Effect of strain rate and structure on the development of cohesion and friction in a sensitive clay

The study was undertaken to investigate the effects of rate of strain and structure on the development of true cohesion and true friction in a saturated cohesive soil. The soil investigated was a locally available sensitive clay. The various methods proposed by different investigators to measure true cohesion and true friction are reviewed. After some trials the method adopted was that proposed by Bishop (1962) in which over-consolidation is used to obtain samples at the same void ratio but different initial effective stress. All samples were tested in undrained triaxial compression. To investigate the effects of rate of strain on the development of true cohesion and true friction, the results were compared for two identical sets of samples tested at two significantly different strain rates. To investigate the effects of structure on the development of true cohesion and true friction, a technique was developed to remold the specimens in place at constant void ratio after they had been sheared in the undisturbed state. Remolding has the effect of breaking-down the structure of a sensitive undisturbed clay. The test results from undisturbed and remolded samples were compared. In general the investigation showed that in the undisturbed clay the strength in terms of maximum (Ϭ₁-Ϭ₃) decreased with decreasing strain rate, however, the strength in terms of (Ϭ⁻₁/Ϭ⁻₃) max, was not significantly affected by strain rate when compared on an effective stress basis. In remolded clays there was a tendency for the strength in terms of maximum (Ϭ₁-Ϭ₃) to increase with increasing time to failure. In the undisturbed clay true cohesion developed rapidly at small strains, attained a peak and decreased to zero at large strains. True friction was mobilized slowly and attained a maximum value at large strains. At strains less than 8% both true friction and true, cohesion increased with increasing strain rate. However, at large strains true friction and true cohesion were not affected by strain rate. In the remolded clay true cohesion developed slowly attaining a small but significant value which remained constant with strain. The maximum value of true cohesion in the remolded clay was very much lower than that for the undisturbed clay. The development of true friction with strain was essentially unaffected by remolding and both the undisturbed and remolded clay developed about the same maximum value of true friction at large strains. The observed behaviour was explained in terms of changes in clay structure with strain rate and remolding.Applied Science, Faculty ofCivil Engineering, Department ofGraduat