Effect of Strain Rate on Stress–Strain and Pore Pressure Response of Very Soft Clay

The strain rate-dependent mechanical behaviour of soils is critical to understand for different geotechnical applications involving both gradual and rapid loading scenarios. The present experimental study evaluates the mechanical response of very soft clay specimens across a broad range of strain rates (ε˙ = 0.005%/min–5%/min), which is scarcely addressed in the literature, through a series of CU triaxial tests conducted under 100 kPa confining pressure. The results were presented in terms of stress–strain and pore pressure behaviour, effective stress path, maximum obliquity response, and pore pressure parameter. The stress–strain curves were rate-dependent at low axial strains (εa < 0.1%) (isotach behaviour), before transitioning into TESRA-type behaviour at larger strain levels. The influence of strain rate on peak deviatoric stress and the corresponding peak axial strain was also examined. Rate sensitivity of undrained shear strength was quantified using the peak strength change rate (R), while soil stiffness variation with changing strain rates was assessed through the secant modulus.