Shear wave velocity, Vs, is a soil mechanical property that can be advantageously measured in both the field and laboratory under real and controlled conditions. The measured Vs values are customarily used in conjunction with other in situ (e.g., standard penetration test blow count, N-SPT, and cone penetration resistance, qc-CPT) and laboratory (e.g., effective confining pressure, [Formula: see text], and void ratio, e) measurements to establish an abundant number of Vs-based correlations that could later be utilized to augment (in some cases, replace) designated testing. An attempt is made here to present the salient features of some existing widely used correlations to provide the reader with a comprehensive understanding about the nature of these correlations and their applicability in geotechnical engineering practices. It is recognized that the reliability of some of these empirical formulations, still in general use today, has been questioned, as they are characterized by their lack of dependence on stress state and particle characteristics. A new Vs1–(N1)60 (where Vs1 is the stress-normalized shear wave velocity, and (N1)60 is the stress-normalized penetration blow count) correlation that accounts for grain sizes is highlighted by combining a recently published Vs1–qc1 (where qc1 is the stress-normalized cone tip resistance) formulation and available (N1)60–qc1 relationships. The new formulation is applicable to uncemented relatively young Holocene-age soil deposits. The estimated Vs1 values based on the proposed correlation are compared with reliable laboratory and field measurements, and the comparison shows that accounting for grain size of granular soils yields more realistic results regarding the Vs values than when particle size is not considered. The prime effect of grain size was to change the range of possible void ratios, which in turn had a substantial impact on Vs values. Moreover, a new Vs1–(N1)60 chart has been proposed, allowing the practitioner to estimate Vs1 values based on a combination of data including N-SPT, e, grain size, and relative density.
Approaches commonly used to assess the seismic stability of slopes range from the relatively simple pseudo-static method to more complicated nonlinear numerical methods, e.g., finite element (FE) and finite difference (FD). The pseudo-static method, in particular, is widely used in practice as it is inexpensive and substantially less time consuming compared to the much more rigorous numerical methods. However, the pseudo-static method is widely criticized because it does not take into account the effects of the earthquake on the shear strength of the slope material nor the seismic response of the slope. Hence, some researchers recommend its use only in slopes composed of cohesive materials that do not develop significant pore pressures or that lose less than about 15% of their peak shear strength during earthquake shaking. However, the use of the pseudo-static method in these soils is also problematic as clayey slopes generally fail in pseudo-static stability analyses (i.e., factors of safety are less than 1) and the failure surface is completely predominated by the thickness of the clayey layer in the slope or foundation. The reliability of the pseudo-static method in natural clayey slopes is examined here based on rigorous numerical simulations with FLAC. The numerical results are compared and verified using available static and dynamic 1g laboratory tests. This article then addresses some of the crude assumptions of the pseudo-static method and provides practical suggestions to be applied to refine the outcomes of pseudo-static analyses not only in terms of the computed safety factors, but also in the prediction of the failure surface through the consideration of additional aspects of the dynamic responses of the clayey slopes.
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