Evaluation of Liquefaction Return Period for Bangalore Based on Standard Penetration Test Data: Performance Based Approach

Size: px

Start display at page:

Download "Evaluation of Liquefaction Return Period for Bangalore Based on Standard Penetration Test Data: Performance Based Approach"

Britney Collins
5 years ago
Views:

1 Amercn J. of Engneerng nd Appled Scences 2 (3): , 2009 ISSN Scence Publctons Evluton of Lquefcton Return Perod for Bnglore Bsed on Stndrd Penetrton Test Dt: Performnce Bsed Approch K.S. Vpn nd T.G. Sthrm Deprtment of Cvl Engneerng, Indn Insttute of Scence, Bnglore Ind Abstrct: Problem sttement: The conventonl lquefcton evluton s bsed on determnstc pproch. However n ths method the uncertnty n the erthquke lodng s not properly tken nto ccount. However recent reserch n ths feld ndctes tht ths uncertnty n the erthquke lodng hs to be consdered n the lquefcton potentl evluton. Moreover the evluton of lquefcton return perod s not possble n the conventonl determnstc methods. Ths study explned the methods for evlutng the probblty of lquefcton nd the return perod of lquefcton bsed on probblstc pproch. Approch: In ths study the geotechncl dt ws collected from 450 bore holes n Bnglore, Ind, coverng n re of 220 km 2. The sesmc hzrd nlyss for the study re s crred out usng Probblstc Sesmc Hzrd Anlyss (PSHA) nd the pek ccelerton t ground surfce ws evluted for ste clss-d fter consderng locl ste effects. For ssessment of ste clss, sher wve velocty profles n the cty hd been obtned usng Multchnnel Anlyss of Surfce Wves (MASW) survey. Bsed on ths dt the probblstc lquefcton nlyss ws done to evlute the probblty of lquefcton n the study re. Bsed on the performnce bsed pproch the lquefcton return perod for the study re ws lso evluted. Results: The results showed the vrton of lquefcton susceptblty for the study re. The corrected stndrd penetrton vlues ured to prevent the lquefcton for return perods of 475 nd 2500 yers were lso presented here. Concluson/Recommendtons: The sptl vrton of probblty of lquefcton nd the fctor of sfety gnst lquefcton evluted usng the two methods mtch well for the study re. Key words: MASW, PSHA, probblstc lquefcton, SPT INTRODUCTION Devsttng effects of sesmc sol lquefcton cses hve stressed the need for ssessment of lquefcton potentl n sesmclly ctve res. The smplfed procedure for the lquefcton evluton ws suggested by Seed nd Idrss [1]. Durng the pst three decdes ths method hs undergone sgnfcnt modfctons nd modfed form of ths method suggested by Youd et l. [2] s beng wdely followed for the evluton of lquefcton potentl. These conventonl lquefcton evluton methods wll gve the fctor of sfety gnst lquefcton. Where s modfed method, suggested by Cetn et l. [3], wll gve the probblty of lquefcton for ny gven ste. However both the bove methods use sngle ground ccelerton vlue ( mx ) nd sngle erthquke mgntude (M w ) nd these methods do not consder the uncertnty n erthquke lodng. It hs been seen from the Probblstc Sesmc Hzrd Anlyss (PSHA) tht the fuency of occurrence of lower ground ccelerton vlues wll be more nd tht of the hgher ccelerton vlues wll be less. Ths vrblty of ground ccelerton vlues were not consdered n the bove mentoned methods. The method suggested by Krmer nd Myfeld [4] to evlute the lquefcton return perod usng probblstc method (performnce bsed pproch), the uncertnty n erthquke lodng s lso tken nto ccount. Ths study explns the sptl vrblty of probblty of lquefcton nd the lquefcton return perod of Bnglore bsed on probblstc methods usng SPT dt obtned from 450 bore holes n n re of 220 km 2. MATERIALS AND METHODS Probblstc Sesmc Hzrd Anlyss (PSHA): The study re selected s Bnglore metropoltn re, whch covers bout 220 km 2 nd s shown n Fg. 1. The sesmc hzrd for Bnglore t rock level ws estmted usng the Probblstc Sesmc Hzrd Anlyss (PSHA). The nnul rte of exceednce of Correspondng Author: T.G. Sthrm, Deprtment of Cvl Engneerng, Indn Insttute of Scence, Bnglore Ind Tel: Fx:

re bsed on MASW survey ws used. Surfce level ccelerton vlues were estmted usng the ttenuton reltons presented by RghuKnth nd Iyengr [7] for South Ind.

