Stress-strain behaviour of unsaturated soil in true triaxial tests

A suction-controlled true triaxial apparatus for unsaturated soil was developed from the existing true triaxial apparatus for sand by attaching a device to supply matric suction to specimens. Using the developed apparatus, true triaxial tests (sigma(1) greater than or equal to sigma(2) greater than or equal to sigma(3); where sigma(1), sigma(2), and sigma(3) are the three different principal stresses) on an unsaturated silty soil were carried out under constant suction using the negative pore-water pressure method (s = -u(w) > 0; u(a) = 0) for applying the matric suction, s (s = u(a) - u(w); where u(a) is the pore-air pressure and u(w) is the pore-water pressure). It was found that the true triaxial test results under three different principal stresses are uniquely arranged on the extended spatially mobilized plane (extended SMP) for frictional and cohesive materials that is modified from the original SMP for frictional materials by introducing a bonding stress, sigma(0) (= c.cotphi, where c is cohesion and phi is the internal friction angle). It was also found that the shear strengths of the unsaturated silty clay obtained by the true triaxial apparatus nearly agree with the extended SMP failure criterion ((I) over cap (1)(I) over cap (2)/(I) over cap (3) = constant, where (I) over cap (1), (I) over cap (2), and (I) over cap (3) are the first, second, and third invariants of the translated stress tensor). The measured stress-strain-strength behaviour of the unsaturated soil in three-dimensional (3D) stresses can be well simulated by an elastoplastic model with the transformed stress based on the extended SMP criterion and a special hardening parameter.