COOLING PRIZE PAPER

Project description The ART project is worth 2134M and involves the construction of twin bored road tunnels each 1.3km in length, and associated portal works IIIlinaI:iOn e located within the airport perimeter for operator BAA (Figure 1).The ART will provide an unrestricted link from the central terminal area (CTA), to both remote aircraft stands on the edge of the western airport

';eIII apron in the short term and to the proposed fifth terminal (T5) in the )orary long term. Construction work commenced on the west portal in January 2001 and project completion is scheduled for 2005. BAA used the integrated team approach to deliver the project, fol- lowing the key ideas formulated by the Construction Task Force in RO ) )IllC LISlllC Rethinking Construction (1998). Mott MacDonald Group was selected as designers responsible for tunneVportal design and subsequently introduced and implemented the OM. Laing O'ourke was responsible for portal construction. I:ie The west portal is divided into four sections, as shown in Figure 2: the TBM chamber (deep box), the cut and cover (top-down construc- tion), the junction and the satellite ramp (retained cut). The site is rea- sonably constrained because of its location, nearby aircraft operation o ~sel va",iona and the Piccadilly underground line running under it. This paper focuses on the construction of the TBM chamber.

Ground conditions IIIeI: iOC Ground conditions for the Heathrow area are well documented and at the west portal site comprise the following [levels are given relative to on the Heathrow airside tunnel datum (TD) 100m below AOD]: Made ground from 123.0m to 122.5m TD Terrace Gravels from 122.5m to 118.0m TD road tunnel project London Clay from 118.0m to 67.0m TD The made ground is generally 0.5m thick which forms the aircraft stands/taxi-ways, and consists of concrete pavement with sub-base formation. The Terrace Gravel deposits are 5m thick, and comprise a Synopsis dense to very dense, sandy gravel with localised clayey silt/fine sand Use of the observational method (OM) on projects where retained deep matrix zones. The London Clay, approximately 50m thick, underlies excavations require temporary propping is well-documented the gravel stratum, which is typically composed of a firm to stiff grey (Nicholson et al, 1999).The requirement to make savings in both pro- clay with occasional shell and fossil fragments, partings of silt and ject cost and programme by modifying the temporary works is a key bands of claystone. driver for OM, and its use on the Airside Road Tunnel (ART) project at An upper weathered clay zone is present, typically 0.5m thick, iden- Heathrow Airport is an example of an instance where considerable tifiable by orange brown, blue grey, mottled colourings. The water table savings are possible. is in the Terrace Gravel layer with a phreatic surface varying from This paper describes how OM was implemented, through the tech- between 120.5m and 119m TD (Mott MacDonald, 2000). nique of progressive modification (Powderham, 2002) on the first sec- tion of the west portal excavation, a 15m deep chamber from which a Base case design tunnel boring machine (TBM) would be launched. It details how an The TBM chamber (Figure 3) is a box structure measuring approxi- innovative contingency measure, laser controlled excavation and com- mately 30m wide by 21.5m long and 15m deep. It was designed using prehensive wall convergence monitoring allowed careful control of the moderately conservative design parameters determined from exten- excavation phase, in which 60t of temporary steel propping was elimi- sive site investigation and sample testing. nated from the TBM chamber alone. This allowed a time saving of four A conventional top down construction sequence (Box 1) was origi- to five weeks from the project programme, with improved site safety nally opted for in which one level of temporary intermediate propping resulting from the need not to handle the heavy steel propping within would have been installed at mid height (Figure 4). Consisting of steel a confined working environment. In addition, an enhanced under- waling beams, tubular props, end plates and hangers, the entire frame standing of the construction process led, through continuous improve- weighed over 60t, the installation of which would have been lengthy ment, to the realisation of further benefits on the remaining sections and posed challenges to the constructor in terms of sequencing and of the west portal and also the east portal. ensuring safety.

