these locations are not the favoured engineering solution dueto the added complications of working in the river and access to sites.Nevertheless, in certain areas, the complexity of the connection to the

main tunnel and availability of suitable sites means that such sites areconsidered the only feasible sites.

Transport of materials and equipment

3.6.33 Construction of the CSO works would require a wide variety of materialsand equipment to be transported to and from the working sites. Thesesmaller sites could also be managed as satellites to main tunnel drive sitelocations, which would minimise the need for offices, stores, and other sitefacilities.

3.6.34 For the purposes of this report, it is assumed that all transport to and fromCSO sites would be by road.

Power supply and site services

3.6.35 The temporary service requirements for CSO sites would be lessdemanding than those for main tunnel drive sites.

Third-party infrastructure impact

3.6.36 The works at CSO sites would have the potential to affect third-partyinfrastructure and buildings, specifically near-surface services and theriver walls that form the River Thames flood defences. Near-surfaceservices would be present at all sites, but the complexity of the existing

layouts and the possibility of diversionary routes would vary. Constructionworks would be designed to avoid or minimise potential impacts on third-party infrastructure and buildings as far as practicable.




i

Thames Tunnel

Engineering options report

Abbey Mills routeList of contents

Page number

1 Executive summary ......................................................................................... 1 2 Introduction ...................................................................................................... 3

2.1 Background ............................................................................................. 3 2.2 Purpose of this report .............................................................................. 4 2.3 Engineering design development ............................................................ 5

3 System design and engineering requirements .............................................. 7 3.1 System design and engineering assumptions ......................................... 7 3.2 Health and safety considerations ............................................................. 7 3.3 System requirements ............................................................................... 7 3.4 Engineering geology .............................................................................. 15 3.5 Tunnel engineering and construction requirements ............................... 20 3.6 CSO engineering and construction requirements .................................. 30

4 Main tunnel drive options .............................................................................. 35 4.1 Introduction ............................................................................................ 35 4.2 Main tunnel engineering: Options preparation ....................................... 35 4.3 Main tunnel engineering: Options assessment ...................................... 49

5 Connection tunnel drive options .................................................................. 57 5.1 CSO connection options ........................................................................ 57 5.2 Connection tunnel: Drive options ........................................................... 63

6 Conclusions and recommendations ............................................................ 69 7 Next steps ....................................................................................................... 71

The following can be found in the accompanying document Engineering optionsreport - Appendices - Abbey Mills route (110-RG-PNC-000000-000827):

Appendix A – Assumptions register

Appendix B – Drawings

Appendix C – Time chainage

Appendix D – Geology



4 Main tunnel drive options


36

52 main tunnel sites and 71 CSO sites on the shortlist for phase oneconsultation. Main tunnel sites could be used either individually or combined with an adjoining site to provide the required site area.

Main tunnel site types

4.2.7 The number of locations from which the individual drives could belaunched and received would vary according to the direction in which thedrives would be constructed. There are potential benefits from reducingthe number of drive sites by selecting double drive sites where two TBMsare driven in opposite directions, which makes site servicing requirementsand logistics more efficient. However, additional space would be required.

4.2.8 Figure 4.1 summarises the possible main tunnel site types that could beused to establish feasible drive options.

Figure 4.1 Main tunnel site types

Single main tunnel drive site

- main tunnel driven in one

direction

- main tunnel received from

another direction

Double main tunnel drive site

- main tunnel driven in two

directions sequentially

Double main tunnel drive site

- main tunnel driven in two

directions concurrently

Main tunnel reception site


one direction

Main tunnel reception site


two directions

Main tunnel intermediate site

- main tunnel drive through

tunnel

Note: any of these main tunnel site scenarios

could include the drive or reception of CSO

connection tunnels

Intermediate/reception site setup

tunnel drive direction

Single main tunnel drive site

- main tunnel driven in one

direction

r

d - r

d + d

r - r

i

d - d

d

r

i intermediate

reception

drive

drive site setup

shaft

d

It may be possible to use

one shaft instead of two





37

Site zones

4.2.9 In order to manage the total number of combinations of main tunnel drivesite options, the shortlisted sites were grouped into a limited number of main tunnel site zones. This was based on the sites’ geographicalproximity. Figure 4.2 illustrates the zones for all three tunnel routes.

Figure 4.2 Main tunnel site zones for all three routes

4.2.10 As Zones S8, S9 and S10 are only associated with the River Thamesroute and Rotherhithe route, they are not considered further in this report.Figure 4.3 illustrates the zones associated with the Abbey Mills route.

Figure 4.3 Main tunnel site zones for the Abbey Mills route

4.2.11 Table 4.1 identifies which zone each of the shortlisted main tunnel sitesbelongs to for the Abbey Mills route (ie, zones S0 to S7 and S11). Theseare illustrated in drawings in Appendix B of the Engineering options report –

Abbey Mills route –

Appendices (Spring 2012).

S0 – Acton

S1 – Hammersmith

S2 – Barn Elms

S3 – Wandsworth Bridge

S4 – Lots Road

S5 – Battersea

S6 – Shad

S7 – Limehouse

S11 – Abbey Mills

S10 – Beckton

S9 – Charlton

S8 – Deptford

S0 – Acton

S1 – Hammersmith

S2 – Barn Elms

S3 – Wandsworth Bridge

S4 – Lots Road

S5 – Battersea

S6 – Shad

S7 – Limehouse

S11 – Abbey Mills





38

4.2.12 The assessments of the available worksites within each specific zone arenot considered in this report. The site specific factors are examined in thesite suitability reports, which address the use of sites as temporaryworksites and in terms of the final permanent works requirements.

4.2.13 Table 4.1 provides the potential usage of the shortlisted main tunnel sites.

Table 4.1 Grouping of shortlisted main tunnel sites for the Abbey Mills routepost-phase two consultation

Sitezone

Site ID Site nameLocal

authoritySite usage

S0

S01EG Acton StormTanks

Ealing reception

S02EG Commercial units,Stanley Gardens

Ealing reception

S03EG Acton Park

Industrial Estate

Ealing reception

S04EG Industrial units, Allied Way

Ealing reception

S1 No shortlisted sites

S2

S17RD Barn Elms Richmond double drive

single drive

reception/intermediate

S3

S18WH Feathers Wharf Wandsworth reception/intermediate

S72HF Fulham Depot,next toWandsworthBridge

Hammersmithand Fulham


S87HF Carnwath RoadRiverside

Hammersmithand Fulham

single drive


S4 No shortlisted sites

S5

S61WH Battersea Park Wandsworth double drive

single drive


S68WH Battersea Power

Station

Wandsworth double drive with S92WH

single drive


S72WH Kirtling Street withCringle Street

Wandsworth spilt double drive withS93WH

spilt single drive with S93WH

split reception/intermediatewith S93WH






39

NB: ‘Split sites’ refers to sites that are too small on their own but could be used in combinationwith another site/sites to form a suitable site.

