Salinity and oven-drying effects on the plasticity of a marine soft clay

Considering the extensive use of plasticity-based correlations in geotechnical practice to estimate soil parameters, this paper evaluates the influence of pore fluid salinity and soil drying on the plasticity of Ballina clay, a estuarine soft clay from northern New South Wales (Australia). A comprehensive experimental study which includes controlled leaching/salinisation paths applied to natural (remoulded) as well as oven-dried clay prior to the estimation of the Atterberg limits is presented. Plasticity tests are complemented with chemical analysis of the pore fluid carried out to evaluate the processes involved in the leaching/salinisation mechanisms for remoulded and oven-dried clay. Strong dependency of liquid limit on pore fluid salinity and oven-drying are observed in Ballina clay. Leaching modifies the soil fabric from an initially saline-sodic flocculated towards a normal flocculated arrangement. The experimental results show that changes in soil plasticity upon leaching are largely reversible upon salinisation paths. Oven-drying promotes the stacking of clay minerals (aggregation) which in turn reduces the water absorption capacity of the clay. The consequences of neglecting both salinity and drying effects in practice when adopting well-established relationships between mechanical parameters and soil plasticity are also briefly discussed.