Recent Advances in Soil Liquefaction Engineering: A Unified and Consistent Framework

Over the past decade, major advances have occurred in both understanding and practice with regard to assessment and mitigation of hazard associated with seismically induced soil liquefaction. Soil liquefaction engineering has evolved into a sub-field in its own right, and engineering assessment and mitigation of seismic soil liquefaction hazard is increasingly well addressed in both research and practice. This rapid evolution in the treatment of liquefaction has been pushed largely by a confluence of lessons and data provided by a series of major earthquakes over the past dozen years, as well as by the research and professional/political will engendered by these major seismic events. The overall field of soil liquefaction engineering is now beginning to coalesce into an internally consistent and comprehensive framework, and one in which the various elements are increasingly mutually supportive of each other. 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 soil liquefaction engineering. This paper will highlight a number of important recent and ongoing developments in soil liquefaction engineering, and will offer insights regarding research in progress, as well as suggestions regarding further advances needed. 1.0 INTRODUCTION additional issues of post-liquefaction strength and stressdeformation behavior also began to attract increased attention. Soil liquefaction is a major cause of damage during earthquakes. “Modern” engineering treatment of liquefactionToday, the area of “soil liquefaction engineering” is emerging related issues evolved initially in the wake of the two as a semi-mature field of practice in its own right. This area devastating earthquakes of 1964; the 1964 Niigata (Japan) and now involves a number of discernable sub-issues or sub­ 1964 Great Alaskan Earthquakes. Seismically-induced soil topics, as illustrated schematically in Figure 1. As shown in liquefaction produced spectacular and devastating effects in Figure 1, the first step in most engineering treatments of soil both of these events, thrusting the issue forcefully to the liquefaction continues to be (1) assessment of “liquefaction attention of engineers and researchers. potential”, or the risk of “triggering” (initiation) of liquefaction. There have been major advances here in recent Over the nearly four decades that have followed, significant years, and some of these will be discussed. progress has occurred. Initially, this progress was largely confined to improved ability to assess the likelihood of Once it is determined that occurrence of liquefaction is a initiation (or “triggering”) of liquefaction in clean, sandy soils. potentially serious risk/hazard, the process next proceeds to As the years passed, and earthquakes continued to provide assessment of the consequences of the potential liquefaction. lessons and data, researchers and practitioners became This, now, increasingly involves (2) assessment of available increasingly aware of the additional potential problems post-liquefaction strength, and resulting post-liquefaction associated with both silty and gravelly soils, and the important overall stability (of a site, and/or of a structure or other built facilites, etc.). There has been considerable progress in 1 Dept. of Civil and Environmental Engineering, University of California, Berkeley. 2 Dept. of Civil Engineering, Middle East Technical University, Ankara, Turkey. 3 Fugro Engineering, Santa Barbara, California. 4 Arup, San Francisco, California. 5 URS Corporation, Oakland, California. 6 U.S. Geological Survey, Menlo Park, California. 1. 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. Fig. 1: Key Elements of Soil Liquefaction Engineering evaluation of post-liquefaction strengths and stability over the past fifteen years. If post-liquefaction stability is found wanting, then deformation/displacement potential is large, and engineered remediation is typically warranted. If post-liquefaction overall stability is not unacceptable, then attention is next directed towards (3) assessment of anticipated deformations and displacements. This is a very “soft” area of practice, and much remains to be done here with regard to development and calibration/verification of engineering tools and methods. Similarly, there are few engineering tools and guidelines regarding (4) the effects of liquefaction-induced deformations and displacements on the performance of structures and other engineered facilities, and criteria for “acceptable” performance are not well established. Finally, in cases in which the engineer(s) conclude that satisfactory performance cannot be counted on, (5) engineered mitigation of liquefaction risk is generally warranted. This, too, is a rapidly evolving area, and one rife with potential controversy. Ongoing evolution of new methods for mitigation of liquefaction hazard provides an ever increasing suite of engineering options, but the efficacy and reliability of some of these remain contentious, and accurate and reliable engineering analysis of the improved performance provided by many of these mitigation techniques continues to be difficult. It is not possible, within the confines of this paper, to fully address all of these issues (a textbook would be required!) Instead, a number of important recent and ongoing advances will be highlighted, and resultant issues and areas of controversy, as well as areas in urgent need of further advances either in practice or understanding, will be noted. 2.0 ASSESSMENT OF SUSCEPTIBILITY 2.1 Liquefiable Soil Types: The first step in engineering assessment of the potential for “triggering” or initiation of soil liquefaction is the determination of whether or not soils of “potentially liquefiable nature” are present at a site. This, in turn, raises the important question regarding which types of soils are potentially 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 significant controversy and confusion regarding the liquefaction potential of silty soils (and silty/clayey soils), and also of coarser, gravelly soils and rockfills. Coarser, gravelly soils are the easier of the two to discuss, so we will begin there. The cyclic behavior of coarse, gravelly soils differs little from that of “sandy” soils, as Nature has little or no respect for the arbitrary criteria established by the standard #4 sieve. Coarse, gravelly soils are potentially vulnerable to cyclic pore pressure generation and liquefaction. There are now a number of well-documented field cases of liquefaction of coarse, gravelly soils (e.g.: Evans, 1987; Harder, 1988; Hynes, 1988; Andrus, 1994). These soils do, however, often differ in behavior from their finer, sandy brethren in two ways: (1) they can be much more pervious, and so can often rapidly dissipate cyclically generated pore pressures, and (2) due to the mass of their larger particles, the coarse gravelly soils are seldom deposited “gently” and so do not often occur in the very loose states more often encountered with finer sandy soils. Sandy soils can range from very loose to very dense, while the “very” loose state is relatively uncommon in gravelly deposits and coarser soils. The apparent drainage advantages of coarse, gravelly soils can be defeated if their drainage potential is circumvented by either; (1) their being surrounded and encapsulated by finer, less pervious materials, (2) if drainage is internally impeded by the presence of finer soils in the void spaces between the coarser particles (it should be noted that the D10 particle size, not the mean or D50 size, most closely correlates with the permeability of a broadly graded soil mix), or (3) if the layer or stratum of coarse soil is of large dimension, so that the distance over which drainage must occur (rapidly) during an earthquake is large. In these cases, the coarse soils should be considered to be of potentially liquefiable type, and should be evaluated accordingly. Questions regarding the potential liquefiability of finer, “cohesive” soils (especially “silts” and “silty clays”) are increasingly common at meetings and professional short courses and seminars. There is considerable new field data regarding this issue from recent major earthquakes, and this is an area in which major changes in both understanding and practice are occurring. Seed et al. (2003) 2 I Q I I IT , , 1. Percent Finer than 0.005mm 15% 2. Liquid Limit (LL) 35% 3. Water Content (W ) 0.9 x LL