Perspectives on Earthquake Geotechnical Engineering

Abstract

Soil liquefaction has been responsible for extensive damage to buildings, bridges, pipelines and other critical infrastructure in many past earthquakes. The need to predict the potential for initiation of liquefaction and of its various effects has led to greatly improved understanding of the mechanics of the liquefaction process. Most of the efforts made toward evaluating liquefaction hazards have been directed toward various aspects of ground failure, including post-liquefaction settlement, lateral spreading, and the development of flow slides. The occurrence of liquefaction, however, can also affect ground surface motions, and hence the seismic response of structures founded at or near the ground surface. This paper reviews the process of liquefaction and the manner in which its occurrence is typically detected. The hydraulic conditions required for the manifestation of surface effects are reviewed, and limitations in the inference of liquefaction triggering from the presence or absence of such effects are discussed. The effects of liquefaction triggering on soil stiffness, as they pertain to ground response, are described and discussed. Procedures for identification of the timing of liquefaction triggering are reviewed, and illustrated using a procedure based on the Stockwell transform. The effects of post-triggering response on ground surface motions are interpreted in terms of response spectra. The abilities of four advanced site response programs to compute ground motions in liquefiable soils are examined and evaluated from the standpoints of pre- and post-triggering response. The programs were applied to the Wildlife case history, and to a hypothetical soil profile containing 7 m of liquefiable soil. The programs all predicted consistent response up to the point of triggering,but their predictions of post-triggering response were highly variable. The implications of post-triggering response for the evaluation of ground surface response spectra are discussed.