Mechanism of deformation during cyclic undrained loading of saturated sands

Abstract

Laboratory cyclic loading tests have provided most of our understanding of the response of saturated sands under earthquake loading. In many cases, however, the interest has centered on relating the resistance to liquefaction to the number of cycles of loading. Few attempts have been made to examine in detail the response within cycles of loading. Such an examination is essential for a fundamental understanding of the processes leading to porewater pressure and strain development or liquefaction. To satisfy the need for practical applications, a close simulation of the stress condition on soil elements in the field, both prior to and during an earthquake, has been emphasized in all cyclic loading studies. Thus, cyclic triaxial tests on isotropically consolidated samples or cyclic simple shear tests on one-dimensionally consolidated samples have been used to simulate stress conditions below level ground. On a soil element below level ground, there are no initial static shear stresses on horizontal planes prior to earthquake loading. On the other hand, soil elements beneath sloping ground or under a loaded structure are subjected to initial static shear stresses on horizontal planes prior to earthquake loading. The stress conditions in these soil elements have been simulated by cyclic loading triaxial tests on anisotropically consolidated samples or cyclic simple shear tests with initial static shear stress prior to cyclic loading. The present study is aimed at obtaining a better understanding of the mechanism of deformation and porewater pressure generation during cyclic undrained loading of saturated sand. The study was performed using the cyclic triaxial test. A range of relative densities and both isotropically and anisotropically consolidated samples were tested in order to simulate the practical stress condition under level and as well as sloping ground. A natural by-product of the study is the basic data on the influence of initial static shear on resistance to liquefaction over a range of relative densities.