Strain-rate effects in Mexico City soil

Mexico City soil has very high specific surface, plasticity and void ratio; its natural structure is preserved until the yield pressure σ′_y, which is typically above the in situ effective stress σ′_v, and the mechanical response changes significantly when the effective confining stress σ′_o exceeds the yield pressure σ′_y. In this study, the effects of strain rate on the undrained response of Mexico City soil are explored using undisturbed specimens subjected to monotonic triaxial compression tests at a constant rate of deformation. Results show that strain-rate effects on undrained strength and mode of failure depend on σ′_y σ′_o, hence, on the degree of natural structure preserved in the specimen. Undrained strength increase with strain rate, particularly in the more structured specimens (i.e., higher σ′_yσ′_o). The role of σ′_yσ′_o on strain-rate effects in this unremolded natural soil resembles the effect of overconsolidation ratio on resedimented specimens. The limitations in using standard triaxial equipment for strain-rate effect studies are discussed.