Viscous behaviour of soil under oedometric conditions: Discussion

Abstract

Mesri and Choi (1985) reported data on the uniqueness of EOP e versus log σ′v relationship and not on “unique e–ln σ lines,” as has been stated by the authors. The end of primary (EOP) e versus log σ′v curve corresponds to the elapsed time tp for incremental loading (IL) and to the imposed vertical strain rate e . vp for a constant rate of strain (CRS); both correspond to the end of primary consolidation defined by near-zero (e.g., 1 kPa) excess pore-water pressure (Mesri et al. 1994). Mesri and coworkers have been aware that secondary compression follows primary consolidation (Mesri and Godlewski 1977, 1979; Mesri and Castro 1987; Mesri 1987; Mesri and Shahien 1993), and therefore any number of e versus log σ′v curves can be constructed at values of t > tp or e . v < e . vp. It is therefore important to appreciate the meaning and significance of the uniqueness of the EOP e versus log σ′v curve (Mesri et al. 1995) and not to confuse it with a set of e versus log σ′v curves at different instances during secondary compression. The authors assert that ψ “is always constant”. This would be true only if the e versus log σ′v relationships were always linear. In fact, the e versus log σ′v relationships of most soils are not linear, even in the compression range completely beyond the preconsolidation pressure σ′p. The Cα/Cc law of compressibility predicts secondary settlement starting from any shape of EOP e versus log σ′v curve and in recompression, compression, as well as following the removal of a surcharge, where Cc = ∆e/∆ log σ′v denotes the slopes of e versus log σ′v curves at various values of t/tp ≥ 1 and Cα = ∆e/∆ log t denotes the slopes of e versus log t curves at various values of σ′v (Mesri and Godlewski 1977, 1979; Mesri and Castro 1987; Mesri 1987). The parameter termed 1/n or Iv by the authors is in fact Cα/Cc, which, incidentally, has quite a reasonable value of 0.054 for the highly organic gyttja, see Table 1 (Mesri et al. 1994; Terzaghi et al. 1996). In general, Cc and therefore Cα are not constants and vary with both σ′v and t; however, Cα/Cc remains constant. The authors have formulated a constitutive model and regard it “as verified” apparently based on one oedometer test on one soil (incidentally, the diameter to thickness ratio of 1.67 is too small and may lead to considerable ring friction during measurements of secondary compression). The significant variable in the authors’ model is OCR; however, it had a single value of 1.24 (or less: 198/178 = 1.11 in “test a,” 338/304 = 1.11 in “test b”) in the authors’ test. This corresponds to a value of effective surcharge ratio, R′s = σ′vs/σ′vf – 1 equal to or less than 0.24. Secondary compression behavior after the removal of surcharge has been explained and predicted by the Cα/Cc law of compressibility (Mesri 1986; Mesri and Feng 1991; Mesri et al. 1994). As the effective surcharge ratio, R′s = σ′vs/σ′vf – 1 = OCR –1, increases, the postsurcharge secondary compression rate decreases; however, not in accordance with the authors’ approach, which was previously used by Johnson (1970) to estimate postsurcharge secondary settlement (Mesri and Feng 1991). The problem is illustrated in Fig. 1. The approach used by the authors is based on the assumption that secondary compression after unloading from B to C can be computed as the continuation of secondary compression along A to C. The expression for OCR = σ′vs/σ′vf resulting from secondary compression is (Mesri and Choi 1979; Mesri 1987)