LECTURE LIQUEFACTION OF SOILS DURING EARTHQUAKES I.M. Idriss Woodward-Clyde Consultants San Francisco, California, U.S.A. DEFINITION OF TERMS The following definitions perta1n1ng to cyclic loading conditions are based on slight modifications of those originally proposed by Lee and Seed (1967). FAILURE: When the induced cyclic: strains become excessive. COMPLETE LIQUEFACTION: When a soil exhibits no resistance to de­ formation over a wide strain range. PARTIAL LIQUEFACTION: When a soil exhibits no resistance to de­ formation over a strain range less than that considered to cons­ titute failure. INITIAL LIQUEFACTION: When a soil first exhibits any degree of partial liquefaction during cyclic loading. Ordinarily, the fir@t condition reached is INITIAL LIQUEFACTION, followed by PARTIAL, and COMPLETE liquefaction, with FAILURE being reached at some stage during the partial or complete liquefaction stages. The following definitions are given by Seed, Arango and Chan (1975) : INITIAL LIQUEFACTION: Denotes a condition where ,during the course of cyclic stress applications, the residual pore water pressure on completion of any full stress cycle becomes equal to the applied 507 J. B. MlUtins (ed.), Numerical Methods in GeomechanicB, 507-530. Copyright e 1982 by D. Reidel Publishing Company. 508 1. M.IDRISS confining pressure; the development of initial liquefaction has no implications concerning the magnitude of the deformations which the soil might subsequently undergo; however,it defines a condition which is a useful basis for assessing various possible forms of subsequent soil behavior. INITIAL LIQUEFACTION WITH LIMITED STRAIN POTENTIAL OR CYCLIC MOBI­ LITY: Denotes a condition in which cyclic stress applications develop a condition of initial liquefaction and subsequent cyclic stress applications cause limited strains to develop either be­ cause of the remaining resistance of the soil to deformation or because the soil dilates, the pore pressure drops, and the soil stabilizes under the applied loads. LIQUEFACTION: Denotes a condition where a soil will undergo con­ tinued deformation at a constant low residual stress or with no residual resistance, due to the build-up and maintenance of high pore water pressures which reduce the effective confining pressu­ re to a very low value; pore pressure build-up may due either to static or cyclic stress applications. The following definitions are given by Seed (1979): LIQUEFACTION: Denotes a condition where a soil will undergo con­ tinued deformation at a constant low residual stress or with low residual resistance due to the build-up and maintenance of high pore water pressures, which reduce the effective confiningpressu­ re to a very low value; pore pressure build-up leading to lique­ faction may be due either to static or cyclic stress applications and the possibility of its occurence will depend on the void ra­ tio or relative density of a sand and the confining pressure; it may also be caused by a critical hydraulic gradient during an up­ ward flow of water in a sand deposit. PEAK CYCLIC PORE PRESSURE RATIO OF 100%: Denotes a condition where during the course of cyclic stress applications, the resi­ dual pore water pressure on completion of any full stress cycle becomes equal to the applied continuing pressure; the development of a peak cyclic pore pressure ratio of 100% has no implications concerning the magnitude of the deformations that the soil might subsequently undergo: however, it defines a condition that is a useful basis for assessing ~arious possible forms of subsequent soil behavior. PEAK CYCLIC PORE PRESSURE RATIO OF 100% WITH LIMITED STRAIN PO­ TENTIAL OR "CYCLIC MOBILITY": Denotes a condition in which cyclic stress applications develop a peak cyclic pore pressure ratio of 100% and subsequent cyclic stress applications cause limited strains to develop either because of the remaining resistance of LIQUEFACTION OF SOILS DURING EARTHQUAKES 509 the soil to deformation