Basement Construction Method Statement 032031

245 Warwick Rd

Basement Construction Method Statement

032031

7 April 2014

Revision 00

Buro Happold

245 Warwick Rd Revision 00 Basement Construction Method Statement 7 April 2014 Copyright © Buro Happold Limited Page 3

Revision Description Issued by Date Checked

00 For Planning VA 07/04/2014 AP

O:\032031 245 Warwick Road Town Planning\F4 Structures\03 Reports\Basement Construction Method

Statement\CMS\140407 245 Warwick Rd Basement Construction Method Statement 00.docx

This report has been prepared for the sole benefit, use and information of Embassy Development Limited for the

purposes set out in the report or instructions commissioning it. The liability of Buro Happold Limited in respect of the

information contained in the report will not extend to any third party.

author Vincent Allott

date 07/04/14

approved Angus Palmer

signature

date 07/04/14

..

Buro Happold


Contents

1 Introduction 7

2 Site Description 8

2.1 Site Constraints 8

2.2 Unexploded Ordinance 8

3 Site History and Geology 9

3.1 Site History 9

3.2 Site Geology 9

3.3 Ground Profile 9

3.4 Groundwater 9

3.5 Preliminary and Detailed Design Parameters 9

4 Geo-environmental Assessment 11

4.1 Background 11

4.2 Preliminary geo-environmental assessment 11

4.3 Analytical results for soils 11

4.3.2 Analytical Results for leachate 11

4.4 Summary of Ground Risks 11

4.5 Further Testing and Considerations 11

5 Basement Design and Construction Methodology 12

5.1 Design 12

5.2 Preliminary pile design 12

1.1 Preliminary retaining wall design 13

5.3 Secant piled wall 14

5.4 Sheet piled wall 14

5.5 Waterproofing 15

5.6 Capping Beam 16

5.7 Movement Joint Strategy 17

6 Drainage strategy 18

6.1 Existing System 18

6.2 Proposed Drainage Strategy 18

Buro Happold


7 Further Considerations 20

Appendix A Indicative Site Plan

Appendix B Preliminary Structural Basement Drawings

Buro Happold


1 Introduction

Buro Happold have been appointed by Embassy Development Limited to provide Engineering Services for the proposed

development at 245 Warwick Rd, Kensington. This report describes the geotechnical, geo-environmental and structural

aspects of the basement design and construction method for the basement. It is to be read with and forms part of the

current S73 application (Ref 14/01234). The current scheme differs from that described in the original application due to a

reduction in basement area on both levels of basement. Refer to the Design and Access Statement issued 28/02/2014.

The design is currently developed to RIBA 2007 Work stage D. Where further detail is to be developed during stage E or

on appointment of a contractor, this is identified within the relevant section of this report (which will be superseded in due

course by a construction method statement provided by the contractor).

The project comprises a residential development of 3 buildings totalling approximately 30,500 square metres above

ground area. Two levels of basement will be provided, both of which link the superstructures’ footprints with the upper

level basement (Basement 1) covering the full site extent and lower level of basement (Basement 2) with a smaller

footprint. The upper basement contains the car parking space, with the remaining basement areas housing plant, storage,

building management facilities and gym space. The development is located within a two acre site including large areas of

landscaping at ground floor level.

Above ground level there is a centrally located building with superstructure 11 storeys high with buildings to the east and

west with superstructures 7 and 8 storeys high, respectively. The sites, both North West and South East of this site are

currently being developed by other parties (Berkeley Homes – St Edwards)

Buro Happold


2 Site Description

2.1 Site Constraints

The site is bounded on the North East by Warwick Road. To the South West is a train line which accommodates the

District Line and London Overground services. Previously the site was a car park and on the South Western boundary is

a substantial number of trees separating the site from the railway lines. The railway operators are to be contacted to

establish asset protection agreements between the parties and initiate appropriate approval/reviewing procedures.

To the North West is the 375 Kensington High Street development which is made up of a number of buildings of circa 13

storeys and includes basement levels. To the South East are the developments of the former Telephone Exchange

(Kensington Row Phase One) site as well as the Homebase site. These developments are also by Berkeley Homes (St

Edwards) and form part of the larger master plan.

Berkeley Homes will be contacted so that interface details and principles can be agreed to help finalise the basement

walls’ design and party wall agreements.

Construction access will be constrained as this can only occur from Warwick Road and is exacerbated by the fact that the

road only carries one-way traffic.

The site has less than a metre drop across it from East to West.

2.2 Unexploded Ordinance

Buro Happold has carried out a preliminary review of on-line bomb damage maps and this shows that it is improbable

that unexploded ordnance landed on the site.

A detailed risk assessment by a UXO specialist is however recommended. This would provide full presentation of the data

together with independent advice regarding the level of risk and the need for and scope of mitigation measures, upon

which separate reliance could be made.

Site Plan – Refer Appendix A

Buro Happold


3 Site History and Geology

3.1 Site History

The historical usage of the site has been traced by the previous ground investigation contractor using old Ordnance

Survey maps dating back to 1869. A summary is presented below.

The earliest available records, in 1869, indicate that the site was a coal depot; which was associated with the adjacent

railway tracks to the west of the site. The surrounding area was primarily residential and occupied by small areas of

residential housing interspersed with open fields and orchards. By 1896 the site remained a coal depot but the nearby

fields and orchards were developed, primarily for residential use. Also to the west of the site the railway sidings were

extended and are known as the Earls Court Junction.

