Tunnel Face Stability Analysis in Soft Ground in Urban Tunneling by EPB Shield. (Case Study: 7 th Line in Tehran Metro)

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1 Austrln Journl of Bsc nd Appled Scences, 5(): , 0 ISS unnel Fce Stblty Anlyss n Soft round n Urbn unnelng by EPB Sheld. (Cse Study: 7 th Lne n ehrn Metro) Jvd Mohmmd, Kourosh Shhrr, Prvz Morefvnd Deprtment of Mnng Engneerng, Islmc Azd Unversty ehrn South Brnch, Irn. Deprtment of Mnng, Metllurgy nd Petroleum Engneerng, Amrkbr Unversty of echnology, Hfez 44, ehrn , Irn. Abstrct: rffc congeston nd envronmentl fctors re cretng demnd for greter utlzton of underground spces n urbn res. In mechnzed excvton of subwy tunnels, the Erth Pressure Blnced Sheld(EPBS) hs been developed n the recent decdes for mngng the nstblty of the excvton profle n complcted geotechncl condtons n urbn res. Durng the dvncement of n EPBS, the fce-stblzng pressure s one of the most mportnt fctors of crtcl nd prncple to be evluted correctly. In tunnelng by EPBS, hgh fce pressure often leds to surfce uphevl wheres low fce pressure leds to sudden collpse of the fce nd ultmtely settlement of the surfce. Both of these msevluted pressures my cuse the dmges whch followed by pert of tme nd fnnce. hs pper s dscussed bout urbn tunnelng by EPBS n soft ground condtons nd focused on clculton of fce-stblzng pressure pplyng to the cse of 7 th lne n ehrn metro project. Fcesupport pressure s estmted by most current methods of lmtng equlbrum nlyss nd lmtng nlyss n excvted prts (two sttons) of drecton wth dfferent nd dffcult geotechncl condtons. Obtned results compred to EPBS opertonl results whch showed the results of Broere nlytcl method s most ttrctve nd relstc mong others. For one of the excvted sttons clculted vlue of the Broere nlytcl method obtned Kp nd EPBS ctul vlue ws equl to 8.0 Kp. Due to results verfcton tht llustrte good djustment wth the ctul vlues, quntty fce-stblzng pressure s predcted for some of the lgnment unexcvted sttons (S7, V7, W7 nd X7) whch cn be used n bult procedure. Mxmum vlue ws quntty Kp for stton V7 nd mnmum vlue obtned quntty Kp for stton W7. Key words: unnelng, fce-stblzng pressure, EPBS, nlytcl method,7 th lne of ehrn metro. IRODUCIO As excvton of tunnels n unfvorble geotechncl condtons nd n hevly populted urbn envronments s becomng prevlent, the mportnce of mntnng tunnel fce stblty s rechng more mportnce. Ensurng tunnel fce stblty s drectly relted to the sfe nd successful constructon of tunnel. In ths context, tunnel fce stblty nlyss drectly reltes to fce-support pressure. he correct evlutng of fce pressure to vod fce nstblty depends on vrous fctors, such s coheson, frcton ngle nd permeblty of the ground, type of the mchne, dvnce rte, unt weght of slurry or condtoned sol, tunnel dmeter, overburden, nd locton of the ground wter tble. Mny reserchers hve proposed nlytcl pproches to determne the requred pressure to stblze the tunnel fce. Most of them re bsed ether on lmtng equlbrum nlyss (Broms nd Bennmrk, 967; Kruse, 987; Jncsecz nd Stener, 994; Angnostou nd Kovár, 996; Broere, 00; Crrnz-orres, 004), or lmtng nlyss (Atknson nd Potts, 977; Dvs, et l., 980; Lec nd Dormeux, 990). In the frst prt of the pper, some referenced methods for evlutng the stblty of fce re presented then project chrcterstcs re descrbed. In the next prt fce-stblzng pressure s clculted n two excvted sttons of 7 th lne of ehrn subwy (sttons 7 nd O7) by lmtng equlbrum nlyss nd lmtng nlyss methods. hen obtned results re compred to fce-stblzng pressures of EPBS opertonl. In fct, purpose of ths comprson s the selecton of the best nlytcl method for the project. Fnlly s logcl consequence, sutble method s selected mong others, nd s predcted fce-support pressure n unexcvted vrous prts (sttons S7, V7, W7 nd X7). Lmtng Equlbrum nd Lmtng Anlyss Methods(LEM nd LAM): Method of Broms nd Bennemrk (967): A well soluton bsed on resc mterl s Broms nd Bennemrk s soluton. hey suggested the stblty rto (Eq. ()) for vertcl openng. hs soluton s ndependent of the overburden-to-dmeter rto. =(q s - σ )/c u +(C+R).Y/c u () Correspondng Author: Jvd Mohmmd, Deprtment of Mnng Engneerng, Islmc Azd Unversty ehrn South Brnch, Irn. E-ml: jmohmmd98@yhoo.com el:

