Effect of increased pore water salinity on the shrinkage behaviour of a fluvial clay soil

Shrinkage and swelling of expansive clay soils is known to cause severe damage to buildings and infrastructure, particularly in regions rich in soft soils. Many deltaic areas, including those near coastlines, are underlain by clay and peat, making them especially vulnerable. Salinisation may typically happen in coastal areas. Changes in pore water composition can alter the shrinkage and swelling behaviour as the soil structure and interaction of aggregates is affected by the pore water composition. This may lead to changes in the physical properties of the soil, to irreversible shrinkage, and thus additional land subsidence. In the Netherlands, large areas with Holocene clay-rich soils are already experiencing intrusion of salt water, making it critical to understand how salinity affects soil volume change. To investigate this, we studied the effect of increased pore water salinity on the shrinkage behaviour of natural, undisturbed Holocene fluvial clay samples from the Netherlands. We saturated natural undisturbed clay samples with solutions of varying composition (tap water, artificial sea water (34.5 g/L) and a mix of calcium with anions (15.6 g/L)). Shrinkage was monitored during evaporation in a climate-controlled room, followed by prolonged re-saturation to distinguish between reversible and irreversible shrinkage. The results indicate that small variations in the sample characteristics, such as Atterberg limits, grain size distribution and clay mineralogical composition, are key factors with respect to the amount and rate of volume loss with a decrease in the soil moisture content. However, increased pore water salinity slows evaporation and consequently soil shrinkage, while it increases irreversible shrinkage. Further, the addition of sea water-potassium results in the transformation from smectite and vermiculite to illite, with irreversible volume loss as a consequence. These findings indicate that salinization of clay-rich soils, may accelerate land subsidence depending on local soil properties. Given the complexity and heterogeneity of natural soils, studying undisturbed samples is essential for accurately predicting the effects of pore water salinity changes.