Definition of drive options

4.2.14 The overall development of options and selection of sites includesconsideration of the following:

a. main tunnel drive options: there are a number of drive options for which the number of TBMs, number of sites and length of drives vary.Main tunnel sites from which tunnels could be driven in one or twodirections are differentiated

S86WH Post Office Wandsworth spilt double drive withS80WH



S92WH Part of BatterseaPower Station Wandsworth

double drive single drive


S93WH Kirtling Street Wandsworth double drive

single drive


S94WH Post Office Way Wandsworth single drive



S95WH Depots, PontonRoad Wandsworth

double drive single drive


S6

S54SK King’s StairsGardens

Southwark single drive


S76SK Chambers Wharf Southwark single drive


S7

S020T Shadwell Basin Tower Hamlets

single drive


S021T King EdwardMemorial Park

Tower Hamlets

single drive reception/intermediate

S024T/S025T

Heckford Streetsites

Tower Hamlets

split reception/splitintermediate

S036T Limehouse Basin Tower Hamlets


S11S84NM Abbey Mills

Pumping StationNewham single drive

reception





40

b. main tunnel site options: there are a number of sites could be used for each drive option where the space, tunnel alignment, and other factorsvary

c. CSO connection options: the type of CSO connection would depend

on flow, geology, proximity of the main tunnel or a main tunnel site,and a number of other factors

d. CSO site options: a number of CSO sites might be available for eachCSO drop shaft, and the type of connection would vary according to anumber of factors, including the proximity of the main tunnel or a maintunnel site.

4.2.15 In order to establish the range of drive options, each drive is consideredbetween two zones, with a drive site in one zone and a reception site inthe other. Combining different zones together yields a number of drive

options. The basic constraints below are applied in order to establish theinitial number of drive options:

a. drive lengths (maximum and minimum)

b. site type (potential as a double drive, single drive or intermediate/reception site).

4.2.16 For the purposes of this report, main tunnel drive options were determinedin terms of zones, which each include a number of sites (see paragraph4.2.9). The individual site options will be considered and assessed as part

of the site selection process and discussed in the Section 48: Report onsite selection process.

4.2.17 Other more unconventional drive options considered include use of convertible TBMs, docking TBMs underground, and TBM abandonment asa means of extending drive lengths or avoiding the need for a main tunnelsite. However, these options were not taken forward in the selection of thetunnel drive strategy for a range of reasons as described in the followingparagraphs.

4.2.18 One such option is to convert a TBM from EPB to slurry (or vice versa)mid-drive, as could be necessary at the eastern end of the proposed route

(just east of Tower Bridge) when traversing the Chalk to ThanetSand/Lambeth Group interface. However, there is no comparableprecedent for full conversion of a large diameter, high pressure TBM fromearth pressure balance mode with conveyor muck handling to slurrypressure balance with slurry pipe muck handling. Such a change, if feasible, would require major structural rebuilding more suited to theconditions of a fabrication factory than hazardous confined spaceconditions under the river with limited access and lifting facilities.Compromises in TBM design, including increased use of boltedconnections, would be required to effect the conversion; however thiswould make the TBM inherently more flexible, affect efficiency, and

threaten the TBM’s ability to cut through flint-bearing Chalk, especiallygiven the longer total drive length. The excavated material handling and





41

disposal systems would also need to be changed. Furthermore, since thesections of tunnel in these conditions would be excavated sequentiallyrather than simultaneously and would require a period of downtime for TBM conversion, the programme would increase beyond the six-year totalconstruction period for the project as set out in paragraph 4.3.34.

4.2.19 Another option is to drive two TBMs to meet at an underground ‘dockingpoint’ where the internal mechanical equipment would be removed fromthe TBM leaving the shell in the ground. This has been done in Tokyo on amuch smaller scale, and also in the Storebælt Tunnel in Denmark.However, the Storebælt Tunnel is under the sea, which precluded other lower-risk options. Stabilising the ground in order to dismantle thecutterheads involves significant health and safety risks and greater riskdue to heavy lifting and flame-cutting underground without support fromabove ground or in a shaft.

4.2.20 Bearing in mind the large internal diameter of the main tunnel, the drive

alternatives discussed above have a number of disadvantages including:

a. the health and safety risks involved in constructing a cavern throughthe body of the TBM within which to dismantle the cutterhead

b. the difficulties associated with stabilising the ground to facilitateconstruction of a ‘docking cavern’ in poor saturated ground usingground treatments such as freezing. This would likely involve using jack up barges in the River Thames to drill holes to pump liquidnitrogen or super cooled brine into the ground for a prolonged periodin order to create a stable section of ground within which to excavatethe cavern

c. the need to convert a TBM’s excavated material handling andprocessing facilities from screw and conveyor, for handling EPB TBMpaste, to slurry pipes and liquid separation plant for a slurry TBM.There is no comparable precedent for this

d. the need to dismantle the innards of two TBMs using flame-cutting inhazardous, confined conditions. The risks cannot be over emphasisedas the components can weigh up to 100 tonnes

e. the need for unusual heavy lifting operations underground in confinedconditions.

4.2.21 The clear outcome from studying these alternatives was that theyrepresent unacceptably high health and safety risks to workers and anunacceptable project risk.

4.2.22 The project has adopted a goal of zero accidents, zero harm and zerocompromise. These alternatives are not compatible with these aspirationsand the project requirements described in Section 3.2 and paragraph3.5.1.

Drive constraint assumptions

4.2.23 The initial list of drive options was established using the following

considerations:





42

a. The construction period shall not exceed six years (see paragraph4.3.33).

b. Initial assessments indicate that to keep to the six-year constructionperiod programme constraint and to reduce programme risk, themaximum drive length should not exceed approximately 12km.

However, for tunnels driven from deep diaphragm wall shafts, themaximum drive length is approximately 8km in order to compensatefor the greater time required to construct this type of shaft. Thedisadvantages of longer drive lengths include greater TBM wear andthe greater risk of major component failure. Some of the longest drivesof comparable diameter in London are on the Channel Tunnel RailLink (HS1) where the 7.5m drive from Stratford International Station toLondon West Portal (King’s Cross) wore out the TBM head and screwconveyor.

c. The minimum drive length is 3km as the set-up costs for an operation

of this scale would be disproportionate to the tunnelling costs for lengths less than 3km.

d. Drive options are constrained by the type of site available in eachzone (potential as a double drive, single drive or intermediate/reception site).

Derivation of the drive options

4.2.24 The main tunnel would be split into a number of drives each constructedusing a separate TBM.

4.2.25 Table 4.2 provides a matrix from which the initial possible drive options

have been established, starting with consideration of drive length and sitetype.

4.2.26 The table illustrates a matrix of possible drive scenarios (that is, whichzones could be connected together), using the zones available for tunneldrive and tunnel reception. The matrix is colour-coded, with colouredsquares denoting possible options for driving the tunnel from a zone thathas an available drive site to a zone that has an available reception site.The lines on the matrix indicate which drive lengths are too short, too longor potentially acceptable. The matrix also indicates the approximateoverall chainage drive lengths in metres (drive lengths are measured from

the average chainage of sites within each zone).

Drive options: Comprehensive list of initial possibleoptions

4.2.27 The information from the matrix in Table 4.2 has been used to identify theinitial provisional main tunnel drive options, which are presented in Table4.3.

4.2.28 Table 4.3 shows that, based solely on consideration of drive length andsite type constraints, there are six drive options for the western zones (S0 Acton to S5 Battersea), which need to be matched to one of the six drive

options for the eastern zones (S6 Shad, S7 Limehouse and S11 AbbeyMills) to create 36 different drive options.





43

Table 4.2 Drive options: Consideration of practical drive lengths

Z o n

e n a m e

A

c t o n

B a r

n E l m s

W a n

d s w o r t h

B

r i d g e

B a t t e r s e a

S

h a d

L i m e h o u s e

A b b

e y M i l l s

Zone number S0 S2 S3 S5 S6 S7 S11

Chainage (m) 0 3,954 6,682 11,543 18,981 20,454 24,064

S0 3954 6682 11543 18981 20454 24064

S2 2728 7589 15027 16500 20110

S3 4860 12298 13772 17381

S5 7438 8912 12521

S6 1474 5083

S7 3609

No sites available in the zone S1 (Hammersmith) or zone S4 (Lots Road)

S11

eydrive length too shortdrive length too longdrive length potentially acceptable

Drive length acceptableDrive length potentially too long from a deep diaphragm wall shaftDrive length too long or too short

Potential to be a double or single drive or intermediate/reception siteotent a to e a s ng e r ve or nterme ate recept on s te

Potential to be an intermediate/reception site





44

Table 4.3 Initial provisional main tunnel drive options

Drive options: Further development4.2.29 Further to the derivation of initial provisional drive options, a further, more

detailed review was carried out to determine what other factors affect or preclude specific options.

