A case study on the bearing characteristics of a bottom uplift pile in a layered foundation

The bottom uplift pile, which has been applied in practical projects, has the following advantages: the pile body is not easy to crack, good bearing characteristics, and small displacement of the pile top. Based on the bearing capacity test of foundation piles in the third stage expansion project of Lanzhou Zhongchuan International Airport, the upper part pile of the self-balancing test method was used to simulate the bottom uplift pile, and the anchor piles in the anchor pile method were regarded as normal uplift piles. The bearing characteristics of the bottom uplift pile in a layered foundation were studied by comparing these two kinds of piles. The results show that under the same displacement of the pile top, the ultimate uplift bearing capacity of the bottom uplift pile can be more than twice that of the normal uplift pile because of the fully exerted frictional resistance of the soil at the bottom of the pile, the Poisson effect of the pile body and the avoidance of the influence of pile body deformation on the pile top displacement. The maximum axial force of the bottom uplift pile appears at the bottom of the pile and gradually decreases from the bottom to the top, which is opposite to that of the normal uplift pile. The properties and thickness of the soil layers around the pile have a great influence on the distribution curves of the frictional resistance along the pile length of the two kinds of uplift piles. With changing soil layer conditions, the distribution curve may be a "parabola", a "straight line" or a "double line". The soil property plays a decisive role in the frictional resistance, which may cause softening. The influence of the pile diameter on the ultimate uplift bearing capacity is greater than that of the pile length, while the elastic modulus of the pile has little influence.