Implications of Surface Hydration and Capillary Condensation for Strength and Stiffness of Compacted Clay

Short-ranged hydration of water on mineral surfaces and capillary condensation in larger pores influences mechanical behavior of unsaturated soil (e.g., strength, stiffness) in different ways. Transition between water retention dominated by surface and cation hydration at high suction into a regime dominated by capillary condensation at low suction corresponds to a transition in soil behavior, yet the occurrence and implications of this transition remain unclear. Results are presented from Brazilian tensile strength (BTS) tests using compacted kaolinite disks equilibrated under controlled relative humidity (RH) conditions. Transition between surface hydration and capillary condensation is evident for specimens compacted to dense and loose conditions as a nonmonotonic relation between tensile strength and RH. Minimum tensile strength occurs in both cases at approximately 80% RH, or corresponding potential of about −14 to −30 MPa. Transitions in strength, stiffness, and strain at failure are consistent with transition from a hydration-dominated water retention regime to a capillary-dominated water retention regime. A semiquantitative framework accounting for evolution of suction stress is applied to interpret the experimental results. Results provide evidence for the hydration–capillary transition in compacted clay and clarify corresponding implications to macroscopic behavior.