Coupled Effects of Dry Density and Initial Water Content on the Swelling Characteristics of Expansive Soils in Southern Shaanxi, China

Expansive soils pose a severe geotechnical hazard to infrastructure construction in the southern Shaanxi region of China. However, the combined effects of initial water content and dry density on the swelling characteristics of this regional soil, and particularly the temporal evolution of swelling, remain insufficiently quantified. The primary objective of this study is to quantify the effects of initial water content and dry density on swelling characteristics, and to characterize the temporal evolution of swelling in expansive soils from southern Shaanxi. In this work, a series of laboratory swelling tests were carried out using a WZ-2 swelling apparatus and a WG type triplex low-to-medium-pressure consolidometer to systematically investigate the swelling characteristics of expansive soils under initial water contents of 15–24% for the free swelling ratio tests and 14–22% for the swelling pressure tests, and dry densities of 1.50–1.65 g/cm3. The temporal evolution of the free swelling ratio and the variation characteristics of swelling pressure were analyzed, and the coupled influencing mechanisms of dry density and initial water content on the swelling properties were discussed. The results indicate that the free swelling ratio exhibits a distinct three-stage temporal evolution: rapid increase (0–10 min), decelerating increase (10–40 min), and onset of stabilization (after 40 min), with stable state reached after 100 min, with the rapid swelling phase contributing 65–75% of the total swelling. At a constant dry density, the free swelling ratio decreases with increasing initial water content, while the swelling pressure also decreases monotonically with increasing initial water content. At a constant water content, the free swelling ratio increases with increasing dry density, whereas the swelling pressure increases linearly (R2 ≥ 0.87). The sensitivity analysis indicates that, within the investigated ranges, swelling pressure shows greater relative sensitivity to dry density than to initial water content, with sensitivity coefficients of 5.47 and 1.48, respectively. These findings demonstrate that dry density governs the swelling behavior of the tested expansive soil, while initial water content modulates the response within the investigated range.