By 1951 the site became a Territorial Army (TA) centre and the buildings to the north west of the site were demolished

and replaced by Charles House. The coal depot still remained to the south east of the site. In the surrounding area there

were buildings labelled as ruins; bomb damage maps obtained for the area show buildings in the area to have been

damaged. The 1953 map showed that the ruins were cleared from the site. The site was occupied by both the TA Centre

and a new building called Warwick House. The site and surrounding areas remained unchanged until 1972 when some of

the surrounding buildings were cleared and replaced with the Empress Telephone Exchange, which was located

immediately to the south east end of the site. The railway sidings to the West, residential houses to the south and the

depot to the South East were also all cleared, and the latter two areas were marked as car parks.

By 1983 the buildings on the site were cleared and the site appeared to be vacant. The most recent map (2005) shows no

significant alteration to the site or its surroundings.

3.2 Site Geology

A historic intrusive ground investigation was conducted by Card Geotechnics Limited on behalf of their client Nalex

Limited in December 2006. This comprised of 12No. window sampler locations and 5No. cable percussive boreholes. The

boreholes were completed to depths of 10m and 30 below ground level. Gas and ground water monitoring stand pipes

were installed in the boreholes upon their completion. Boreholes were logged and representative soil samples were

retrieved for laboratory analysis. The window sampler holes were completed to depths of 0.3-5m below ground level, into

the Kempton Park Gravels. Eight of the window sampler locations did not penetrate the Made Ground and were

abandoned due to what was assumed to be concrete obstruction. Dual gas and water stand pipes were installed in the

remaining four locations.

3.3 Ground Profile

The exploratory holes completed during the historic ground investigation generally encountered Made Ground overlying

Kempton Park Gravels and London Clay. A summary ground profile is presented in Table 3.1 below:

Stratum Typical thickness (m) Depth to top of stratum (mbgl)

Dark brown/black sandy slightly clayey fine to coarse gravels. Becoming more clayey with depth with fragments of tarmac, stone, concrete, ash, clinker. glass, wood, brick, and granite blocks [MADE GROUND]

1.4 – 3.2 0

Soft to firm orange brown sandy CLAY with occasional fragments of black carbonaceous gravel fragments, and sub-rounded to sub-angular fine to coarse gravels and occasional pockets of medium dense brown sand. [KEMPTON PARK GRAVEL/LANGLEY SILT)

0.8 – 1.9 1.4 – 2.4

Loose to dense orange brown. occasionally clayey, SAND and GRAVEL., Gravel is sub-rounded to Sub-angular, fine to coarse. [KEMPTON PARK GRAVEL]

1.15 – 4.7 1.7 – 4.1

Stiff to very stiff fissured locally orange mottled grey silty sandy CLAY with occasional claystone and selenite fragments. [LONDON CLAY]

Proven to 22.4 6.3 – 7.9

Table 3.1 – Ground profile

3.4 Groundwater

During drilling of the boreholes groundwater was encountered at approximately 5.5m below ground level in the Kempton

Park Gravels, which then rose between 0.23m — 0.65m after 20 minutes. During window sampling no groundwater was

encountered. During the monitoring visits groundwater was encountered in all five borehole locations and in one of the

window sampler locations (WS1). The groundwater was approximately 4.09 — 5.93 m bgl.

3.5 Preliminary and Detailed Design Parameters

Conservative geotechnical design parameters have been derived from the historic ground investigation and laboratory

testing. Further ground investigation and laboratory testing is being undertaken with greater relevance to the current

proposed scheme, details of which will be submitted to discharge the pre-commencement condition attached to the

planning permission for the site.

A summary of the design parameters derived from the historic investigation is presented in figure 3.1 below:

Buro Happold


Figure 3.1 – Design Parameter Summary

Buro Happold


4 Geo-environmental Assessment

4.1 Background

The Envirocheck report suggests there are no significant environmental constraints present on the site. The site’s past

use as a coal depot gives rise to the potential for some contamination within the Made Ground. The contamination is likely

to be in the form of heavy metals, hydrocarbons and possibly some organic contamination associated with the historic

uses of the site. The previous construction/ demolition activities on the site also give rise to a potential for various

contaminants (including asbestos) to be present in the fill / Made Ground.

4.2 Preliminary geo-environmental assessment

The chemical results from the Card investigation have been reviewed by Buro Happold and the soil test results have been

compared with available Soil Guideline Values (SGVs), Buro Happold and other industry Generic Assessment Criteria

(GAC). Consideration of the most sensitive receptors in the respective CLEA standard land- uses scenarios (the 0 to 6

year old child for the residential with and without plant uptake scenarios and the adult for the commercial/ industrial land-

use scenario) using the software model “CLEA 1.06” and associated handbook. These scenarios are suitably

conservative for the proposed development which includes residential apartments with limited soft landscaping.

Leachate samples have been assessed using UK Drinking Water Standards (DWS) and Freshwater Environmental

Quality Standards (EQS). The ground gas data has been assessed with reference to CIRIA report C665 NHBC guidance

(Ref.35) and the relevant British Standard.

A total of 13 soil samples, including eight in the Made Ground and five in natural strata, were subject to analysis. A

majority of samples recorded concentrations of determinands below screening thresholds. The following exceedences

were identified:

4.3 Analytical results for soils

Lead

The sample range of Lead was 7mg/kg - 800mg/kg. Three samples of the Made Ground (23% of dataset) exceeded the

residential threshold (450mg/kg) and a single sample (8% of dataset) exceeded the commercial threshold (750mg/kg). No

natural samples exceeded the screening thresholds. [Note: Defra is currently considering thresholds for lead which are

understood to be more conservative (i.e. lower) than the values referred to above].

