Abstract
A type of pre-failure instability that occurs for medium loose to dense sand under fully drained conditions is studied in this paper. It is observed experimentally that when a specimen is sheared along a drained stress path involving a decrease in the effective mean stress, it becomes unstable after the stress path crosses an instability line (IL). The IL specifies a minimum stress ratio under which instability may occur. It is not unique but changes with the void ratio of the soil and the applied effective stresses. The instability occurring under drained conditions is different from that under undrained conditions. The differences and similarities between the two types of instability are pointed out. Practical implications of the study in analysing the failure mechanisms of granular soil slopes under various drainage conditions are discussed.
Highlights
Failure of geotechnical structures can be initiated by instability of soil
Instability line and instability under undrained conditions Firstly, the results of some isotropically consolidated drained (CD) and undrained (CU) tests on loose sand are presented in Figure 3(a) to define the failure line (FL) and the instability line (IL)
The FL determined for loose sand is the same as the critical state line (CSL) or the steady state line (SSL) (Chu, 1995; Leong et al, 2000; Wanatowski and Chu, 2007)
Summary
Failure of geotechnical structures can be initiated by instability of soil. Instability is normally considered to have taken place when the stress state of a soil element satisfies a failure criterion, as in the conventional stability analysis. A typical example is static liquefaction, which occurs before the effective stress path reaches the failure line (FL) or the steady state line (SSL). This so-called pre-failure instability has been studied mainly for saturated loose sand under undrained conditions Using a fully instrumented model slope of a loose granular material, Eckersley (1990) observed that the failure in his model tests was initiated under essentially static, drained conditions, and the pore
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