2 re bsed on MASW survey ws used. Surfce level ccelerton vlues were estmted usng the ttenuton reltons presented by RghuKnth nd Iyengr [7] for South Ind. Probblstc evluton of lquefcton potentl: Whle evlutng the lquefcton potentl, the erthquke lodng Cyclc Stress Rto (CSR) s evluted usng the followng expresson: mx vo d CSR = 0.65 g vo MSF r (2) Fg. 1: Study re (Bnglore cty) long wth borehole loctons Pek Horzontl Accelerton (PHA) from n sources s evluted n PSHA usng the followng equton: N ν (z) = λ (m ) V(z) P (R r m )P (Z z m,r ) m m u r r = = mx n = (1) n n= 1 m m 0 r = r > mn = = The men nnul rte of exceednce of ground moton prmeter Z, wth respect to z λ n (m ) = Fuency of occurrence of mgntude m t the source n P n(r = r m ) = Probblty of occurrence of n erthquke of mgntude m t dstnce r from the ste for sesmc source n P(Z > z m,r ) = Probblty t whch the ground moton prmeter Z exceeds predefned vlue of z when n erthquke of mgntude m occurs t dstnce of r from the ste In ths nlyss the uncertntes nvolved n mgntude recurrence rte [5], hypocentrl dstnce [6] nd the ttenuton of sesmc wves [7] were consdered. Whle evlutng the lquefcton susceptblty, the surfce level Pek Ground Accelerton (PGA) hs to be clculted. The PGA vlues were clculted from the rock level PHA vlues, usng the mplfcton fctors suggested by Rghu Knth nd Iyengr [7]. To estmte surfce level PGA vlues, the ste clss presented by Anbzhgn nd Sthrm [8] for the study 538 mx = Pek ground ccelerton g = Accelerton due to grvty (n the sme unt s mx ) vo = Totl vertcl stress t the gven depth vo = Effectve vertcl stress t the gven depth r d = Stress reducton fctor MSF = Mgntude sclng fctor, whch depends on the moment mgntude of erthquke After nlyzng sgnfcntly lrge dtbse of feld hstores of sesmc sol lquefcton, Cetn et l. [3] suggested new procedure for evlutng the probblty of lquefcton potentl. The relton suggested s: (N 1) 60(1 + θ1fc) θ2 ln CSR eq θ3 ln Mw θ4(ln( / P ) + θ 5FC + θ6 PL = Φ ε (3) P L = Probblty of lquefcton (s frcton) Φ = Stndrd norml cumultve dstrbuton functon (N 1 ) 60 = Corrected N vlue FC = Fneness content n percentge CSR eq = Cyclc stress rto wthout MSF Mw = Moment mgntude of erthquke ' = Effectve vertcl pressure t the gven depth P = Atmospherc pressure (n the sme unt s θ 1- θ 6 E ) ' = Regresson coeffcents = Model uncertnty Both the bove methods consder sngle level of ground ccelerton nd sngle erthquke mgntude for evlutng the lquefcton potentl.

