Use of Cyclic Simple Shear Testing in Evaluation of the Deformation Potential of Liquefiable Soils (original) (raw)
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SHEAR STRAIN DEVELOPMENT IN LIQUEFIABLE SOIL UNDER BI-DIRECTIONAL LOADING CONDITIONS
A comprehensive testing database composed of modeling-quality multi-directional cyclic simple shear testing on medium to high relative density, fully-saturated samples of Monterey 0/30 sand has recently been developed. This testing program incorporated a variety of multi-directional stress paths, including a large number of stress paths never before examined. Results from these tests have proven useful for enhancing current understanding of liquefaction behavior by allowing for a more complete theory to emerge. This new 3-dimensional theory greatly expands current understanding of liquefaction behavior and elucidates some areas in which current theory—which has been based principally on uni-directional laboratory testing—can be misleading or unconservative. Of particular interest are the topics of pore pressure generation and softening, the relationship between pore pressure and strain capacity, and the dilational lock-up in medium density sands that acts to limit large free-flow type deformations. Insight has also been gained on the complex effects caused by an initial static shear stress such as would be imposed by sloping ground conditions or the presence of a structure.
Static Liquefaction Assessment Combining Shear Wave Velocity, Peak Strength and Soil Grading
A large set of undrained compression triaxial tests was carried out on different types of cohesionless soils, from sands to silty-sands and silts. Shear wave velocity measurements were carried out alongside. These tests exhibited distinct state transitions ranging from flow liquefaction to strain softening or strain hardening. With the purpose of defining a framework to assess soil liquefaction, it was found that the ratio between the shear wave velocity (VS0) and the peak undrained deviatoric stress, qpeak, VS0/qpeak, could be accurately used to define a boundary between liquefaction and strain hardening for sands, and between strain softening and strain hardening for silty-sands and silts. Since this ratio is a function of the tested material, it was also discovered that the prediction of these boundaries could be made as a function of soil grading, namely via the coefficient of uniformity, CU. Despite not being regarded as a strong geomechanical parameter, CU is easily determined...
The effects of liquefaction on sloped ground generally include failure due to abrupt flow sliding or a more progressive accumulation of residual deformation. Although such phenomena have been often observed following major earthquakes, their mechanisms are not fully understood yet. In this paper, based on results of a series of large-strain undrained cyclic torsional simple shear tests with initial static shear (i.e. sloped ground conditions) conducted on loose Toyoura sand specimens (relative density 44-48%), an attempt is made to address this issue. Three distinct failure mechanisms that sandy sloped ground may experience during earthquakes have been identified from the tests: cyclic liquefaction, rapid flow liquefaction and shear failure. Of these, rapid flow liquefaction is the most critical since it produces sudden development of large shear deformation (i.e. flow sliding). Alternatively, when the offset of liquefaction is not achieved, a more progressive accumulation of large ...