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I.'on",racI; ~ .li',i-ibEs r 'ore I:unne s

Maidstone Cattiemarketto Beaver Road

Flgnre 1:Plan of CTRL C430.

Greensands Gasworks Footbridge

Footbridge Footbridge Beaverpropped cut Road cut

4 track 2 trackAshford International Station

~ Tunnel Q Retained cut ~ Propped retained cut

By Fleur Loveridge, Mott MacDonald

Conventional design methods for braced excavations are commonlyfound to overestimate the prop loads measured during subsequentconstruction (Powrie & Batten, 2000, Twine 8r Roscoe, 1999).This mayresult in the over-design of support systems. The distributed prop load(DPL) method provides an alternative method of estimating temporaryprop loads based on case histories.

This paper compares loads found using the DPL method and thosegiven by conventional wall analysis with loads measured during theearly stages of construction of the Channel Tunnel Rail Link (CTRL)Contract 430. It demonstrates that the DPL loads are greater than thosemeasured, but significantly less than those found by analysis. The DPLmethod is then applied to forthcoming areas of construction. Thesubsequent reduction in prop load predictions allowed review of themethod of propping and resulted in cost savings of approximatelyf.175,000.

Project contextSection 1 of CTRL will connect the Channel Tunnel to existing Railtrackinfrastructure near Gravesend. Rail Link Engineering awardedContract 430, the largest contract on Section 1 worth 2150M, to Skanska(then Kva.rner Construction) in 1998. C430 comprises 14.4km of highspeed rail link through east Kent. Some 1.8km of cut and cover tunnelsand retaining structures are needed to pass through central Ashford toconnect to Ashford International Station. Structures include 570m offour track tunnel, 420m of two track tunnel and 800m of two trackpropped retained cut and retained cut (Figure 1).

Flgnre 2: Fabric In Weald Clay.

Ground conditionsGround conditions comprise three main units from the LowerCretaceous system. The succession with typical depths below groundlevel is given below: Geotechnical parameters are given in Table 1.Hythe Beds fromgmto6mAtherfield Clay from 6mto 19mWeald Clay from 19m

The Hythe Beds consist of loose silty and clayey fine sand and soft tofirm sandy clay with occasional weak calcareous sandstone beds andthin limestone bands. Sandstone boulders were also encountered duringpiling. The Hythe Beds exhibit a large scatter in their geotechnicalparameters, including a range of plasticity from low to very high.

The Atherfield Clay is stiff to very stiff closely fissured sandy clay.The deposit is divided into two materials according to its plasticbehaviour. The upper deposit is predominantly of very high plasticity,having a plasticity index of 54%. The lower deposit is of intermediateplasticity with a plasticity index of 32%.

The Weald Clay underlies the site and is of considerable thickness.The deposit is stiff to very stiff thinly laminated closely fissured clay ofintermediate to high plasticity with numerous silt laminae andpartings. Silt pockets and bands of fissured siltstone up to 200mm thickare also present.

The water table is 1m to 2m below ground level. Although all thedeposits are principally fine grained, field trials showed that ejector

Table 1:Design soil parameters

Sos type Yonng's IgodnhtskN/m(and

gradient)

c',Acthre R PasshreTkn/m'plain kP, 8/$ .=+2/3 8rg =-2/3

strain)Rail Unk Engineering design soll paramehnsHythe Beds

Atherfield Clay (high plasticity)

Atherfleld Clay (Intermediate plasticity)

Weald Clay

1800(4860)3600(3640)3600(3640)18200(3200)

20

20

20

20

32

26

25

0.26

0.33

0.38

0.35

5.8

3.75

3.1

3.5

Skanska design soll parameters for obsertragonal analyshtHythe Beds 1800

(4860)Atherfield Clay (high and 3600intermediate plasticity) (3640)Weald Clay 18200

(3200)

20

20

20

35

25

10

0.23

0.37

0.35

7.3

3.3

3.5

38 GROUNI) RNGINRRRING AUGUST 2001

COOLING PRIZEl Construction with onelevel of temporary props

A thertl eldclay I

Water levelin Atherfieldclay

Atherf laid clay

Construction with onelevel of temporary props

VOUS

Atherfieldclay

Construction with onelevel of temporary props

I=

Water level

lttreald ofay, iso

fn

A therftetdclay

Full roof slab

Water levelrn Atherfieldclay h Reinforced concrete roof

beams at 4.5m centres

Atherceldclay

Water level

lirWI ~Figure 3:Section at Greensands Way. Figure 4: Secgon at Gasworks ianL Figure 5:Section at Beaver Road Cut.

