Design and Performance of Retaining Walls

Proceedings of the Institution ofCivil EngineersGeotechnical Engineering 163October 2010 Issue GE5Pages 279–290doi: 10.1680/geng.2010.163.5.279

Paper GE-D-09-00052Received 26/06/2009Accepted 06/07/2009

Keywords: design methods & aids/field testing & monitoring/retainingwalls

Howard RoscoePrincipal GeotechnicalEngineer, Atkins DesignEnvironment & Engineering,Risley, UK

David TwineDirector, Ove Arup andPartners, London, UK

Design and performance of retaining walls

H. Roscoe MSc, DIC, CEng, MICE and D. Twine MSc, DIC, CEng, MICE

Embedded retaining walls for 1.8 km of cut-and-cover

tunnels and earth-retaining structures were redesigned

during the construction period to optimise construction

methods and temporary propping. The design approach

included many of the developments now recommended

in Ciria report C580, and the paper summarises the

experience gained on this contract. The site team made

detailed observations of the performance of all 14

structures during construction, and this paper gives an

overall summary of the wall movements and prop loads

that were measured. The walls were surcharged, and

none of the props was preloaded. Despite this, the

maximum movements were within those estimated

from published correlations. Wall movements were time

dependent, and occurred at rates of up to 0.2 mm/day.

Prop loads were generally about 40% of the values

obtained from moderately conservative calculations.

Reducing the prop stiffness assumed in calculations

improves agreement, and measurements are reported

that provide a basis for closer appraisal of this aspect in

future designs.

NOTATION

c9 effective cohesion

E Young’s modulus of concrete

E9h horizontal drained Young’s modulus of soil.

I second moment of area

K0 coefficient of earth pressure at rest

s9 mean principal stress

t9 shear stress

�9 effective angle of friction

1. INTRODUCTION

Geotechnical engineers have long recognised the importance of

field observations to provide insight into behaviour

mechanisms, and as a guide to the selection of design

parameters (Burland et al., 1979). The cut-and-cover tunnels at

Ashford were carefully monitored during construction,

providing the opportunity to compare the design of embedded

retaining walls with their performance.

The tunnels were designed during the period 1998–2002.

Various different standards were current (BSI, 1994; Highways

Agency, 1994; Padfield and Mair, 1984), and further changes

were being introduced to bring geotechnical design within the

Eurocode framework (BSI, 1997). A consistent approach was

developed for use throughout the Channel Tunnel Rail Link

(CTRL) project, now known as High Speed 1 and much of this

is incorporated in recent guidance on the design of embedded

walls (Ciria report C580: see Gaba et al., 2003).

This paper describes the geometry of 14 retaining structures

that make up the tunnel complex, and the soil and water

conditions in which they were constructed. It sets out the basis

of the design, and comments on those aspects that affected

work on this contract.

Detailed monitoring measurements were made during the

construction of all 14 structures (Holmes et al., 2005). The

paper summarises the wall movements and prop loads that

were measured, and compares these with the design

calculations. Wall movements continued after excavation, and

the paper gives the rates of movement that were measured.

Prop load measurements are reported that identify the

important effect of prop stiffness. They show that the prop

stiffness values that are often assumed in design are unrealistic.

Alternatives are suggested that would improve the accuracy of

design calculations.

2. ASHFORD TUNNELS

CTRL passes through Ashford, Kent in two cut-and-cover

tunnels that, together with approaches and linking earth-

retaining structures, amount to 1.8 km of contiguous bored

piled retaining structures. The tunnels were constructed by

Skanska Construction (UK) in partnership with engineers and

project managers Rail Link Engineering (RLE) on behalf of

Union Railways.

Figure 1 shows the common features of the embedded retaining

walls at Ashford. The tunnel site sloped down from north to

south, so that preliminary excavation to the level of the tunnel

roof generated greater surcharges on the north walls than on

the south. The walls were formed with contiguous bored piles

of between 900 mm and 1350 mm in diameter (Roscoe et al.,

2002). Cementation Foundations Skanska bored the piles dry,

but added bentonite slurry to stabilise the bores during cage

installation and concreting. A deep dewatering system using

ejector wells was installed to control the water pressures in the

underlying Weald Clay.