Hoathrow Airport illy Tunnel der excavation) Satellite ramp (retained cut)

Future extension to T5 West @st Soir)ham runway Portal 30m

Airside road Junction (retained cut) er tunnel (ART)

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30 GROUND ENGiNEERiNG MAY 2003 COOLING PRIZE PAPER

TBM chamber Base Case TBM chamber Observational excavation sequence Method excavation sequence (section A-A) (section A-A)

holes psta 1 wotNa Casl retN shb O wetNa Excatfah lo haporarv prop hwl (11LQn TB) 1 week s— hshB fanporarr ~ns (ON) ~htalel 3 weeks Temporary prop level Exctwah le sess ssssnsss hwN 1 week 1500mm diameter C— base shh 4 wetNa .ales Ihntota sssasassmar PrePPhs sesesssssssss 1 week

1. Excavate to temporary prop level ae~1. Excavate to level of 'first cut (114.5m TD) (114.5m TD) BoxE:TBNc -ON-

Temporary steel props hshi pass CnnnNa Wall convergence Caslreof slab Bweeks instrumentation E hlolewleffhefcta(1145aTB) 1 week halos wos ~tsssssssasss esesess eats hshlssssesseeesssn Excaealelefonnaeonleeelh 08eesata cf week ConchntNbaseslab Sweeks

Tbhl cTBweela 2. Install temporary steel propping 2. Install monitonng instrumentation (115.0m TD) (115.0m TD) and excavate to formation level in 0.5m slices Observational method design Assessment of the base case led to the realisation that the observation- al method, formulated by Peck (1969) and subsequently developed through progressive modification by Powderham (1994), could be used to significantly improve project value through modification of the tem- porary works. Base slab The decision taken for the TBM chamber was to excavate to forma- tion level, without installing any temporary propping (Figure 4). However in order to maintain a high level of safety during excavation, wall movements would need to be monitored and a pre-designed con- tingency measure provided. OM was targeted at creating significant time savings (Box 2), but to succeed required a combination of both 3. Excavate to formation level 3. Cast blinding strut followed by technical innovation and team commitment. (108.0m TD) base slab It was crucial to control ground movements during the excavation phase, and therefore predefined limits for wall convergence (trigger A levels) based on the conventional red/amber/green traffic light system were used. The trigger levels were determined from the allowable wall deflections calculated in the base case design and past experience of other projects using OM. This required careful judgement, since if set Base slab too high, the trigger levels may not have highlighted significant move- ment, and if set too low, the site team could have been continually on a state of high alert unnecessarily. It was not possible, due to the nature of the excavation, to implement the progressive modification approach through a linear sequence (Powderham, 2002), in which propping is gradually omitted. The use of 4. Cast base slab and remove temporary Plan of TBM chamber the original steel propping system as a contingency measure was a non- props starter, due to the length of time it would take to install; thus an inno- RBnte ra Comparison of Base Case anN ON oxcatfallon seqnencsa

c/SVS.. aaasaarfuearn vative approach was adopted. In this case the contingency was the i- a ~ ~.. casting concrete -I,' s of a blinding strut across the entire area of the chamber, which would in effect laterally restrain the piles at whatev- er depth of excavation was reached, if a red trigger level was exceed- ed. The original temporary steel props could then be installed, the blinding broken out and excavation continued. Constrffction sequence Contiguous bored pile walls formed the three main sides of the TBM chamber (details summarised in Table 1), with the fourth being left as a battered slope which extended into the adjacent cut and cover section. Soft bored piles (500mm diameter) were installed between the harder piles, with the toes extending into the London Clay to prevent ground water ingress from the Terrace Gravels. Unlike the side walls, the headwall piles did not form part of the permanent works, with selected piles being broken out before the start of tunnelling to form the tunnel eyes. The embedment depth of these piles was reduced to reflect this. Removal of the pile tops by the Elliott method preceded construc- RBtne3:TON chaaher(nachheopenhB In reel shh). tion of the roof slab. The initial phase of excavation proceeded through two openings in the roof slab, to a depth of approximately

