Salinity effect on the liquid limit of soils

Abstract

Previous studies stated that, with increasing salinity, the decreased liquid limit for expansive soils was attributed to the shrinkage of diffuse double layer, while the increased liquid limit for non-expansive soils was explained by the growing particle flocculation. These two mechanisms seem to be controversial and it is difficult to understand how an increasing salinity can promote soil flocculation. This study aims at clarifying the mechanism controlling liquid limit by conducting cone penetration test, sedimentation and rheological tests on MX80 Na-bentonite and silty soil. Results showed that the liquid limit and yield stress of MX80 increased then decreased with increasing salinity, while they increased slightly for the silty soil. In sedimentation test, faster settling rate and smaller sediment volume were identified for MX80 and silt suspensions in NaCl solution, evidencing the shrinkage of diffuse double layer and the formation of denser aggregated structure. This suggested that the change of liquid limit is the result of two mechanisms in competition, which depend on the compression rate of diffuse double layer: (1) the water storage in nano-fissures which correspond to the nanoscale spaces among interlayers of clay particles resulting from the slight shrinkage of diffuse double layer, and (2) the water expulsion into larger pores with significant shrinkage of diffuse double layer. For the MX80 at low salinity and for the silt soil, the first mechanism prevailed and the increasing liquid limit was attributed to the requirement of more water to fill the nano-fissures. In contrast, for MX80 at high salinity, the second mechanism prevailed—the diffuse double layer was compressed significantly, changing the double-layer water to free water and giving rise to the decrease in liquid limit.