桩筏连接形式对劲芯复合桩地震响应影响试验研究

为揭示桩筏连接形式对可液化地基中劲芯复合桩地震响应的影响规律，采用1g振动台开展了连接式和非连接式复合桩桩筏基础的模型试验研究，对比分析了2组试验工况中模型体系的固有频率和阻尼比、试验场地宏观现象、土体的超孔压和加速度、上部结构的加速度和位移、桩身弯矩等动力响应。2种工况中均输入不同地震动强度的El Centro地震波。试验结果表明：非连接式桩筏（DPR）工况中模型体系的固有频率小于连接式桩筏（CPR）工况的固有频率，而DPR工况的阻尼比大于CPR工况；采用DPR基础可减轻地基土体的液化程度，其超孔压比峰值比CPR工况最大可减少19.3%；在不同地震动强度下，中粗砂垫层的隔震效应使得DPR工况中地基土体和上部结构的加速度反应均低于CPR工况，0.4g El Centro波输入时，DPR工况中上部结构加速度放大系数比CPR工况减少13.5%；DPR基础的整体性和刚度相对较差，导致DPR工况上部结构侧向位移和筏板沉降均较CPR工况增加50%以上；地基液化导致桩身弯矩急剧增大，CPR工况中复合桩的最大弯矩出现在桩头，而DPR工况中桩身弯矩峰值出现在距桩头1/3~1/2桩长处，且DPR工况中桩基弯矩峰值较CPR工况减少近50%。因此，桩筏连接形式差异对劲芯复合桩地震响应影响显著，研究成果可为液化土层中劲芯复合桩桩筏基础的抗震设计与研究提供参考。

Experimental Study on Effect of Pile-raft Connection Type on Seismic Response of Piles Improved with Cement-treated Soil

To reveal the effect of pile-raft connection types on the seismic response of piles that are improved with cement-treated soil in liquefiable soils, 1g shaking table tests of connected and disconnected piled raft foundations were performed. In the two scenarios, the natural frequency and damping ratio of the model system, the macro-phenomenon of the tests, the excess pore pressure and acceleration response of the soils, the acceleration and displacement response of the superstructure, and the moment response of the piles were evaluated and compared. Both models were subjected to the El Centro earthquake wave with different peak accelerations. Results show that the natural frequency of the model system in the disconnected piled raft (DPR) case is lower than that in the connected piled raft (CPR) case, whereas the damping ratio is larger in the DPR case. The use of the DPR foundation can mitigate the degree of soil liquefaction, and the maximum reduction in the excess pore pressure ratio can reach 19.3% compared to the CPR case. Due to the seismic-isolation effect of the cushion, the acceleration responses of the soils and superstructure in the DPR case are clearly lower than in the CPR case. Under the 0.4g El Centro wave conditions, the acceleration amplification of the superstructure in the DPR case is 13.5% less than in the CPR case. The integrity and rigidity of the DPR foundation are relatively poor, which results in an increase of more than 50% in the lateral displacement of the superstructure and the settlement of raft compared with those of the CPR foundation. Soil liquefaction leads to a sharp increase in the bending moment of pile. In the CPR case, the peak bending moment generally occurs at the top of the pile improved with cement-treated soil. However, the peak is located near a depth of a 1/3 to a 1/2 pile length beneath the pile head in the DPR case. Moreover, the peak pile moment in the DPR case is approximately 50% lower than that in the CPR case. Therefore, the difference in the pile-raft connection type has a significant influence on the seismic response of piles improved with cement-treated soil. The results of the study may provide evidence for the seismic design and research of pile-raft foundations improved with cement-treated soil in liquefiable soil.