The effect of calcium ions in pore water on the residual shear strength of kaolinite and sodium montmorillonite

This paper reviews the literature concerning the effects of pore water chemistry on residual shear strength (tau(r)'). It presents the results of index limit and residual shear strength testing undertaken on kaolinite and sodium montmorillonite, using concentrations of calcium chloride similar to the calcium levels measured in groundwaters obtained from landslipped calcareous mudrock of the Fuller's Earth Formation. The residual shear strength of the sodium montmorillonite increased markedly for the sample treated and tested with 80 mg/l Ca2+ compared with that tested with deionized water. Greater concentrations of Ca2+ resulted in only a marginal further increase in the residual shear strength. These changes were also reflected in the interpreted parameters, phi(r)' and c(r)'. The liquid limit of the montmorillonite decreased dramatically when the samples were treated with 80 mg/l Ca2+, and this effect persisted and/or was slightly enhanced when the concentration of Ca2+ was increased. The residual shear strength and Atterberg limits of kaolinite were much less sensitive to Ca2+. The paper briefly discusses the implications of the findings for slope stability and the importance of using an appropriate chemical solution in the water bath when undertaking residual shear strength testing. As the laboratory-measured I: is shown to vary with the chemistry of the pore water, this provides a possible explanation for the difference between results calculated by back-analysis and those obtained in the laboratory using deionized water. Attention is drawn to the necessity of defining the complete residual failure envelope as, if this is curved, the residual shear strength parameters (tau(r)', phi(r)' and c(r)') will vary with sigma(n)'.