Cyanide

The samples range of Cyanide was <1mg/g – 13.5mg/kg. A single sample of Made Ground (8% dataset) was identified

above the residential screening criteria (4.6mg/kg). No samples exceeded the commercial screening thresholds.

Total Petroleum Hydrocarbons (TPH)

The TPH has been assessed with TPH C6-C8 and C10-C40 carbon bands. Since the Card report the technique for

assessing hydrocarbons is established on the TPH Criteria Working Group (TPH CWG) which is based on an

Aromatic/Aliphatic split and smaller carbon banding. All C6 – C8 results were below detection limits and relevant

screening thresholds. A total of five Made Ground samples were tested for TPH C10 - C40, the sample range was

<10mg/kg – 600mg/kg. Two samples (40% of dataset) exceeded the Aromatic C10-C12 screening threshold (290mg/kg).

Polyaromatic hydrocarbons (PAHs)

A majority of PAHs from the Made Ground were below detection limits and screening thresholds. Samples from locations

in the Made Ground exceed the residential thresholds for benzo(a)pyrene (3 samples, 33% dataset), benzo(a)anthracene

(2 samples, 22% dataset), chrysene (1 sample, 11% dataset), benzo(b/k)fluoranthene (2 samples, 22% dataset),

indeno(123-c,d)pyrene (1 sample, 11% dataset) and dibenzo(ah)anthracene (1 sample, 11% dataset). One sample of

benzo(a)pyrene exceeded the commercial threshold (14mg/kg) which is 11% of the dataset. No samples of the natural

strata exceeded the residential or commercial thresholds.

Asbestos

No asbestos testing has been undertaken at the site.

4.3.2 Analytical Results for leachate

Some environmental samples in the Made Ground have recorded concentrations above screening criteria, including

barium, PAHs, benzo(a)pyrene and TPH. No exceedences are recorded within the natural strata. Some leachate samples

were identified above Environmental Quality Standards (EQS) and Drinking Water Standards (DWS). No groundwater

samples have been tested.

4.4 Summary of Ground Risks

Considering the proposed development, including a site wide basement and therefore removal of material, there is

negligible risk to human health at the site. Appropriate PPE and welfare facilities will provide mitigation of any residual

risks to groundworkers. Where soft landscaping is required, imported material should meet human health screening

criteria and be suitable from horticultural purposes.

4.5 Further Testing and Considerations

Considering the excavation required for the basement and the removal of material off site further chemical classification of

the Made Ground and natural strata will be required. Recent guidance (Waste Management 2, August 2013) will have to

be referred to for further waster classification, based on this most samples were non-hazardous, however WS5 identified

elevated zinc concentrations that may classify the material in this area as Hazardous Waste. This will require confirmation

with further chemical analysis (to include asbestos and WAC testing) as a part of the planned site Investigation.

Imported materials for soft landscaped areas will need to meet human health screening criteria and meet requirements for

horticultural purposes.

It is anticipated that the basement will be tanked to prevent the ingress of water and also ventilated. This is likely to

provide suitable gas protection measures, however this will need confirmation.

Buro Happold


5 Basement Design and Construction Methodology

5.1 Design

Preliminary basement and foundation design has been conducted and is presented here. The design makes use of the

conservative parameters derived in section 3.

The project consists of 3 separate buildings, with two levels of basement. The B1(upper) level basement extends the

width of the site whist the B2 (lower) basement is proposed to be reduced to about half the width of the site as part of the

current S73 application, as shown in Figure 5.1.

The basement walls to the South East and North West of the site are immediately adjacent to the neighbouring

basements. Communication is ongoing with the neighbours design team and Engineers specifically to determine specific

details of construction sequence and final condition of all structures. One of these structures has already been

constructed and the other is due to be constructed before the excavation works for this project begin. Both basements are

understood to be 2 levels deep. It follows that the South East and North West walls do not serve a retaining purpose and

therefore can be built using traditional reinforced concrete methods in open cut excavations. Based on this, a standard

reinforced concrete basement wall is currently planned and this can be constructed close to the site boundary by using

sacrificial formwork along the boundary, outside of the wall or the wall can be constructed with precast units. These could

be solid precast units or a hybrid solution using a combination of pre-cast and in-situ concrete in which the pre-cast

elements act as permanent structural formwork, such as the “twin-wall” solution.

To avoid structural interaction between neighbouring buildings, the 245 Warwick Road scheme currently shows piles

along the South Eastern single storey basement perimeter that are sleeved from pile cap level to approximately 2 metres

below the neighbour’s basement formation level. In the North West, with the double storey basement, the scheme has

piles that are set back from the boundary with ground beams cantilevering passed the pile line, out to the basement

perimeter to support the basement wall which in turn supports a certain extent of the ground and Basement B1 slabs.

In the current proposal the B2 basement is significantly smaller than the B1 basement. This will require a traditional

retaining wall, in open excavation. This retaining wall has not been considered here, but will be designed for the earth

pressure acting behind the wall.

The South West and North East walls adjacent to the railway and road respectively will both require a retaining wall. The

lengths and retained heights of these walls are fairly similar and therefore the forces they impart on the basement ‘bath

tub’ are well balanced. Any out of balance in the total force will be catered for by the shear capacity at the foundation pile

heads.

In recognition of the water table level and uplift forces, the Basement B1 (upper level) and Basement B2 (lower level) ,

slabs will be provided with different structural thicknesses to adequately resist the uplift at each level. These depths have

been based on a permanent ground water level at approximately +0.25 m AOD (from Geotechnical Investigation) and

assuming an accidental “burst main” scenario with a raised water table that dissipates relatively quickly.

