The drained residual shear strength of overconsolidated clays is an important parameter in assessing the stability of slopes that contain a preexisting shear surface. The main issue influencing a laboratory testing program to measure the drained residual strength is whether a natural or laboratory-formed shear surface will be used. A multistage test procedure using a modified Bromhead ring shear apparatus and an overconsolidated, precut, remolded specimen is described that provides a reliable and practical method for measuring the drained residual shear strength. Results of ring shear tests on 32 clays and clay shales reveal that the drained residual strength is controlled by clay mineralogy and the quantity of clay-size particles. The liquid limit is used as an indicator of clay mineralogy, and the clay-size fraction indicates the quantity of clay-size particles, which are particles smaller than 0.002 mm. Therefore, increasing the liquid limit and clay-size fraction decreases the drained residual strength. The ring shear tests also reveal that the drained residual failure envelope is significantly nonlinear for overconsolidated clays with a clay-size fraction greater than 50 percent and a liquid limit between 60 and 220. Analysis of several case histories shows that this nonlinearity should be incorporated into a slope stability analysis. Previous correlations do not provide an accurate estimate of the drained residual strength because they (a) are based on only one soil index property, for example, clay-size fraction or plasticity; and (b) do not provide an estimate of the stress-dependent nature of the residual failure envelope. A new correlation is presented that is a function of the liquid limit, clay-size fraction, and effective normal stress and can be used to estimate the entire nonlinear residual failure envelope.
|Number of pages
|Transportation Research Record
|Published - Jul 1995
ASJC Scopus subject areas
- Civil and Structural Engineering
- Mechanical Engineering