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 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. 1.0 INTRODUCTION Soil liquefaction is a major cause of damage during earthquakes. “Modern” engineering treatment of liquefactionrelated issues evolved initially in the wake of the two devastating earthquakes of 1964; the 1964 Niigata (Japan) and 1964 Great Alaskan Earthquakes. Seismically-induced soil liquefaction produced spectacular and devastating effects in both of these events, thrusting the issue forcefully to the attention of engineers and researchers. Over the nearly four decades that have followed, significant progress has occurred. Initially, this progress was largely confined to improved ability to assess the likelihood of initiation (or “triggering”) of liquefaction in clean, sandy soils. As the years passed, and earthquakes continued to provide lessons and data, researchers and practitioners became increasingly aware of the additional potential problems associated with both silty and gravelly soils, and the important additional issues of post-liquefaction strength and stressdeformation behavior also began to attract increased attention. Today, the area of “soil liquefaction engineering” is emerging as a semi-mature field of practice in its own right. This area now involves a number of discernable sub-issues or subtopics, as illustrated schematically in Figure 1. As shown in Figure 1, the first step in most engineering treatments of soil liquefaction continues to be (1) assessment of “liquefaction potential”, or the risk of “triggering” (initiation) of liquefaction. There have been major advances here in recent years, and some of these will be discussed. Once it is determined that occurrence of liquefaction is a potentially serious risk/hazard, the process next proceeds to assessment of the consequences of the potential liquefaction. This, now, increasingly involves (2) assessment of available post-liquefaction strength, and resulting post-liquefaction overall stability (of a site, and/or of a structure or other built facilities, etc.). There has been considerable progress in