Geotechnical Engineering 163 Issue GE5 Design and performance of retaining walls Roscoe • Twine 279

The tunnel complex was divided for design purposes into 14

different structures.

Figure 2 gives the local names of the structures, the method of

construction that was used, and the extent of deep dewatering

in the Weald Clay, and Table 1 gives the key dimensions of the

walls.

3. GROUND CONDITIONS

Figure 1 shows typical ground conditions at this site, and Table

2 gives the overall succession and summary descriptions of the

soils. Table 1 summarises the stratum boundaries at each

structure.

3.1. Investigations

Over 80 cable percussion borings and 15 rotary-cored borings

were made prior to construction. Standard penetration tests

were performed at regular intervals, and undisturbed 102 mm

samples and rotary cores were obtained. The rotary cores were

split, examined and photographed. The undrained strength and

effective stress shear strength parameters of the Atherfield and

Weald Clays were measured in laboratory triaxial tests.

3.2. Permeability

One of the key issues facing the designers was the permeability

of the Atherfield and Weald Clays. The site investigations

showed that the Weald Clay contained silt partings and some

bands of siltstone, but the results of permeability tests carried

out in standpipes were inconclusive, and to some extent

contradictory. Full-scale pumping tests carried out in the early

stages of the contract (Roscoe and Twine, 2001) showed that

the Weald Clay was strongly anisotropic, with vertical and

horizontal permeabilities of around 10�9 m/s and 10�7 m/s

respectively.

The permeability of the Atherfield Clay also influenced the

design, but was less well understood. Further dewatering trials

made during the construction of the Gasworks Lane propped

cut showed that Atherfield Clay should be treated as a drained

material, but that dewatering the Atherfield Clay was

impractical.

3.3. Design parameters

The designs generally adopted moderately conservative peak

values for soil properties (Table 3). The strength parameters for

the Atherfield and Weald Clays were derived from the triaxial

test results plotted in Figures 3 and 4. Angles of friction were

increased by 28 to reflect the difference between triaxial and

plane-strain stress states.

Back-analysis of the first structure to be excavated justified the

use of ‘most probable’ soil properties in conjunction with the

observational method. The values shown in Table 3 were

modified by increasing the effective cohesion of Atherfield

Clay to 10 kPa.

There was a wide scatter of undrained strength measurements

in both the Atherfield and Weald Clays, and so undrained

strengths were not used (Roscoe and Twine, 2001). Stiffnesses

were assumed to increase linearly with depth below ground

surface. Horizontal drained Young’s modulus, E 9h, was taken as

about 600 times undrained strength.

The coefficient of earth pressures at rest, K0, was taken as 1.0

to allow for the effects of pile installation. Research work

during the contract (Clark et al., 2004) shows that this was a

HytheBeds Overburden

thickness

Surcharge height

Retained height

Thickness:passive

Surchargeoffset

Ejector wells

Atherfield Clay

Thickness: total

Weald Clay

Responsezone

Penetration10 m

Temporarysand drain

Figure 1. Common features of embedded walls

Constructionmethod

Top down

Permanentpropped

Bottom up

Retainedcut

Chainage88 700� 88 900� 89 100� 89 300� 89 500� 89 700� 89 900� 90 100�

Scale

0 100 200 m

Deep dewatering in Weald Clay

2 Prop1 prop Chord

walls2 props

Utilitiesbridge

Box names: ChartRoad

Advance Chart Roadto Maidstone

Railway

MaidstoneRailway

Greensands Way

Pro

p’d

4B 4A 3 2 1

Gasworks Lane Cattlemarketto Beaver

Road

BeaverRoad

Prop’d RetainedCut

N

Figure 2. Schematic plan of Ashford tunnels

280 Geotechnical Engineering 163 Issue GE5 Design and performance of retaining walls Roscoe • Twine

reasonable assumption, but that a somewhat lower value of K0

might have been adopted.

4. GROUNDWATER

Groundwater level prior to construction was between 1 m and

2 m below ground surface. Water levels in the Hythe Beds

remained high during the construction period, but were

reduced to base of capping beam level by site drainage.

