The post-peak response of soils governs the behavior of soil masses subjected to moderate to high levels of strain; this occurs during earthquake-induced flow failures, such as slope instabilities or bearing capacity failure. Examples of earthquake-induced flow failures are the failure of the Lower San Fernando dam in 1971, failure of tailings dams in Chile in 1965 and 1985, and the bearing capacity failures in Niigata, Japan, in 1964. The objective of this research program is to provide experimental evidence of the nature of the post-peak response of soils and to cast these results into a manageable theoretical framework. The concept of an undrained steady- state strength as a limiting shearing resistance for a soil undergoing steady-state deformations provides a framework for evaluating liquefaction and flow slide potential. It is imperative that uniform strains develop in laboratory tests used to define this relationship; thus the uniformity of deformation is being examined experimentally by shearing specimens of saturated sands in both servo-controlled, heavily-instrumented plane strain compression and a standard triaxial device.
