Based on the upper plastic limit analysis theory

the kinematically admissible velocity field of a capped rigid pile is obtained through both numerical and model tests. According to the energy balance principle and unified strength theory

an equation is developed for the ultimate bearing capacity limit solution of the pile

in which the effect of the intermediate principal stress is considered. In addition

the variations of the pile ultimate bearing capacity with the cap size

soil cohesion and internal friction angle are analyzed through parametric analysis. It is found that the upper limit solution increases more significantly for the pile with considering the intermediate principal stress than those without. A reasonable range of the unified strength parameter is from 0.1 to 0.2 in calculating the rigid piles bearing capacity. With the increase of the cap size

the pile bearing capacity increases. When the ratio of cap diameter D to pile diameter d is greater than 2

the bearing capacity increases more significantly. With the increase of soil cohesion and internal friction angle

the bearing capacity increases drastically. By comparison with experimental results

it is found that the proposed method yields better results than the existing theoretical methods.