Discussion: Influence of structure on the time-dependent behaviour of a stiff sedimentary clay

Abstract

The authors have decided to study the influence of microstructure on the time-dependent response of undisturbed and reconstituted London clay using drained and undrained triaxial compression tests (CIU and CID) with step changes in the applied strain rate. The paper presents interesting behaviour, and the authors should be commended for demonstrating the viscous response of London clay.The primary influence of microstructure on the engineering response of London clay can be seen in Fig. 18(a), which compares the stress-strain response in the undisturbed and reconstituted states. Fig. 18(b) shows similar behaviour from triaxial compression tests on Rosemère clay from Eastern Canada (Philibert, 1976). From Fig. 18, it can be seen that there are similarities in the relative stress-strain response of the two materials, in spite of their vastly different index properties (e.g. I L % 0 compared with I L % 1.2). The stress-strain response of both clays during triaxial compression is characterised by: (a) a peak shear strength followed by post-peak strength reduction with large strain; (b) a predominantly strain-hardening response of the reconstituted or disturbed materials; and (c) at large strain, a post-peak strength of the undisturbed clay that approaches that of the reconstituted and 'cut' materials respectively. The difference in behaviour (the shaded areas in Figs 18(a) and 18(b)) is typically attributed to the effects of microstructure or weak bonding between the clay particles and aggregates of clay particles. Such behaviour is analogous to that typically observed in oedometer consolation tests on undisturbed and reconstituted materials (Burland, 1990).Regarding the time dependence or rate sensitivity of London clay, the authors quantify viscous effects using the jump in deviatoric stress induced immediately after changing the axial strain rate. Although such an approach has merit, the following presents an alternative interpretation of the rate-sensitive response of London clay using the theory of overstress viscoplasticity (Perzyna, 1963). The current authors hope that this alternative interpretation will provide additional insight into the viscous response of undisturbed and reconstituted London clay.

Evaluation of the viscous behaviour of clay using generalised overstress viscoplastic theory

“…In addition, since most natural clays are structured to some degree, a few comments on the influence of structure are warranted. First, studies showing the results of CRSN triaxial tests with stepchanges in the strain rate performed on undisturbed structured clays indicate that AE is the same for both the structured and destructured material (see Hinchberger & Qu, 2007). Consequently, structure is not expected to effect the evaluation of AE from CRSN tests, which should be confirmed by the inclusion of data for remoulded clay in the assessment.…”

Constitutive approach for rate‐sensitive anisotropic structured clays

“…It is widely recognized [18,19] that both the undrained shear strength, s u , and preconsolidation pressure, p , of clay increase by 5-15% per order of magnitude increase in strain-rate. Figure 3(a) illustrates the effect of strain-rate on the stress-strain response of Belfast and Winnipeg clays [19][20][21][22] and Figure 3(b) summarizes the influence of strain-rate on the undrained shear strength of both clays corresponding to axial strains of 1.8, 10 and 15% [22]. Referring to Figure 3(b), it can be seen that there is a linear variation of mobilized shear strength versus strain-rate when plotted on a log-log scale.…”

“…Referring to Figure 3(b), it can be seen that there is a linear variation of mobilized shear strength versus strain-rate when plotted on a log-log scale. Such behavior can be described using a power law [21,22]. In addition, the rate-sensitivity, which is characterized by the slope in Figure 3(b) is the same at the peak strength and large-strain post-peak strength corresponding to axial strains of 10 and 15%.…”