or because the soil dilates, the pore pres­ sure drops and the soil stabilizes under the applied loads. Cyclic mobility may also be used in a bro.ader sense to describe the cyclic straining that may occur even with pore pressure ratios less than 100% in which case the actual peak value of pore pressure ratio may simply be stated. FAILURE CRITERION It follows from the definitions summarized in the previous pages that for each situation, a FAILURE CRITERION must be defined and adopted. The failure criterion should be set based on: (A) desired perfor­ mance; and (B) consequences of not achieving this performance le­ vel How stringently the failure criterion is to be developed depends on: (A) the naure of the soil; (B) How this soil behaves under cyclic loading conditions; (C) degree of conservatism desired; and (D) degree of conservatism (explicit and/or implicit) that exists in the other parameters that also enter in the evaluation of liquefaction potential. Aspect.s (A) and (B) are illustrated in Sheets n9 1 and 2. 510 I. M. JDRISS Table 1 - Estimated susceptibility of sedimentary deposits to liquefaction during strong seismic shaking T,pt 0' Otpo, It co."orAI dhVlbwtlo" Llktll~ood that CO~f\IO"lo" u;j!~ru, wh." 0' COhUIO"I." UlwrAIU, wOwld be 'w".ptlDIt 10 Ilq•• - Udl"","U I" dcpo,l\, hctlo" (D, Ai' 0' dopo,lt) <S·Ja Jr +10 ocrr., Plrt\!ocrrp =",.",\ rt \ llCtl'l, eo"tl",l\ul dO!!!!'''' AI.,,, ''',,,,nt'l LOCAll, .Irhbl. V0rt hIgh "'g" Low Vor, I .. 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''Ii' rrrriJ' I"n i" LOw 'V l L.;-::Jn.l lee,1 'I y.r1.bl, M'1" .McI1r,.. t! low 1:.". fort ~norc Loc::.l" 'f.r1.bl, /11 ~n t'c.J~rl tf Lo. 'fir,"'''' 10. ArtlflelAI Uo::,:!ttod fill Va.IAbl. V.r, ~19ft . - - W;.C:tj '.11 'w'.nlibh LO. - - From Youd and Perkins (1917) 1: I • e -- .-.-. _I / . - _e c. • -.-. 11 ., ,. , / -! i \ , I • ~ . • i ,. 1.1 • .• I'• , 1 , • C : ,_I \ ....- .- 0 i ~-.-.-. 1.' ,,- ", I.' ,,' !G' F:"9- 7 Ef!ect of Geological ~ge on the Vo~d Ratio and Dry Dens~ty, FROM TOLNa (1975) Sheet n9 1 LIQUEFACTION OF SOILS DURING EARTHQUAKES 511 20 0:: 0 Sacramento River Sand V> 03 = 10 Kg per sq em V> f QJ S- o. S 0 u 10 / ..... V 0:: QJ U S- / / QJ "... 0- C ) ~ 0 L- .....'"S- V> \ ';;; ~--~ I-.... «X "'Test N9 137 el=O,61 Dr=100~ 10 r\ oop=O,95 Kgf/em2 :\; liquefaet. 8cycl 0:: 0 Test N9 114 Test N9 119 el =087 Dr =38% e =071 D =78% V> " 0:: 1 QJ oop=O 39 Kg f/cm2 0 =0 70 K~ f/cm2 ..... ycl X 1iquefact. 9t . 1i~uefrt. lleycl . QJ 20 3 10 30 100 300 1000 Number of Cycles FIG. 9.-AXIAL STRAIN AFTER Th1TIAL lJQUEF ACTION FOR PUL­ SA TING LOAD TESTS AT 3 DE~SIT1ES FROM SEED AND LEE (1966) FACTORS AFFECTING LIQUEFACTION CHARACTERISTICS OF IN·SITU DEPOSITS 1. MAGNITUDE OF CYCLIC STRESS (I.E., INTENSITY OF SHAKING) 2. NUMBER OF STRESS CYCLES (I.E., DURATION OF SHAKING) 3. INITIAL RELATIVE DENSITY (OR INITIAL VOID RATIO) 4. METHOD OF SOIL PLACEMENT (SOIL STRUCTURE) 5. PERIOD UNDER SUSTAINED LOAD 6. PREVIOUS STRAIN HISTORY 7. INITIAL EFFECTIVE CONFINING PRESSURE 8. LATERAL EARTH PRESSURE COEFFICIENT AND OVER CONSOLIDATION 9. FAILURE CRITERION (INITIAL LIQUEFACTION OR MAXIMUM PERMISSIBLE STRAIN) 10. GRAIN SIZE, GRAIN SHAPE AND GRADATION OF SOIL Sheet n9 2 U> 0.4 \ : I \ 1 \ \ i -'" I; ,I ! I Monterey No 0 Sand I I i I I :. I I ~II j I -t::? 03 - .- --- 1--- r-i • _.4- t--- ...... ! I : tJ o o ex OIl o 2 ~-.- -- ::-.....+- OIl .. (I)- : '0 , ;, Ii; , 1 I .. I I ' I I I . I 1 I -.., I .. 01 .. ___ + +1 , -~-+-I1:J:-JrH+L---i--.~-t-1-t. ~ L o.. ~-l---t ,- I I : I I' , U I ' iii , I I I ,. I I I ' o 3 5 10 30 50 100 300 WO iOJO Number of Cycles, Nc tJ> l:J' rD 1"'_ 0. AIM. Own M1d 1SMd. 1117., rtrD f' ::l S -0 Figure 2·3. Cf)R RECTED 'CICTo VS Nc FOR INITIAL LIQUEFACTION ;:tI w Ui en o.~ i < z C Or ~oo/o H.Qh fr'Qu'ncy vlbrotlona = t"1 "r1 on mOl,t lompl .. CT~:8p" Lo", freQuency i!) vlbrO',ona .. y 0 Sand '"'I .Monl' No o Z 0.41 ~ o "r1 oen ~ o • 0 c t? := .., 0.3 t --~. Z Cl o PluvloI,d -ojr I t"1 o > G: a~ .