2 Aust. J. Bsc & Appl. Sc., 5(): , 0 Where Y=the totl unt weght of the ground, c u =undrned sher strength of the ground, q s =surchrge, R=rdus, σ =the mnmum fce support pressure nd C=overburden (cover depth). Fg. : he tunnel-fce stblty model of the method of Broms nd Bennemrk. Emprclly, the nstblty condtons re ssocted wth vlue of 6. herefore, the fce-stblzng pressure σ s: σ =Y.(C+R)+q s -.c u () where =6. Method of (Atknson nd Ptts, 977): Plstc lmt nlyss pproches employ plne strn condton. Atknson nd Potts nvestgted the requred support pressure for n unlned tunnel cross secton wy from tunnel hedng n cohesonless sol. he mnmum support pressure(σ ) s : σ = k p YR/(k p -) (3) where k p =(+snø)/(-snø) nd ø=the sol frcton ngle. Fg. : he tunnel-fce stblty model of the method of (Atknson nd Potts, 977). Method of (Dvs, et l., 980): Lke prevous soluton, ths method employs plne strn condton too. Dvs et l. nvestgted fcestblzng pressure for purely cohesve mterl. hs method llows the stblty nlyss of tunnel wth rdus R, n cohesve sol, where rgd support s nstlled t dstnce P from the fce. Hence these reserchers presented dstnce P (Fg.3) nd the stblty rto. he stblty rto n the two cses of cylndrcl (Eq.(4))nd sphercl (Eq.(5)) s clculted. 590

3 Aust. J. Bsc & Appl. Sc., 5(): , 0 Fg. 3: he lodng schemes of the method of (Dvs, et l., 980). =+ln(c/r+) (4) =4+ln9C/R+) (5) Method of (Kruse, 987): he mnml support pressures needed for sem-crculr nd sphercl lmtng equlbrum mechnsm whch hve been clculted by Kruse n lmtng equlbrum nlyss usng the sher stresses on the sldng plnes. Of the three mechnsms proposed, the qurter crcle (Fg.4b) wll lwys yeld the hghest mnml support pressure: σ =(DY/3-c/)/tnø (6) As Kruse lredy ndctes ths my not lwys be relstc representton of the ctul flure body. In mny cses the hlf-sphercl body (Fg.4c) wll be better representton. In tht cse the mnml support pressure cn be found from: σ =(DY/9-c/)/tnø (7) where D= tunnel dmeter nd c= sol coheson. Fg. 4: Crculr nd sphercl flure mechnsms. Method of (Lec nd Dormeux, 990): he purpose of lmtng nlyss s to provde n estmte of stblty condtons for mechncl system regrdless of the behvour of the mterl. Lec nd Dormeux used the lmtng nlyss concept to evlute the stblty of tunnel fce drven n frctonl sol nd compred these results wth centrfuge tests performed by (Chmbon nd Corte, 994). A resonble greement ws found between the theoretcl upper bound estmtes nd the fce pressures mesured t flure from the tests. herefore, by modfyng the upper bound soluton suggested by Lec nd Dormeux the three-dmensonl nlytcl model. hree flure mechnsms hve been consdered. hey ll nvolve the movement of sold concl blocks wth crculr cross-sectons. he openng of ech cone s equl to ø nd ts velocty V s prllel to ts xs (Fg.5). 59

