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 ${\sigma }_y^{\prime }$ , which is typically above the in situ effective stress ${\sigma }_v^{\prime }$ , and the mechanical response changes significantly when the effective confining stress ${\sigma }_{\mathrm{o}}^{\prime }$ exceeds the yield pressure ${\sigma }_y^{\prime }$ . 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 ${\sigma }_y^{\prime }∕{\sigma }_{\mathrm{o}}^{\prime }$ , 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 ${\sigma }_y^{\prime }∕{\sigma }_{\mathrm{o}}^{\prime }$ ). The role of ${\sigma }_y^{\prime }∕{\sigma }_{\mathrm{o}}^{\prime }$ 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.