A Study of the Drained Strength Characteristics of Overconsolidated Clays at Low Effective Stresses

This paper presents the results of a study to determine the drained strength parameters of overconsolidated clays at low effective stresses. Overconsolidated clays experience dilation when sheared under undrained conditions in conventional triaxial compression (TC) and direct simple shear (DSS) tests. Moreover, the effective stresses at failure can be significantly larger than the preshear stresses. Consequently, interpretation of the results of conventional triaxial and DSS tests to define the failure envelope at low effective stresses involves considerable extrapolation of the data. Therefore, obtaining reliable interpretations of the drained strength parameters at low effective stresses (${\sigma }_{\mathrm{v}}^{\prime }<100\mathrm{kPa}$) is difficult. In this study, a new approach in performing TC and DSS tests, which involves shearing anisotropically consolidated specimens by increasing the pore water pressure at a constant shear stress, is presented. This procedure enables a reliable interpretation of the drained strength parameters at effective stresses as low as 10 kPa. This paper presents the conceptual test methodology, typical results of TC and DSS tests, and statistical summaries of the drained strength parameters of inorganic clays determined from numerous tests performed in this study. The results of the tests performed in this manner show consistent mobilization of significant effective cohesion, ${\mathrm{c}}^{\prime }$, which is related to the preconsolidation stress, ${\sigma }_{\mathrm{p}}^{\prime }$. Effective cohesion values normalized with respect to the preconsolidation stress (${\mathrm{c}}^{\prime }/{\sigma }_{\mathrm{p}}^{\prime }$) generally range from 0.3% to 6.1% for lean clays and from 1.7% to 7.1% for inorganic fat clays. The mean ${\mathrm{c}}^{\prime }/{\sigma }_{\mathrm{p}}^{\prime }$ values determined from triaxial tests were 2.5% for lean clays and 3.4% for fat clays. Similarly, the mean values from the DSS tests were 2.8% for lean clays and 3.5% for fat clays. The effective friction angles for lean and fat inorganic clays ranged between 23° and 37°, with mean values of 31° and 30° from the triaxial and DSS tests, respectively.