Study On The Bearing Characteristics Of Jacked Piles Under Vertical Cyclic Loading

Jacked piles have been widely used in China’s construction with high construction speed, low cost, no noise and pollution. In some engineerings such as wind power, high-speed traffic and oil platforms, the pile foundations are not only under static loads, but also the vertical cyclic loads caused by trains, winds, and waves. Bearing capacity and settlement of piles under cyclic loads has become an important control parameter for engineering design. However, the current pile bearing behavior under cyclic loads is still not clear, and therefore theoretical and experimental research conducted in this project has significant meanings.The bearing characteristics of pile under vertical cyclic loads are researched by physical model tests, field full-scale tests and numerical simulation methods, the results are as follows:The results of field test indicate that the penetration resistance is mainly provided by the main pile end and the shaft resistance is relative small; the bearing capacity of pile decreases under cyclic loading with the increasing settlement. And the rapid changes occurs in the early period and gradually becomes slower in the later, and trend to keep stable after about 15 cycles.(2) pile-soil interface cyclic shear physical model tests were conducted by cyclic shear test apparatus. It was found that the pile-soil interface shear stress decrease obviously with increase of cyclic shear number of index trrnd, and remain stable after about 20 shears, and the decreasing amplitude becomes greater with the higher vertical stress.(3) It is found in numerical simulation analysis that plastic deformation will occur in the soil and pile under cyclic loads under cyclic loads. During uninstall process, the elastic deformation of the soil and the pile will recover, plastic deformation is not restored; under cyclic loads deformation of soil around the pile gradually reduce with the increase of soil depth; vertical displacement decreased with the increase of distance away from the pile in the horizontal direction.(4) pile top displacement and maximum displacement in pile bottom reach a stable value under cyclic loads; the relationship between displacement of pile bottom and pile top growth linearly finally, indicating that the pile reaches maximum compression; significant residual stress occurs in the pile after unloading and residual stress are mainly distributed in the lower part of the pile, particularly in the largest base; some soil will be squeezd out under cyclic loads with annular distribution along the pile diameter direction.(5) The vertical displacement of the pile is no longer with the increase of the number of cycles under 1.5 times the bearing capacity of the standard value, and this change reaches a steady state in the first loading, and unloading process; the pile vertical stress increases with the increase of depth, and the maximum residual stress occur in the bottom of the pile.