大间距桩筏基础地震响应离心模型试验研究

Centrifuge tests on seismic response of piled raft foundation with large spacing

针对饱和软土地基中大间距摩擦型桩筏基础抗震问题,采用离心振动台开展了桩筏基础地震响应的试验研究。为模拟实际软土场地浅部超固结黏土和深部正常固结黏土的地层特征,试验在高岭土土样表面预铺砂层,然后在50g离心加速度下进行固结。上部结构简化为质点和杆构件,基础形式包括桩头刚接和桩头自由2种类型的大间距桩筏基础模型。试验分析了模型的加速度和位移、土层内部孔隙水压力以及桩身应变等响应。试验结果表明:在软土场地自振频率范围内,结构–基础–地基三者相互作用十分明显,基础与结构加速度放大系数高于其它频率范围;桩头刚接与桩头自由的基础在地震时均产生了较周围地表土体更大的竖向沉降,但震后较长时间内基础与地表沉降速率基本一致;地震结束时,桩头刚接的上部结构侧向位移与基础倾斜值均较桩头自由时减少一半以上,但上部结构加速度放大作用更加明显;桩筏基础承载能力因土体软化行为有所降低,震后部分上部荷载由群桩转移到筏板,但桩筏荷载分担比例总体变化不大。试验揭示了软土震陷时桩基的变形控制机理,为软土地基减沉桩基础的抗震设计提供参考。

Centrifuge tests are performed to study the seismic response of piled raft foundation with large spacing in saturated soft clay. By laying sand on clay surface before centrifuge consolidation under 50g, the soil layer with upper over-consolidated clay and lower normally-consolidated clay is simulated. Structural models are simplified to mass points and members. Two foundation types of connected and non-connected piled rafts are considered in tests. The responses of earthquake acceleration, displacements, pore water pressures and pile strains are investigated. The test results show that in the natural vibration frequency range of soft clay ground, the interaction of superstructure-foundation-soil is very remarkable, and the acceleration amplification factors of structure and foundation are higher than those in other frequency ranges. The earthquake induced instant settlements of the connected and non-connected piled rafts are larger than those of the surrounding ground, but a long time after the earthquake the settlement velocities of foundation and ground are almost the same. At the end of the earthquake, more than half of lateral displacement of superstructure and foundation inclination of the connected piled raft are reduced compared with those of the non-connected piled raft, but with higher acceleration amplification effects of superstructure. After the earthquake some loads transfer from pile group to raft on account of soil degradation and loss of bearing capacity of piled raft. However, there are generally few changes of load sharing ratio between piles and raft. The research has revealed the deformation control mechanisms of pile foundation under seismic subsidence in soft clay, and it may provide evidence for the design of settlement-reducing piles.