The slabs will be formed on 75mm void-former on a blinding, to avoid the need for the slabs to be designed to resist

heave forces in addition to the water uplift loads.

Figure 5.1 – SE-NW Basement Section

5.2 Preliminary pile design

It is intended to carry vertical loads from the structure by rotary bored piles. These piles will typically have cut off levels at

B2 however, there will be piles with cut off at B1 where no B2 basement is currently planned. Preliminary pile design has

been conducted for pile sizes from 600mm diameter to 1500mm diameter. The piles have been designed using the LDSA

method. The following factors of safety has been used; FOS for shaft friction = 1.5, FOS for base capacity = 2.5, FOS for

overall capacity = 2.6. Figure 5.2 shows the safe working load (SWL) for various pile diameters with increasing depth.

Since net heave has not be appropriately considered at this stage, the piles have not been considered for tension

capacity. However, it is not thought the tension capacity requirement for piles with tower blocks above will govern the

geotechnical design. Areas of the site will experience a net unloading where there is no structure above ground level and

an excavation for either one or two levels of basement. It follows therefore that piles in these location will be subject to

heave and will be reinforced accordingly. Reinforcement for the piles subject to tension will need to be sufficiently long to

ensure the mass concrete portion of pile is not subject to tension and pile cracking does not occur. If tension

reinforcement is required at depth lower than the cage toe a single central bar shall be specified.

B1

B2 Retaining Wall

Buro Happold


Figure 5.2 - Pile SWL for various diameters with increasing depth

1.1 Preliminary retaining wall design

On the North East and South West boundaries with Warwick Road and the railway respectively adjacent to them, ground

retention is necessary during construction and permanently. This must be carefully achieved to high tolerances and strict

displacement criteria. The final design will ensure the highway and railway authorities’ displacement limitations are met.

Two retaining wall options have been considered. The first a rotary bored secant piled wall and the second a pressed in

sheet piled wall. Both of which use a similar bottom up construction sequences. The wall is designed to not required an

intermediate prop in both temporary and permanent conditions. This will avoid the need to use smaller excavators

underneath temporary props. The retaining wall allowing excavation to B2 is presented as it represents the worst case. A

shorter wall will be used in areas where excavation to B1 formation is required. Some consideration to the stiffness and

hence deflection compatibility of the two wall lengths will need to be conducted in detailed design. The sequence is as

follows:

1. Install wall 2. Conduct bulk excavation in majority of site leaving berms at either end 3. Install top temporary props 4. Excavate berm to formation level and install piles 5. Install basement slab and B2 retaining wall (refer figure 5.1) 6. Install ground floor slab 7. Remove top temporary props and install intermediate upper basement slab

The sequence is illustrated in the following sketches:

Stage 1 and 2: Install wall and bulk excavation leaving berms

Stage 3: Install temporary props

Stage 4: Excavate berm to formation level and install piles

Stage 5: Install basement slab and B2 retaining wall

Stage 6 and 7: Install ground floor slab, remove top temporary props and install intermediate slab

Figure 5.3 – Retaining wall construction sequence

Buro Happold


5.3 Secant piled wall

The secant pile wall has been designed to used 1050mm diameter hard male piles at 1200mm centres, with firm female

piles in between. The following list details inputs and outputs from the preliminary analysis and design process:

• A 16.25 kPa surcharge has been applied to the retained surface to represent 37.5 units of HB loading.

• Temporary prop stiffness assumed to be k= 50,000 kN/m/m.

• The permanent slab stiffness is assumed to be k=100,00 kN/m/m.

• Piled wall stiffness taken as EI=1390000 kNm2/m.

• Pile toe at -11.00mOD

• Pile cut off level at +5.5 mOD (worst case)

• The maximum bending moment in the long term case is 522 kNm/m [SLS] or 940 kNm/pile [ULS]

• The males piles require a cage of 8B32’s to full depth (Concrete design strength assumed to be 35 N/mm2.

• The maximum horizontal displacement in analysis is 25mm, this can be assumed to give a vertical settlement behind the wall of 13mm.

The secant wall will need to be stiff enough to limit lateral movements such that the resulting settlement trough behind the

wall does not adversely affect the railway or the road. Typically the aim is to limit the potential damage to Category 1, very

slight, in accordance with the CIRIA guidance (contained in “Building Response to Tunnelling”, 2001by Burland et al. first

put forward by Burland in 1977) however this is yet to be agreed with the relevant authorities and stricter requirements

may have to be met.

It is intended to provide a Grade 3 basement in accordance with BS8102. This provides a watertight environment with no

damp. This is appropriate and necessary for the gym, storage and many of the plant spaces. To achieve Grade 3, a high

grade of waterproofing is required. The secant wall will prevent flowing water and provides an effective first barrier

however additional waterproofing is essential and is discussed in more detail in a later section.

A 1050 diameter secant wall has been chosen within the scheme. This ensures minimal propping is required during

construction of the basement and with a carefully planned construction methodology and sequencing, propping can

potentially be eliminated. This diameter secant pile requires propping only at the top when the basement is fully

excavated over both storeys. If the ground floor is constructed with adequate restraint through cores/other prior to full

excavation, then propping can be avoided completely. Furthermore, this pile diameter also ensures that no prop at B1

level is necessary in the permanent condition.