3 m = Incrementl nnul fuency of exceednce for ccelerton nd mgntude m The condtonl probblty n Eq. 4 cn be evluted by [4] : P[FS < FS,m ] L L * (N 1) 60(1 + θ1fc) θ2 ln(csr eq,fs L) θ3 ln(m ) θ4(ln( / P ) + θ 5FC + θ6 = Φ ε (5) Fg. 2: De-ggregted hzrd curve wth respect to mgntude Hence the uncertnty n erthquke lodng s not ccounted n these methods. A typcl deggregted sesmc hzrd curve, the vrton of nnul fuency of exceednce gnst the PHA, wth respect to mgntude s shown n Fg. 2. The Fg. 2 clerly ndctes tht prtculr ground ccelerton s not contrbuted by sngle mgntude; nsted t s beng contrbuted by dfferent mgntudes. More over the fuency of occurrence of ccelerton vlues lso vres; lower ccelerton vlues wll occur more fuently thn the hgher ccelerton vlues. These uncertntes n erthquke lodng re not consdered n the conventonl lquefcton nlyss. Krmer nd Myfeld [4] modfed the probblstc method suggested by Cetn et l. [3] to evlute the return perod of sesmc sol lquefcton bsed on probblstc pproch. In ths pproch, the contrbutons from ll the mgntudes nd ll the ccelerton levels hve been consdered. Thus, the uncertnty nvolved n evlutng the erthquke lodng for the ntton of lquefcton s tken cre off. The nnul probblty of exceednce for gven fctor of sfety vlue s gven by: NM N P[FS FS,m ],m FS * L L L = 1 = 1 Λ = < λ (4) Λ FS * = Annul rte t whch fctor of sfety wll be L less thn L FS L = Fctor of sfety gnst lquefcton FS L FS Ф = Stndrd norml cumultve dstrbuton = Effectve over burden pressure = Trgeted vlue of fctor of sfety gnst The vlue of N s the corrected N vlue ured lquefcton N M = Number of mgntude ncrements to prevent the lquefcton wth n nnul fuency of N = Number of ccelerton ncrements λ exceednce of λ *. N 539 P = Atmospherc pressure n the sme unt s vo CSR eq, = 0.65 r d g vo (6) CSR eq, = The CSR vlue clculted for n ccelerton r d = Stress reducton fctor nd ths cn be clculted usng the new method suggested by Cetn nd Seed [9] In smlr wy the lquefcton potentl cn lso be chrcterzed by lquefcton resstnce ured t ste for gven return perod. The lquefcton resstnce s chrcterzed by SPT resstnce t the gven locton nd t the ured depth. The probblstc method cn be ppled to get the nnul fuency of exceednce for N : N M N * P[N N N = 1 = 1,m ],m λ = > λ (7) P[N > N,m ] N (1 + θ1fc) θ2 ln(csr eq,) 3 ln(m ) 4(ln( / P ) 5FC θ θ + θ + θ6 = Φ ε (8)

The correctons ppled to SPT vlues nclude the correcton for overburden pressure, short rod length, smpler, bore-hole dmeter nd hmmer energy effcency. MASW survey ws crred out t 58 loctons (Fg.

4 RESULTS The geotechncl dt for the study re were collected from 450 bore holes whch were spred cross the study re (Fg. 1). The depth of boreholes were up to mxmum of 35 m nd the verge depth of borehole ws bout 20 m. The SPT vlues obtned were corrected to get the (N 1 ) 60. The correctons ppled to SPT vlues nclude the correcton for overburden pressure, short rod length, smpler, bore-hole dmeter nd hmmer energy effcency. MASW survey ws crred out t 58 loctons (Fg. 3) n Bnglore nd bsed on equvlent sher wve velocty obtned, the study re ws clssfed s ste clss D [8]. The 6 vulnerble sesmc sources n the study re, whch ws dentfed by Anbzhgn et l. [10], were used n evlutng the sesmc hzrd. The PGA vlues t surfce level were clculted for ste clss-d usng PSHA method. In order to consder the worst scenro for the lquefcton nlyss, the wter tble ws ssumed t the ground surfce. The probblty of lquefcton t depths of 3 nd 6 m for return perod of 475 yers re shown n Fg. 4 nd 5. The evluton of return perod of sesmc sol lquefcton ws done usng the Eq. 4 nd 7. The curves showng the vrton of fctor of sfety gnst lquefcton return perod t depth of 3 m for four dfferent loctons n Bnglore re shown n Fg. 6. The 4 loctons selected, vz. Indrngr, Kormngl, Hudson Crcle nd Hebbl re promnent plces n the study re nd they re locted t four dfferent regons of the study re. These curves re smlr to the de-ggregted sesmc hzrd curves (Fg. 2) nd both the curves show the vrton of the prmeters consdered, pek ground ccelerton or fctor of sfety, gnst the nnul fuency of exceednce. The mn dvntges of these curves re, the fctor of sfety gnst lquefcton for ny gven return perod cn be obtned drectly. Such evluton s not possble n ny of the determnstc lquefcton evluton methods. In smlr wy, the curves between the corrected N vlues ured to prevent lquefcton nd return perod re shown n Fg. 7. The (N 1 ) 60 ured to prevent lquefcton for ny specfed return perod cn be obtned from these curves. If the corrected N vlue (obtned from the ste nvestgton) t the ste s less thn the vlue obtned from ths curve, then the ste s vulnerble to lquefcton for tht return perod. Fg. 4: Probblty of lquefcton t depth of 3 m for return perod of 475 yers Fg. 3: MASW testng loctons n Bnglore cty, Ind 540 Fg. 5: Probblty of lquefcton t depth of 6 m for return perod of 475 yers

Fg. 6: Fctor of sfety Vs return perod t 3 m depth for three loctons n Bnglore Fg. 8: Fctor of sfety gnst lquefcton for return perod of 475 yers t 3 m depth Fg.

depths of 3 nd 6 m for the entre study re were evluted nd the results re shown n Fg. 8 nd 9. The (N 1 ) 60 ured to prevent lquefcton (Fg.

