AN EFFECTIVE STRESS UNDERSTANDING OF LIQUEFACTION BEHAVIOR

The state of knowledge with regard to the static and cyclic liquefacti on behavior of sands (sands according to the Unified Soil Classificati on System) has progressed tremendously in the last twenty years (Natio nal Research Council, 1985), In fact, based upon the so-called steady state concepts of Casa-grande (1975), Castro (1975) and Poulos (1981), it is generally accepted that the end or steady state condition of a liquefied loose sand is the same whether due to static, dynamic, or cy clic undrained loading. Even so, there is still much work to be done, particularly in regard to the post peak (or strain softening) response that follows the initiation of liquefaction, It is necessary that the post-earthquake stability of earth embankments, in which there is suc h liquefiable material, be assessed based on the residual strength of that type of material, At present, the two approaches to the evaluatio n of residual strength are less than adequate for the task, One approa ch is based on an empirical relationship as derived from a limited num ber of field case studies involving the failure of embankment slopes v ia liquefaction. Alternatively, a laboratory approach for evaluating r esidual strength involves running static consolidated-undrained triaxi al tests on both reconstituted and undisturbed samples. Such testing r equires a degree of sophistication and experience that is beyond the c apability of the great majority of geotechnical firms. It is shown her e, however, that the laboratory determined undrained steady state (or residual) strength and the associated critical confining pressure can be assessed in a straightforward manner from drained triaxial tests wi th volume change measurements based upon a proposed effective stress i nterpretation of such undrained behavior. Accordingly, it should be po ssible for all geotechnical firms with the standard capability to perf orm drained triaxial tests on sands (with associated volume change mea surements) to accurately assess the residual strength of such liquefyi ng material. The same procedure also allows the prediction of the whol e undrained stress-strain curve and the corresponding effective stress path.