A robust connection between the basement B2 slab and the secant wall is required. This ensures that the two elements

do not pull apart from each other horizontally and also that there is no differential vertical movement between the slab and

the secant wall. Large forces can potentially be generated horizontally and vertically due to shrinkage affects and

settlement of either the slab or the secant wall as live and permanent loading on the two vary and also during

construction, as the building size increases in varying stages.

The image below within the waterproofing section gives an indication of how this connection is achieved. Reinforcement

is cast into the male piles of the secant wall, amongst the main pile reinforcement. After the piles have been installed and

the wall face exposed, reinforcement that is later cast into the basement slab, will be attached to the pile connector

reinforcement by mean s of couplers. Alternatively, propriety void formers can be cast into the pile around the main pile

reinforcement. These are removed from the pile face when casting the slab and slab reinforcement can be hooked around

the main bars in the pile, all to be cast together.

Figure 5.4 – Diagram showing typical section through secant pile wall and achievable installation tolerances to be accounted for when

setting out the wall line (note: applies in opposite direction from centreline as well (away from basement))

5.4 Sheet piled wall

In general the alternative solution using a sheet piled wall is similar to the secant pile solution described above. Following

are specific parameters relating to sheet piled wall preliminary design:

Buro Happold


The sheet piled wall has been designed as PU32 high yield (S355) ‘U’ profiled sheets, with fully welded clutches. The

following list details inputs and outputs from the preliminary analysis and design process:

• A 16.25 kPa surcharge has been applied to the retained surface to represent 37.5 units of HB loading.

• Temporary prop stiffness assumed to be k= 50,000 kN/m/m.

• The slab stiffness is assumed to be k=100,00 kN/m/m.

• Piled wall stiffness taken as EI=148000 kNm2/m.

• Pile toe at -11.00mOD

• Pile cut off level at +5.5 mOD (worst case)

• The maximum bending moment in the long term case is 260kNm/m [SLS] or 390 kNm/m [ULS]

• The section modulus required is given by: z = (390*10^3)/(355/1.2) = 1318 cm3/m.

• The section of a PU32 wall is 3200 cm3/m. Giving a FOS of 2.4.

• The maximum horizontal displacement in analysis is 39mm, this can be assumed to give a vertical settlement of 20mm

5.5 Waterproofing

The basement waterproofing strategy for the elements below the ground floor, is discussed and summarised in this

section. BS 8102 2009 ‘Code of practice for protection of below ground structures against water from the ground’ was

used to assess the available waterproofing options.

The ground water table is above the level of the lowest basement slab, BS 8102 classifies this case as ‘High risk’ to the

ingress of ground water.

As discussed previously, grade 3 is a high specification that is suitable for habitable space and no water penetration is

acceptable.

Basement Walls

With a combination of a high risk basement and a high demand for the exclusion of water, a robust waterproofing strategy

that combines two types of protection, is required. Along the secant wall a drained cavity is to be provided. This is one

type of protection. The other, the passive protection, is to be provided by the secant pile wall itself with a waterproofing

and vapour resisting liner attached – a proprietary ‘egg crate’ drainage product. Water that does penetrate the basement

wall will run down all the way passed B1 level to B2 level, within the cavity and be drained away.

The drained cavity will be closed off from the internal spaces with a blockwork inner leaf. At B2 level there will be a

channel formed to falls leading to gullies and eventually being discharged to the Thames Water combined sewers. The

discharge of groundwater to the Thames Water combined sewers is subject to agreement and licensing with Thames

Water and will be based on an estimation of flows from the cavity drains. These are typically low and therefore typically

acceptable to Thames Water.

Along the North West and South East in-situ reinforced concrete walls, the use of waterproofing additive is proposed for

the concrete. This renders the reinforced concrete wall watertight. This is an alternate to more traditional, externally or

internally applied, water barriers which are also an option. The benefit with using the additive and waterproof concrete is

that it eliminates the need for sealing joints in membranes that are prone to leaks and there is no risk of damage to the

waterproofing as there is when pouring concrete against membranes.

The additive is included under strict manufacturer guidelines and it is often the case that the manufacturer would typically

have a representative on-site for any initial training and guidance that the contractor may require. This ensures a high

level of quality control to lead to the necessary waterproofing warranties.

Basement B2 Slab

Similar to the North West and South East walls, watertight concrete using waterproofing additive is proposed to seal the

basement slab. This waterproof concrete is only typically required for the upper 200-300mm layer of concrete. The final

depth will depend on the contractor responsible for the waterproofing, in agreement with the additive manufacturer. The

rest of the slab depth can be poured using ordinary concrete.

The layer of watertight concrete is sealed against the secant wall with hydrophilic strips within the layer depth, all to the

waterproofing manufacturer’s guidance and details.

Fig 5.5 - Secant pile wall cavity drain plan detail and waterproofing at basement slab level

Buro Happold


Fig 5.6 - Secant pile wall cavity drain section detail and waterproofing at basement slab level

5.6 Capping Beam

The capping beam to the perimeter secant piled wall will typically be a 1200 deep x 1550 wide reinforced concrete

section. The capping beam has two main functions:

1 In the temporary case, during excavation and basement construction, if the wall is to be propped (as

opposed to the ground floor being constructed prior to full excavation, then the capping beam acts as a

waler beam to spread the earth pressure from the piled wall onto the temporary lateral propping system.

Fig 5.7 Capping beam acting as waling beam

2 In the permanent case the capping beam allows the spread of load from superstructure columns through

transfer beams onto a sufficient number of male piles below. This is needed on the South Western secant

wall.

