CONTRIBUTION BY P. W. MAYNE While Vardanega et al. (2012) present an interesting empirical power law relationship for representing the stress– strain response of clays under constant volume, they must take care in the mixing and matching of different strength modes. In particular, the observed increase of normally consolidated undrained shear strength ratio (cu=s’v0)nc with plasticity index (Ip) given by equation (4) was specifically developed on the basis of raw (uncorrected) vane shear strengths measured in the field on natural clays. The general trend for the shear strength ratio from field vane tests (FVTs) varying with Ip is reported by Bjerrum (1972), Larsson (1980), Jamiolkowski et al. (1985), Chandler (1988) and Leroueil & Hight (2003), albeit using non-linear relationships such as power law rather than the original linear form. On the other hand, the general trend for the strength ratio (cu=s’v0)nc from triaxial compression (TC) tests on clays shows essentially no dependence on Ip (Larsson, 1980; Jamiolkowski et al., 1985; Ladd, 1991; Ladd & DeGroot, 2003). As a consequence, this fact led Chandler (1988) to recommend an Ip expression that inter-relates FVT strengths to equivalent TC values. Figure 9 shows the trends of (cu=s’v0)nc versus Ip using the original FVT dataset of Skempton (1957) and the laboratory dataset from TC, direct simple shear and triaxial extension modes presented by Ladd & DeGroot (2003). While the FVT data show a strong correlation with Ip (r 2 5 0?946), the TC data do not (r 5 0?0002). These trends have been supported by larger datasets including over 200 vane shear test (VST) data and over 200 laboratory strength data (Mayne, 2012). On the other hand, as per the theoretical link established by critical state soil mechanics, the ratio (cu=s’v0)nc for TC mode increases with the effective stress friction angle w9 of the clay, as verified elsewhere (Mayne, 1988, 2012).