Double drive site in Zone S2 Barn Elms

4.2.30 Zone S2 Barn Elms contains only one site, S17RD (Barn Elms), which hasbeen identified as a possible drive site (refer to Table 4.1). Table 4.3identifies one drive option using a double drive site in this zone (optionW2). The suitability of S17RD as a double drive site has been reviewed,considering in particular the ability to transport double the quantity of

excavated material by barge.

A c t o

n

B a r n E

l m s

W a n d s w o r t h

B r i d g e

S h a

d

L i m e h o u s e

A b b e y

M i l l s

S0 S2 S3 S6 S7 S11

W1 r d-r - d

W2 r d-d - r

W3 r - d-r d

W4 r - - d

W5 r - i d

W6 r i - dE1 d - r-r d

E2 d - r-d r

E3 d r-r - d

E4 d r-d - r

E5 r d-r - d

E6 r - d-r d

No site

required

Single

Reception

Double

ReceptionIntermediate Single Drive

Double

Drive

- r r-r i d d-d

Zone

Drive option

Drive and

Reception

r-d

B a t t e r s e a

S5

E a s t e r n

The site type for the Zone S5 (Battersea) depends on which eastern drive option is matched

with which western drive option. There are no sites available in Zone S1 (Hammersmith) and

Zone S4 (Lots Road).

Legend: The following nomenclature/legend is used in the table to define the types of site

required in the defined zones. Where 'd' denotes drive site, 'r' denotes reception site and 'i'

denotes intermediate site. The tunnel is driven from a ‘d’ drive location to a ‘r’ reception

location and through an 'i' intermediate location.

W e s t e r n





45

4.2.31 The available river depth and width, coupled with the maximum barge sizeable to service this site, makes transporting the quantity of excavatedmaterial required for a double drive site highly unlikely. There is a limitedtidal window in which to load and move barges, and this may beinsufficient to meet the predicted demands. The conclusion is that this

double drive option should remain on the list of options and the concernsnoted here will be taken into account when the multidisciplinary teamcompares the drive options to select the proposed drive option for Section48 publicity.

Long tunnel drives through different geological strata

4.2.32 Table 4.3 shows a number of initial provisional drive options, includingdriving a tunnel from Zone S5 Battersea to either Zone S6 Shad or S7Limehouse. All of these drives would start in London Clay and traverse theLambeth Group and Thanet Sand Formation into Chalk. Because thetunnel drops on a continuous gradient towards the east, longer drives

would also be deeper and therefore subject to higher groundwater pressure.

4.2.33 As noted in Section 3.5, different types of tunnelling machines arepreferred for different ground conditions. Since these drives wouldtraverse mainly London Clay, the Lambeth Group and Thanet SandFormation, an EPB TBM would most likely be used. However, while thistype of TBM is most suited to London Clay and the Lambeth Group, whichmake up approximately 7.5km of the drives, it is less suitable for use inChalk. For this reason, a specific risk assessment is necessary todetermine the viability of longer drives from Zone S5 Battersea to the east,

terminating in Chalk.4.2.34 This risk assessment identified a number of consequences associated

with driving an EPB TBM into Chalk, including:

a. reduced tunnel advance rates: the assumed long average advancerate for an EPB TBM in the Lambeth Group and Thanet SandFormation is taken to be 90m/week. However, it is considered that theadvance rate for this TBM in Chalk should be reduced to 50m/week,due to inefficient working and additional maintenance

b. increased health and safety hazards for work required to maintain theTBM

c. increased risk of mechanical TBM failure (seals, bearings and screwconveyor)

d. increased risk of wear on the cutterhead

e. increased risk of excavated material transfer problems due togroundwater content

f. increased risk mitigation costs resulting from the factors above.

4.2.35 In order to reduce the risks associated with tunnelling across the changefrom the Lambeth Group and Thanet Sand Formation to Chalk, it is

preferable to keep the final length of tunnel bored in Chalk at the end of along EPB drive to a minimum. However, it is considered that for both drive





46

options from Zone S5 Battersea to Zone S7 Limehouse (E1 and E2) andboth drive options from Zone S5 Battersea to Zone S6 Shad (E3 and E4),the distance in Chalk is not long enough to remove them from the list of feasible drive options.

4.2.36 Similar risks and assumptions apply to drive options that include driving a

tunnel from either Zone S6 Shad or S7 Limehouse to Zone S5 Battersea(E5 and E6).

Deep diaphragm wall shafts

4.2.37 Sites in Zone S5 Battersea, S6 Shad, S7 Limehouse and S11 Abbey Millsrequire deep diaphragm wall shafts due to the depth of the tunnel. Thedrive length for drives from these shafts is restricted to approximately 8kmdue to the longer duration of construction of this type of shaft and the needto keep to the six-year overall construction period programme constraint.

4.2.38 The drive length is potentially too long for the following drive options,

which are over 8km in length (see Table 4.2):a. Zone S5 Battersea to Zone S0 Acton (ie, drive options W4, W5 and

W6)

b. Zone S5 Battersea to Zone S7 Limehouse (ie, drive options E1 andE2)

c. Zone S7 Limehouse to Zone S5 Battersea (ie, drive option E6).

4.2.39 The drive lengths are potentially too long; however, drive options W4, W5,W6, E1, E2 and E6 will not be removed from the list of feasible options for drive length reasons alone. The 8km constraint is approximate; therefore

further programme assessment will be undertaken (see Table 4.7).Access points

4.2.40 Main tunnel drive shafts and CSO drop shafts that are on-line (the maintunnel passes directly through the shaft bottom) would be the designatedaccess points to the tunnel system. The spacing between thesepermanent access points should not exceed 9km.

4.2.41 The drive length for drive option W4 (Zone S5 Battersea to Zone S0 Acton) is over 9km; therefore an intermediate site needs to be considered. Although the W4 main tunnel drive length is too long without anintermediate site for access purposes, it will not be removed from the listof feasible options for that reason, in case a CSO drop shaft can insteadbe incorporated on-line to provide the required access point.

Tunnel vertical alignment and gradient

4.2.42 The western drive options involve drives between Zone S5 Battersea andZone S0 Acton. However, there is a vertical tunnel alignment constraintimposed by the London Ring Main and other existing tunnels in thissection of tunnel; therefore the tunnel vertical alignment needs to changealong the route. The change in tunnel vertical alignment can beaccommodated at a shaft in either Zone S2 Barn Elms or Zone S3Wandsworth Bridge.





47

4.2.43 Drive options W1 to W3 have drive/reception shafts in either Zone S2 BarnElms or Zone S3 Wandsworth Bridge to accommodate the verticalalignment change.

4.2.44 Drive options W5 and W6 have an intermediate shaft in Zone S2 BarnElms or Zone S3 Wandsworth Bridge to accommodate the vertical

alignment change.

4.2.45 However, drive option W4 has no shafts in Zone S2 Barn Elms or Zone S3Wandsworth Bridge to accommodate the vertical alignment change andtherefore was removed from the list of feasible drive options.

Drive options: Interim list of options

4.2.46 Having reviewed the drive options from Table 4.3, an interim list of driveoptions is presented below in Table 4.4.