During construction, the water pressures in the underlying

Weald Clay were lowered by over 10 m using vacuum ejector

wells installed by WJ Groundwater Limited. The extent of deep

dewatering is indicated in Figure 2. Permanent gravity wells

were provided to relieve pore pressures beneath the base slabs

once the ejector system was removed, and the design water

pressures were modified to allow for their effect.

Water levels in the Atherfield Clay were recharged from the

Hythe Beds, and influenced by drainage to the underlying

Weald Clay. Temporary sand drains were installed to prevent

pore pressure building up beneath tunnel formation, and a

hydrostatic profile was adopted in design. Measured pore

pressures were somewhat lower than this, but were

inconsistent, and were not incorporated in the design.

5. CONSTRUCTION METHODS

Figure 5 illustrates the methods of excavation and temporary

propping. The structures were built bottom up wherever

possible and eight of the structures were redesigned during

construction to use this method. Two of the structures carried

existing road and rail links that could not be severed and were

constructed top down.

Figure 6 shows bottom-up construction in progress at

Greensands Way. There is open access for excavation and base

slab construction, and tubular steel props have been installed

to support the walls in their temporary condition. The roof slab

was cast on falsework after the base slab was complete. The

retained cuts at the east end of the complex (Figure 2) used the

same temporary support method.

The permanently propped structures were designed with

reinforced concrete props at capping beam level (Figure 5).

Following value engineering, the designers increased the prop

spacing to allow excavators and cranes to work between the

props, and these structures were built bottom up (Roscoe and

Twine, 2001). The much larger Chart Road to Maidstone

Railway box was completely redesigned for bottom-up

construction using permanent reinforced concrete props

(Loveridge et al., 2008). The roof slab was supported from the

props using precast beams and an in situ concrete infill.

The Chart Road box was originally a top-down structure, but

was redesigned for bottom-up construction, as shown in

Figure 5.

The advance box (Figure 5) and the Maidstone Railway box

(Figure 7) were built top down. The reinforced concrete roof

slabs were constructed prior to excavation, and supported on

plunge columns or on central piles. Soil was then excavated

beneath the roof slab, and temporary props were installed to

support the walls until the base slabs were cast. Figure 7

Box

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shows excavation approaching formation level in the

Maidstone Railway box. Ventilation, lighting, cranage

restrictions and limited access all add to the cost of top-

down construction.

6. PROPPING

The structures with permanent concrete props are shown in

Figure 2. Table 4 gives prop dimensions and spacing. The

concrete props and roof slabs were supported on corbels to

form pinned connections with the walls. Exceptionally, a full

moment connection was provided between the roof and walls

of the Greensands Way structure.

Temporary tubular steel props were purpose-made for the

contract, and Figure 8 shows the prop and waling system. Each

prop could carry a constant-temperature load of 5400 kN, with

a further 1895 kN due to temperature.

The system allowed for multiple use of the props, and both

length and spacing could be adjusted. Props in the four-track

structures (Figure 7) were assembled from two of the props

from the two-track structures (Figure 6) and a shorter make-up

piece. Prop spacing could be increased from 4.5 m to 6 m by

extending the walers during installation.

Concrete-filled bags packed the walers from the wall piles, and

in situ concrete thrust blocks were cast between props and

walers. The props were not preloaded. Hydraulic jacks were

inserted during prop removal so that the thrust blocks could be

lifted or broken out and the prop loads reduced incrementally.

The system proved robust in practice, and rapid to install. In

normal production a gang of four could prop 18 m of tunnel in

three shifts.

At Beaver Road retained cut the calculated prop loads exceeded

the capacity of proprietary propping systems (Loveridge, 2001).

Lower loads were estimated using the distributed prop load

(DPL) method (Twine and Roscoe, 1999), in conjunction with

load measurements from adjacent sections of the work

(Loveridge, 2001). This justified the use of hired props and

saved over £150 000.

Temporary props were installed at about 400 locations but

none was damaged or displaced by the ongoing construction

activities. With appropriate site control and up-to-date safe

working practices it is not considered necessary to treat the

loss of a prop as one of the design cases.

