Assessments on the swelling potential of an expansive clay and change in shear strength due to swelling

Expansive soils are composed of soil materials whose total volume increases during swelling or decreases during shrinkage in response to variations in water content. In this study, it was aimed that the swelling properties and the effect of swelling on effective shear strength of an expansive clay gathered from a site where Ankara clay is located. A comprehensive testing program, including X-ray diffraction (XRD) analysis, physical and index property determination, swelling tests and consolidated and drained direct shear tests were conducted on the clay samples. Mineralogical analyses revealed that Ankara clay contains relatively high proportions of smectite, contributing to its high plasticity, with an average plasticity index of approximately 50 %. From indirect evaluations methods, swelling potential of the clay samples was found to be very and extremely high. The results of swelling tests showed that both swelling percentage and swelling pressure decreased with increasing initial water content of undisturbed clay samples, and that there was a relationship between them. Although similar results from tests on some of recompacted samples were obtained, the determined swelling percentages on mostly recompacted samples were found to be incompatible with those determined on undisturbed samples because their initial dry densities and water contents were different from those of undisturbed samples. The results of the consolidated and drained direct shear tests on undisturbed samples with different initial dry densities showed that as the initial dry density increases effective shear strength also increases. Multiple regression analysis was performed considering the normal stress used in the experiments and the initial dry densities of the samples determined before the experiment as input parameters. The regression model obtained from this analysis and the input parameters were found to be statistically significant at a 95% confidence level. In addition, dry densities at the time of the experiment were also calculated, and it was shown that regression model from multiple regression analysis repeated using this data was also statistically meaningful. Finally, the change in the effective shear strength of the clay in the active zone before and after swelling was investigated by considering this relationship, and it was concluded that the decrease in strength would vary from over 90% to 0% from surface to the bottom of active zone.