Table 4.4 Interim main tunnel drive options

4.2.47 Table 4.4 shows that the interim list of potentially feasible drive optionsincludes five drive options for the western zones (S0 Acton to S5Battersea), one of which needs to be matched to one of six drive optionsfor the eastern zones (S5 Battersea to S11 Abbey Mills) to create 30different drive options. The full list of interim drive options is providedbelow in Table 4.5.

A c t o n

B a r n E l m s

W a n d s w o r t h

B r i d g e

S h a d

L i m e h o u s e

A b b e y M i l l s

S0 S2 S3 S6 S7 S11

W1 r d-r - d

W2 r d-d - r

W3 r - d-r dW5 r - i d

W6 r i - d

E1 d - r-r d

E2 d - r-d r

E3 d r-r - d

E4 d r-d - r

E5 r d-r - d

E6 r - d-r d

W e s t e r n

Zone

Drive option

B a t t e r s e a

S5

E a s t e r n

The site type for the Zone S5 (Battersea) depends on which eastern drive option is matched

with which western drive option. There are no sites available in Zone S1 (Hammersmith)and Zone S4 (Lots Road).





48

Table 4.5 Interim list of main tunnel drive options

4.2.48 Table 4.5 lists the 30 potentially feasible drive options and indicates that:

a. 18 options use four TBMs and 12 options use three TBMs

b. Four options use four drive sites and one reception site; 14 optionsuse three drive sites and two reception sites; four options use threedrive sites, one intermediate site and one reception site; and eightoptions use two drive sites, one intermediate site and two receptionsites.

c. All options require a main tunnel reception site in Zone S0 Acton, ie, atone end of the main tunnel.

d. All options require a main tunnel site (drive or reception) in Zone S11

Abbey Mills, ie, at the other end of the main tunnel.

A c t o n

B a r n E l m s

W a n d s w o r

t h B r i d g e

S h a

d

L i m e h o u s e

A b b e y

M i l l s

Drive option S0 S2 S3 S6 S7 S11

W1/E1 r d-r - d d - r-r d 3 0 2 4

W1/E2 r d-r - d d - r-d r 3 0 2 4

W1/E3 r d-r - d d r-r - d 3 0 2 4

W1/E4 r d-r - d d r-d - r 3 0 2 4

W1/E5 r d-r - d r d-r - d 4 0 1 4

W1/E6 r d-r - d r - d-r d 4 0 1 4

W2/E1 r d-d - r d - r-r d 3 0 2 4

W2/E2 r d-d - r d - r-d r 3 0 2 4W2/E3 r d-d - r d r-r - d 3 0 2 4

W2/E4 r d-d - r d r-d - r 3 0 2 4

W2/E5 r d-d - r r d-r - d 3 0 2 4

W2/E6 r d-d - r r - d-r d 3 0 2 4

W3/E1 r - d-r d d - r-r d 3 0 2 4

W3/E2 r - d-r d d - r-d r 3 0 2 4

W3/E3 r - d-r d d r-r - d 3 0 2 4

W3/E4 r - d-r d d r-d - r 3 0 2 4

W3/E5 r - d-r d r d-r - d 4 0 1 4

W3/E6 r - d-r d r - d-r d 4 0 1 4

W5/E1 r - i d d - r-r d 2 1 2 3

W5/E2 r - i d d - r-d r 2 1 2 3

W5/E3 r - i d d r-r - d 2 1 2 3

W5/E4 r - i d d r-d - r 2 1 2 3

W5/E5 r - i d r d-r - d 3 1 1 3

W5/E6 r - i d r - d-r d 3 1 1 3

W6/E1 r i - d d - r-r d 2 1 2 3

W6/E2 r i - d d - r-d r 2 1 2 3

W6/E3 r i - d d r-r - d 2 1 2 3

W6/E4 r i - d d r-d - r 2 1 2 3

W6/E5 r i - d r d-r - d 3 1 1 3

W6/E6 r i - d r - d-r d 3 1 1 3

Zone

N u m b e r o f d r i v e s i t e s

N u m b e r o f r e c e p t i o n

s i t e s s i t e s

N u m b e r o f T B M s

N u m b e

r o f

i n t e r m e d i a t e s i t e s

B a t t e r s e a

S5





49

e. All options require a main tunnel site in Zone S5 Battersea, which isapproximately halfway along the main tunnel.

f. Not all drive options require a main tunnel site in Zone S2 Barn Elmsand not all drive options require a main tunnel site in S3 WandsworthBridge. However, a main tunnel site is required in one of these two

zones.

g. Not all drive options require a main tunnel site in Zone S6 Shad andnot all drive options require a main tunnel site in S7 Limehouse.However, a main tunnel site is required in one of these two zones.

4.3 Main tunnel engineering: Options assessment

4.3.1 This section describes the engineering-related factors that affect thedesirability of the tunnel drive options.

4.3.2 All the options presented in Table 4.5 are considered potentially feasible in

engineering terms. The areas for engineering assessment are risk(comprising engineering and health and safety risks), programme, cost,transport and energy.

4.3.3 Other factors that are specific to each drive option, including planning,community, environment, and property, will also be compared in the nextsite selection process to determine which drive option is selected for theproposed scheme and presented in the Section 48: Report on siteselection process.

Health and safety and engineering risk considerations

4.3.4 The following risk criteria are considered relevant for the comparison of drive options. Most of the risk criteria can be considered in terms of healthand safety risk and/or engineering construction risk.

Health and safety issues: General

4.3.5 Overall health and safety risks were considered in relation to the overallextent of work and the total quantity of man hours required. Specific healthand safety risks are considered along with other hazards and risksdetailed below.

4.3.6 In general terms, the effort required and the risks associated with buildingthe tunnel would be proportional to the length of the tunnel. The relativebenefits or adverse effects are similarly proportional. The scale of differences was relevant when considering all three tunnel routes, but isnot discussed further in this report, which only considers one route (the Abbey Mills route) for which all drive options have essentially the sametunnel length.

Health and safety issues: Access

4.3.7 Access and egress from the main tunnel would be via main tunnel shaftsand CSO drop shafts directly on the line of the tunnel. Paragraph 4.2.40identifies which drive options are of concern in terms of access points. The

distance between shafts would be minimised as far as practicable;however, for comparison purposes, the relative benefits or adverse effectsfrom a long-term inspection and maintenance perspective would be





50

proportional to the number of access shafts provided. There are differentnumbers of sites associated with the Abbey Mills route depending on thedrive options; therefore this is an issue for consideration.

Geology

4.3.8 Flints and flint bands cause wear to TBM cutters and TBM cutterhead faceprotection coatings, which increases the likelihood and frequency of interventions at the excavation face in front of the TBM to replace worncomponents. Face interventions involve sending workers in front of theTBM cutterhead, often in compressed air or oxygen enriched air, and areconsidered to be hazardous operations. Although face interventions areessential and normal procedure for tunnelling, the number should beminimised in order to reduce the associated safety risk of entering theTBM cutterhead in proximity to unsupported ground, and potential delay.The likelihood and frequency of face interventions for each option isrelated to the tunnel length. The principle consideration is the drive length

in flint-bearing Chalk formations as well as the type and design of theTBM. Designing the cutterhead to incorporate wear indicators andendoscope inspection and to allow rear replacement of discs reduces theneed to carry out face interventions. Extending the drive length increasesthe risk of the need for major refurbishment of the cutterhead to replaceworn armour plating that protects the body of the machine.