Fig 5.8 Capping beam acting to spread vertical load

Under this circumstance the capping beam will need to be significantly deeper than the typical 1200 deep section. A

depth in order of 1800mm may be required and this can be determined through detailed analysis in the next design stage.

Buro Happold


5.7 Movement Joint Strategy

No movement joints are proposed in either of the basement slabs as these are likely to be relatively thermally stable. An

appropriate pouring sequence is necessary though to ensure that most curing shrinkage has occurred before the full slab

extent has been poured and is connected up to restraining points such as the cores.

There will be two movement joints in the ground level slab running roughly parallel to Warwick Rd. The figure below

shows how ground level slab diaphragm action is used to transfer propped retaining wall loads to transverse walls and the

foundations:

Fig 5.9 Load path for retaining wall forces

Buro Happold


6 Drainage strategy

6.1 Existing System

Figure 6.1 Existing public sewer (combined) record from Thames Water

Details of the existing sewer sizes and locations at the Warwick Road have been taken from information provided by

Thames Water. Figure 1 above shows the location of the existing external sewer network around the proposed site.

A combined sewer is identified to be running in a north west- south east direction along Warwick Road. The combined

sewer size is 2591mm x 2438mm, and an invert level is recorded as IL 0.34m next to the north corner of the project site.

The road level is +5.5m approximately.

Within the project site, no existing sewers have been identified.

6.2 Proposed Drainage Strategy

Proposed Surface Water Drainage

A new surface water drainage system will be provided to collect surface water run-off from the proposed site.

The surface water run-off will drain from hard impermeable surfaces such as: the roof areas, access road and any other

hard standing areas. The proposed drainage strategy and surface water disposal is intended to mimic the existing

strategy and where possible, the existing connection point(s) will be reused and maintained.

Attenuation will need to be provided to reduce the risk of flooding to the development and downstream flooding of

neighbouring properties.

To minimise on-site and downstream flooding of neighbouring properties, surface water flows will be controlled at source.

This will involve on-site attenuation of storm flows and restricted discharge rates into the existing sewer combined in line

with the requirements of the London Plan where a reduction in the peak flows and volume of surface water from new

developments is expected.

As the proposed development site is fully occupied by the extent of the basements, the form of the Sustainable Drainage

System (SuDS) that can be considered able to deal with the quantity of surface water discharge quite limited.

Below ground attenuation in the forms of void formers, tanks or others of proprietary storage system will be considered to

provide on-site storage for severe storm flows. An alternative could be pre-formed GRP tanks with pumping units built in.

Storage above podium slab . The podium system is designed to accommodate any surface water run-off using

interlocking plastic storage system through a combined drainage component and sub base replacement system.

Design Criteria

The proposed surface water drainage system will be designed to the following criteria:

• No surcharge of the below ground network for a 1 in 2 year storm event;

• No flooding at ground level in a 1 in 30 year storm event;

• Storm events up to a 1 in 100 year storm shall be contained within the attenuation tank(s) and consideration

needs to be given in the event of a pump failure by providing additional storage capacity; and

• Climate change factor of 20% will be applied to the peak rainfall intensity.

Proposed Discharge Rate

The total site area is 0.842 hectares (ha) and existing impermeable area is approximately 75%.

The existing discharge is estimated to be:

Existing flow, Q = 2.78 x 50mm/hr x 0.842 ha x 75%

= 87.78 L/s

Based on the London Plan, the discharge is restricted to 50% of the existing rate. Therefore, the discharge rate for the

proposed development is restricted to 43.9 L/s for all storm events.

Table 6.1 - total flow from existing and new development

Buro Happold


Attenuation Calculations

Table 6.2 below summarises the volume of the below ground storage required to achieve the restricted discharge rates 43.9 L/s. The impermeable area for the project site is the total site area, 0.842ha. The storage volume have been calculated using the Windes Quick Storage Estimate Programme and 20% climate change (CC) has been applied, attached in Appendix B.

Table 6.2 - attenuation design to restrict expected runoff

Proposed Foul Water Drainage

A new and separate foul water drainage system will be provided to collect foul water generated from the new buildings. It is understood the project comprises a development of about 174 units for sale and 81 affordable housing units, along with 481 sqm of A1, A2, A3 or D1 uses.

Estimated Foul Loads

The calculations for the proposed foul loads are summarised in the next table.

Table 6.3 - foul water load estimate for the intended development

Buro Happold


7 Further Considerations

The following points address aspects specifically mentioned in the Royal Borough of Kensington and Chelsea

Subterranean Development SPD document:

Suitability of local geology to support loads and construction techniques imposed

The new basement structure has been designed based on site specific geological parameters. Refer section 5.

Impact of the subterranean development, and associated construction and temporary works, on the structural

integrity and natural ability for the movement of existing and surrounding structures, utilities, infrastructure and

man-made cavities, such as tunnels

Network rail have confirmed no underground services in the vicinity of the site. Overground services exist in close

proximity to the site and this has been taken into account in the basement design (refer section 5). Communication is

ongoing with Network rail in terms of approvals. Local service providers have been contacted and site surveys undertaken

to identify any existing or planned underground services in the area as part of the ongoing design process.

Whether the development will initiate slope instability which may threaten its neighbours

Coordination is on-going with the design teams of adjacent developments to the North West and South East boundaries

and this has determined the foundation strategy in these areas. Retaining walls to the North East and South West

boundaries are being designed to criteria acceptable to the transportation authorities affected in either case. Refer section

5.

