Study of wetting and drying cycle effect on the clayey sandy soil permeability

Drought and moisture reduction can weaken and crack soil structure, altering its shear strength and permeability. This paper investigates changes in permeability of clayey sandy soil undergoing drying and wetting cycles, which are often overlooked during the design and construction of earth structures such as embankments and dams. To conduct the investigation, a sandy soil called Firoozkooh sand No. 161 and a kaolinite clay soil were selected and mixed in varying ratios to produce soil samples. Two mixtures of sand and clay were prepared with 20% and 30% clay and 80% and 70% sand by weight, respectively. The sand used was poorly graded and the clay was low plastic. Test samples were created using the maximum dry unit weight and optimum moisture content obtained from standard compaction tests. Permeability tests were then conducted on the prepared samples in both drying and wetting directions using a common triaxial apparatus. In addition, some Scanning Electron Microscope (SEM) images were taken of the soil combinations. Results from the permeability tests revealed that the drying and wetting process initially has a significant effect on the water entrance to the sample due to soil cracking, and the soil permeability reduces over time as water passes through the soil and is absorbed by clay minerals, closing voids and cracks. Eventually, the permeability of all samples, including those that underwent drying and wetting, reached a relatively constant value, and seepage became stable. Moreover, the samples that underwent the drying and wetting processes showed higher permeability than the initial samples prepared with optimum moisture content. The test results also indicated that completely dried samples do not necessarily have the highest permeability. Furthermore, SEM images revealed that samples with 20% clay content had more voids compared to those with 30% clay.