Effect of Capillary and Cemented Bonds on the Strength of Unsaturated Sands

Summary

The cohesive interactions between grains play a prevailing role in the mechanical behaviour of unsaturated granular materials such as fine sands. These interactions are generally bonds of various natures that evolve according to the surrounding hygrothermic conditions. We study the case where the liquid present in the material is a water solution saturated with sodium chloride. The bonds are then of capillary type and the cohesive interactions are mainly attractive. In this case, the mechanical strength in an unconfined compression test is relatively low. At low relative humidity, the phase change of water involves a crystallization of salt at the contact points between grains generating thus bonds of solid type. The mechanical strength of the material is thus enhanced. An experimental study of the variation of the mechanical strength during the crystallization of salt allowed us to show two distinct cohesive regimes: capillary and cemented. The transition between these two regimes does not seem to be correlated with the mass of the crystallized salt, but rather with the residual degree of saturation. An analysis of these results is proposed by comparison with numerical simulations based on the discrete element approach.