砂土中群桩的压拔加载试验研究

进行砂土的单桩和2×2群桩的压、拔试验，测定桩身轴力、桩顶荷载、位移以及桩端阻力。试验研究结果表明，群桩的抗压群桩效应系数 >1，最优桩间距为4D(D为直径)，对应的 约为1.2；桩端阻力群桩效应系数 大于桩侧摩阻力群桩效应系数 ，其最优桩间距为5D，对应的 值约为1.3；桩侧摩阻力群桩效应系数在3D时约为1.2，但随桩间距的增大而减小。与前人理论分析的结果不同，由于砂土的挤密效应，试验测得抗拔群桩效应系数也大于1，最优桩间距在4D～5D之间，系数约为1.2。桩间距为7D时压、拔群桩效应系数均趋近于1，可基本忽略群桩效应。

Experimental study of pile group in sand under compressive and tensile loadings

Model tests have been conducted on a single pile and pile group in sand under both compressive and tensile loadings. Axial forces, loads and displacements at the pile head, and pile tip resistance were measured. Experimental results show that the compressive group pile effect coefficient ? is greater than 1. The optimal pile spacing is 4D(D is pile diameter), at which η is about 1.2. The pile tip resistance coefficient is greater than the lateral friction coefficient . The pile tip resistance coefficient reaches maximum at the spacing of 5D with a value of about 1.3. The lateral friction coefficient is about 1.2 at pile spacing of 3D, but it decreases with the increasing of pile spacing. Different from the results revealed by previous theoretical analysis, the test results show that the pullout group effect coefficient is greater than 1, due to sand densification during pile driving. The optimal pile spacing is between 4D-5D, with a maximum coefficient of about 1.2. The group coefficient is close to 1 when the pile spacing is greater than 7D and the group pile effect can be ignored.