GROUND ENGINEERING MAY 2003 31 COOLING PRIZE PAPER

lbble 1:TOMchamber OSe~~ ment. Table 2 gives details of the OM excavation. rate was increased to three slices per 24-hour period res aamasesa $$os lhagorary Ogos since monitored wall movements werer booth smallsm and ivell within the IocaOan Oerlh aml sonlh wage green zone. Ne4amehfr(m) 18 18 NesDachg(m) 18$ IAS Instriimentatlon 24 20 of im lementing OM is the need for a compre- Ne hmglh(m) ' res saaesaeaa hanO (m 1D) %Hog aeaaema 107.12 gs1N.N 107.12 ni em. Three separa e eclmi o of lao Iewg(mTD) SMO 10L30 utomated mini prism survey o Ssrhnl e wall movement within ll e TBM ll b aOeDOf (m) ~~74OL7 KS ''l ~ A manually read digital tapee exextensometerensom pprovided the primary OM monitoring of horizon'ntal wall convergence. It was se case tempo- which the final decision to continue excava tionion was made. Readings rary proppinging would have been installe,'llealled thisis wasw deemed the level of wereere taken at four locations between oppop g p "first cut" (114.5m TD). ' Since it mar e t ispoj, ~ utomated surveying of optica mini p d the as the primary back-up. 'red byy a Leica ' deepest p o to introduce special exca- TCa 1800 Total Station mounted on a rac e tiaL roof slab (Figure 7). Readin ith the f ation level data transmittei ed back to a computer a a adient of 5 3 ('owards the headwall. ~ ManuManuallyy read inclinometers provi e re 5) was taken so as to deflection throughou e e ess, an in a ivel flat forma ion, e tion of the were used as a secondary ch ' re uired. Two 0.5m slices per 24-hour period To establish instrumentation relia y, a e was set as e maxim ith th view to changing it u within e ts. (Figure 8) to comp Leveiling the sloping temporaryor formation was carri'ed out usingus a nometer tubes (pre-inre-installed), and a tape ex ens ' ' bracckete mmounted dual grade level, with a receiv rismfixedatalevelof 115m w for laser, ' inside the chamber. As eac . m below the Ieve of u . back up ' f wall pile, was lowered by a sys tems were mstalled on 13 Augu st 2001, after which 24 hour m p' 0.5m increment. A long reac h 3800 excavatore positioned on e su

a ion of "first cut" on 15 August MOhttOylllg IOSIIults 'iafter 114 ~ Convergence of the side w p oui s 0 e 6). A concrete blinding strut tape extens ained the piles, wi'th th e permanent 15mm movement over reinforced base slab being susubsequentlyse constructe,d 11us p t'l 'l'e' in the een zone wi no a

Ground level Top of roof slab (1.5m thicld 123.0m TD A(rpolt pavement (0.5m (hicthick) V

Excavation below level of first cut in Level of wallI 13no 05msllcesover 114hours convergence instrumentation ) mmmm L el of 'first cut'15 Om TD Terrac gravel

n r r '.Ogham Rgmo WgrosooecSen gM oaOOmo1).

Rgme 6:TSM chamber foSowing Shll casOng ef Day 0700-1NO los ~Iarf efsMII eng ef sMII bSndlnS slrnL Olghl1$ NHI7Nlns (mlD) (mll) (m)

ISISNSI Oay 11450 11400 LS l I( sL Wghl 1NAW 11LN ILS I Ii s 1SNSISI Oay 11%00 11LSS LS Wghl 11LSS IN 11'12AM 17INNI Day 11LSO IN Wghl 112.N 111AM 1.0 1SIN/01 Oay 111.N 11LSO ILS Wghl 11MO 10MO 1AI 10INISf Day 10MO 100AM ILS Wghl IOMS 10MO 1.0 20NSN1 Oay gxssaase ann oml Cool ~shnf OIA