4.3.9 High groundwater pressures at the face might increase the programmerisk arising from failure of TBM bearings, which occurs when ground isforced past the seals into the main bearings under pressure. It is alsomore complex to undertake routine inspection and maintenance

interventions and might result in longer periods between inspections,which increases the risk of unexpected component failure. There are alsoincreased health and safety hazards associated with face interventions.The risks to tunnelling are therefore proportional to the maximumgroundwater pressures likely to be encountered and the length over whichthey would occur.

4.3.10 Tunnel face interventions and appropriate face control become moredifficult where there are mixed geological conditions at the face; suchconditions can vary over short distances. The level of risk is higher inChalk where there is little or no cover below the interface with the ThanetSand Formation because Chalk is less stable under these conditions.

4.3.11 The risk of delay due to disturbed ground conditions and suddengroundwater ingress increases at geological faults. The major geologicalstructures identified by the site investigations are described in paragraph3.4.8. The impacts are likely to be minimal for closed-face tunnelling, ieboth EPB TBM and slurry TBM tunnelling methods. The level of risk for each drive option is related to the number of likely fault zones along eachroute.

4.3.12 This was a relevant factor when comparing the three tunnel routes but isnot discussed further in this report, which only considers one route (the Abbey Mills route) as the geology is essentially the same across all thedrive options.





51

Third-party assets

4.3.13 The excavation required for the project, including the deep shafts andmain tunnel, would result in ground movements that have the potential toaffect adjacent properties and infrastructure. The number of assetspotentially impacted is related to the length of the tunnel. The magnitude

of the influence is related to the extent and depth of the excavation,ground conditions, the geometric relationship to the infrastructure, and themethod of construction.

4.3.14 The level of risk to major infrastructure for each option, including bridgesand tunnels, would depend on how many structures are within the tunnels’range of ground movement influence.

4.3.15 The presence of unknown obstructions or future planning proposals alongthe tunnel route presents a risk to the delivery of the project. The level of risk is reduced where the tunnel follows the line of the river.

4.3.16 This was a relevant factor when considering the three tunnel routes, but isnot discussed further in this report, which only considers the Abbey Millsroute, which encounters essentially the same third-party assets for alldrive options.

Site requirements

4.3.17 The risks associated with drive sites may include carrying out works inproximity to major utility services or railways and completing associateddevelopment works such as temporary jetties or cofferdams. The level of risk is related to the number of worksites required.

4.3.18 Servicing the tunnel drive sites presents risks in terms of establishingtransport links to and from sites to deliver construction materials andremove excavated material. Levels of risk would increase where there areno established connections to main roads or existing wharf facilities.Larger sites would offer more flexible worksite arrangements and thereforepresent lower risk.

4.3.19 The drive options for the Abbey Mills route require different numbers of drive sites.

Tunnel alignment

4.3.20 Construction risks associated with tunnelling using a TBM are proportional

to the total length of the tunnel. General tunnelling risks are associatedwith working with heavy machinery and handling heavy structuralelements at depth in a confined environment.

4.3.21 This was a relevant factor when considering all three tunnel routes, but isnot discussed further in this report, which only considers the Abbey Millsroute as all the drive options are essentially the same length.

TBM

4.3.22 The potential for unplanned interventions due to mechanical breakdownsor cutterhead/tool wear presents a health and safety and construction risk.This risk reduces with shorter drive lengths and can be mitigated moreeffectively where there are opportunities to provide ground treatment from





52

surface locations such as roads, canals or river courses, where there areno buildings or other significant structures.

4.3.23 There is a further risk of additional interventions where a tunnel drivepasses from the Thanet Sand Formation into Chalk compared to tunnellingthrough the geological units above the Thanet Sand Formation. Tunnelling

through Chalk, especially Chalk that contains a high percentage of flintboulders up to 800MPa strength, is likely to increase the frequency andduration of interventions for inspection and maintenance.

4.3.24 Mixed-face conditions might cause the TBM to run less efficiently, withmore delays and possible breakdowns, which increases the risk of over-excavation due to balancing the different support requirements for firmground overlaid with softer material, which results in increased groundmovement. This level of risk would be higher when the tunnel drive followsthe interface boundary between two geological strata (either clay/sand or sand/chalk interfaces).

4.3.25 The various drive options associated with the Abbey Mills route requiredifferent numbers of TBMs.

Constructability

4.3.26 The risks associated with long tunnel drives are discussed in Section 4.2.It is most preferable to reduce the risks associated with tunnelling acrossthe change from the Lambeth Group and Thanet Sand Formation to Chalkin order to keep the final length of tunnel bored in Chalk at the end of along EPB TBM drive to a minimum. It is therefore considered that, basedon engineering risk, options with drives from Zone S5 Battersea to ZoneS7 Limehouse are not favoured and, where possible, should be avoided.

4.3.27 Failure of construction contractual arrangements is a project risk. Dividingthe main tunnel works into packages of already proven physical andfinancial scale reduces the overall risk to the project.

4.3.28 There might be an opportunity for savings where double drive sites areused. Larger contracting organisations might be able to construct twodrives from a single shaft and omit a shaft by sharing some worksitefacilities.

CSO connections

4.3.29 The health and safety and construction risks associated with CSO sitesand interception structures are proportional to the number of drop shaftsrequired for each option and the depth of the shafts. Some drop shaftsneed to be as deep as the main tunnel and it is desirable to minimise thenumber of such drop shafts.

4.3.30 Similarly, the health and safety and construction risks associated with theconstruction of the connection tunnels are related to the connection tunnellength for each option and the predicted ground conditions.

4.3.31 Furthermore, the health and safety and construction risks associated withthe construction of connections to the main tunnel are proportional to the

number of connections. It is inherently less risky, and therefore preferable,to make connections to main tunnel shafts rather than directly to the main





53

tunnel. This is because maintenance and inspection access to aconnection point located in a shaft is more straightforward than aconnection point located inside a tunnel. It is also more difficult toconstruct a connection directly into the tunnel. Moreover, connectionworks at a shaft would not interfere with the progress of the main tunnel

construction. Options that require construction of a junction with largediameter open face excavations in deep, water-bearing ground wouldcarry higher risk.

4.3.32 While the relative benefits or adverse effects of each tunnel route are notexamined as part of this report, it is noted that each of the main tunnelroutes would provide different storage volumes and different systemperformances (ie, volume and frequency of spills into the river). This was arelevant factor when considering all three tunnel routes, but is notdiscussed further in this report, which only considers the Abbey Mills routefor which the CSO connections are essentially the same for all drive

options.Programme considerations

4.3.33 The overall project programme is based on a construction period of sixyears, which includes local site mechanical and electrical testing andcommissioning but does not include system-wide testing andcommissioning. These construction programme activities follow on fromthe overall project design, planning and procurement activities.

4.3.34 A maximum six-year construction period has been assumed in order toconstruct the project in an efficient manner and to enable it to becompleted as early as possible given the legal drivers for the project, asset out in the Needs Report . A six-year period would allow the TBMs to bematched to the geology in order to maximise tunnelling production rates. Itwould also ensure that drive lengths are reasonable (ie, the risk of interventions to repair the face of TBMs is not excessive) and that the sizeof the construction contracts is viable (ie, they can be financed and thereare contractors in the market that are large enough to take on thecontracts). Drive options that would extend construction beyond six years,require longer drives through variable geology, require larger contracts,and increase the risk of fines from the European Union for breaching theUrban Waste Water Treatment Directive will be avoided.

