Shear Strength of Prestressed Cohesive Soils

The results of an experimental investigation to determine the effects of preloading on the Mohr-Coulomb failure envelopes of cohesive soils are described. In situ shear tests using the Borehole Shear Test (BST) were performed on a number of natural soil deposits. Initially, conventional stage tests were conducted to obtain the failure envelope. Subsequent tests were conducted by consolidating to a higher normal stress than was used in the shear phase of the test. Results are evaluated by examining the relative increase in shearing resistance between the original failure envelope and the prestressed failure envelope. By defining the Preloading Stress Ratio (PSR) as the ratio of the normal stress where shear strength comparisons are being made to the preloading stress, the relationship between relative strength increase and PSR could be determined. The data indicate that this increase may be described as a power function of the form y = ax^b, where a and b are regression coefficients which indicate the rate of increase in strength with increasing PSR. The results for all soils tested displayed a hysteresis between the original and the prestressed Mohr-Coulomb envelopes, with the prestressed envelopes indicating higher cohesion and lower friction angles.