tnt Mlrttr% ~~J VrtL".".itule;";., "iaa

I swr

sn .wl,l

Figure Band 7:Conslructlon at Gaswottts Lana

system dewatering was an effective method of groundwater control inthe Weald Clay. This is due to anisotropic permeability resulting fromthe natural fabric of the deposit, which includes silt partings andfissured siltstone bands (Figure 2). Dewatering of the Weald Claygenerally maintained a water level of a few metres below tunnelformation.

The majority of the excavations take place within the Atherfield Clay,with the Weald Clay generally lying below the tunnel formation. TheHythe Beds do not occur throughout the site. Where present, they areencountered at the very top of the excavation, commonly forming thebatter above the capping beam.

The two track structuresThe two track tunnel includes three types of structure, those with a fullroof slab, propped retained cuts and retained cuts. The first structures

to be constructed were Greensands Way and Gasworks Lane.Greensands Way is shown in Figure 1 with a cross-section through thestructure in Figure 3. The initial 90m of the tunnel was constructedusing two levels of temporary props. The remaining 250m of tunnel wasconstructed with one level of temporary props using the observationalmethod (Nicholson et al, 1999).

A cross-section through Gasworks Lane is shown in Figure 4. It wasconstructed with permanent discrete reinforced concrete roof beams at4.5m spacing and one level of temporary props (see Figures 6 and 7).Construction of Gasworks Lane forms the only two track excavation inthe Hythe Beds. Measurements of prop loads from both structures aregiven below.

Work began on the Cattlemarket to Beaver Road section in spring 2001.The section includes three main structures, Footbridge Propped Cut,Footbridge Cut and Beaver Road Cut. Footbridge Propped Cut is an

GROUND ENGINEERING AUGUST 2001 39

COOLING PRIZE

extension of the Gasworks Lane structure(Figure 4). Footbridge Cut and the smallerBeaver Road Cut are open structures(Figure 5). One level of temporary propswas used in construction.

Construction follows a 'bottomup'equence.

Greensands Way~ Excavate to piling platform, install pilesand construct capping beams~ Excavate to temporary prop level andinstall props~ Where required excavate to second proplevel and install props~ Excavate to formation and cast base slab~ Constructroof slab~ Remove temporary props

Gasworks Lane and FootbridgePropped Cut~ Excavate to piling platform, install pilesand construct capping beams~ Construct permanent reinforcedconcrete props~ Excavate to temporary prop level andinstall props~ Excavate to formation and cast base slab~ Remove temporary props

Table 2:Measured prop loads

Construetion detailsClltL cbalnage ebsenraeonat

nleelod Usedin desi go V

Greensamls Way89+37089+43489+46589+53289+550upper89+550 lower

YesYesYesYesNoNo.

YesYesYesYesYesYes

Gasworks Lane89+67589+76089+80089+82589+850

NoNoNoNoNo

YesYesYesNoNo

*third prop strain gauges damaged during construction

2170 25472159 28441847 37781859 21241464 14821869 1627

152926802000185816181691

202825612808194715211729

2222 17411816 16812126 13511664 25141870 2123

2243 1789 19992032 1843* 1739789 16591420 1804

Maximum measured prop hade kN

Prop t Prop 2 Plop 3 Prop 4 ittnnngeload

Footbridge Cut and Beaver Road Cut~ Excavate to piling platform, install pilesand construct capping beams~ Excavate to temporary prop level andinstall props~ Excavate to formation and cast base slab~ Remove temporary props

Temporary props were fabricated fromsteel tubes of 1016mm external diameterand 83,116mm'ross-sectional area. Propinstallation details are shown in Figures 8and 9. The props were not pre-loaded andwere installed at a 4.5m spacing for allstructures

