Experimental research on influences of suction on deformation characteristics of loess under unloading-reloading conditions

In order to explore the influences of matrix suction and net confining pressure on the deformation-strength and unloading-reloading modulus of loess, three groups of 27 tests are performed using the unsaturated triaxial apparatus for undisturbed Q3 loess and remolded loess, namely, 9 triaxial consolidated drained shear tests on undisturbed Q3 loess under controlled suction and net confining pressure, 9 unloading-reloading triaxial tests on undisturbed Q3 loess and 9 unloading-reloading triaxial tests on remoulded loess under controlled suction and net confining pressure. The research results show that the matric suction and net confining pressure have great influences on the strength and deformation characteristics of the soils. The behavior of deviatoric stress-strain curve of the undisturbed Q3 loess turns from the ideal elastic-plastic model into hardening type, and the curves cross at the initial stages because of their constitutive property, and the volume of the samples decreases during shearing. And the behavior of the remoulded samples exhibits gradually strong dilatancy when the net confining pressure is small, and shear contraction only occurs when the net confining pressure is larger. Apparent cohesion, tangent Young's modulus and unloading-reloading modulus increase with the increase of the suction. The influences of suction on the friction angle and parameter (the index that the initial Young's modulus of undisturbed Q3 loess changes with the increase of net confining pressure), (the index that the unloading-reloading modulus of undisturbed Q3 loess changes with the increase of net confining pressure), and (the index that the unloading-reloading modulus of remoulded Q3 loess changes with the increase of net confining pressure) are insignificant and can be regarded as constants, respectively. and then, the modified formulae for the apparent cohesion, tangent Young's modulus and unloading-reloading modulus are proposed based on the test results considering the influences of suction and net confining pressure, so the incremental nonlinear model for unsaturated soils is improved.