Cyclic Pore Pressure Generation, Dissipation and Densification in Granular Mixes

Introduction

Soil liquefaction and its post-liquefaction response have been of great interest in geotechnical engineering for more than three decades. Liquefaction induced failures include landslides, sand boils, cracks, excessive ground settlements, lateral spreading, and foundation failures. Pore water pressure builds up in loose saturated soil deposits due to cyclic shearing. At the same time, dissipation and redistribution of this shear induced pressure take place at a rate depending on the hydraulic conductivity and volume compressibility characteristics of the soil deposit and drainage conditions. When the rate of pore pressure generation and build up is significant, a non-plastic soil temporarily loses a large portion of its strength, which may lead to liquefaction and breakdown of the soil structure. Pore pressure dissipation will usually be accompanied by rearrangement of particles, reconsolidation and a reduction in volume of voids, hence settlement of ground surface. Current knowledge on pore pressure generation and post-liquefaction dissipation and volume change characteristics of granular soils rely primarily on data from clean sands (Ishihara & Yoshimine, 1992; Lee & Albaisa, 1974; Pyke et al., 1975; Silver & Seed, 1971a,b; Seed et al., 1976; Tatsuoka et al., 1984; Tokimatsu & Seed, 1984, 1987; Yoshimi et al., 1975). However, recent earthquake case histories indicate that sites containing a significant percentage of fine grains, mostly non-plastic, also liquefy due to seismic loading (Seed et al. 1983, Youd et al. 2001). Only a limited amount of research information is available for silty soils. Therefore, not surprisingly, evaluation of liquefaction characteristics of silty soils has recently attracted attention of researchers. Recently there has been advances in the understanding of the effects of silt content on monotonic and cyclic strength and liquefaction resistance of silty soils and to a lesser extent on post-liquefaction response of silty soils (Andrews & Martin 2000, Chang 1990, Georgiannou et al. 1990, 1991, Ishihara 1993, Koester 1994, Pitman et al. 1994, Shenthan 2001, Singh 1994, Thevanayagam et al. 2001, Thevanayagam and Martin 2002, Thevanayagam et al. 2002, Thevanayagam et al. 2007a-b, Vaid 1994, Yamamuro & Lade 1998). Data on post-liquefaction characteristics of such soils are scarce.

This article presents results from an experimental study of pre- and post-liquefaction characteristics of non-plastic sand-silt mixes at silt contents from 0% to 100% by weight, and three natural non-plastic silts. Undrained cyclic triaxial tests followed by dissipation of cyclic-induced pore pressures were carried out in order to determine pore pressure generation, pre- and post-liquefaction compressibility, pre- and post-liquefaction coefficient of consolidation, and post-liquefaction densification characteristics of these soils. Findings from this study are summarized. The influence of silt content on these characteristics is examined in the context of intergranular void ratio and intergranular contact density concepts (Vaid 1994, Thevanayagam et al. 2002, Thevanayagam 2007a).