32 GRouND ENGINEERING MAY 2 3 COOLING PRIZE PAPER

North wall Roof slab Inclinometer (in pile) O 122 ~ Pile 15007 ~ -e Tape extensometer fxxxx mx~ 120 Mini prism (fixed to pile) I Q Total station (fixed to 118 I roof slab soffit) Inclinometer 0144.22-12 014)4.24-15 lie ~------w Pile 15055 10mm displacement at 115m TD I Headwall 114 'evel of first cut (114.0m TD) 014)424.12 i Q,'-0 Pile 15058 e=e 112 014)424 14 E c 110 'Top, of base slab (109.0m TD) ; 014)4.2413 Convergence measurement section (CMS) at 115.0m TD w 108 'op of blinding strut (107.9m TD) between: l'Formation level (107.6m TD) [Inn nnaKIxTX'I Pile 15033-Pile 15007 |oe Pile 15055 - Roof slab soffit I Pile 15033 lee Pile 15058 - Roof slab soffit South wall I 102

Pile toe level (99 3m TD) yeecccee $$ ———— -) 98 -20 2 4 6 8 101214161820 ee eesaeeee7 M e ~,dnlse kmn perlmt Cumulative A displacement (mm) ~ eeeeee~ - ——,dxiSHampertad —15/08/01 OM excavation started ---- 21/08/01- Blinding strut cast -- - - 24/09/01 - Base slab poured ~ Inclinometer 01-04-22-12 (Figure 10) indicated that pile 15007 move- ment at the same level was approximately 10mm, which when com- Note: Assumed top fixity after lmm top deflection occurred up to 15/08/01. bined with a similar value for the south wall (pile 15033)gives wall con- Base readings taken on 20/07/01. vergence of 20mm. The discrepancy with the tape reading has been attributed to reading error from the inclinometer probe, and was noted at other CMSs during west portal construction. ptpme104 Wall dfcplacennmt meacmed tr)f Imdlnemeter 01~22-12. ~ Measured wall movements by the automated mini prism survey were also small and confirmed the readings taken by the tape exten- the TBM chamber construction. someter. Reliable performance of this system allowed it to be selected ~ Improved site safety and working conditions within the TBM cham- as the primary method for later OM phases on the east portal. ber due to prop elimination. ~ Realisation of additional time and material savings on the other Conclttslons west portal sections through continuous improvement of the OM Optimisation of temporary works through the implementation of OM process (time savings of 24 weeks). can allow considerable savings to be made. On the ART project, using ~ Modifications to the temporary and permanent works design of the it led to the following benefits: ART east portal, resulting from enhanced understanding of soil-struc- ~ Excavation of the TBM chamber without the need to install tempo- ture interaction and construction techniques yielded further time and rary steel propping weighing approximately 60t. cost savings. ~ Time savings of four to five weeks on the project programme from Knowledge gained from construction of the ART portals is also help- ing to improve the design efficiency for some aspects of the TS under- 75 ground works. 70 65 113 Acknowledgmnonls 60 The author would like to thank the BAA design team, his Mott 55 ill Excavation level in m TD his ----— Amber Zone MacDonald colleagues and EngD 50 014)4-24-13 - 109 supervisors Alan Powderham and E -- - E Increase in excavation rate William Powrie for their continued 45 CI lee — 107 i— support and guidance throughout 40 E Ill the project. 00 35 0 - 105 O ) eee O 30 ) ee eee LIJ Construction Task Force (1998).Chairman Sir Qnen Znae - 103 25 John Egan, Rethinking Construction. DETR, 20 London. - 101 Mott MacDonald (2000). T5 geotechnical 15- design basis report. - 99 Nicholson et al (1999). The observational 10- method in ground engineering: principles and I applications. CIRIA Report 185, London. 5' ee I Peck, RB (1969).Advantages and limitations of 00"--- I I the observational method in applied soil -5 e I I e ~ e I I 95 mechanics. Geolerhnique 19, No. 2, pp 171-187. 42 -30 -18 -6 6 18 30 42 54 66 78 90 102 114 126 138 150 162 174 Powderham, AJ (1994). An overview of the observational method: development in cut and Excavation Excavation cover and bored tunnelling projects. commenced completed Georechnique 44, No. 4, pp 6194)26. Powderham, AJ (2002). The observational Time since excavation below level of 114.5m TD commenced (hrs) method —learning from projects, Geotechnica I Engi neeri ng 155, No 1, pp 594)9. plgme fk Wall coneersence meaemedby tape extencemeter 01~24-13.

GROIIND ENGINEERING MAY 2003