Drained residual shear strength measured by multiple reversal direct shear or ring shear tests has been successfully used for over four decades for stability analyses of reactivated landslides in stiff clays and clay shales; A body of literature has accumulated in recent decades, claiming that “healing” or “strength regain” is realized in time on preexisting slip surfaces already at residual condition. In other words, the shear stress required to reactivate a landslide is claimed to be larger than the drained residual shear strength determined using laboratory tests. This article presents (1) a comparison of secant residual friction angle determined from laboratory tests and secant mobilized friction angle back-calculated for reactivated landslides; (2) explanations that field evidence used to claim “healing” can be attributed to alternative factors, and the laboratory evidence on “strength regain” upon reshearing is the result of either the testing apparatus or testing procedure, or is inapplicab...

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 s_uo(mob) is reviewed to s_uo(FV), s_uo(UC), s_uo(TC), s_uo(TE), s_uo(DSS), and also to N_k from push cone penetration test.

The spatial correlation length (SCL), or the scale of fluctuation, is a parameter for describing the spatial variability of soil and one of the important parameters used in random field theory. Studies reporting the spatial correlation length based on real field data of offshore/nearshore sea bottom soils are rather limited in the literature, so in this study, the vertical spatial correlation length is determined using site investigation data from two sites of the southern coast of Turkey. Based on quite extensive data, the vertical spatial correlation length is estimated using four different autocovariance functions. The values are within typical ranges reported in the literature for similar soil groups, both onshore and offshore. It is also noted that the widely-used exponential function almost always gives the lowest value of spatial correlation length. The results of this study add to the database of spatial correlation lengths based on real data and could be useful for future studies on reliability assessment of offshore foundations using random finite element method.