Modeling critical-state shear strength behavior of compacted silty sand via suction-controlled triaxial testing

Most of the recently postulated unsaturated shear strength models have been calibrated only for a short variety of soils. In addition, these models are yet to be extended and calibrated over a wider range of matric and total suction states. The present work focuses on further refinements of previously proposed shear strength equations in light of newly obtained experimental evidence of shear strength behavior of compacted silty sand at a critical state from suction-controlled triaxial tests conducted between 0.05 MPa to 300 MPa suction range. A refined and rather simple equation comprising two independent functions, is introduced and validated, including a thorough parametric investigation, to predict the unsaturated shear strength of compacted silty sand at a critical state for a wide range of matric and total suction states. The experimental program included a total of 21 consolidated drained (CD) triaxial tests conducted on statically-compacted specimens of silty sand under strain- and suction-controlled conditions. Experimental results show that the angle of internal friction (ϕ′) remained virtually constant over the entire range of induced suction states; however, the shear strength increased while the angle of internal friction with respect to suction (ϕb) decreased with increasing suction, with both varying non-linearly. Finally, a gradual increase in brittleness of the test soil at peak-failure condition, as well as an increasingly marked strain-softening post-failure, was observed with increasing suction.