Model tests on bearing characteristics of pile foundation in expansive soil foundation under vertical loadings

Based on the self-developed multi-directional load system for piles a series of model tests are carried out to study the behaviors of compressive and uplift loaded piles in expansive soils with two states of optimal water content and full saturation. The test results show that the change of water content of expansive soils has more significant influences on the behaviors of compression piles than those of tension piles, but more considerable on the deformation of tension piles. The ultimate bearing capacity of compression piles are always greater than that of tension piles. With the increase of water content, the ultimate skin frictions of compression piles and tension piles both decrease, but the ultimate skin friction under uplift load is obviously smaller than that under downward compressive load. Under different water contents, the ultimate skin friction of compressive piles is parabolically distributed, while that of uplift piles is linearly distributed. This is due to the fact that the displacement of the top of the tension piles is larger than that of the compression piles when the pressure piles and the uplift piles reach the ultimate bearing capacity, which makes the deep side friction of the tension piles play more fully. With the increase of water content, the ultimate bearing capacities of compression piles and tension piles decrease significantly, and the decrease of compression piles are even greater, but the ultimate bearing capacity of compression piles are always larger than that of tension piles.