In the recent yers the buldng codes re revsed to consder the 2% probblty of exceednce of PGA n 50 yers, whch corresponds to return perod of 2475 yers.

The fctor of sfety gnst lquefcton t depths of 3 nd 6 m re shown n Fg. 12 nd 13. The (N 1 ) 60 ured to prevent lquefcton t depths of 3 nd 6 m re shown n Fg. 14 nd 15. 541 Fg.

5 Fg. 6: Fctor of sfety Vs return perod t 3 m depth for three loctons n Bnglore Fg. 8: Fctor of sfety gnst lquefcton for return perod of 475 yers t 3 m depth Fg. 7: (N 1 ) 60 ured to prevent lquefcton versus return perod t 3 m depth for four loctons n Bnglore The vrton of fctor of sfety gnst lquefcton for return perod of 475 yers t depths of 3 nd 6 m for the entre study re were evluted nd the results re shown n Fg. 8 nd 9. The (N 1 ) 60 ured to prevent lquefcton (Fg. 10 nd 11) ndcte the corrected SPT vlues ured to prevent lquefcton t the respectve loctons. These vlues ncrese slghtly wth depth,.e., from 3-6 m, due to the ncrese n overburden pressure wth depth. In the recent yers the buldng codes re revsed to consder the 2% probblty of exceednce of PGA n 50 yers, whch corresponds to return perod of 2475 yers. Hence the fctor of sfety gnst lquefcton nd the (N 1 ) 60 ured to prevent lquefcton for return perod of 2500 yers were lso evluted. The fctor of sfety gnst lquefcton t depths of 3 nd 6 m re shown n Fg. 12 nd 13. The (N 1 ) 60 ured to prevent lquefcton t depths of 3 nd 6 m re shown n Fg. 14 nd Fg. 9: Fctor of sfety gnst lquefcton for return perod of 475 yers t 6 m depth Fg. 10: (N 1 ) 60 ured to prevent lquefcton for return perod of 475 yers t 3 m depth The FS L vlues re showng decresng trend n both the Fg. 14 nd 15 becuse of ncrese n return perod.

Fg. 11: (N 1 ) 60 ured to prevent lquefcton for return perod of 475 yers t 6 m depth Fg. 14: (N 1 ) 60 ured to prevent lquefcton for return perod of 2500 yers t 3 m depth Fg.

13: Fctor of sfety gnst lquefcton for return perod of 2500 yers t 6 m depth The (N 1 ) 60 ured to prevent lquefcton lso shows n ncresng trend n Fg. 14 nd 15 becuse of the ncrese n return perod.

4 nd 5 tht for most of the loctons n the study re the probblty of lquefcton decreses wth depth. Ths s due to the ncrese n SPT vlues wth depth for those loctons. The fctor of sfety (Fg.

6 Fg. 11: (N 1 ) 60 ured to prevent lquefcton for return perod of 475 yers t 6 m depth Fg. 14: (N 1 ) 60 ured to prevent lquefcton for return perod of 2500 yers t 3 m depth Fg. 12: Fctor of sfety gnst lquefcton for return perod of 2500 yers t 3 m depth Fg. 15: (N 1 ) 60 ured to prevent lquefcton for return perod of 2500 yers t 6 m depth DISCUSSION Fg. 13: Fctor of sfety gnst lquefcton for return perod of 2500 yers t 6 m depth The (N 1 ) 60 ured to prevent lquefcton lso shows n ncresng trend n Fg. 14 nd 15 becuse of the ncrese n return perod. For the morty of the study re, the probblty of lquefcton s less tht 10 % (Fg. 4 nd 5) nd these ndcte tht most prts of the study re re sfe gnst lquefcton. Moreover t s cler from Fg. 4 nd 5 tht for most of the loctons n the study re the probblty of lquefcton decreses wth depth. Ths s due to the ncrese n SPT vlues wth depth for those loctons. The fctor of sfety (Fg. 8 nd 9) rnge of 0-1 ndcte tht these loctons re hghly vulnerble to lquefcton; the rnge of 1-2 re modertely vulnerble nd the fctor of sfety hgher thn 2 ndcte tht these loctons re sfe gnst lquefcton. At most of the loctons, the fctor of sfety gnst lquefcton t 6 m depth s hgher thn tht t 3 m depth nd t s due to the ncrese n SPT vlues wth depth. 542