4 Aust. J. Bsc & Appl. Sc., 5(): , 0 Fg. 5: Concl blocks nd knemtc condtons used n M nd M. herefore condton s stsfed long the flure surfces between the movng blocks nd the rest of the ground. he two mechnsms M nd M re shown n Fg.6 nd Fg.7 respectvely. M s collpse mechnsm, wheres M refers to blow out flure. Flure s due to the collpse of one concl block n M. Fg. 6: Flure Mechnsm M (Collpse). Fg. 7: Flure mechnsm M (uphevl). Lec nd Dormeux obtned fce-support pressure (σ ) for upper bound : σ = S q s + Y DY (8) where q s =surchrge, D=dmeter, Y=unt weght of ground nd S nd Y re non dmensonl weghtng coeffcents. 59

5 Aust. J. Bsc & Appl. Sc., 5(): , 0 (9) (0) () () (3) (4) =49-ø/ (6) ß=/4+/ (7) where R A, R B, R C, R D nd R E re non dmensonl fctors, ø=frcton ngle of the sol, =ngle of sldng plne, nd ß=flure ngle(slp ngle). ote: S s lmost lwys smller thn Y nd s equl to zero for vlue of ø when C/D0.6. Method of (Jncsecz nd Stener, 994): So you see Fg.8, tht shows the three dmensonl flure scheme tht conssts of sol wedge (lower prt) nd sol slo (upper prt). Jncsecz nd Stener nvestgted the effects of sol rchng bove the tunnel hedng, nd suggested three-dmensonl erth pressure coeffcent K A3 n the ble for dfferent vlues of the frcton ngle. (5) Fg. 8: hree dmensonl lmtng equlbrum method. hese reserchers presented equtons for clcultng the fce-stblzng pressure(σ ): σ = 4E/D (8) where D=tunnel dmeter nd E=erth pressure(eq.(9)). 593

6 Aust. J. Bsc & Appl. Sc., 5(): , 0 E cos.tn sn. K W sn.tn cos S (9) Where ß=slp ngle (ble ), ø=ngle of nternl frcton, S =slo weght (Eq.(0)), w =weght of sol wedge(eq.()), =sher force of sol(eq.()), nd K=cohesve force(eq.(3)). S W B. D.cot. B. D..cot V (0) () D.cot c B. D. c K sn K y. V.tn Where bove B=wedge wdth, k y =erth pressure coeffcent, nd Ý=submerged unt weght. () (3) ble : Vrton of slp ngle. C D he vertcl pressure s clculted ccordng to erzgh s soluton, so tht n dry sol condtons (Eq.()) nd when there s underground wter (Eq.(3)). V k.tn. C. K.tn C. K.tn d c e q e 0. (4) V c e k.tn H. K.tn Vd. e H. K. tn (5) where Y d =dry unt weght, =hlf of wedge wdth, H= heght of ground wter, C=overburden, c=sol coheson, q 0 =surchrge, nd =vertcl pressure n dry condtons(eq.(6)). Vd Vd d c e k.tn C H. K. tn q0. e C H. K. tn (6) ble : hree dmensonl erth pressure coeffcent. C D K A

7 Aust. J. Bsc & Appl. Sc., 5(): , 0 Method of (Angnostou nd Kovr, 996): hs three-dmensonl sttc system (Fg.9) s bsed upon the slo theory(jnssen, 895) nd ws frst ppled by (Horn, 96) to the nvestgton of tunnel fce stblty. he nlyss s performed n drned condton, nd dfference between the stblzng wter pressure nd the effectve pressure n the plenum of n EPBS s presented. Fg. 9: he tunnel-fce stblty model of the method of Anognostou nd Kovr. At lmtng equlbrum, the effectve stblzng pressure (Eq.(7)) depends on the tunnel dmeter D, on the overburden H, on the pezometrc hed h F, on the elevton of the wter tble h O, on the sher strength prmeters c nd ø, on the submerged unt weght Ý (for the sol beneth the wter tble), nd the dry unt weght Y d (for the sol bove the wter tble). Fg. 0: omogrms for the dmensonless coeffcents F 0 to F 3. σ =F 0 ÝD-F c+f Ý h-f 3 c h/d (7) where F 0, F, F nd F 3 re non dmensonl coeffcents (Fg.0) nd h=h O -h F. 595

