Shear Strength Mobilized in Undrained Failure of Soft Clay and Silt Deposits

In the early days of Soil Mechanics and Foundation Engineering, two important assumptions were made as a part of the limiting equilibrium stability analysis of undrained failure of soft clay and silt deposits. It was assumed that undrained shear strength is independent of the laboratory or in situ testing methods used to determine it, and that the shear strength mobilized in a full-scale undrained failure in the field is equal to the undrained shear strength measured by a laboratory or in situ test. However, during the latter part of Geotechnical Engineering, it was realized that these two assumptions, in general, are not correct because undrained shear strength is influenced by mode of shear, shearing time to failure, progressive failure, and soil disturbance. As these controlling factors are rarely the same for any two testing methods or for any test and a full-scale field failure situation, undrained shear strengths determined by various tests are expected to be different; and the laboratory and in situ test measurements must be corrected before they are used for stability analysis of full-scale field undrained failures. Using undrained shear strength data back-calculated from failures of embankments, footings, and excavations, and data from in situ and laboratory tests, the relationship of suo(mob) is reviewed to suo(FV), suo(UC), suo(TC), suo(TE), suo(DSS), and also to Nk from push cone penetration test.