大规模超长群桩基础工作性能的数值模拟

超长桩基础在土木工程中的应用越来越多，但对其工作性状的研究相对滞后。以某超长群桩基础工程为例，用三维非线性有限元方法分析了超长单桩及群桩的工作性能。计算中桩、土和承台的有限元模型均用8节点六面体等参单元，桩-土界面用有厚度的接触面单元模拟；混凝土的应力-应变关系用线弹性模型，土体用非线性Duncan-Chang弹性模型模拟；承台施工过程用分级加荷的方法模拟，承台混凝土的硬化过程用变化模量的方法模拟。计算结果表明，超长单桩的承载性状属摩擦桩，其桩身失稳的原因是桩侧土体的破坏；在群桩基础中，不同位置基桩的工作性能不完全相同，承台周边桩的工作性能与单桩类似，而与内部桩的差别较大；现行规范中推荐的群桩效应系数和有限元计算结果间存在较大差别，表明现行规范可能不适用于超长群桩基础，对超长群桩基础的工作性能进行深入研究是必要的。

Numerical simulation on behavior of large-scale overlength pile group foundation

Overlength pile foundation is successfully used in more and more civil engineering, but the study on its behaviour is still very little. Based on an example of a large-scale overlength pile group foundation engineering, the behaviours of overlength single pile and pile group foundation are investigated using nonlinear three-dimensional finite element method (3D FEM). A hexahedron isoparametric element with 8 nodes is employed in the finite element model of pile, foundation slab and soil. A special thin element with a very little thickness is used to simulate possible nonlinear deformations between soil and pile. Linear elastic-model and nonlinear Duncan-Chang elastic model are used to simulate the constitutive relation of concrete of pile and foundation slab, and soil mass respectively. The course of building foundation slab is simulated by adding elements in calculating, and that of hardening concrete of foundation slab is simulated using the method of increasing its modulus. Calculating results indicate that the behaviour of overlength single pile is frictional, and its failure is the result from the failure of soil around pile firstly. In group pile foundation, many clear differences in behaviours of piles can be found among the piles located in different positions under the foundation slab, especially between the piles near the edge of foundation and ones in the centre of the foundation. The behaviour of side pile near the edge of foundation slab is almost same with that of single pile, but apparent difference from that of centre pile. Reduction factor of pile group can be obtained from both current technical code for building pile foundation and calculating results of 3D FEM. The large difference in the reduction factors of pile group from the two methods indicates that it is necessary to investigate further the behaviour of overlength pile group foundation, because the current technical code isn’t enough for the designation and construction of overlength pile group foundation, especially for larger-scale overlength pile group foundation.