Stratum Description Typicalthickness: m

Fill and alluvium Loose silts and clayey sands 0–6Hythe Beds Silty and clayey fine sand with sandstone bands 0–8Atherfield Clay Stiff or very stiff clay, frequently closely fissured with intermittent thin partings of silt 0–13Weald Clay Stiff or very stiff clay containing many silt partings and laminations and thin bands of siltstone Up to 120

Table 2. General succession

Soil Plasticity index: % �9: degrees c9: kPa K0 E9h, initial (gradient): MPa

Hythe Beds 22 32 0 1.0 1.8 (4.86)Atherfield Clay (HP) 54 26 0 1.0 3.6 (3.64)Atherfield Clay (IP) 32 23 4 1.0 3.6 (3.64)Weald Clay 36 25 0 1.0 18.2 (3.20)

Initial values of horizontal stiffness E9h are taken at ground surface.‘Most probable’ analyses used a value of c9 ¼ 10 kPa in Atherfield Clay but no other soil properties were altered.

Table 3. Soil properties: moderately conservative values used for serviceability limit state design

200

150

100

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ar s

tres

s,: k

Pa

t �

0 50 100 150 200 250 300 350 400 450Mean principal stress, : kPas�

Most probable( 10 kPa, 24°)c� � � �φ

Moderately conservative( 0, 24°)c� � � �φ

Figure 3. Triaxial test results: Atherfield Clay

250

200

150

100

50

0

She

ar s

tres

s,: k

Pa

t �

0 100 200 300 400 500

Mean principal stress, : kPas�

Moderately conservative( 0, 26°)c� � � �φ

Figure 4. Triaxial test results: Weald Clay


7. DESIGN

The designs followed the procedure developed by RLE for use

throughout the CTRL project. BS 8002 (BSI, 1994) was adopted

as the design standard for cantilever and singly propped

embedded walls. Many of the structures supported road and

rail loads, and the procedure included additional steps to show

compliance with BD 42/94 (Highways Agency, 1994). The

Ashford walls fall outside the scope of these documents, and

further reference was made to Eurocode 7 (BSI, 1997) and to

other current developments. This approach has since been

Relief dig

Temporaryberm

Temporaryberm

Retainedheightassumed

Chart Road(bottom up)

Roof slab

Plungecolumns

Advance box(top down)

Greensands Way(bottom up)

RC permanent prop

Gasworks Lane(permanent propped)

Figure 5. Temporary propping arrangements

Figure 6. Bottom-up construction: Greensands Way Figure 7. Top-down construction: Maidstone Railway box

Structure Prop size:m

Prop spacing:m

Chart Road to Maidstone Railway 1 3 2.3 6Greensand Way propped cut 1 3 1 6Gasworks Lane 1 3 1 4.5Cattlemarket to Beaver Road 1 3 1 4.5

Table 4. Permanent prop dimensions


incorporated in published guidance on the design of embedded

walls (Gaba et al., 2003).

Reinforced concrete design was carried out to BS 5400 (BSI,

1990).

7.1. Analysis

With few exceptions, the bending moments, shear forces and

prop loads were determined using the Oasys computer program

Frew. Separate analyses were made for each wall of each

structure. The soil was modelled as an elastic-plastic material,

and the soil stiffness matrix was derived from the input

stiffness and dimensionless results of finite-element

calculations included in the software.

The Greensands Way propped and Greensands Way section 4B

structures are complicated by additional retaining walls known

as chord walls (Figure 2). They were analysed with Oasys’s

finite-element computer program Safe. The soil was modelled

as an elastic-perfectly plastic Mohr–Coulomb material. Each

analysis modelled a complete cross-section, allowing directly

for the interaction between all four walls.

7.2. Limit states

The walls were checked for both serviceability limit state (SLS)

and ultimate limit state (ULS).

The SLS analyses used the design values of soil properties

(Table 3) without modification, and the structural capacity of

the piles was determined by calculating crack widths.

ULS was checked in two ways.

(a) The bending moments and shear forces from the SLS

analysis were multiplied by 1.35.

(b) A separate analysis was made using factored soil properties

obtained by dividing the SLS design values by a

mobilisation factor of 1.2 (BSI, 1994).

In each case, the resulting bending moments and shear forces

were multiplied by a partial load factor of 1.1 (BSI, 1990) and

compared with the ultimate structural capacity of the piles.

