VARIATION OF THE CRITICAL-STATE BOUNDARIES OF AN AGRICULTURAL SOIL

The critical-state theory can be applied profitably to analyse the mec hanical behaviour of agricultural soil. Critical-state parameters and other soil properties are affected by the microstructure and unsaturat ed nature of agricultural soils. We determined the critical-state boun daries of an agricultural soil in both saturated and unsaturated triax ial tests and examined the effects of matric suction and initial struc ture on critical-state boundaries. On the compression plane, the prese nce of air and matric suction in the pores of unsaturated soil signifi cantly affected critical-state boundaries by increasing compressibilit y, gamma. On the deviatoric stress-mean net stress plane, the strength increased with matric suction. On this plane, the critical-state line s for the unsaturated tests had non-zero intercepts. For a given soil structure, the frictional parameter M remained fairly constant with ma tric suction change. However, a change in the initial microstructure r esulted in a change in M, causing the position of the critical-state l ine to 'pivot' in state space.