8 Aust. J. Bsc & Appl. Sc., 5(): , 0 Method of (Broere, 00): hs method modfed some mportnt lmttons of the current nlytcl methods such s the heterogenety of the ground t the fce (Fg.). Broere method s bsed wedge nd slo theory tht some forces re ctng on the wedge (Fg.). Fg. : Wedge nd slo model(left) nd forces ctng on the wedge(rght) n Broere method. hs wedge s ssumed to be rgd body, loded ts effectve weght w nd the overburden resultng from the sol slo, s. On the trngulr sde plnes of the wedge the full cohesve-frctonl forces re tken nto ccount, derved from the horzontl effectve stress. he sher force ctng on the slnted front plne of the wedge, resultng from the norml force ctng on ths plne, s splt n two prts. he frctonl prt R depends only on the ngle of nternl frcton (R = tn Ø). he force K depends only on the coheson of the sol or, n n undrned nlyss, on the undrned sher strength. Equlbrum of these forces results n n effectve erth force E, whch hs to be countered by the effectve support force S. hs s the dfference between the totl support force S nd the wter force W tht results from the pore pressure. For gven wedge ngle θ the resultng erth force E cn be clculted. he mnml support pressure cn be found by tertng over the ngle θ nd mxmzng E. Fg. : Defnton of symbols n the multlyered wedge model. Fg. shows the flure wedge s subdvded n smller bodes, possbly of dfferent thckness, nsde ech of whch the sol condtons re homogeneous. he sol condtons my vry between these slces, s my the wedge ngle θ () between the th slce s slnted flure plne nd the horzontl. Ech slce s loded by the resultng forces from the slce bove ( ) nd below ( + ), () nd b () respectvely, the effectve weght of the slce tself w () nd n overburden force s (). At the slnted flure plne there s cohesve force K () ctng prllel to the plne, s well s frcton force R (), whch results from the norml force (), workng perpendculr to the flure plne. he sde fces of the wedge re ech ssumed to be loded by the sher forces (), whch ct n the sme drecton s K (), gnst the deformton drecton of the wedge. Force equlbrum wll yeld the effectve erth force E () t the fce whch, combned wth the wter force W (), s equl to the support force S (). 596

9 Aust. J. Bsc & Appl. Sc., 5(): , here s vertcl nd horzontl equlbrum n condton of: 0 sn cos cos R K E (8) 0 cos sn sn b w s R K (9) R tn (30) Combnton of equtons (8) nd (9) led to: 0 b w s E K (3) Becuse of shorthnd notton: cos sn tn (3) sn cos tn (33) Ech slce hs to stsfy the equlbrum s well s the contnuty condton: b (34) Boundry condtons: 0 b nd 0 For slce : w w s s b (35) hs result cn be combned wth the equlbrum relton for slce -: W W S S b E E K K (36) Where W S K E (37) S=E+W (38) And so on the bove equtons, fnlly mnmum fce-stblzng pressure( ) s equl to: =4S/D (39) Method of (Crrnz-orres, 004): Crrnz-orres ntegrted method of (Cqout-Kersel, 956).Crrnz s model consders the equlbrum condton for mterl undergong flure bove the crown of shllow crculr (cylndrcl or sphercl) cvty. he mterl hs unt weght γ nd sher strength defned by Mohr-Coulomb prmeters c nd φ, the coheson nd the frcton ngle respectvely. A support pressure P s s ppled nsde the tunnel, whle surchrge q s cts on the ground surfce. For the stuton presented n the Fg.3, Crrnz s soluton defnes the vlue of fce-stblzng pressure (P s ) s the mnmum or crtcl pressure below whch the tunnel wll collpse:

