Effects of air coupling on triaxial shearing behavior of unsaturated silty specimens under constant confining pressure and various drained and exhausted conditions

A simulation of triaxial shear tests on unsaturated silty specimens was performed under constant confining pressure and various drained and exhausted conditions starting from a single initial condition and passing through processes of suction variation and isotropic consolidation considering the triaxial test as an initial and boundary value problem. The simulation made use of a soil-water-air coupled finite deformation analysis code incorporating the SYS Cam-clay model as the constitutive equation of the soil skeleton and applying the average soil skeleton stress in the stress equation. The results showed that the coupling effect of the highly compressible air has a significant effect on the mechanical behavior. In addition, the results also explained other behaviors that would have been outside the capability of an analysis model that did not take account of factors other than air coupling such as suction. The authors wish to place special emphasis on the following findings:(1) For simulation purposes, even using a soil water characteristic curve determined uniquely by the relationship between the degree of saturation and suction, it is still possible to calculate the increase in the degree of saturation for the drained and exhausted shear tests (under constant suction).(2) A comparison of the experimental and calculated initial stiffnesses, volumetric strains, etc. showed that suction has an inhibiting effect on plastic deformation not only in drained and exhausted, but also in undrained and unexhausted conditions.(3) For tests in which the volumetric restraint conditions of specimens were varied by controlling the air pressure under undrained conditions, results suggested that an introduction of gas leading to a rise in gas pressure within a highly structured ground may produce a risk of the ground displaying strain softening behavior. In the simulation, it was shown that this softening behavior can be represented in terms of structure decay as described by the SYS Cam-clay model.