4.3.35 The main factors that would affect the duration of the constructionprogramme include the following:

a. Location of drive shafts: the time it takes to construct a shaft fromwhich to launch a TBM is critical to the duration of the programme.Therefore, deep shafts in more difficult ground conditions that requiredewatering activities and diaphragm wall methods would add time tothe programme, compared to shallower shafts in more favourableground.

b. Length of drive: the duration of a drive is generally proportional to thelength, although average drive rates would reduce for very short drives

where the proportion of time taken to establish the full TBM back-up islonger. The geological conditions also affect the rate of tunnelling.





54

c. CSO connection works to main tunnel shafts: some CSO connectionsto drive shafts could only be constructed on completion of the maintunnel drive, which could affect the time required to complete theproject. At main tunnel reception shafts, there would likely be moretime to complete any CSO connections before the arrival of the TBM.

4.3.36 The programme assumptions regarding the essential constructionactivities used for the comparison of drive options are provided in Table4.6.

Table 4.6 Programme assumptions for comparison of options

Zones Zones

S0 to S4 S5 to S11

Key Activity Comment

Mobilise shaft site 20 wks 26 wks Includes dewatering for sites in the east

Build and excavate

shaft20 wks 50 wks

Based on segment, SCL or caisson for

zones S0 to S4 and diaphragm wall for

zones S5 to S11

Base slab to shaft 4 wks 6 wksBased on permanent base slab of

reinforced concrete

Tunnel eye 10 wks 10 wks

Based on opening in segment shafts

and internal collar arrangement for

d'wall shafts

Tunnel worksite

setup2 wks 2 wks

Transform the site from shaft

construction setup to tunnel

construction setup

TBM installation 12 wks 15 wksMain body only. Excludes backup

which goes in during the slow start

200m drive for

TBM burial and

backup installation

22 m/wk 22 m/wk200m slow start based on no

backshunt being provided

Main tunnel drive 100 m/wk 80 m/wk

Long average excludes 200m long

TBM installation length. 90m/week for

EPB TBM when in Lambeth

Group/Thanet Sand. 50m/week for

EPB TBM when in Chalk.

Tunnel strip out 4 wks 4 wksFor removal of conveyor and for

extraction of CSO TBMs if necessary

Main tunnel

secondary lining140 m/wk 140 m/wk Based on reinforced in situ lining

Shaft lining 5 wks 10 wks In si tu concrete lining

Shaft internal

structures25 wks 30 wks Internal slabs and cover structures

Local M&E testing

and

commissioning

8 wks 8 wksExcludes project-wide M&E testing and

commissioning

Duration/rate





55

4.3.37 Table 4.7 summarises the potential construction durations for each driveoption in weeks, based on the assumptions set out in Table 4.6.

Table 4.7 Summary of construction durations for main tunnel drive options

4.3.38 Table 4.7 identifies the duration of the drive options presented in Table 4.5and highlights the drive options that would take longer than six years(312 weeks) to construct. The longest options would take 356 weeks,which is significantly longer than the programme requirements and, for thisreason, all 12 W5 and W6 drive options were removed from the list of feasible options. The options that exceed the six-year period by up to ten

weeks were considered close enough to keep on the list of feasibleoptions. For the remaining options, the difference in duration was smallenough to conclude that programme risk is not a significant differentiatingfactor.

4.3.39 Time-chainage diagrams have been produced for five representative driveoptions (W1/E1, W1/E3, W2/E6, W3/E4 and W3/E5). The diagrams wereonly provided for five drive options as all the other drive options essentiallyhave the same overall construction duration as one of those five. Thesediagrams provide more detail on the overall duration given in Table 4.7(refer to Appendix C of the Engineering options report – Abbey Mills route – Appendices (Spring 2012)).

Cost considerations

4.3.40 The Engineering options report (Spring 2010) included a comparison of relative costs using key quantities of work. It considered cost differencesacross the three different tunnel routes and across drive options withdifferent numbers of TBMs. The costs proved very similar where thenumber of TBMs was the same.

4.3.41 This report is only concerned with the Abbey Mills route and therefore therelative cost comparison has only been applied to drive options that usethree or four TBMs. The drive options that use only three TBMs would cost

less than those with four TBMs due to the saving on the manufacture of one TBM.

Drive Weeks Drive Weeks Drive Weeks

W1/E1 321 W3/E1 321 W6/E1 356W1/E2 321 W3/E2 321 W6/E2 356

W1/E3 294 W3/E3 286 W6/E3 330

W1/E4 294 W3/E4 286 W6/E4 330

W1/E5 284 W3/E5 284 W6/E5 330

W1/E6 319 W3/E6 319 W6/E6 330

W2/E1 321 W5/E1 356

W2/E2 321 W5/E2 356

W2/E3 286 W5/E3 330

W2/E4 286 W5/E4 330

W2/E5 284 W5/E5 330W2/E6 319 W5/E6 330

Drive options with durations over 312 weeks (6 years)





56

Transport considerations

4.3.42 Transport considerations are not discussed in this report, but areexamined on a site-by-site basis in the site suitability reports.

Energy considerations

4.3.43 Energy was a relevant factor when considering all three tunnel routes, butis not discussed further in this report, which only considers the Abbey Millsroute, as all the drive options have similar energy needs.

Drive options: Final list

4.3.44 Table 4.8 shows the final list of 18 feasible main tunnel drive options to betaken forward to the next stage of the site selection process for multidisciplinary consideration.

Table 4.8 Final list of main tunnel drive options

A c t o n

B a r n E l m s

W a n d s w o r t h B r i d g e

S h a d

L i m e h o u s e

A b b e y M i l l s

Drive option S0 S2 S3 S6 S7 S11

W1/E1 r d-r - d d - r-r d 3 0 2 4

W1/E2 r d-r - d d - r-d r 3 0 2 4

W1/E3 r d-r - d d r-r - d 3 0 2 4

W1/E4 r d-r - d d r-d - r 3 0 2 4W1/E5 r d-r - d r d-r - d 4 0 1 4

W1/E6 r d-r - d r - d-r d 4 0 1 4

W2/E1 r d-d - r d - r-r d 3 0 2 4

W2/E2 r d-d - r d - r-d r 3 0 2 4

W2/E3 r d-d - r d r-r - d 3 0 2 4

W2/E4 r d-d - r d r-d - r 3 0 2 4

W2/E5 r d-d - r r d-r - d 3 0 2 4

W2/E6 r d-d - r r - d-r d 3 0 2 4

W3/E1 r - d-r d d - r-r d 3 0 2 4

W3/E2 r - d-r d d - r-d r 3 0 2 4

W3/E3 r - d-r d d r-r - d 3 0 2 4

W3/E4 r - d-r d d r-d - r 3 0 2 4

W3/E5 r - d-r d r d-r - d 4 0 1 4

W3/E6 r - d-r d r - d-r d 4 0 1 4

N u m b e r o f d r i v e s i t e s

N u m b e r o f

i n t e r m e d i a t e s i t e s

N u m b e r o f r e c e p t i o n

s i t e s s i t e s

N u m b e r o f T B M s

S5

B a t t e r s e a

Zone



5 Connection tunnel drive options


57


5.1 CSO connection options

5.1.1 For the purposes of this report, five different connection types have beenidentified for connecting the existing CSO sewers to the main tunnel, asfollows:

a. Type A: connection tunnel to main tunnel shaft connection

b. Type B: connection tunnel to main tunnel connection

c. Type C: two or more CSOs connected by connection tunnels prior toconnection to main tunnel or main tunnel shaft

d. Type D: drop shaft adjacent to main tunnel (no connection tunnel)

e. Type E: connection culvert to main tunnel shaft (or to drop shaft online of the main tunnel) connection (no connection tunnel).