10 Aust. J. Bsc & Appl. Sc., 5(): , 0 Fg. 3: Mn desgn of Cqout-Kersel. ps qs c h. tn fs K K FS h FS K c tn (40) Where =the tunnel rdus, h=xs depth below the surfce, FS=fctor of sfety(eq.(4)). sntn FS sntn c tn FS cr c tn tn FS tn FS cr Project chrcterstcs: ehrn Metro lne 7 n Irn hs 7 km length nd s dvded nto two prts.e. orth-south Lot nd Est- West Lot.(Fg.4). 7 th lne of ehrn metro s consst of 6 sttons tht orth-south Lot hs 3 sttons nd Est-West Lot hs 3 sttons too. he feld explortons nd surveyng were performed by borng 37 boreholes. Dstnce between boreholes s generlly n the rnge of between 800m nd 50m pproxmtely. ble 3: mn dt of 7 th lne of ehrn metro. me of tunnel 7 th lne of ehrn metro orth-south secton Purpose/functon of the tunnel Metro lne otl length About 4 Km umber of sttons 3(7,O7,P7,7,R7,S7,7,U7,V7,W7,X7,Y7,Z7) Mxmum slope 5.0% Mnmum overburden 9.5 meter to prevent encounterng cvl lmttons Mnmum tunnel crown dstnce 5.0 meter from other metro lnes Foreseen EPB BM m (00% of totl) Constructon methods Cutterhed dmeter 9.6 meter BM tunnel 8.5 m nternl dmeter lned wth precst elements typcl secton (4) (4) 598

11 Aust. J. Bsc & Appl. Sc., 5(): , 0 Fg. 3: Drecton of 7 th lne of ehrn metro. At frst the excvton strted n orth-south Lot from stton 7. here re mn dt of 7 th lne of ehrn metro, orth-south secton n tble 3. ble 4: Sol clssfcton of orth-south Lot of ehrn metro lne 7. Stton BSCS 7 ML,ML O7 WM,ML,ML P7 WM,ML,CI 7 CL,CI,CL,CI R7 ML,CL PC,PM,WC S7 PC,PM,WC,WM PC,PM WM,ML 7 WM,WC,PC,ML U7 WM,ML WM,ML,CL,CI CL,CI,SMI,WM V7 ML,CL CL,MIS W7 CL,CL,CL CL,CL X7 CL,CL,CL,CL CL ML,CL,CL,CL CL,CL Y7 ML,CL Z7 WC,PC Chnge ble 5: Sol groupng for geotechncl desgn purpose. Sol Clssfcton Symbols Ctegory roup Subgroup I -F S-F WM, PM WC, PC SWM, SPM SWC, SPC II F SF ML, MI CL, CI SML, SMI SCL, SCI III F FS ML, MI CL, CI MLS, MIS CLS, CIS IV F ML, MI CL, CI 599

12 Aust. J. Bsc & Appl. Sc., 5(): , 0 bles 4, 5 nd 6 show sol clssfcton, chnge, sol ctegory nd fnlly geotechncl dt of drecton. ble 6: eotechncl desgn dt n the project re. Clcultons nd comprng to EPB ctul result: In ths pper fce-stblzng pressures re clculted for excvted sttons (7 nd O7) by nlytcl methods whch wth EPB ctul results re wrtten n the tble 7. ble 7: Fce-stblzng pressure of nlytcl methods nd EPB sheld. Fce-stblzng pressure( ) (kp) Stton Chnge (Km+m) Surchrge(qs) (kp) Overburden (m) Atknson & Ptts Broms Kruse O ble 7 shows the dfference between the EPBS ctul results nd the nlytcl methods results. As ths s cler tht condtons of some nlytcl methods such s Kovr nd Broms dffer from geotechncl condtons of sttons 7 nd O7, hence the boxes re flled by puttng crosses ( n the tble nd f for these sttons fce-support pressure clculte by them, obtned qunttes wll be llogcl. Predcton of quntty fce-stblzng pressure for some unexcvted sttons: he condtons of selected sttons dffer from together, for exmple n tunnel depth, coheson, frcton ngle, surchrge nd etc. Hence dfferent qunttes fce-stblzng pressures ( ) obtned for these sttons by lmtng equlbrum nlyss method of Broere whch re wrtten n the tble 8. Conclusons: Accordng to the comprson of nlytcl methods results to EPB opertonl results, for ths project wth these geotechncl propertes, lmtng equlbrum nlyss method of Broere s logcl nd relstc mong others. It s mpossble tht ll the nlytcl methods such s Anognosou nd Kovr, Crrnz-orres, Broms nd etc use for ll prts of ehrn metro lne 7 project, becuse condtons of methods dffer together, for exmple method of Crrnz-orres uses for shllow tunnels nd method of Anognostou nd Kovr do not use for cohesve sol. Lec & Dormeux Crrnz-orres Anognostou & Kovr Broere Actul result of EPB sheld 600

