Characterization Of Swelling Stress And Soil Moisture Deficiency Relationship For Expansive Unsaturated Soils

Abstract

Expansive soils vary in volume, in relation to water content. Volume changes when wetting (swelling) and drying (shrinkage). Lightweight structures in construction are the most vulnerable structures experiencing severe defects when built on these soils. In South Africa, expansive soils are the most problematic which impose challenges to civil engineers. The prediction of the swelling stress has been a concern to the construction industry for a long time. The swelling stress is generally ignored in engineering practice. Nonetheless, the swelling stress can develop significant uplift forces detrimental to the stability of foundations. Considering the swelling stress in foundation design in expansive soils enhance the durability, the service life, and reduce the cost of assessment and repair works to be undertaken in the future. Mathematical models are offered as an alternative to direct oedometer testing. Mathematical models are a useful tool to assess swelling stress. The aim of this study was to characterize the relationship between the swelling stress, the soil suction, and other soil parameters. Moreover, develop mathematical models to predict the swelling stress of field compacted expansive soils. Laboratory tests have been performed such as particle size distribution, Atterberg limits, linear shrinkage, specific gravity, free swell ratio, X-ray diffraction, soil suction measurement, modified Proctor compaction test, and zero-swell test (ZST). Multiple regression analysis was performed using software NCSS11 to analyze the data obtained from the experiments. The relationships between the swelling stress and other soil parameters were established. It was observed that, at the optimum moisture content (OMC), the swelling stress values are within the range of 48.88 kPa to 261.81 kPa, and the matric suction values are within the range of 222.843 kPa to 1,778.27 kPa. The swelling stress values on the dry side of the OMC are higher than values on the wet side. In addition, compaction at the OMC can reduce the swelling stress by 15%. Furthermore, the geotechnical index properties, the swelling parameters, affect the swelling stress of compacted expansive soils. Nevertheless, there is a key impact of the type of clay mineral on swelling stress. Six predictive mathematical models were developed. These models were validated using soil samples collected from various areas across the province of Free State (Petrusburg, Bloemfontein, Winburg, Welkom, and Bethlehem). Lastly, good correlations between predicted values and values obtained from experimental works confirm the reliability of the multiple regression analysis. The data points are very close to the line 1:1. Furthermore, the graphical analysis shows that the correlation of the values obtained from the models developed in this study are more precise than the values obtained from other models. Therefore, the predictive models developed in this research work are capable to estimate the swelling stress with acceptable accuracy.