Geoscience Reference
In-Depth Information
CHAPTER 1
SPT- AND CPT-BASED RELATIONSHIPS FOR THE RESIDUAL SHEAR
STRENGTH OF LIQUEFIED SOILS
I.M.Idriss
1
and Ross W. Boulanger
2
1
Department of Civil and Environmental Engineering, University of California
at Davis, California, USA
2
Department of Civil and Environmental Engineering, University of California
at Davis, California, USA
Abstract.
Anevaluationofpost-earthquakestabilityofearthembankmentsorslopesthatcontain,
or are founded on, soils that may liquefy requires estimating the liquefied soil's residual shear
strength, S
r
. Decisions regarding the need for expensive mitigation efforts, including ground
improvement work, often hinge on the selected S
r
values. This paper presents recommended SPT-
and CPT-based relationships for estimating the residual shear strength ratio, S
r
/
σ
vo
, of liquefied
nonplastic soils in the field based on a review of prior case history studies, laboratory testing stud-
ies,andrecentfindingsregardingvoidredistributionmechanisms.Therecommendedrelationships
provide guidance regarding the unavoidable task in practice of having to extrapolate beyond the
available case historydata. Limitations in the state of knowledge are discussed.
1. Introduction
Proceduresforestimatingtheresidualshearstrength,S
r
,ofliquefiedcohesionlessornon-
plasticsoilshaveevolvedconsiderablyoverthepast25years.Proceduresthatrequirelab-
oratorytestingoffieldsampleshavebeendevelopedthatusesamplesobtainedbyfrozen
sampling techniques (e.g., Robertson et al., 2000) or samples obtained by high-quality
tubesamplingtechniquescoupledwithproceduresfor“correcting”theshearstrengthfor
theestimatedvolumechangesthatoccurduringsamplingandtesting(e.g.,Castro,1975;
Castro and Poulos, 1977; Poulos et al., 1985). Procedures based on using case histories
for estimating the in-situ S
r
of liquefied soils have been developed by back-analyses of
liquefactionflowslides,asfirstpresentedbySeed(1987)andsincemodifiedbyanumber
of investigators (e.g., Davis et al., 1988; Seed and Harder, 1990; Ishihara, 1993; Wride
et al., 1999; Olsonand Stark, 2002).
Whitman (1985) described situations where pore water seepage driven by earthquake-
inducedexcessporewaterpressuregradientscouldleadtothelocalizedlooseningofthe
liquefiedsoil,or“voidredistribution”,thatcouldresultinS
r
beingmuchlowerinthefield
thanwouldbeobtainedfromlaboratorytestsofsamplesatthepre-earthquakevoidratio.
These situations require the presence of a soil layer of significantly lower permeability
overlying the liquefied soil layer, thereby impeding the outward seepage, as illustrated
for an infinite slope in Figure 1.1. Physical and analytical modeling studies by Kokusho
(2000),Kulasingametal.(2004),andMalvicketal.(2004)haveillustratedandevaluated
