Critical-state parameters for an unsaturated residual sandy clay

This paper presents experimental data from constant water content triaxial tests on a residual soil from Singapore, tested under unsaturated conditions with measurements of matric suctions. The data for critical-state conditions from these tests are presented within a framework previously proposed by the first author. The unsaturated critical state requires five parameters: M_a, M_b, Γ_ab, λ_a and λ_b. At high degrees of saturation (in a non-aggregated condition) M_a = M_b = M_s (the saturated critical-state stress ratio). At lower degrees of saturation, the value of M_b drops considerably and reaches values near to zero at degrees of saturation below 60%. Conversely, the value of M_a appears to increase as the degree of saturation reduces. A similar pattern is observed for λ_a and λ_b. The functions relating the critical-state parameters to degree of saturation (or volumetric water content) can be expressed in normalised form by referencing them to the saturated state. The same normalised functions for M_a/M_s, M_b/M_s, λ_a/λ_s and λ_b/λ_s fit the experimental data for the residual sandy clay from Singapore and a lateritic gravel from Kenya previously tested. This suggests that the form of these functions may be common to a range of soil types.