58

Type A CSO connection

5.1.2 The Type A connection is illustrated schematically in Figure 5.1. This typeof connection would be used where a connection tunnel is requiredbetween a CSO interception point and a main tunnel shaft. The

interception point would be on a site some distance from the main tunnelsite where the two could not be connected by a connection culvert. Aninterception chamber would be built around the existing CSO sewer andconnected to a drop shaft by a connection culvert. The drop shaft wouldthen be connected to a main tunnel shaft by a connection tunnel.

5.1.3 In some cases, the connection tunnel might need to be driven from theCSO site, which has implications for CSO site selection because the sitewould have to be large enough to support the necessary tunnelling plantset-up. Where possible, the connection tunnel would be driven from themain tunnel site.

Figure 5.1 Type A CSO connection

Interceptionchamber

Connectionculvert

Drop shaft

Connection Tunnel

MainTunnel

MainTunnelShaft

Vortex/direct drop

PLAN VIEW

SECTION VIEW





59

Type B CSO connection

5.1.4 The Type B connection is illustrated schematically in Figure 5.2. This typeof connection would be used where a connection tunnel is requiredbetween the CSO interception point and the main tunnel, and where the

main tunnel is located in competent ground, such as London Clay, so thata direct tunnel-to-tunnel connection could be made. In other lessfavourable ground conditions, depending on the nature of the ground andgroundwater, this method might require ground treatment. In deep, water bearing ground such as Chalk, it is preferable to avoid this type of connection. All four other connection types are easier to construct in poor ground conditions than Type B.

5.1.5 An interception chamber would be built around the existing CSO sewer and connected to a drop shaft by a connection culvert. The drop shaftwould then be connected directly to the main tunnel by a connectiontunnel.

5.1.6 In most cases, the connection tunnel would have to be driven from theCSO site, which has implications for CSO site selection as the site wouldhave to be large enough to support the necessary tunnelling plant set-up.

Figure 5.2 Type B CSO connection

Connectionculvert

Drop shaft

Connection Tunnel

MainTunnel

SECTION VIEW

PLAN VIEW

MainTunnel

Vortex/directdrop

Interceptionchamber





60

Type C CSO connection

5.1.7 The Type C connection is illustrated schematically in Figure 5.3. This typeof connection would be used where two or more CSOs are interceptedand brought together before they are connected to the main tunnel, either

directly or at a main tunnel shaft.5.1.8 An interception chamber would be built around the existing CSO sewer

and connected to a drop shaft by a connection culvert. The drop shaftwould then be connected to a second drop shaft by a connection tunneland the second drop shaft would be connected to the main tunnel or amain tunnel shaft by a connection tunnel.

5.1.9 In some cases, the connection tunnel would need to be driven from one of the CSO sites, which has implications for CSO site selection as the sitewould need to be large enough to support the tunnelling plant set-up.

Figure 5.3 Type C CSO connection

Second CSOConnection

Connectionculvert

Drop shaft

Connection Tunnel

Main

Tunnel

ConnectionTunnel

Dropshaft

Vortex/direct drop

Vortex/direct

PLAN VIEW

SECTION VIEW

Either directly tomain or a

main tunnelshaft

Interception

chamber





61

Type D CSO connection

5.1.10 The Type D connection is illustrated schematically in Figure 5.4. This typeof connection would be used where the drop shaft is located directlyadjacent to the main tunnel.

5.1.11 An interception chamber would be built around the existing CSO sewer and connected to a drop shaft by a connection culvert. The connectionbetween the drop shaft and the main tunnel could be via a single or multiple cell junction detail, depending on hydraulic flow requirements andground conditions. This connection type is easier to construct in poor ground conditions than a Type B.

Figure 5.4 Type D CSO connection

Connectionculvert

Dropshaft

Multiple/singleconnection junction

MainTunnel

Vortexdrop

PLAN VIEW

SECTION VIEW

Interceptionchamber





62

Type E CSO connection

5.1.12 The Type E connection is illustrated schematically in Figure 5.5. This typeof connection would be used where the CSO interception point can beconnected directly to a shaft located on the line of the main tunnel. An

interception chamber would be built around the existing CSO sewer andconnected to a shaft by a connection culvert. The shaft could be either aCSO drop shaft or a main tunnel shaft. If it is a drop shaft, it would be builtbefore the main tunnel, which would be driven through the drop shaft. Itwould therefore need to be large enough to allow the main tunnel to passthrough the bottom.

5.1.13 This type of connection is considered easier to build in poor groundconditions at depth. This arrangement provides an opportunity to inspectand possibly maintain the main tunnel TBM and, once the project isoperational, a possible additional means of main tunnel access, ventilationand overflow (depending on the location).

Figure 5.5 Type E CSO connection

Connectionculvert

MainTunnel

Main TunnelShaft or CSO

Drop Shaft

VortexDrop

PLAN VIEW

SECTION VIEW

MainTunnel

Interceptionchamber





63

5.2 Connection tunnel: Drive options

5.2.1 The main tunnel shortlisted sites have been grouped into zones. The CSOconnection type selected for individual CSOs is dependent on theproximity of the main tunnel and main tunnel sites to the CSO sites. For

this reason, the selection of the appropriate connection type for most CSOsites is not considered in this report, but is discussed in the Section 48: Report on site selection process. However, the Type C CSO connection,where two or more CSOs are intercepted and brought together prior toconnecting to the main tunnel, is discussed in this report because theremay be more than one connection tunnel drive option and it may impacton the main tunnel drive options.

5.2.2 Engineering factors to be considered when selecting the CSO connectiontypes for each CSO site include:

a. main tunnel drive strategy and site selection

b. hydraulic system preferences

c. location of the CSO interception site and whether it could beconnected to a main tunnel shaft by a connection culvert

d. distance between the CSO interception site and the main tunnel or main tunnel shaft

e. whether two or more CSOs could be connected before connecting tothe main tunnel

f. local ground conditions – in poorer ground conditions, junctions wouldbe more difficult and tunnel-to-shaft connections (Types A, C, D and

E) may be preferred over tunnel-to-tunnel connections (Type B)

g. potential for impacts on existing or planned infrastructure

h. maximum flow rates – for larger flows, the connection tunnel may betoo big to connect directly to the main tunnel

i. overall number and size of shafts required

j. cost and programme.

Type C CSO connection options

5.2.3 There are two examples of Type C CSO connections associated with the

Abbey Mills route:

a. the Frogmore connection tunnel: a connection tunnel that bringstogether flows from the Frogmore Storm Relief – Bell Lane Creek CSO(CS07A) and Frogmore Storm Relief – Buckhold Road CSO (CS07B)before connecting to the main tunnel

b. the Greenwich connection tunnel: a connection tunnel that bringstogether flows from the Greenwich Pumping Station CSO (CS33X),Deptford Storm Relief CSO (CS32X) and Earl Pumping Station CSO(CS31X) before connecting to the main tunnel.





64

Frogmore connection tunnel

5.2.4 Table 5.1 below presents the list of Frogmore connection tunnel driveoptions to be taken forward to the next stage of the site selection processfor multidisciplinary consideration.