7.3. Wall stiffness

The Young’s modulus of the uncracked Grade 40 concrete was

taken as 31 3 106 kPa and multiplied by the second moment

of area (I ) of the piles to give an uncracked short-term wall

stiffness (EI ). Wall stiffness varies during the life of the

structure, and design values were derived as shown in Table 5.

It was assumed that the walls of top-down structures would not

crack during construction, but that for bottom-up structures

cracking would reduce the second moment of area of the walls

(I ). For long-term conditions the wall stiffness was reduced to

50% of the uncracked value to allow for creep and relaxation

of the concrete.

7.4. Prop stiffness

Initial analyses used the elastic stiffness of the props without

reduction, assuming them to be 100% efficient. Back-analysis

of the first structure to be excavated showed that observed

1016 22·2 CHS prop�

Twin610 305 238 UB

Waler� �

Twin254 254 167 UC

Extension waler� �

Detail ‘A’

0 5 m

Scale

Detail ‘A’Walingbeam Concrete

bags750 950 in situ

concrete thrust block�

35 mm thickend plate

1016 22·2 CHS�

Stiffeners

Jacking pointsfor prop removal

Figure 8. Temporary propping system

Stage of analysis Bottom-upconstruction

Top-downconstruction

During construction 0.7EI 1.0EILong term 0.5EI 0.5EI

Table 5. Derivation of wall stiffness


performance could be matched only if a lower prop stiffness

was assumed. In later analyses a prop efficiency of 50% was

adopted. The effect of this change is discussed in Section 8.3.

In contrast, loads used to model temperature effects (expansion

and contraction of the props) were calculated using a prop

efficiency of 60% (Twine and Roscoe, 1999) from the outset.

7.5. Comments

Design work at Ashford provided early experience of current

guidance on the design of embedded retaining walls (Gaba et

al., 2003). This approach was used in the initial designs for the

bored piled walls, and by the site team to optimise the

temporary works and construction sequence.

7.5.1. Limit states. In the optimised designs, bending

moments and/or shear forces were close to the limiting

structural capacities determined as described in Section 7.2.

Twenty-one sections were analysed. The calculated bending

moments were within 95% of the limiting capacity in nine

cases, and in a further nine cases the calculated moment lay

between 85% and 95% of the limiting capacity. The most

critical limit states were distributed as follows

(a) SLS: 15 cases

(b) ULS determined from an SLS analysis: two cases

(c) ULS analysis: three cases

(d ) ULS and SLS equally critical: one case.

Analysing each of the walls for both limit states increases

design costs, but this distribution shows that the most critical

case could not be identified in advance.

Once the governing limit state was clearly established,

subsequent amendments to the construction sequence were

justified for that limit state only.

7.5.2. Wall stiffness. The uncracked wall stiffness was used to

analyse the top-down structures, but when excavation was

complete, flexural cracks were seen at about the level of the

maximum bending moment. Clearly, the vertical load on the

walls had not been sufficient to prevent cracking. Using the

cracked modulus in the analyses would have reduced the

calculated bending moments and increased the calculated wall

movements. The temporary props could have been set at lower

levels to accelerate excavation and reduce costs.

7.5.3. Relaxation. Frew can model the relaxation of concrete,

taking account of the reduction in bending moment that results

from the lower long-term stiffness (Gaba et al., 2003), but

initial analyses for the Gasworks Lane propped cut did not use

this facility, overestimating long-term moments by about 30%

of their ‘relaxed’ values. This led to an unnecessarily slow and

cautious approach during the first excavations. Subsequent

designs took benefit from relaxation to reduce calculated long-

term moments.

Concrete starts to relax from the time that load is applied. This

effect is allowed when considering long-term cases, but may

also influence the temporary stages. Design of the Maidstone

Railway box allowed 25% relaxation in the later stages of

construction following prop removal.

8. PERFORMANCE DURING CONSTRUCTION

Over 700 instruments were installed during the contract, and

all 14 structures were carefully monitored during construction

(Holmes et al., 2005). This paper summarises the maximum

wall movements and prop loads that were measured. Wall

movements during the cantilever stage of the excavations and

the effects of asymmetric loading (sway) are described

elsewhere (Loveridge et al., 2008; Roscoe, 2003).

