Discussion of “Multivariate distribution for undrained shear strengths under various test procedures”

Abstract

Care needs to be exercised when examining the empirical correlations for undrained shear strength proposed by Ching and Phoon (2013) because the manuscript does not adequately define the overconsolidation ratio (OCR), and the data in Table 2 that were used to develop the empirical correlations include values of OCR, as small as 1.1, 2.2, and 3.3 and as large as 50, 60, and 80. A clear distinction needs to be made among (i) OCR = p ′ / vo ′ , where for a natural soil deposit sublayer p ′ is the preconsolidation pressure and vo ′ is preconstruction effective vertical pressure; (ii) OCR = p ′ / vc ′ , where vc ′ is a consolidation pressure in the recompression range of a natural soil deposit sublayer with a preconsolidation pressure p ′ ; and (iii) OCR = vm ′ / vc ′ , where vm ′ , for a soil sublayer in the field or in the laboratory, is the maximum effective vertical pressure after loading and primary consolidation, and vc ′ is a consolidation pressure after unloading and primary rebound. In addition, the data in Table 2 include natural soil deposits as well as specimens reconstituted in the laboratory. In geotechnical engineering, there is a separate body of empirical literature on undrained shear strength of soft clay and silt deposits with values of p ′ / vo ′ typically less than 3 and rarely greater than 6 (Terzaghi et al. 1996, Arts. 20.1 to 20.7), and on undrained shear strength of stiff clays and shales (Terzaghi et al. 1996, Art. 20.8). This discussion only concerns soft clay and silt deposits because for these soils exists an adequate body of laboratory andfield undrained shear strengthdata as a functionof composition (e.g., expressed in termsof plasticity index, Ip), mode of shear (e.g., field vane (FV), direct simple shear (DSS), triaxial compression and extension (TC and TE, respectively), unconfined compression (UC), and mobilized in full scale construction), and rateof shearor time to failure (e.g., FVandUC fast, and field construction much slower).