13 Aust. J. Bsc & Appl. Sc., 5(): , 0 In Broere method overburden ffects on the quntty fce-stblzng pressure more thn other prmeters lke ngle of nternl frcton. ble 8: Predcted qunttes of fce-stblzng pressure by Broere method. Stton Chnge (Km+m) Overburden (m) Surchrge(q s) (kp) Fce-stblzng pressure( ) (kp) S V W X REFERECES Mohmmd, J., 00. unnel Fce Stblty Anlyss n Soft round by EPB Method(Cse Study: ehrn Metro Lne 7), M Sc hess, ehrn, Irn. Angnostou,. nd D. Rzos, 008. eotechncl nd contrctul spects of urbn tunnelng wth closed shelds. Swtzerlnd. Angnostou,. nd K. Kovr, 993. "Fce stblty condtons wth Erth- Pressure- bssure- blnced Shelds". unnelng nd Underground Spce technology, (): Angnostou,. nd K. Kovr, 994. " he fce stblty of slurry- sheld drven tunnels", unnelng nd Underground Spce echnology, 9(): Angnostou,. nd K. Kovár, 996. Fce stblty n slurry nd EPB sheld tunnelng. eotechncl Aspects of Underground Constructon n Soft round, Rotterdm, Blkem., pp: Broere W., A.F. vn ol, 00. me-dependnt nfltrton nd groundwter flow n fce stblty nlyss. Modern unnelng Scence nd echnology, Kyoto, Jpn. Broere, W., 00. unnel Fce Stblty nd ew CP Applctons, Dssertton, U Delft 00. Broere, W., 00. Influence of excess pore pressures on the stblty of the tunnel fce. eotechncl Aspects of Underground Constructon n Soft round, oulouse, Frnce. Crrnz-orres, C., 004. " Computton of fctor of Sfety for Shllow unnels usng Cquot's Lower Bound Soluton. " In Publcton. Chmbon, P. nd J.F. Corte, 994. Shllow unnels n Cohesonless Sol: Stblty of unnel Fce, Journl of eotechncl Engneerng, ASCE, 0(7): uglelmett, V., P. rsso, A. Mhtb nd Sh. Xu, 007. Mechnzed unnellng n Urbn Ares, Desgn Methodology nd Constructon Control. eodt S.p.A., urn, Itly Jncsecz, S. nd W. Stener, 994. " Fce support for lrge Mx- Sheld n heterogeneous ground condtons". unnelng 94. London. Km, S.H. nd F. onon, 00. Fce stblty nd requred support pressure for BM drven tunnels wth del fce membrne-drned cse. unnelng nd Underground Spce echnology. Kovr, K. nd M. Rmon, 006. Urbn tunnelng n soft ground usng BMS ". nt Conf. & exhbton on tunnelng nd trenchless technology, Subng Jve- Selngor Drul Ehsn:7-3; the nsttuton of Engneers. Mlys Lece, E. nd L. Dormeux, 990. "Upper nd lower Bound solutons for the fce stblty of shllow crculr tunnerl n frctonl", eotechnque", 40: Muller-Krchenbuer, H., 97. Stblty of slurry trenches Proc, 5 th. Europ. Conf. SMFE, Mdrd., pp: Muller-Krchenbure, H., 977. Stblty of slurry trenches n nhomogeneous subsol. In... edtor 9 th Interntonl Conference on Sol Mechncs nd Foundton Engneerng. Repetto, L.V. unnett, V. uglelmett nd. Russo, 007. Sheld tunnelng n senstve res: new desgn procedure for optmzton of the constructon-phse mngement. EODAA S.p.A., orno, Itly. Rbcch, R., 994. " Recent orenttent nell progettzone sttc delle gllere". Att XVIII Convegno zonle d eotecnc. Russo,., 003. "Evluton the requred fce- support pressure n EPBs dvnce mode. " llere e rnd Opere Sotterrnnee n.7-dcembre 003. Sngh, B. nd R.K. oel, 999. Rock Mss Clssfcton, Unversty of Roorkee nd Centrl Mnng Reserch Insttute, Ind. Wng, H. nd J. J, 009. Fce Stblty Anlyss of unnel wth Ppe Roof Renforcement Bsed on Lmt Anlyss. EJE, 4. 60