8.1. Maximum wall movements

Figures 9 and 10 show typical deflected shapes for top-down

and bottom-up construction. Maximum movement occurred

close to the base of the top-down excavations. For bottom-up

construction cantilever movements at the top of the wall

(Roscoe, 2003) governed both deflected shape and maximum

movement.

Figure 11 plots the maximum wall movements at the end of

excavation against retained height H. These measurements

include toe movements of about 5 mm (Holmes et al., 2005) as

indicated on the figure. Also shown are the linear relationships

often used to estimate the movement of embedded walls (Gaba

et al., 2003).

Wall movements for the top-down and permanent propped

structures are generally close to the 0.15%H line, with an upper

bound at about 0.2%H. Results from the structures that

included an initial cantilever stage are sensibly bounded by the

0.4%H line. The Greensands Way structures were constructed

using the observational method to utilise more of the walls’

capacity, and movements were proportionately greater than

elsewhere. Larger movements were recorded at inclinometer CR

IC1 in the Chart Road box and at the utilities bridge. These are

explained by a long delay in construction, and inundation

during regional flooding.

The Ashford walls support significant surcharges (Figure 2),

and at Gasworks Lane they retain up to 5.8 m of overburden

rather than stiff clay. None of the props was preloaded during

installation. Despite these factors, the wall movements lie

within the expected range.

Figure 12 summarises the ratios between measured and

calculated movements. Measured movements were about 40%

of those calculated using a moderately conservative approach,

but for finite-element analyses were grouped about the

expected value of 70%. The ratios for ‘most probable’ analyses

cover a wide range, but only rarely approached 100%. Many of

these analyses were critical only for the initial cantilever stages

of the excavation.

8.2. Time-dependent movement

It has long been recognised that ground movements around

excavations in stiff clay have a significant time-dependent

component (Burland et al., 1979). This aspect is particularly

important where an observational method is being used and

controls are set on the rates of movement (Holmes et al., 2005),

and at Ashford results from the first structures to be

constructed were used to optimise the subsequent excavations.

Wall profiles at different stages of construction (Figures 9 and

10) show that movement continues during periods when there


is no excavation or change to the propping. This is also seen in

Figure 13, a graph of maximum movement against time in the

Maidstone Railway box.

Figure 14 summarises all the movements that were recorded

between the end of excavation and base slab completion. Rates

of movement were generally less than 0.2 mm per day, and

Chart RoadAdvanceChart Road/Maidstone RailwayMaidstone RailwayGreensand Way – proppedGreensand Way – 4BGreensand Way – 1 to 4AGasworks LaneCattlemarket to Beaver Road

(2 props)

0·15%

(2 props)

0 5 10 15 20

Retained height, : mH

5 mm

(2 props)

Utilitiesbridge

(2 props)

0·4%

Formationleft 11

months

CR IC1

Utilitiesbridge

60

50

40

30

20

10

0

Max

imum

mov

emen

t at d

ig to

form

atio

n st

age:

mm

Figure 11. Wall movement against retained height

0

5

10

15

20

25

30

Dep

th: m

0 20 40 60 80 100

Wall movement: mm

Latest 18 April 2001

Top of wall

Temporary prop

Base slabFormation

Key dates

Max. cantilever 13 May 2000

Dig below prop 30 May 2000

Dig to formation 5 June 2000

Cast slab 21June 2000

Toe of wall

Figure 10. Wall movement profiles: bottom-up construction,Greensands Way box

Roof slab

Temporaryprop

Base slabFormation

Key dates

Excavate to temporary prop23 February 2001

Toe of wall

Excavate to formation15 May 2001

Cast base slab 28 July 2001

Destress props 4 August 2001

Latest 12 September 2001

�20 0 20 40 60 80 100

Wall movement: mm

40

35

30

25

20

15

10

5

Dep

th: m

Figure 9. Wall movement profiles: top-down construction,Maidstone Railway box


were approximately linear over the period of observation (up to

75 days). Negative movements were measured on bottom-up

structures, where maximum deflections at the tops of the walls

decreased as the walls rotated about the props in response to

increasing movements lower down.