7 CONCLUSION The evluton of lquefcton potentl for Bnglore ws crred out bsed on two dfferent methods-probblty of lquefcton nd the performnce bsed pproch. However the generl pttern of lquefcton susceptblty mtches well n both the methods. For most of the study re, the probblty of lquefcton for return perod of 475 yers s less thn 10%. The fctor of sfety vlues obtned from the performnce bsed (probblstc) pproch show tht the fctor of sfety vlues re hgher thn 1 for most of the study re. In evlutng the lquefcton return perod usng the probblstc pproch, the uncertnty n erthquke lodng ws tken nto ccount n better wy thn n the conventonl nlyss methods. ACKNOWLEDGEMENT Reserchers thnk ISRO-IISc Spce Technology Cell, Indn Insttute of Scence, Bnglore, Ind for fundng the proect ttled Assessment of Sesmcty of Pennsulr Ind-Usng Remote Sensng nd GIS, (Ref. No. ISTC/CCE/TGS/195 Dted 7 Mrch 2007). REFERENCES 1. Seed, H.B. nd I.M. Idrss, Smplfed procedure for evlutng sol lquefcton potentl. J. Sol Mech. Found., 97: Youd, T.L., I.M. Idrss ndrus, D. Ronld, I. Arngo nd G. Cstro et l., Lquefcton resstnce of sols: Summry report from the 1996 NCEER nd 1998 NCEER/NSF workshops on evluton of lquefcton resstnce of sols. J. Geotech. Geoenvron. Eng., 127: &dtype=cvps&gfs=yes 3. Cetn, K.O., R.B. Seed, D.A. Kureghn, K. Tokmstu, L.F. Hrder, R.E. Kyen nd R.E.S. Moss, Stndrd penetrton test-bsed probblstc nd determnstc ssessment of sesmc sol lquefcton potentl. J. Geotech. Geoenvron. Eng., 130: DOI: /(ASCE) (2004)130:12(1314) 4. Krmer, Steven, Myfeld nd T. Roy, Return perod of sol lquefcton. J. Geotech. Geoenvorn. Eng., 133: DOI: /(ASCE) (2007)133:7(802) 5. McGure, R.K., Probblstc sesmc hzrd nlyss nd desgn erthqukes: Closng the loop. Bull. Sesm. Soc. Am., 85: Kureghn, D.A. nd A.H.S. Ang, A fultrupture model for sesmc rsk nlyss. Bull. Sesm. Soc. Am., 67: evnce&uthor1=&fulltext=&pubdte_yer=1977 &volume=67&frstpge= Rghu Knth, S.T.G. nd R.N. Iyengr, Estmton of sesmc spectrl ccelerton n pennsulr Ind. J. Erth Syst. Sc., 116: Anbzhgn, P. nd T.G. Sthrm, Mppng of verge sher wve velocty for bnglore regon: A cse study. J. Envron. Eng. Geophys., 13: DOI: /JEEG Cetn, K.O. nd R.B. Seed, Non lner sher mss prtcpton fctor (rd) for cyclc sher stress rto evluton. Sol Dyn. Erthquke Eng., 24: DOI: /.soldyn Anbzhgn, P., J.S. Vnod nd T.G. Sthrm, Probblstc sesmc hzrd nlyss for Bnglore. Nt. Hzrds, 48: DOI: /s

13 Design of Revetments, Seawalls and Bulkheads Forces & Earth Pressures

13 Design of Revetments, Seawalls and Bulkheads Forces & Earth Pressures 13 Desgn of Revetments, Sewlls nd Bulkheds Forces & Erth ressures Ref: Shore rotecton Mnul, USACE, 1984 EM 1110--1614, Desgn of Revetments, Sewlls nd Bulkheds, USACE, 1995 Brekwters, Jettes, Bulkheds nd