—Concrete bag—Double I beam waier

Concrete thrust block (one end of prop only)

f

�1/Tubular steel prop

Strain gauges offset 3mto avoid end effects

Comparison of prop loadsDesign predictionsThe cut and cover designs were carried out using the flexible earthretaining wall program FREW (Oasys, 1998). During all stages ofconstruction the ground was assumed to be fully drained. Themoderately conservative geotechnical parameters determined by RailLink Engineering following ground investigation and testing (Londonand Continental Engineering, 1997) were adopted in design. Improved

i ~Contiguousbored pilewall

Strain gauges equidistantaround prop

Figure 8:Prop installation details

Measurement of prop loadsProp loads were monitored duringconstruction by vibrating wire straingauges. Three or four adjacent props wereeach fitted with four gauges at equalspacing around the prop (Figure 5). Thisprovides an average load for each prop thatis independent of any bending that mayoccur (Batten et al, 1999, Richardson et al1999).

Base readings were taken before castingof the concrete thrust block. Followinginstallation, readings were taken by a data Figure 9:PropplngatgreensandsWay.logger every ten minutes. Built-inthermistors allowed temperature variations to be measured and henceremoved (Batten et al, 1999, Richards et al 1999).The resulting loads aredue to soil and water pressures only.

All prop load measurements were considered before construction ofthe base slab. Prop loads were found to vary by up to 1000kN in each set ofmonitored props (Table 2). Typical prop load fluctuations are shown inFigure 10.

The loads developed in a set of three reinforced concrete roof beamswere also measured at one location in Gasworks Lane. The maximumaverage load recorded before casting of the concrete base slab was607kN.

40 GROUND ENGINEERING AUGUST 2001

COOLING PRIZE

Table 4:Prodhfhns of prop hads at CafgemarkofIo Romrer Road

Groensands Way89+37089+43489+46589+53289+550 upper89+550 lower

YesYesYesYesNoNo

YesYesYesYesYesYes

202825612808194715211729

351035103300330023313325

Dbseneflonal Dewalwfngof ghasmud Dotdgn propNelbodused Weald clayy maximum hadskNIn design? prop hade kN

DPL Prodhhdprop loads Idl

465144754751390817992243

CTRL

cbalnage

Foofbrldge Propped Cul89+87089+90089+930

4133205226462408

Prodlcled prop loads kN

Design DPL

prop loads predhgons(FRGW)

Gasworks Lane89+67589+76089+80089+82589+850

NoNoNoNoNo

YesYesYesNoNo

19991843173916591804

44504450445035103510

28192819281921632163

Footbridge Cul89+940

Ranrer Road Cul90+08090+11090+150

3571

180011481512

2271

14051298807

2500geotechnical parameters were derived by Skanska after back-analysis ofconstruction monitoring. The design soil parameters are detailed inTable 1.

2000

1500

0~1000'00!

tLf W '~ P ]'A

',

~lrd0~'28 Nov 8 Dec 18 Dec 28 Dec 7 Jan 17 Jan

1999 2000Year

Section at chainage 89+830

44.5

'-;~I.',,r;-~?'-i'~ 43.4

=.~I

42.9

38.0U

Temporary prop

il

Il

,ii 32.8'7

r rim.. ~ nor

0.5TH

0.2TH

+0.3P

I I I C m ~ iiiie uII IM~mu—

UIW sun==i.=.c„; Qt

All elevations in m AOD

Prop spacing = 4.5mHeight = 11.7m 7 = 20kN/m2

AS = soft to firm clay8S = stiff to very stiff clay

Prop load = prop spacing x height spanned x distibuted prop load

= 4 5 x ( 03TH [40 45 - 40 4] + 0 5TH [40 4 - 36 6 ] [=2163 kN

Dos 1:DPL diagram for Gasworks Lane.

GRQUND ENGINEERING AUGUST 2001

Figure 10:Prop load fluctuations at Greensands Way, temperature corrected.

Distributed Prop Load(DPL) methodThe DPL method is based on case histories of deep braced excavationsworldwide. Box 1 gives a typical DPL diagram for Gasworks Lane andillustrates prop loads derived by the method. Where the two sides of thestructure were asymmetrically loaded, the DPL method was applied tothe side with the more onerous loading. The excavations at GreensandsWay and Gasworks Lane are generally 13m wide and llm to 15m deep.This falls well within the range of excavation sizes of the case histories(Twine & Roscoe, 1999).10kPa integral live load is allowed for in the DPLmethod. No additional surcharges were imposed in the analysis.

