Twin tunnel construction - Ground movements and lining behaviour

Transcription

1 Geotechncal Aspects of Underground Constructon n Soft Ground Mar & Ta y/or (eds) 1996 Ba/kema, Rotterdam. ISBN Twn tunnel constructon - Ground movements and lnng behavour T I.Addenbrooke & D. M. Potts Imperal College of Scence, Technology ana' Medcne, London, UK J' / ABSTRACT: The numercal analyss of twn tunnel constructon has shown the shortcomngs n the desgn assumpton of superposton for both sde by sde and pggy back tunnel geometres. The nteracton between two tunnels passng at depth has been shown to depend on relatve tunnel poston and spacng. l INTRODUCTION Modem metro constructon n congested urban envronments often nvolves the excavaton of new tunnels n close proxmty to each other. In addton mprovements to an exstng network oftenrequres new constructon adjacent to exstng tunnels. At the desgn stage t s therefore mportant to address the nteracton of the ttmnels and ther effects on overall ground response. Unfortunately current desgn practce, whch s based on emprcal methods and rules of thumb, gves lttle gudance n ths respect. 2 The ground surface settlement above multple tunnel constructon s commonly obtaned by the superposton of ndvdual settlement troughs obtaned from emprcal Gaussan dstrbuton desgn curves fer each tunnel n the geometry (New and O Relly, 1992). Tls mplctly assumes that the presence of an exstng tunnel does not alter the expected ground movements nto a new ttmnel, and also does not accotmt for any consoldaton movement occurrng when no engneerng actvty s takng place between constructon of each tunnel. The dstorton to an exstng tunnel lnng nduced by adjacent new tunnellng may be sgnfcant. Such consderatons are not addressed by emprcal desgn, and can only be assessed by ntunercal methods such as the fnte element method. 1 A numercal parametrc study has been undertaken to 441 nvestgate tunnel behavour. Two dstnctly dfferent geometres are consdered, one wth the tunnels rtmnng sde by sde at the same horzontal axs depth, and the other wth the tunnels rtmnng one above the other, pggy back fashon, along the same vertcal axs lne. The spacng between the two tunnels s vared. The responses of both the ground surface, and the tunnel lnngs are consdered. The valdty of the assumpton of superposton s addressed, and the dfferences n tunnel lnng response are hghlghted. 2 ANALYSIS DETAILS 2.1 Geometry and sequence of excavaton. The fnte element code ICFEP (Imperal College Fnte Element Program) was used to carry out ten separate analyses. In seven cases the excavaton of two m dameter runnng tunnels n London Clay was modelled, wth a 22 day rest perod between excavaton of the frst and the second. The sde by sde tunnels were all modelled at a depth of 34 m below ground level wth centre lne to centre lne spacngs of 8,l2, 16, and 32 m. The lower tunnel of the pggy back tunnels was constant at 34 m below ground level, 'wth upper tunnel axs levels of 24, 20, and I6 m below ground level, resultng n centre to centre spacngs of 10, 14, and 18 m respectvely. The lower tunnel was excavated frst n each nstance. Two tunnels excavated n close proxmty wth a rest perod of 22 days are lkely to be part of a sngle programme of works, and commonly the lower tunnel would be excavated frst. Indeed ths s the case wth the Jublee Lne Extenson work outsde the Insttuton of Cvl

