Critical-state behaviour of an agricultural soil

An unsaturated soil mechanics approach is used to define some critical-state relations for an agricultural soil. The unsaturated critical-state equations are presented in terms of four state variables, namely, mean net stress, deviatoric stress, matric suction and specific volume. A series of experiments was carried out using a triaxial system modified for unsaturated soils testing. Experimental data were used to determine simple critical-state relations for an agricultural-soil. All specimens attained a critical-state under continuous shearing. Unique critical-state lines were established for fixed soil suctions of zero (saturated series) and 50 kPa (unsaturated series). Shearing under confinement stresses greater than the magnitude of the matric suction of the soil, however, caused the pore-water pressure to increase (decreasing suction) even under constant water content conditions, with a corresponding loss in strength at critical-state. Uniqueness of the critical-state line could therefore not be established for a fixed water content in these experiments. The study showed that the range of applied stresses and the matric suction are factors that must be considered in the formulation of constitutive relations for agricultural soils. (C) 1998 Silsoe Research Institute.