Calculation Method For Effective Pile Length Of Super Long Pile Group With Consideration Of Pile Group Effect

With the economic development and the continuous progress of foundation construction technology, structures like high-rise buildings, super high-rise buildings and large bridges are developing rapidly. Due to the fact that the tolerated settlement for such large-scale construction is rigorous, the super-long pile group foundation is extensively used for its high bearing capacity at small settlement. However, in order to increase the structural reliability, the designers often increase the length of pile group foundation blindly which causes waste and construction difficulty inevitably. The effective length of the super-long pile has been identified in many experimental works and it becomes a key issue for super-long pile design. Supported by National Natural Science Foundation "the test and numerical simulation of effective length in the bridge super-long pile group foundation (51278184)" and "The research of design and construction technology of Super-long pile groups in the deep soft soil area(No.CXKJSF0108-2)" of the demonstration project of the Ministry of Transport, theoretical study and numerical simulation were performed to study the influence of the effective length. A calculation method the effective length for super-long pile group foundation with account for pile group effect was proposed in this paper.At first, the definitions and research situation about effective length of pile group foundation under vertical load are discussed. On this basis, the shear displacement method was used to consider the lateral stress transfer of soil around piles, and the load transfer method was employed to analyze the vertical load transfer of pile. Meanwhile, the blind-effect between piles was also considered. Then the influence of pile-pile interaction and pile-soil interaction of pile groups was incorporated into the basic differential equation of vertical load transfer. A calculation method of effective length for super-long pile group foundation was put forward and it was validated using well-documented case histories. It shows that vertical load and pile-soil stiffness coefficient have a significant influence on the effective length of pile group while pile spacing has a relatively minor influence. In addition, it can be found that the influence of Poisson ratio can be negligible.At last, Finite element analysis software ADINA was applied to establish the three dimensional calculation models of pile group. On this basis, the influencing factors of the effective length of super-long pile group were studied, including ratio of length and diameter for the pile, pile spacing, soil frictional angle and cohesion. The result shows that the effective length of pile group is sensitive to vertical load, pile-soil stiffness coefficient, soil frictional angle and cohesion. However, The pile radius and the pile spacing have a slight influence on the effective length and the influence of Poisson ratio can be negligible. Thus these conclusions can provide a certain theoretical reference for practical engineering.