2 Engneers where two tunnels were drven n pggy back fashon wthn a few months of each other, the lower tunnel beng _constructed frst. Three further analyses were carred out to model sngle tunnel constructon at axs depths of 24, 20, and 16 m belowgrotmd level, to provde comparsons wth the twn tunnel pggy back results. For comparson wth the ground surface response to the second of the sde_ by sde tunnels, the excavaton of the frst tunnel provded the greenfeld sngle tunnel behavourf Fgure lshows the relevant geometres and sol profle modelled. The pllar wdth and pllar depth (for sde by sde and pggy back respectvely) are defned as the extrados to extrados dmenson, that s the spacng less 2 tunnel rad. ' R? 2 f 8 ' Sand - gozvé Thames Gravel 7m.Cosa c' = 0 kpa _ ' U U _ ' = 0 _ c' = 5 kpa n 34m pllar G London Y Clay 1- ' 0 depth ` X spacng: y=l0,l-1 or 18m x = 8, l2, 16, or 32m D = 4.l46m C) 'O pllar wdth Fgure 1. Twn ttmnel geometry. 2.2 F ne element analyss. The sand stratum at the surface was modelled as beng lnear elastc wth a modulus of 5000 kpa. In the pre yeld regon, the Thames Gravel and the London Clay were 'both modelled wth non lnear elastc behavour usng a consttutve model of the form descrbed by Jardne et al (1986); the model and parameters are detaled n Appendx A. The Thames Gravel and London Clay were attrbuted the strength parameters shown n Fgure 1. Plastc flow was defned by an angle of dlaton equal to half the angle of shearng resstance,. The sand and gravel were modelled as non consoldatng materals whlst the London Clay was attrbuted wth a lnear sotropc permeablty of lx10" m/s. ' _ The ntal stresses prescrbed a hydrostatc pore water pressure profle from a water table located 2 m below the ground surface, and a Coeffcent of Earth Pressure at Rest, Ko of 0.5 n the sand and gravel, and 1.5 n the London Clay. All the sols had a bulk unt weght of 20 kn/m3. Coupled consoldaton analyss was employed to model the constructon of the tlmnels and the rest perods. The sol was modelled by 8 noded soparametrc quadrlateral elements. The tunnel lnngs were modelled by 3 noded Mndln beam elements (Day and Potts, 1990). Excavaton was modelled by removal of the sold elements wthn the crcular ttmnel boundary over a smulated tme perod of 8 hours. That s the stresses that the sol wthn the tunnel appled to the tunnel boundary were evaluated and then appled n the reverse drecton over several ncrements. Durng ths procedure the sol elements wthn the tunnel were not ncluded n the analyss. The volune loss nto the frst ttmnel was prescrbed to be 1.4%. Volume loss was montored aganst ncremental progress and when the prescrbed value was acheved the beam elements representng the tunnel lnng were constructed. The beam elements modelled a 168 cm thck concrete lnng, wth a unt weght of 24 kn/m3. The YO\ll1g S Modulus was 28 x 106 kn/mz, and the Posson s Rato was As the excavaton contnued the remanng ground stresses were transferred _ nto the tunnel lnng resultng n stresses and bendng moments exstng on completon of excavaton. A number of tme ncrements were then modelled to represent the rest perod. The excavaton of the second ttmnel followed the same pattem as the frst, wth the tunnel lnng beng constructed at the same relatve ncrement of unloadng. For the second excavaton percentage unloadng was therefore the controlled parameter, rather than the volume loss. 3 RESULTS 3.1 Surface settlement. Sde by sde; n Fgure 2 the shape of the short term surface settlement profle n response to the second tunnel excavaton alone s plotted. The settlement, S, s normalsed by the maxmum settlement, Sm, and s plotted aganst the dstance from the centre lne of the tunnel beng excavated. A nunercally predcted greenfeld settlement

