Small-strain deformation characteristics of frozen clay from static testing

Abstract

Small-strain stiffness characteristics of frozen clay as measured in laboratory static tests are reported in this paper. Accurate strain measurement, which has rarely been attempted for frozen soils, was achieved with local gap sensors that function stably in a triaxial cell filled with refrigerant at freezing temperatures, assisted by an extremely high-resolution servomotor for axial loading. High-plasticity clay samples were frozen directly under confining pressure after being consolidated to different isotropic effective stresses, and loaded at three different rates and three different temperatures. The importance of avoiding ram–bush friction and a way to achieve it are discussed. The observed stress–strain relationships for strains of less than 0·01% indicated features surprisingly similar to those seen for unfrozen states, such as independence of the initial elastic modulus from the loading rate, onset of non-linearity (or yielding) at smaller strains for slower loading, while the linearity threshold strain was greatly increased at lower temperatures. These similarities are made clear through parallel tests on the same clay in unfrozen states. An interesting feature was seen for frozen samples; those consolidated to higher effective stress exhibited lower stiffness when being frozen. A simple model taking into account the volumetric ice content explains this feature.