Impact on drainage, sewage, surface water and ground water, flows and levels

The development will not have a significant effect on the perched water table. The preliminary design drainage strategy

conforms to local statutory requirements (refer section 5.5 and section 6).

How any geological, hydrological and structural concerns have been satisfactorily addressed

Refer to section 5

Engineering details of the scheme, including proposals for excavation and construction

Refer section 5 and Appendix B

Impact on the structural stability of existing and adjoining buildings

Refer above and section 5

Impact on existing and proposed trees

On the Western boundary is a substantial number of trees separating the site from the railway lines. The railway

operators are to be contacted to establish asset protection agreements between the parties and initiate appropriate

approval/reviewing procedures. The trees are not protected and do not have conservation area status.

The sequence for the temporary works and details and design of preferred method of temporary works

Section 5 describes the principles of the proposed temporary works method. Details will be finalised in conjunction with a

temporary works designer and the contractor in the more detailed phase of the design process.

Buro Happold

245 Warwick Rd Revision 00 Basement Construction Method Statement 7 April 2014 Copyright © Buro Happold Limited

Appendix A Indicative Site Plan

M

A

T

H

E

S

O

N

R

O

A

D

L

IS

G

A

R

T

E

R

R

A

C

E

L

B

5

.

2

m

Block B

Block F

Block E

Block G

Block C

Block A

Block D

INDICATIVE SITE PLAN ONLY

Buro Happold

245 Warwick Rd Revision 00 Basement Construction Method Statement 7 April 2014 Copyright © Buro Happold Limited

Appendix B Preliminary Structural Basement Drawings

UP

UP

UP

UP

UP

UP

C1 C2 C3 C4 C5 C6 C7 C8 C9 C10

CA

CB

CC

CD

CE

CF

CH

CI

CJ

WA

WB

WC

WD

WE

WF

WG

W1 W2 W3

AA

AB

A1 A2

A3

1

S-S1095

1

S-S1096

© Buro Happold Limited or its group companies.All Rights reserved. Buro Happold and its group companies assert (unless otherwise agreed inwriting) their rights under s.77 to 89 of the Copyright, Designs and Patents Act 1988.

DO NOT SCALE THIS DRAWING.

Notes

Rev Description Date Drn Ch'd

Status of drawing

Buro HappoldConsulting Engineers

Architect

Project

Drg Title

Scales@A0

Drawn By

Checked By

Date

Job No.

Drawing No.

Rev

Email:Web: www.burohappold.com

HEALTH AND SAFETY INFORMATION

IN ADDITION TO THE HAZARDS/RISKS NORMALLYASSOCIATED WITH THE TYPES OF WORK DETAILED ONTHIS DRAWING, NOTE THE FOLLOWING

CONSTRUCTION.

MAINTENANCE/CLEANING/OPERATION.

DECOMMISSIONING/DEMOLITION.

17 Newman StreetLondonW1T 1PDUK

Tel: +44 (0)20 7927 9700Fax: +44 (0)870 787 4145

1 : 200

S-S0998

032031

245 WARWICK ROAD, LONDON

SWANKE HAYDEN CONNELL ARCHITECTS

BASEMENT 2

STAGE D

[email protected]

SA

GF

14.02.2014 00

REFERENCE PLAN

SCALE 1 : 200

1 BASEMENT 2

00 STAGE D ISSUE 14.02.14 SA GF

1. FOR GENERAL NOTES REFER TO

DRAWING NUMBER 032031 / S-S0001

UP

DN

UP

DN

DN

UP

UP

UP

DN

DN

DN

UP

C1 C2 C3 C4 C5 C6 C7 C8 C9 C10

CA

CB

CC

CD

CE

CF

CH

CI

CJ

WA

WB

WC

WD

WE

WF

WG

W1 W2 W3

AA

AB

A1 A2

A3

1

S-S1097

1

S-S1098



Notes


Status of drawing


Architect

Project

Drg Title

Scales@A0

Drawn By

Checked By

Date

Job No.

Drawing No.

Rev




CONSTRUCTION.




Tel: +44 (0)20 7927 9700Fax: +44 (0)870 787 4145

1 : 200

S-S0999

032031



BASEMENT 1

STAGE D

[email protected]

SA

GF

14.02.2014 00

REFERENCE PLAN

SCALE 1 : 200

1 BASEMENT 1




UP

DN

UP

DN

DN

DN

DN

DN

C1 C2 C3 C4 C5 C6 C7 C8 C9 C10

CA

CB

CC

CD

CE

CF

CH

CI

CJ

WA

WB

WC

WD

WE

WF

WG

W1 W2 W3

AA

AB

A1 A2

A3

1

S-S1099

1

S-S1100



Notes


Status of drawing


Architect

Project

Drg Title

Scales@A0

Drawn By

Checked By

Date

Job No.

Drawing No.

Rev




CONSTRUCTION.