3 profle, whch represents a no nteracton stuaton s also plotted for drect comparson. lt s evdent that the shape of the settlement troughs above each of the second tunnels s very smlar to the greenfeld profle, but the lateral poston of` the maxmum settlement s- offset wth respect to the tunnel centre lne, towards the exstng tunnel. The assumpton of superposton would attrbute a greenfeld settlement profle to the second turmel excavaton, centred on the tunnel axs. The magntude of Sm depends upon the volume loss nto the second excavaton. The reduced stffness of the ground n the regon of the second excavaton nfluences the volume loss. For example, Sm, above the second tunnel excavaton located at a spacng of 8m from the frst was 5.6mm, whlst for a spacng of l6m, Smax above thc_ second 40 ttumel was 5.05mm. zo o zo _ dstance from tunnel centre lne (m) Fgure 3 shows how, for the settlement profle formng above the second excavaton, the dstance of the poston of maxmum settlement wth respect to the tunnel centre lne (the eccentrcty of SML) vares wth pllar wdth. In the nstance of zero nteracton, the eccentrcty s 0.0. A sngle avalable feld data pont s plotted for comparson. It s based on Bartlett and Bubbers (1970) nterpretaton of settlement measurements above a sde by sde, twn tunnel excavaton whch formed 'part of the Vctora Lne constructon. The data pont les on the trend ndcated by the numercal study. Wth.a pllar wdth less than l dameter, the eccentrcty of Smax s tendng toward a value equal to double the pllar wdth. The eccentrcty (as a multple of pllar wdth) reduces wth ncreased spacng, down to less than 0.25 for a pllar wdth greater than 7 dameters.» I I ' - I I Pggy back; Fgure 4 compares the shape of the ` ` ` *":'?Q>\ short term surface settlement profle as a result of "` \ 0_2_ the upper, second excavaton wth the numercally `$:\_ A predcted greenfeld settlement for a ttumel at the -,$s\ \ \ s/s I 04_ same axs ' depth..'. The settlement, S, has been '~ \ normalsed by the maxmum settlement, Sm. For t \ 06 _ each depth of tunnel, the dstance from the tunnel YN ' _ 711' centre lne, X, has been normalsed by the trough <\ wdth parameter for a greenfeld profle above a _-sreeneld fs " J# tunnel of the same depth, g. The trough wdth.sm Spacng ` ` /Qff parameter, g, s the dstance from the tunnel centre _ -12mIIP2 I1g ' "T lne to the pont of nflecton of the settlement Spacng 1Sf1 11H=1 profle. Ths form of normalsed plot results n the Q...32m spacng SP2II=I1g concdence of the numercal greenfeld profles for all three depths of tunnel. Fgtu'e 2. Settlement above 2nd of two sde x/_ by sde excavatons ' ' ' ' ' ' 0.2. _-2" /_7>/ 2_0 eccentrcty of Smax S/Sm _.- ' 4/ ' 1_0 (multple of pllar wdth) _ ICFEP dm 0_6.'/f 1_5 o Bartlett&Bubbers _- (1970) ' /_, 0.8 greenfeld - -/f -' 4' //I ' 0_5 1.0 <5/. 14m=pacIng lom _.Q/,... l8mspacng I 6 I 3 I 10 I spacng I I I I I I A pllar wdth (multple of dameters) O **O lst tunnel Fgufe 3~ E enu ty of Smax Varyng wth Fgure 4. Settlement above the 2nd of two pllar wdth (sde by sde tunnels). pggy back excavatons.

4 All three comparsons (10 m, 14 m, and 18 m spacngs) show the twn tunnel settlement to be wder than the greenfeld profle. The_ shape of' the settlement profle above the second tunnel excavaton s dfferent to the greenfeld profle. The closer 'the spacng, the more flat bottomed the profle becomes above the tunnel centre lne. The magntude of Sm s dependent on _the depth of the tunnel and the volume loss, whch s nfluenced by the sols reduced stffness n the zone of the second excavaton. In Fgure 5 the dstance- from the turmel centre lne to the pont of nflecton of the settlement profles above the second tunnel excavatons, t s plotted as a rato to the greenfeld g value, aganst the pllar depth. In the nstance of zero nteracton, /g equals " - ICFEP data /, 'I 2 I I 4 I I 6I Fgure 5 _ \ pllar depth (multple of dameters) ' Poston of nflecton pont varyng `wth pllar depth (pggy back tunnels). The trend shows an ncrease n trough wdth as the spacng reduces. For a pllar depth of less than 1 tunnel dameter, the profle can be twce as wde as the greenfeld profle whch would be assumed n superposton. 