The time-dependent movements at this site cannot be

explained by consolidation of the Weald and Atherfield Clays.

The Weald Clay has high horizontal permeability, and responds

to changes of load in a drained manner. The consolidation

period for a 13 m thick layer of Atherfield Clay is only a few

days, and layer thickness in the passive zone is generally much

less (Figure 1). Further work is needed to quantify the relative

importance of soil creep and relaxation of the concrete

structure.

8.3. Prop loads

Loads were measured in 36 sets of three props (Holmes et al.,

2005). Variations in load within each set were generally within

�25% of the average, and all load measurements in steel props

were corrected for temperature effects during processing. The

results plotted in Figure 15 are typical. The loads dropped as

the base slab expanded after being cast, but then increased

with time by between 10% and 30% of the load at the end of

excavation.

Figure 16 compares the average load for each set measured

at the end of excavation with the calculated value at that

stage. The calculations were performed on several different

bases. The data include results from both Frew and Safe

analyses, using either moderately conservative or most

probable soil properties, and assuming the props were either

100% or 50% efficient.

Measured loads were about 40% of those calculated in

moderately conservative analyses, significantly lower than

the expected ratio of about 70%. The Greensands Way

structures were analysed using more probable soil properties,

and measured loads were closer to 70% of the calculated

values.

Prop stiffness has an important effect on the accuracy of the

calculations. The loads from analyses that assumed a prop

efficiency of 50% are marked with a tick in Figure 16. They are

more realistic than the loads calculated by assuming the props

were fully efficient.

Table 6 summarises the load data for structures that were

propped at two levels. Without exception the upper props

attracted a much greater percentage of their calculated load.

This effect is most marked where the upper props are stiff

reinforced-concrete permanent props. Loading during the first-

stage excavation tightens the upper props and increases their

efficiency. They then attract a greater share of load during the

later stages.

These data show that props are not 100% efficient. Structural

engineers commonly take a prop efficiency of 60% when

calculating temperature loads. This approach may be

appropriate when selecting a prop stiffness for wall analysis,

but would not allow for the ‘hardening’ of props after first

loading, as evidenced in Table 6. This could be modelled by

allowing initial movement before the prop is activated, or by

increasing prop stiffness at an intermediate stage of the

calculation.

9. CONCLUSIONS

Some 1.8 km of cut-and-cover tunnels and earth-retaining

structures were redesigned during the contract period to use

bottom-up methods of excavation wherever possible, and to

take benefit from deep dewatering in the Weald Clay.

This contract provided early experience of the approach now

incorporated in CIRIA Report C580 (Gaba et al., 2003). Both

ultimate and serviceability states were analysed, and in 85% of

cases the serviceability analyses were the more critical.

Designs that assumed that the walls of top-down structures

would not crack during construction were found to be unduly

conservative.

Modelling relaxation of the concrete structure gave significant

reductions in calculated bending moments. Relaxation may in

some circumstances apply to temporary situations, as well as in

the long term.

Many of the walls at this site were subject to significant

surcharges. None of the props was preloaded prior to

70%

10

5

0

No.

of

case

s

20 40 60 80 100

5

0

No.

of

case

s

20 40 60 80 100

5

0

No.

of

case

s

20 40 60 80 100

Measured movementCalculated movement

(a)

: %


(b)

: %


(c)

: %

Figure 12. Measured and calculated movements:(a) moderately conservative (Frew); (b) moderatelyconservative finite element (Safe); (c) most probable (Frew)


20 30 50 70

Chart RoadAdvanceChart Road/Maidstone Railway

Maidstone RailwayGreensand Way – propped

Greensand Way 4B–Greensand Way 1 to 4A–Gasworks LaneCattlemarket to Beaver Road

0·1 mm/day

0 10 40 60 80Time: days

Utilitiesbridge

0·2 mm/day

90% consolidation2½ days

25

20

15

10

5

0

Tim

e de

pend

ent m

ovem

ent:

