Factors Affecting Steady State Locus in Triaxial Tests

Abstract

Models based on critical state soil mechanics ("CSSM") require careful calibration of laboratory parameters in order to ensure accurate prediction of the behavior of soil under in situ conditions. This paper presented research on the testing of two distinct materials, a silt and a silty sand. Several triaxial tests were conducted under drained and undrained conditions, with low to very high confining pressures. Distinct mechanical responses were observed: some samples liquefied, some softened, and others hardened when loaded under undrained conditions. The degree of saturation, the induced anisotropy and the induced stress-path were found to influence the state limits between these distinct behaviors. This affected the steady state line ("SSL") position defined by specimens showing strain softening in the experiment. Furthermore, the "critical state line" was only achieved when the specimens experienced strain hardening despite the stress-path during shear and the initial state conditions. Therefore, as these factors can impact the steady state line (SSL) position, it is essential when assessing the geomechanical behavior of the soil that these factors are monitored and controlled under laboratory conditions according to the in situ state condition and stress-path during shear. In the future, constitutive models have therefore to be adapted in order to determine the influence of these conditioning factors in the ultimate CSSM reference lines positions.