The shear strength of soil is a key input parameter required for the satisfactory geotechnical design of any structure. The existence of most residual soils in unsaturated conditions highlights the importance of studying the shear strength of unsaturated soils. In this study, a modified triaxial apparatus was used to examine the mechanical behavior of 5 types of silica sands with mean grain sizes varying from 0.15 to 1.5 mm. The experimental test results of soil shear strength determination were compared with the shear strength estimated through analytical methods. To study the mechanical behavior of test soils at optimal and residual moisture contents, the test samples were initially prepared at the optimal moisture content. To study the effect of the initial moisture content on the mechanical behavior of test soils at the residual moisture content, the test samples were initially prepared at the optimal and residual moisture contents. Residual moisture content samples exhibited higher shear strength compared to optimal moisture content samples. Furthermore, the residual moisture content samples prepared at the optimal moisture content exhibited stronger mechanical behavior compared to the sample prepared at the residual moisture content. The analytical methods employed in this study were not able to accurately predict the shear strength of test soils at residual moisture content.

The soil moisture content at shearing and other factors, including dry density and grain size, influence its shear strength. This study investigated the effect of moisture content at compaction and grain size distribution on the unsaturated soil shear strength. Triaxial compression tests were performed in the laboratory using the modified triaxial apparatus on silica sands No. 3 and 6 without fines and with 20% fines to explore the unsaturated soil shear strength characteristics. Test samples were compacted and sheared at various combinations of the soil’s optimum and residual moisture content. The analysis of the triaxial compression test results shows that moisture content at compaction and the grain size distribution influence the unsaturated soil shear strength. The test samples compacted at optimum moisture content showed higher peak shear strength when sheared at residual moisture content. Further, test results show that the test samples of soil without fines, when compacted at residual moisture content, show higher peak shear strength at optimum moisture content. The finding of this study endorses considering the moisture content at compaction for the geotechnical design of structures while predicting the soil shear strength.

Soil exists mainly in unsaturated conditions. Therefore, accurate prediction of the soil shear strength for unsaturated conditions also becomes equally important for the geotechnical design of earth structures. This study primarily investigates the effect of the moisture content of unsaturated soil on its shear strength. The strength characteristics of silica sands with different grain sizes were studied using the modified triaxial apparatus and analytical methods. For this purpose, four series of triaxial compression tests on silica sands were performed by varying the moisture content of the test sample at compaction and shearing as optimum or residual moisture content. The test results showed that the test sample sheared at optimum and residual moisture content exhibited higher shear strength when the sample was initially compacted at residual and optimum moisture content, respectively. The moisture content at compaction and the soil grain size considerably influence the shear strength of unsaturated sandy soils. Furthermore, the analytical method used in this study for unsaturated soil shear strength prediction does not account for the effect of initial moisture content in predicting unsaturated soil shear strength.