Table 5.1 Frogmore connection tunnel: Drive options

Frogmore SR -

Buckhold Road

Frogmore SR - Bell

Lane CreekMain tunnel

Connection tunnel drive option

FA d r-d r

FB r d then d r

FC d r-r d

FD r d-r d

Frogmore SR -

Buckhold Road

Frogmore SR - Bell

Lane Creek

Zone S3 main

tunnel site


FE d r-d r

FF d through r

FG d r-r dFH r d then d r

FI r d-r d

FJ r through d

Single reception Single driveSequential double

drive

r d d then d

Double reception Drive and receptionTunnel drive through

CSO drop shaft

r-r r-d or d-r through

Connected directly to the main tunnel

Connected to the Zone S3 main tunnel shaft

Legend: The following nomenclature/legend is used to define the types

of site required. Where 'd' denotes drive site, 'r' denotes reception site

and 'through' denotes the tunnel drives through a CSO drop shaft

CSO

CSO





65

Greenwich connection tunnel

5.2.5 The potentially feasible drive options for the Greenwich connection tunnelare presented below in Table 5.2. All the Greenwich connection tunneldrive options connect to the main tunnel via a main tunnel shaft in either Zone S6 Shad or Zone S7 Limehouse. As this would be a long connection

tunnel, the drive options need to be considered in conjunction with themain tunnel drive options concerning Zone S6 Shad and Zone S7Limehouse. The location of Zones G1, G2 and G3 are illustrated in FigureB5 in Appendix B (refer to the Engineering options report – Abbey Millsroute – Appendices (Spring 2012)).

Table 5.2 Greenwich connection tunnel: Initial drive options

5.2.6 If the Greenwich connection tunnel connected to the main tunnel in ZoneS7 Limehouse, the flows would join the main tunnel along with flows fromthe intercepted North East Storm Relief CSO. The engineering would becomplex and challenging as there are both hydraulic and pneumatic (air movement) concerns surrounding introducing too much flow at a singlelocation. Therefore, all the drive options associated with Zone S7Limehouse were removed from the list of feasible options.

5.2.7 To drive the connection tunnel from the main tunnel site in Zone 6 Shad toGreenwich PS after the main tunnel has been driven from Zone 6 Shad tothe main tunnel site in Zone 11 Abbey Mills, and receive the TBM fromZone S5 Battersea at Zone S6 Shad is estimated to require a totalconstruction period of at least 350 weeks. This is 38 weeks longer than themaximum six-year construction period and, for this reason, Option GE1(seq – tunnel driven sequentially in two directions) associated with ZoneS6 Shad was removed from the list of feasible options.

5.2.8 The programme for the other options will need to be checked inconjunction with the preferred main tunnel option at the next stage of evaluation.

Z o n e G

3

G r e e n w i c h P S

D e p t f o r d

S R

Z o n e G

2

Z o n e G

1

E a r l P S

Z o n e S 1 1

A b b e y M

i l l s


GA r through n/a n/a through d r-r d

GB d through n/a n/a through r r-r d

GC r through n/a d then d through r r-r d

GD r through d then d n/a through r r-r d

GE (seq) r through n/a n/a through d after MT r-d r

GF (con) r through n/a n/a through d with MT r-d r

GH d through n/a n/a through r d r

GI r through n/a d then d through r d r

GJ r through d then d n/a through r d r

Z o n e S 6 S

h a d

o r Z o n e

S 7

L i m e h o u

s e

Greenwich connection tunnel

CSO or Zone

Main tunnel

Main tunnel





66

5.2.9 Table 5.3 presents the final list of Greenwich connection tunnel driveoptions to be taken forward to the next stage of the site selection processfor multidisciplinary consideration.

Table 5.3 Greenwich connection tunnel: Final drive options

North East Storm Relief CSO connection options

5.2.10 The post phase one consultation site selection back-check associated withNorth East Storm Relief CSO sites identified two feasible CSO connectiontypes, as follows:

a. The King Edward Memorial Park Foreshore (C29XA) and King EdwardMemorial Park (C29XB) shortlisted sites could be connected to themain tunnel via a CSO drop shaft constructed on the line of the maintunnel. This would be a Type E CSO connection and no connectiontunnel would be required.

b. The King Edward Memorial Park Foreshore (C29XA) and King EdwardMemorial Park (C29XB) shortlisted sites could be connected to themain tunnel via a connection tunnel, and an intermediate shaft locatedon one of the shortlisted Zone S7 Limehouse main tunnel sites. Thiswould be a Type A CSO connection.

Z o n e G 3

G r e e n w i c h P S

D e p t f o r d S R

Z o n e G 2

Z o n e G 1

E a r l P S

Z o n e S 1 1

A b b e y M i l l s


GA r through n/a n/a through d r-r d

GB d through n/a n/a through r r-r d

GC r through n/a d then d through r r-r dGD r through d then d n/a through r r-r d

GF (con) r through n/a n/a through d with MT r-d r

GH d through n/a n/a through r d r

GI r through n/a d then d through r d r

GJ r through d then d n/a through r d r

CSO or Zone

Main tunnel

Greenwich connection tunnel Main tunnel

Z o n e S 6 S h a d





67

5.2.11 Table 5.4 presents the two NESR connection tunnel drive optionsassociated with the Type A CSO connection to be taken forward to thenext stage of the site selection process for multidisciplinary consideration.

Table 5.4 North East Storm Relief Type A CSO connection tunneldrive options matrix

KEMP (C29XB)/

KEMP Foreshore

(C29XA)

Main tunnel site zone:

S7 Limehouse


NA d r

NB r d

CSO site/Zone





68



6 Conclusions and recommendations


69


6.1.1 This report outlines the drive options that are available for the main tunnel(refer to Table 4.8); for the connection tunnels where two or more CSOs

would be intercepted and brought together prior to connecting to the maintunnel (refer to Table 5.1 and Table 5.3); and for the connection tunneloptions where the connection tunnels connect to the main tunnel via anintermediate shaft on the main tunnel (refer to Table 5.4). It supports thesite selection process by providing options for evaluation and selection.

6.1.2 The other four CSO connection types identified for connecting the existingCSO sewers/outfalls to the main tunnel are dependent on the selection of the proposed main tunnel drive strategy and proposed main tunnel sites.Therefore, further work will be carried out on CSO connections once themain tunnel proposals have been identified. System hydraulic preferences

for CSO connections will also be considered at this time.6.1.3 The review of the engineering criteria that affect the various main tunnel

drive options is discussed in Sections 4.2 and 4.3. The results show thatthere are advantages and disadvantages associated with each driveoption. These will be used as a basis for the engineering assessments inthe Section 48: Report on site selection process.

6.1.4 The report has demonstrated that there are appropriate engineeringoptions available to drive the main tunnel that meet the required criteria. Italso provides the basis on which to evaluate and determine the proposedsites and proposed drive option for the main tunnel and connection tunnels

for Section 48 publicity.





70



7 Next steps

7 Next steps

7.1.1 The information in this report, along with the assessments in the sitesuitability reports, will be brought together and discussed at a series of

optioneering workshops attended by the five disciplines (engineering,planning, environment, community and property). The process for theseworkshops for main tunnel sites and CSO sites is outlined below. Thedrive options will be considered and the direction of the individual TBMdrives determined based on the site assessments carried out as part of the site suitability reports in order to determine the use of each site.

7.1.2 For the main tunnel sites, this will involve workshops for the fivedisciplines:

a. firstly, to consider the site suitability reports in order to determine themost suitable site in each zone

b. secondly, to consider this Engineering options report – Abbey Millsroute (Spring 2012) in order to select the preferred drive optionassociated with the most suitable sites in each zone.

7.1.3 The purpose of the workshop is to identify the proposed main tunnel sitesand proposed drive strategy (ie, sites, types of site, and drive direction).

7.1.4 For the CSO sites, this will involve workshops for the five disciplines:

a. firstly, to consider the site suitability reports in order to determine theproposed site to intercept each CSO

b. secondly, to consider this Engineering options report –

Abbey Millsroute (Spring 2012) in order to select the proposed drive strategyassociated with proposed CSO sites that have more than one driveoption.

These considerations and assessments will be presented in the Section48: Report on site selection process.

Documents

110 RG PNC 00000 000826 EOR Abbey Mills Route