THE UNDRAINED SHEARING RESISTANCE OF COHESIVE SOILS AT LARGE DEFORMATIONS

The correct representation of the post-peak undrained shearing resistance of a soil is central to the large-strain deformation analysis of an earth structure. This study examines the ultimate undrained residual shearing resistance of cohesive soils, and the reduction in shearing resistance on a failure surface associated with a slope failure.; Twenty vane shear tests on San Francisco Bay Mud were conducted on samples consolidated to different values of liquidity index ranging from about 0.2 to 1.0. The test results indicate that the ultimate undrained residual strength,c(,u(,r)), of a cohesive soil can be related to the effective consolidation pressure, p', by the ratio; (DIAGRAM, TABLE OR GRAPHIC OMITTED...PLEASE SEE DAI); in much the same way that peak undrained strength is related to consolidation pressure. The; (DIAGRAM, TABLE OR GRAPHIC OMITTED...PLEASE SEE DAI); ratio for San Francisco Bay Mud, was found to range from 0.08 to 0.13.; The reduction in shearing resistance on a failure surface associated with a slope failure was investigated by stability analyses of the slide mass involved in 32 slope failure case histories. The slope failures examined included failures of compacted clay, failures of natural clay slopes, and foundation failures of embankments. The shearing resistance remaining on the failure surface after failure is expressed by the shear stress ratio, defined as the ratio of average shear stress required for equilibrium after failure to the value before failure. No tests were conducted on soils from the slope failures, but a comparison of the computed shear stress ratios with shear strength ratios for a number of clay soils indicated that on the average, the shearing resistance remaining on the failure surface after the failure was significantly greater than the ultimate undrained residual strength. The magnitude of slide mass movement that occurred in the slope failure case histories examined was found to have a reasonably well defined relationship with the reduction in shearing resistance that occurred as a result of failure. This relationship was established using a normalized slope deformation, defined as the absolute deformation times the slope inclination, divided by the height of the slope.; The existence of a relationship between slope deformation and shearing resistance (expressed as a ratio to peak shearing resistance) provides a basis for simple slope deformation calculations. This relationship also provides a guide in assessing the magnitude of the reduction of shearing resistance that might be associated with deformation in cases of seismically induced slope failure.