Tel: +44 (0)20 7927 9700Fax: +44 (0)870 787 4145

1 : 200

S-S1000

032031



GROUND FLOOR

STAGE D

[email protected]

SA

GF

14.02.2014 00

REFERENCE PLAN

SCALE 1 : 200

1 GROUND FLOOR




BASEMENT 1 SSL

+0.750 m

BASEMENT 2 SSL

-2.300 m

W1 W2 W3

W-1ST FLOOR SSL

+9.475 m

W-GROUND FLOOR SSL

+4.650 m

54003600

SSL +4.300 m

100 WIDE BLOCK WORKWALL TO ARCHITECTSDETAILS


CAVITY DRAIN BETWEENUPSTAND AND SECANTWALL AT B2 LEVEL ONLY

SITEBOUNDARY

2000 THK. R.C.FOUNDATION SLAB


900Ø R.C. PILE

500 THK. R.C. BASEMENTSLAB (250 WATERPROOF

CONCRETE)

1050 DIAMETER SECANTPILE WALL

250

1450x1200 DEEPR.C. CAPPING BEAM

HALFEN CHANNELS AT 900CENTRES WITH SLEEVEDWALL TIES ALLOWINGVERTICAL MOVEMENT

SOFT JOINT AT LEVELB1 TO ALLOW VERTICALMOVEMENT


225 THK. R.C. WALL225 THK. R.C. WALL

225 THK. R.C. WALL

500

TOP 250mm OF 500mmCONCRETE SLAB TO BEWATERPROOF CONCRETE

150

300

100 WIDE R.C. UPSTAND

100 WIDE R.C.UPSTAND

350

75mm THICK VOID FORMER ON50mm CONCRETE BLINDING(TYPICAL UNDER 500 THK AND 350THK BASEMENT SLABS)

BASEMENT 1 SSL

+0.750 m

BASEMENT 2 SSL

-2.300 m

W3

W-GROUND FLOOR SSL

+4.650 m






SOFT JOINT AT LEVELB1 TO ALLOW VERTICALMOVEMENT





SSL +4.300 m

TOP 250mm OF CONCRETESLAB TO BE WATERPROOFCONCRETE

250

500

350

SITEBOUNDARY


BASEMENT 1 SSL

+0.750 m

BASEMENT 2 SSL

-2.300 m

A-1ST FLOOR SSL

+7.225 m

A-GROUND FLOOR SSL

+4.050 m

A1 A2

SSL +3.700 m




100 WIDE BLOCKWORK WALL TOARCHITECTSDETAILS


300

TOP 250mm OF CONCRETESLAB TO BE WATERPROOFCONCRETE

650x650 R.C. COLUMN

8000

900Ø R.C. 3-PILE CAP -1800 DEEP

900Ø R.C. PILE

500

250

700x900 DPR.C. BEAM

700x900 DPR.C. BEAM

900x1200 DPR.C. BEAM


300x475 DPR.C. BEAM

200 THK. R.C. WALL

200 THK. R.C. WALL


350

SITEBOUNDARY


CACB

BASEMENT 1 SSL

+0.750 m

BASEMENT 2 SSL

-2.300 m

C-GROUND FLOOR SSL

+4.650 m

300 THK. R.C.WALL

(WATERPROOFCONCRETE)

250 THK. R.C.WALL



500

250

SSL +4.300 m

1800

450 THK. R.C. SLAB

300 THK. R.C. SLAB

300

450

350

900Ø R.C. PILE900Ø R.C. PILE900Ø R.C. PILE



1000x1200 DPR.C.GROUND

BEAM

450

375x1100R.C.

COLUMN

375x1100R.C.

COLUMN

300 THK. R.C. SLAB


CICJ

BASEMENT 1 SSL

+0.750 m

BASEMENT 2 SSL

-2.300 m

C-GROUND FLOOR SSL

+4.650 m

4450

SSL +4.300 m

250 THK. R.C.WALL



350

250

900Ø R.C. PILE900Ø R.C. PILE 900Ø R.C. PILE





300x1100R.C.

COLUMN

450 THK. R.C. SLAB

375x1300R.C.

COLUMN

375x600R.C.

COLUMN

350

450


CECF

BASEMENT 1 SSL

+0.750 m

BASEMENT 2 SSL

-2.300 m

C-GROUND FLOOR SSL

+4.650 m

1500x1600 DPR.C.BEAM

1800


275x1100R.C.

COLUMN

450 THK. R.C. SLAB

300 THK. R.C.WALL




250

350

500

250

900Ø R.C. PILE900Ø R.C. PILE



BASEMENT 1 SSL

+0.750 m

BASEMENT 2 SSL

-2.300 m

C-GROUND FLOOR SSL

+4.650 m SSL +4.300 m

250 THK. R.C.WALL


300 THK. R.C.WALL


400x400R.C.

COLUMN

400x400R.C.

COLUMN

300 THK. R.C. SLAB

900Ø R.C. PILE


1000x1200 DPR.C.GROUND

BEAM

500

250


450 THK R.C. SLAB

350




Notes


Status of drawing


Architect

Project

Drg Title

Scales@A0

Drawn By

Checked By

Date

Job No.

Drawing No.

Rev




CONSTRUCTION.




Tel: +44 (0)20 7927 9700Fax: +44 (0)870 787 4145

1 : 50

S-S5000

032031



SUBSTRUCTURE SECTIONS

STAGE D

[email protected]

SA

GF

14.02.2014 00

SHEET 1

SCALE 1 : 50

1 SUBSTRUCTURE SECTION 1-1SCALE 1 : 50

2 SUBSTRUCTURE SECTION 3-3SCALE 1 : 50

3 SUBSTRUCTURE SECTION 2-2

SCALE 1 : 50


SCALE 1 : 50


SCALE 1 : 50


SCALE 1 : 50



1. FOR GENERAL NOTES REFER TODRAWING NUMBER 032031 / S-S0001

FOR WATERPROOFING MEASURES AT B2 LEVELAND RETAINING WALLS REFER TO STAGE D

ENGINEERING REPORT

Vincent Allott Buro Happold Limited 17 Newman Street London W1T 1PD UK

Telephone: +44(0) 207927 9700 Facsimile: +44(0) 870787 4145 Email: VA

Documents

Basement Construction Method Statement 032031