Effect of Potassium Nitrate on the Potential Swelling and Shrinkage of an Expansive Soil

The effect of distilled water and potassium nitrate solutions at varying concentrations as inundating fluids on the swelling–shrinkage behavior of an expansive soil was studied through a series of wetting and drying tests. Soil samples were prepared by static compaction and then tested in both modified and conventional oedometers under a surcharge pressure of 10 kPa, while being flooded with the different fluids. The wetting and drying tests were conducted at 25°C and 45°C, respectively. Vertical deformation of the samples was measured at the end of each test. The pH and electrical conductivity (EC) of the flooding fluid were determined at the end of the wetting stage. Scanning electron microscopy (SEM) analyses were conducted on the samples after the first and third cycles. The results indicated that during wetting and drying cycles, sample deformation reduced as the number of cycles increased until it reached a constant value (equilibrium condition). The equilibrium condition for distilled water reached after four cycles, with a deformation of 6.35%, but for potassium nitrate solutions at concentrations of 6, 9, 15, and 30 g/L, the equilibrium was reached after the third cycle, with deformations of 5.15%, 5.35%, 4.76%, and 7.3%, respectively. The results of the tests with different inundating fluids indicated that deformation decreases with increasing cycles of wetting and drying. In addition, for potassium nitrate, the deformation depended on its concentration. Higher concentrations of potassium nitrate resulted in greater reductions in soil deformation. The results also showed that, under equilibrium conditions, the wetting–drying paths exhibited a nearly S-shaped curve in the space of void ratio versus water content. SEM results revealed that the structure of the soil was changed with an increasing number of cycles. The values of pH and EC of the different inundating fluids were statistically analyzed, and a regression equation was developed for them. Sensitivity analysis showed that the concentration of potassium nitrate had a greater influence on pH and EC values than the number of wetting and drying cycles.