A new correlation relating the shear strength of reconstituted soil to the proportions of clay minerals and plasticity characteristics

It has been recommended to use “fully softened shear strength” while conducting slope stability analysis of first time slides. Studies pertaining to the influence of the mineral composition of a soil mass on the residual shear strength are available in the literature. However, very limited studies are available regarding the effect of clay mineralogy on the fully softened shear strength. This study addresses the effect of mineral composition of a soil mass on the fully softened shear strength and provides correlations between the geotechnical properties of different clay minerals and the fully softened shear strength. In order to accomplish this, thirty six samples were prepared in the laboratory from different pre-defined mixtures of montmorillonite, kaolinite and quartz. The index properties as well as the fully softened friction angles of these mixtures were measured. The fully softened friction angle was largely dependent on the dominating clay mineral and contained good correlations with the clay content, the liquid limit, and the plasticity index. The difference between the fully softened and residual shear strength was normalized, individually with: i) the fully softened friction angle, and ii) the residual friction angle. These values were then related to the corresponding liquid limits and plasticity indices. Using the mineralogical information as well as the fully softened friction angles obtained, a triangular correlation chart was developed by plotting at two sides of the triangle the proportions of montmorillonite and kaolinite, and the third side with the proportions of quartz plus other minerals. The fully softened friction angle was used to develop contours of the fully softened friction angle for various proportions of constituent minerals. These correlations were then verified with the results of the fully softened friction angles of more than eighty natural specimens demonstrating good agreement with the estimated friction angles.

► Information on the fully softened shear strength (FSSS) of soil.
► Even small proportions of montmorillonite increase the plasticity & reduce the FSSS.
► The FSSS can be estimated if the proportion of each clay mineral is known.
► If the mineralogical composition is unknown, the plasticity index can be used to estimate the FSSS.
► The residual shear strength of a soil with known FSSS can be estimated from the plasticity.