Conceptual model of residual friction in Montmorillonitic soils at large void ratios

D. W. Smith and M. G. Reitsma

In soil mechanics, friction is usually included in a mathematical model as a ‘friction coefficient’ (or angle of internal friction). Conventional geotechnical testing equipment is employed to experimentally define the friction coefficient for a soil. A friction coefficient estimated in this way is a macroscopic parameter representing the integrated effects of many dissipative processes occurring at the microscale. For this reason, conventional geotechnical testing cannot be expected to give any substantial insight into the microscale processes leading to the frictional behaviour observed for soils. Traditional ideas advanced about the frictional behaviour of soils are motivated by observations of the shear strength of dry particle assemblies like sand and gravel. It is shown here that these ideas are not applicable to platy clay soils with well-developed diffuse-double layers at a residual friction state. This provides the foundation for the presentation of a new conceptual theory of energy dissipation in saturated montmorillonite clay soils at the residual friction angle. While the proposed theory is still in its infancy (and there are a number of unresolved issues), it is clear that the theory provides a basis for a deeper understanding of the behaviour of saturated montmorillonitic soils at the residual friction angle. The new theory is based upon a number of assumptions and hypotheses that can be systematically studied experimentally and numerically, and so the theory provides a framework for a systematic experimental and numerical program of investigation.