Comparison of prop loadsThe measured and predicted prop loads for Greensands Way andGasworks Lane are shown in Table 3.The measured loads decrease upchainage as the size of the structures decreases. Where two temporaryprops were used instead of one, the sum of the loads is greater than theload in a single prop.

The moderately conservative design prop loads are up to twice thecorresponding measured loads. This is independent of the number ofprops used in construction and whether the ground was subjected totemporary dewatering.

The DPL method generally provided conservative predictions of proploads compared to the measured loads. However, three DPL predictionsof prop loads were greater than twice the measured load. The temporarydewatering at these locations may have caused this effect.

The DPL predicted loads were less than the moderately conservativedesign loads and greater than the observational design loads. Based onback-analysis the observational method made every effort to match bothmeasured deflections and prop loads. To achieve this the prop stiffnesswas reduced by a factor of two in the observational analysis. Thisresulted in the observational design loads providing the closestpredictions of measured prop loads.

Predictions of prop loadsThe measured prop loads at Greensands Way and Gasworks Laneprovided a calibration for the DPL method and confirmed the reliabilityof the predictions. The DPL method was then used to predict loads forthe propped retained cuts and retained cuts at Cattlemarket to BeaverRoad.

Moderately conservative soil parameters have been used in the designfor Cattlemarket to Beaver Road. No dewatering has taken place duringconstruction. Prop load estimates from FREW are compared with DPLpredictions in Table 4.The DPL predictions are significantly lower thanthe design loads.

The DPL loads for Beaver Road Cut, the smallest 100m of thestructure, are within the capacity range of hired props. Due toprogramme restraints, construction at Cattlemarket to Beaver Road wasscheduled for a time of maximum prop usage. Use of hired props insteadof fabrication of additional props provided a significant cost saving.

Construction at Beaver Road Cut was planned to proceed using Storey

41

COOLING PRIZE

Super Props. Each prop comprised four chords of twin rolled steelchannels. Two Super Props at 4m centres were installed on 5m walers. asshown in Figure 11.The cost of this method of propping is outlined inBox 2. It was estimated that it would provide savings of E175,000.

ConclusionsDesign analyses tend to overestimate prop loads. At C430 moderatelyconservative design prop loads were up to twice those measured duringconstruction.

The DPL method provides rapid conservative determinations of proploads that are generally less than conventional design predictions.

Use of DPL prop load predictions in prop design at C430 resulted in asignificant cost saving by allowing the use of hired props instead ofspecially fabricated props.

Box 2:Cost of propping syslam

1. Faintoated propping systemlength of sectionnumber of props(steel props at 4.5m c/c)cost per propfabrication cost

2 Blred propping systemlength of sectionnumber of props sets(two props on 5m water)lump sum hire(for 20 prop sets for four weeks)additional hire (per week)construction periodadditional hire(for eight weeks)

= 100m=23

=210,000= 2230,000

= 100m=20

= 217,200

= 2860= 12 weeks

total cost = E230.000

sub-total = 217,200

sub-total —234,400

total cost = E51.600e

cx

0

Screw head plates 4m

I beam walers

Storey super props

5m

6) ~

ReferencesBatten et al (1999).Use of vibrating wire strain gauges to measure loads in tubular steelprops supporting deep retaining walls. Proc Inst Civil Engineers GeotechnicalEngineering 137Jan pp3-13.London and Continental Engineering (1997).Central Ashford geotechnical design basisreport.Nicholson et al (1999).The Observational Method in ground engineering: Principles andapplication. CIRIA Report R185.Oasys(1998) Flexible Retaining Wall Analysis. Oasys FREW Versions.11.Powrie & Batten (2000). Comparison of measured and calculated temporary prop loads atCanada Water Station. Geotechnique 50 No. 2 pp127-140.Richards et al (1999).Measurement of temporary prop loads at Mayfair Carpark. Proc. InstCivil Engineers Geotechn ical Engineering 137July pp185-174.Twine & Roscoe (1999) Temporary propping of deep excavations —guidance on design.CIRIA Report C517

Flgnre11: Propping arrangement at Beaver Road Cnt.

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