mm

�5

Figure 14. Wall movement against time: all structures

4000

3000

2000

1000

Load

: kN

11-09-00 01-10-00 21-10-00 10-11-00 30-11-00 20-12-00 09-01-01 29-01-01 18-02-01

Date

Excavateto formation

Castbase slab

Figure 15. Prop loads against time: Maidstone Railway box

Firstexcavation

Dig toprop

Centre

Intermediate dig

Chords

Dig toformation

Cast baseslab

Internal structure

Prop out

50 days

0·1 mm/day

0·2 mm/day

50

40

30

20

10

0

Max

imum

wal

l mov

emen

t: m

m

04-01-01 23-02-01 14-04-01 03-06-01 23-07-01 11-09-01 31-10-01

Date

Figure 13. Wall movement against time: Maidstone Railway box


installation. Despite this, the overall wall movements fall

within the conventional relationships used to estimate wall

movements.

Measured movements at the end of excavation were only about

40% of the values calculated on a moderately conservative

basis. This ratio increased to around 70% where finite-element

analyses were carried out.

Wall movements continued after excavation at rates of up to

0.2 mm/day. This is not explained by consolidation of the stiff

clay soils. Further research is needed into the effects of soil

creep and concrete relaxation.

The rates of movement measured on the first structures to be

constructed were used to optimise the construction sequence

for the excavations that followed.

2000 3000 5000 7000

Chart Road/Maidstone RailwayAdvance

Maidstone RailwayGreensand Way propped–

Greensand Way 4B–Greensand Way 1 to 4A–Gasworks LaneCattlemarket to Beaver Road

70%

0 1000 4000 6000

Calculated load: kN

Utilitiesbridge

40%

6000

5000

4000

3000

1000

0

Mea

sure

d lo

ad: k

N

100%

Calculated using 50%theoretical stiffness

7000

2000

Greensands Waycalculations (x) used‘most probable’ soilproperties

Figure 16. Measured and calculated prop loads. Results for 36 sets of props (117 props in total)

Prop location Prop type Measured load: kN Calculated load: kN Measured load/calculated load: %

Chart Road to Maidstone RailwayCh. 88 + 972: upper Concrete 6215 5576 111Ch. 88 + 972: lower Steel 1827 4565 40Maidstone RailwayCh. 89 + 164: upper Steel 4525 3500 129Ch. 89 + 164: lower Steel 2197 2688 82Greensands proppedCh. 89 + 207: upper Concrete 3033 2100 144Ch. 89 + 207: lower Steel 2312 3960 58Greensands Way 4BCh. 89 + 240: upper Steel 2200 2340 94Ch. 89 + 240: lower Steel 1025 2340 44Ch. 89 + 285: upper Steel 1890 2340 81Ch. 89 + 285: lower Steel 817 2340 35Greensands Way 1Ch. 89 + 555: upper Steel 1356 2331 58Ch. 89 + 555: lower Steel 903 3325 27Gasworks LaneCh. 89 + 675: upper Concrete 303 429 70Ch. 89 + 675: lower Steel 1814 4450 40Cattlemarket to Beaver RoadCh. 89 + 900: upper Concrete 2223 1518 146Ch. 89 + 900: lower Steel 1015 4133 25

All tabulated loads taken at completion of excavation.Measured loads are average values from three adjacent props.

Table 6. Loads in paired props


Temporary props were installed at some 400 locations. None

was damaged or displaced by subsequent site activities, calling

into question the need to design for the loss of a prop.

Loads were measured in 36 sets of three or more props, and

were about 40% of those predicted by moderately conservative

calculations. This was lower than expected. The distributed

prop load (DPL) method was used in conjunction with field

measurements to justify a reduction in the loads used for prop

design.

Prop stiffness had an important effect on the designs. It is not

realistic to assume that props are 100% efficient. Reducing

prop efficiency to 50% improved agreement between calculated

and measured values.

Where two levels of propping were used, the upper props

attracted a much greater proportion of their calculated load.

Loading during the first-stage excavation tightens the upper

props and increases their stiffness during subsequent stages.

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BSI (British Standards Institution) (1990) BS 5400 Parts 1 to 4:

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Burland JB, Simpson B and St John HD (1979) Movements

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Roscoe H and Twine D (2001) Design collaboration speeds

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gateway to Europe. Proceedings of the 9th International

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Design and Performance of Retaining Walls