Influence of Salinity-Based Osmotic Suction on the Shear Strength of a Compacted Clay

As most previous studies have neglected the positive influence of salinity (osmotic suction) on most coastal soils in Australia, the design of transport infrastructure involving these soils has often been overly conservative. In this study, a laboratory approach based on direct shear testing was explained to determine the stress–strain behavior of compacted coastal silty clay (CL) at different levels of osmotic suction generated by various salinity (NaCl) concentrations. A broad data set for a total of 147 direct shear tests conducted on remolded and recompacted test specimens at seven different initial matric suction conditions was analyzed to develop a semiempirical model that captures the effect of osmotic suction on the soil shear strength. The results suggested that the greater the initial matric suction, the more pronounced the role of osmotic suction. The proposed semiempirical model was governed by an electrical conductivity relationship with the osmotic suction generated by soil salinity. A new parameter χ2 was introduced to quantify the role of soil salinity in the apparent soil shear strength corresponding to different levels of osmotic suction. When this novel relationship was coupled with the conventional matric suction theory, the overall unsaturated shear strength of a saline soil could be properly evaluated, as proven by the close proximity of the predictions to the measurements.