3.2 Tunnel lnng response. Fgure 6 consders the,nfluence of the excavaton of the second turmel onffle lnng to the frst. For two sde by sde tunnels the exstng tunnel lnng s forced to squat on drvng of the second ttmnel. The horzontal dameter ncreases n length, whlst the vertcal dameter reduces n length. For two pggy back tunnels the lnng to the frst tunnel elongates vertcally on drvng of the second tunnell That s a lengthenng of the vertcal dameter, and a shortenng of the horzontal dameter. Fgure 6 shows how the magntude of these two dstortons vary wth pllar wdth and depth: 0. _. _ change n dameter (A of mmal value) I sde-by-sde; horzontal dameter 0 3_ sde-by-sde; vertcal dameter ' lengthenng n pggy-back; horzontal damater 0 2. pggy-back;vertea1 dameter, sde-by-sde data 0 1 (Ward, 1969) \K f 0 22: _ shortenng _ V'0'3"l'l'l'l'lI S 10 pllar dmenson (multple of dameters) Fgure 6. Response of lst lnng to passage of 2nd tunnel. (0.1»% change n dameter s equal to 4mm over 4m.) Sde by sde; when the pllar wdth s less than 1 dameter, the lengthenng of the horzontal dameter s just under 0.2% of the ntal dameter. Ths lengthenng reduces wth ncreased turmel spacng. The shortenng of the vertcal dameter s also just tmder 0.2% of the ntal dameter for the mnmum pllar wdth analysed. The shortenng of the vertcal dameter reduces as spacng ncreases. Included on Fgure 6 s data from the constructonuof the London Underground Vctora Lne where two sde by sde tunnels were drven very close to one another (pllar wdth equal to 0.2 dameters) and the dametrc varatons n the frst lnng were recorded. The data ponts plotted here are the average of three very smlar recordngs reported by Ward (1969). Squattng -dstortons were recorded at all three locatons, plotted as a lengthenng of the horzontal dameter, and a shortenng of the vertcal dameter. f he rest perod was 35 days. The data le outsde the extent of the numercal analyses, but close to extrapolated trend lnes ndcated by the numercal data. The shortenng of the vertcal dameter s

5 trend. _ slghtly less than that ndcated by the numercal The mplcaton s that wth a pllar wdth of greater than 7 dameters." the passage of a second tunnel at the same axs level as the exstng tunnel could be consdered not to dstort or nfluence the exstng tunnel lnng constructed a few weeks prevously. Pggy back; none of the analyses exhbted a dametrc dstorton greater than 0.04% of the ntal dameter. The lengthenng of the vertcal dameter (0.04% for a pllar wdth of just over 1 dameter) reduces as spacng ncreases. The shortenng of the horzontal dameter (equal n magntude to the vertcal lengthenng) also reduces as spacng ncreases. _ In ths case, the mplcaton s that wth a pllar depth greater than 3 dameters the passage of a second tunnel drectly above the exstng tunnel could be consdered notto dstort _or nfluence the exstng tunnel lnng constructed a few weeks prevously. 4 CONCLUSIONS Ths paper has shown the nfluence of tunnel poston on the nteracton between two tunnels constructed wthn a month of each other; The ground surface response s dependent on whether the two tunnels are sde by sde, or one above the other. The spacng of the two tunnels also has a bearng on the ground surface response for both geometres. The nfluence that the drvng of a second tunnel has on the lnng prevously nstalled n the frst tunnel also depends upon relatve tunnel poston (to the sde or vertcally _above) and on the spacng. Drvng a new ttmnel above an exstng tunnel has sgnfcantly less nfluence on the exstng tunnel lnng than drvng a new tunnel to the sde. 4.1 Sde by sde. The assumpton of superposton fals to account for the offset of the settlement profle from the centre lne of the second tunnel excavaton whch was evdent from the ntunercal results (Fgure 2). The poston of the maxmum settlement above the second excavaton can be estmated from Fgure 3, whch relates the eccentrcty of Sm to pllar wdth. Ths curve _ggay be dependent on tunnel depth, as all the numefcal analyses modelled 34m deep tunnels. However, Bartlett and Bubbers (1970) data pont fts the trend lne, and s for two tunnels 21m below grotmd level. The horzontal dameter of the frst tunnel lengthens, and the vertcal dameter shortens on passage of the second tunnel. The magntude of ths nduced squattng dstorton reduces wth ncreasng ttmnel spacng, and s neglgble for pllar wdths greater than 7 tunnel dameters (Fgure 6). 