CRITICAL-STATE BEHAVIOR OF AN AGRICULTURAL SOIL

An unsaturated soil mechanics approach is used to define some critical -state relations for an agricultural soil. The unsaturated critical-st ate 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 modi fied for unsaturated soils testing. Experimental data were used to det ermine simple critical-state relations for an agricultural-soil. All s pecimens attained a critical-state under continuous shearing. Unique c ritical-state lines were established for fixed soil suctions of zero ( saturated series) and 50 kPa (unsaturated series). Shearing under conf inement stresses greater than the magnitude of the matric suction of t he soil, however, caused the pore-water pressure to increase (decreasi ng suction) even under constant water content conditions, with a corre sponding loss in strength at critical-state. Uniqueness of the critica l-state line could therefore not be established for a fixed water cont ent in these experiments. The study showed that the range of applied s tresses and the matric suction are factors that must be considered in the formulation of constitutive relations for agricultural soils. (C) 1998 Silsoe Research Institute.