Mechanical Behavior of Pile Foundation Constructed in Composite Ground and its Evaluation

This paper describes a foundation design method in which the ground is improved around the heads of pile foundations in soft ground or loose sandy ground and its practical effectiveness. The shear strength increased due to ground improvement is reflected in the horizontal resistance of piles. In this design method, the influence range of the horizontal resistance of piles and the necessary range of ground improvement are determined by taking account of three-dimensional domain formed with the gradient of the surface of passive failure. The horizontal subgrade reaction of piles is evaluated by converting the shear strength of improved ground to the modulus of deformation. In this study, the validity of design method for the pile foundation with ground improvement was confirmed through an in-situ horizontal loading test. The dynamic behavior of pile foundation constructed in improved ground was also investigated through a series of centrifuge model tests and numerical analyses. The influence of the difference in strength between the original and improved grounds on piles during an earthquake was also confirmed based on the numerical analyses. The cost performance of the proposed method was discussed by comparing with the case without ground improvement.