An Experimental Work into the Effect of Soil Saturation on the Bearing Capacity of Under-reamed Piles

Abstract

This study examines the behavior and load-bearing capacity of under-reamed piles embedded in cohesive soil at different degrees of saturation (100%, 80%, 60%, and 40%). The major objective is to examine the impact of matric suction (MS), cohesion (Cu), and saturation ratio (Sr) on the axial response of both conventional and under-reamed foundations under unsaturated conditions. Three different pile configurations were tested in laboratory models: a single under-reamed pile (SURP), a double under-reamed pile (DURP), and a straight pile without under-reaming (NP). The ultimate bearing capacity and settlement behavior were evaluated using load-displacement curves and axial compression tests conducted inside a steel soil chamber. The results show that decreasing the degree of saturation significantly increases matric suction, thereby improving the bearing capacity of all pile configurations. For the straight pile (L/D = 22.5), the load capacity increased by 259%, 348%, and 48% at saturation rates of 80%, 60%, and 40%, respectively. The increases observed for the single under-reamed pile were 260%, 346%, and 64%, respectively. Under the same conditions, the double under-reamed pile showed increases of 210.5%, 238%, and 24%. When comparing the different types of piles, it was found that the single under-reamed pile achieved capacity improvements of 42-57% compared to the straight pile. On the other hand, the double under-reamed pile provided even more significant improvements, which ranged from 58% to 109% depending on the degree of saturation. The study highlights the strong interaction between saturation, suction, and pile geometry, offering useful insights for foundation design in partially saturated cohesive soils.