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
nam
eC
har
tR
oad
Adva
nce
Char
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oad
toM
aidst
one
Rai
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Mai
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nds
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oad
Bea
ver
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ps
2Pro
ps
Pro
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Ret
ained
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Ret
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89. 8
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14
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10. 2
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83. 8
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Geotechnical Engineering 163 Issue GE5 Design and performance of retaining walls Roscoe • Twine 281
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
50
0
She
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
282 Geotechnical Engineering 163 Issue GE5 Design and performance of retaining walls Roscoe • Twine
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
Geotechnical Engineering 163 Issue GE5 Design and performance of retaining walls Roscoe • Twine 283
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
284 Geotechnical Engineering 163 Issue GE5 Design and performance of retaining walls Roscoe • Twine
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
Geotechnical Engineering 163 Issue GE5 Design and performance of retaining walls Roscoe • Twine 285
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
286 Geotechnical Engineering 163 Issue GE5 Design and performance of retaining walls Roscoe • Twine
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)
: %
Measured movementCalculated movement
(b)
: %
Measured movementCalculated movement
(c)
: %
Figure 12. Measured and calculated movements:(a) moderately conservative (Frew); (b) moderatelyconservative finite element (Safe); (c) most probable (Frew)
Geotechnical Engineering 163 Issue GE5 Design and performance of retaining walls Roscoe • Twine 287
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
288 Geotechnical Engineering 163 Issue GE5 Design and performance of retaining walls Roscoe • Twine
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
Geotechnical Engineering 163 Issue GE5 Design and performance of retaining walls Roscoe • Twine 289
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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290 Geotechnical Engineering 163 Issue GE5 Design and performance of retaining walls Roscoe • Twine