Is Wetting Collapse an Unstable Compaction Process?

This paper provides an interpretation of the phenomenon of wetting collapse in fine-grained soils based on principles of material stability. For this purpose, classic experiments displaying the accumulation of irreversible compaction have been reinterpreted through a plasticity model for unsaturated soils. The resulting simulations have been inspected mathematically, with the goal of detecting a possible loss of control of the wetting process. The reanalysis of the experiments suggests that, if the wetting-induced deformations are interpreted as homogeneous, the considered compaction events do not tend to be affected by a loss of material stability. As a result, such well-known phenomena have been explained as controllable modes of plastic deformation. This feature has been found to be valid regardless of the mode of saturation (i.e., it is valid for both controlled suction removal and water volume injection). Indeed, parametric analyses have shown that the compaction process tends to become unstable only in the presence of highly water-sensitive media exposed to a drastic loss of suction (e.g., soaking or injection), i.e., circumstances during which the soil undergoes large strains and suffers a marked deterioration of its mechanical properties. In these cases, the theory predicts a limit threshold of the water content at which the control of the injection process is lost, the applied stress is no longer sustainable, and even infinitesimal alterations of the moisture content can cause a sharp accumulation of compaction. These findings impact the interpretation of wetting-induced settlements predicted via computational models, and are of general relevance for the design, maintenance, and safety assessment of unsaturated earthen systems interacting with hydrologic processes.