Paper No. SPL-2 1RECENT ADVANCES IN SOIL LIQUEFACTION ENGINEERING AND SEISMIC SITE RESPONSE EVALUATIONSeed, R. B. Cetin, K. O. Moss, R. E. S.University of California Middle East Technical University Kammerer, A. M.Berkeley, California 94720 Ankara, Turkey Wu, J.Pestana, J. M.Riemer, M. F.University of California,Berkeley, California 94720ABSTRACTOver the past decade, major advances have occurred in both understanding and practice with regard to engineering treatment ofseismic soil liquefaction and assessment of seismic site response. Seismic soil liquefaction engineering has evolved into a sub-field inits own right, and assessment and treatment of site effects affecting seismic site response has gone from a topic of controversy to amainstream issue addressed in most modern building codes and addressed in both research and practice. This rapid evolution in thetreatment of both liquefaction and site response issues has been pushed by a confluence of lessons and data provided by a series ofearthquakes over the past eleven years, as well as by the research and professional/political will engendered by these major seismicevents. Although the rate of progress has been laudable, further advances are occurring, and more remains to be done. As we enter a“new millenium”, engineers are increasingly well able to deal with important aspects of these two seismic problem areas. This paperwill highlight a few major recent and ongoing developments in each of these two important areas of seismic practice, and will offerinsights regarding work/research in progress, as well as suggestions regarding further advances needed. The first part of the paper willaddress soil liquefaction, and the second portion will (briefly) address engineering assessment of seismic site response.INTRODUCTIONSoil liquefaction is a major cause of damage duringearthquakes. “Modern” engineering treatment of liquefaction-related issues evolved initially in the wake of the twodevastating earthquakes of 1964, the 1964 Niigata and 1964Great Alaska Earthquakes, in which seismically-inducedliquefaction produced spectacular and devastating effects.Over the nearly four decades that have followed, significantprogress has occurred. Initially, this progress was largelyconfined to improved ability to assess the likelihood ofinitiation (or “triggering”) of liquefaction in clean, sandy soils.As the years passed, and earthquakes continued to providelessons and data, researchers and practitioners becameincreasingly aware of the additional potential problemsassociated with both silty and gravelly soils, and the issues ofpost-liquefaction strength and stress-deformation behavioralso began to attract increased attention.Today, the area of “soil liquefaction engineering” is emergingas a semi-mature field of practice in its own right. This areanow involves a number of discernable sub-issues or sub-topics, as illustrated schematically in Figure 1. As shown inFigure 1, the first step in most engineering treatments of soilliquefaction continues to be (1) assessment of “liquefactionpotential”, or the risk of “triggering” (initiation) ofliquefaction. There have been major advances here in recentyears, and some of these will be discussed.Once it is determined that occurrence of liquefaction is apotentially serious risk/hazard, the process next proceeds toassessment of the consequences of the potential liquefaction.This, now, increasingly involves (2) assessment of availablepost-liquefaction strength and resulting post-liquefactionoverall stability (of a site, and/or of a structure or other builtfacility, etc.). There has been considerable progress inevaluation of post-liquefaction strengths over the past fifteenyears. If post-liquefaction stability is found wanting, thendeformation/displacement potential is large, and engineeredremediation is typically warranted.If post-liquefaction overall stability is not unacceptable, thenattention is next directed towards (3) assessment of anticipateddeformations and displacements. This is a very “soft” area ofpractice, and much remains to be done here with regard todevelopment and calibration/verification of engineering toolsand methods. Similarly, relatively little is known regardingPaper No. SPL-2 2Fig. 1: Key Elements of Soil Liquefaction Engineering1. Assessment of the likelihood of “triggering” or initiation of soil liquefaction.2. Assessment of post-liquefaction strength and overall post-liquefaction stability.3. Assessment of expected liquefaction-induced deformations and displacements.4. Assessment of the consequences of these deformations and displacements.5. Implementation (and evaluation) of engineered mitigation, if necessary.(4) the effects of liquefaction-induced deformations anddisplacements on the performance of structures and otherengineered facilities, and criteria for “acceptable” performanceare not well established.Finally, in cases in which the engineer(s) conclude thatsatisfactory performance cannot be counted on, (5) engineeredmitigation of liquefaction risk is generally warranted. This,too, is a rapidly evolving area, and one rife with potentialcontroversy. Ongoing evolution of new methods formitigation of liquefaction hazard provides an ever increasingsuite of engineering options, but the efficacy and reliability ofsome of these remain contentious, and accurate and reliableengineering analysis of the improved performance provided bymany of these mitigation techniques continues to be difficult.It is not possible, within the confines of this paper, to fullyaddress all of these issues (a textbook would be required!)Instead, a number of important recent/ongoing advances willbe highlighted, and resultant issues and areas of controversy,as well as areas in urgent need of further advances either inpractice or understanding, will be noted.ASSESSMENT OF LIQUEFACTION POTENTIALLiquefiable soils:The first step in engineering assessment of the potential for“triggering” or initiation of soil liquefaction is thedetermination of whether or not soils of “potentiallyliquefiable nature” are present at a site. This, in turn, raisesthe important question regarding which types of soils arepotentially vulnerable to soil liquefaction.It has long been recognized that relatively “clean” sandy soils,with few fines, are potentially vulnerable to seismically-induced liquefaction. There has, however, been