EFFECT OF VOID REDISTRIBUTION OR WATER FILMS ON SHEAR STRENGTH OF LIQUEFIED DEPOSITS

摘要

In liquefied ground, lateral flow is sometimes immensely larger than surface settlement and exceeds several meters even in a gentle slope of less than a few percent. It occurs not only during but also after earthquake shaking. Conventional laboratory soil tests using uniform sand cannot reproduce this phenomenon. Its mechanism is still poorly understood. In this paper, recent findings obtained by various tests on void redistribution and associated lateral flow movement which potentially occurs in layered sand deposits are discussed. 1G shake table tests, 1-D tube tests, torsional simple shear tests, in situ soil investigations, etc. are carried out to understand the undrained shear mechanism during liquefaction. A major finding is that sand deposits in the field consist of sublayers with different particle size and permeability and readily develop water films by post-liquefaction void redistribution at sublayer boundaries which serve as sliding surfaces. The water films also serve as shear stress isolators, limiting the shear stress transmitted into underlying sand and realizing stable post-shaking slides. Basic shear tests indicated that this type of flow failure may occur in loose sand with relative density around 40% or less.