Validation of semi-empirical models for the prediction of swelling stress for compacted unsaturated expansive soils.

Unsaturated swelling soil behaviour is governed by the matric suction, thus the predetermination of swelling stress for expansive unsaturated soil requires rigorous procedures. However, some swelling stress equations assume full saturation of the soil, which leads to the over-design of infrastructure. This study focused on the validation of predictive swelling stress models that correlate suction with other soil properties. Three models were developed, and independent data were used for the validation of the developed models. The predicted swelling stress values were compared to the values predicted by the randomly selected predictive models from the literature. Series of initial studies include the determination of basic soil characterization and swelling stress along with soil mineralogical compositions were conducted to determine their geotechnical properties with their corresponding degree of expansiveness. The replicated expansive soils were also studied for suction, using filter paper techniques to obtain the soil’s unsaturated parameters. Based on the obtained experimental results, three models were developed using a mathematical software package (NCSS11). Independent data obtained from a group of final year students working on the swelling stress response of expansive unsaturated soils at the geotechnical engineering Laboratory from the University of Johannesburg were used for the validation. The developed models showed good agreement with the independent data, having a coefficient 5, and 6, respectively. Compared to models selected from literature, which recorded R2 values of 0.796 and 0.636 with an average variance of 0.097 and 0.257 respectively. The correlation variables showed that the degree of expansion represented by swelling stress demonstrated a proportionality with the moisture capacity along the drying and wetting path of the suction curves. Results suggest that the developed models can reasonably predict the swelling stress of compacted expansive soils at high suction values.