4.2'Pggy back. The assumpton of superposton does not recognse the wder, flatter shape of settlement profle formng above the second, shallower tunnel excavaton whch was evdent from the numercal results (Fgure 4). The settlement above the second excavaton s not of a Gaussan fonn, as assumed for greenfeld stuatons. The dstance to the pont of nflecton of the settlement profle as a result of nteracton has been shown n Fgure 5 to depend on the tunnel spacng. The horzbntal dameter of the frst tumel shortens, and the vertcal dameter lengthens on passage of the second ttmnel. The magntude of ths nduced elongated dstorton reduces wth ncreasng tunnel spacng, and s neglgble for pllar depths greater than 3 tunnel dameters (Fgure 6). 5 ACKNOWLEDGEIVHENTS The work presented here has been sponsored by London Underground Lmted as part of a wder research project nto subsdence due to tunnellng and ts nteracton wth buldngs. 6 REFERENCES Bartlett, J. V. & Bubbers, B. L. (1970). Surface movements caused by bored tunnellng. Conkrence on subway constructon, Budapest- Balatonfured, pp Day, R. A. & Potts, D. M. (1990). Ctu'ved Mndln beam and ax-symmetrc shell elements - a new approach, Int. Jnl. Num. Meth. Eng., Vol 30, pp Jardne, R. J., Potts, D. M., Foure, A. B. & Burland, J. B. (1986). Studes of the nfluence of nonlnear stress-stran characterstcs n sol-structure nteracton, Geotechnque, Vol. 36, No. 3, pp New, B. M. & O Relly, M. P. (1991). Tunnellng nduced ground movements; predctng ther

6 .E magntude and effects, Proc. of the 4th Int. Conf on Ground Movements and Structures, Cardff, Ed. Geddes, pp O Relly, M. P. & New, B. M. (1982). 'Settlements above. _tunnels.n the Unted Kngdom - ther magntude and predcton, Tunnellng '82, The Instn. of Mnng and Metallurgy, pp Ward, W. H. (1969). Deep excavatons and tunnellng' n soft ground - dscusson of paperby Peck, R. B.. Proc. of the 7th Int. Conf on Sol Mech. and Found Engng., Mexco Cty, Vol. 3, pp APPENDIX A / Small stran non -lnear styfess. The secant stffness expressons that descrbe the varaton of shear and bulk modul are: p T -g = A+Bcos[a(log10(-\%'))Y] 2 = R+Scos [5 (log1om) ~] where, G s the secant shear modulus, K s the secant bulk modulus, p' s the mean effectve stress, E s the stran nvarant used n ICFEP, and s related to a, (the axal stran observed n undraned traxal tests) by the expresson: mean effectve stress at that pont. Untl a specfed mnmum stran, Em, s exceeded the shear stffness vares only wth the mean effectve stress. Ths condton also apples once a specfed upper. stran lmt s exceeded, Emu. Lkewse, tutl a specfed mnmum volumetrc stran, em, s exceeded the bulk stffness vares only wth the mean effectve stress, and ths condton apples once a specfed upper stran lmt, emu s exceeded. In the analyses the magntude of the -stffness s prevented from fallng below specfed mnmum values (Gm or Km) Coeffcents and Lmts for Non-Lnear Shear and Bulk Modulus Expressons: Strata Thames London Gravel Clay A B C (%) 5.0 x x 10" on y E mn (%) xl0' xl E mx (%) G mn (kpa) R S T 2.0 x 10'3 1.0 x l0'3 5 Z ev mn (%) 2.1 x 10'3 5.0 x 10'3 av ma, (%) Km," (kpa) I \ - 5 E E E E =,/3 ea where, E = 2 ((e1-e2)2 + (el-e3)2 + (ez-e3)2 e,, ez, and e3 are prncpal strans, ev s the volumetrc stran, A, B, C, R, S, T, 5, ot, 'y and A are all constantsfn Throughout the analyses the stffness at a partcular pont s contnually changng. It depends on both the current stran and the current 446