非连接式桩筏基础水平特性试验

为了给非连接式桩筏的设计与施工提供参考,针对大型桥梁工程采用的减隔震基础形式--非连接式桩筏,考虑竖向荷载、垫层厚度等因素的影响,进行了非连接式桩筏水平承载性能的室内模型试验研究。采用粒子图像测速法跟踪了土体水平位移路径,利用数字图像关联技术获得了土体位移场与剪切带,分析了竖向荷载、垫层厚度等因素对筏板水平位移、桩身弯矩和剪力分布的影响规律,提出了非连接式桩筏基础水平承载力的简化模型及计算方法,并用试验数据进行了验证。研究结果表明：竖向荷载可有效限制筏板水平位移,设置垫层可显著减小桩身弯矩及剪力,并且桩身弯矩和剪力随垫层厚度的增加而减小;土体在垫层顶部形成位移集中区,产生局部塑性变形并不断扩大,最后局部塑性变形区贯通形成完整的剪切带;非连接式桩筏基础水平承载力与竖向荷载相关,当竖向荷载较小时,其水平承载力由筏板与垫层界面摩擦力控制,当竖向荷载较大时,其水平承载力取决于垫层的水平承载力,这与图像分析获得的垫层破坏结论一致。因此,对于非连接式桩筏基础的设计及具体工程应用,可以通过调整垫层的厚度来减小桩身弯矩及剪力;所揭示的非连接式桩筏基础水平承载力变化规律可为今后大型桥梁隔震基础的设计与研究提供参考。

Test on the Horizontal Characteristics of Disconnected Piled Raft Foundation

To provide reference for the design and construction of disconnected piled rafts (DPRs), specifically aiming at the seismic isolation foundation of DPRs, which has been widely adopted in large scale bridge construction, by considering the influences of vertical load and cushion thickness, the horizontal bearing characteristics of DPRs were studied by conducting a laboratory test. Particle image velocimetry was used to track the horizontal soil displacement path, and the displacement field and shear zone were obtained using the digital image correlation (DIC) technique. The influences of vertical load and cushion thickness on the horizontal displacement of the raft, pile bending moment, and shear force were analyzed. A simplified model and calculation method for obtaining the lateral bearing capacity of DPRs were proposed and verified with the test data. Results demonstrate that the vertical load on raft could limit the horizontal displacement of the rafts, whereas the pile bending moment and the shear force reduced with a reduction in the thickness of the cushion, which was positioned between the raft and the pile head. The soil concentration zone formed at the top of the cushion and the local plastic deformation zone then appeared and expanded continuously. Toward the end, the local plastic deformation zones penetrated to form an overall shear zone. The lateral bearing capacity of DPRs was dependent on the friction between the raft and the cushion or the horizontal bearing capacity of the cushion, which was related to the vertical load on the raft. This conclusion corresponds to the cushion failure mode obtained by the DIC technique. Therefore, the bending moment and the shear force can be reduced by adjusting the cushion thickness during the design and engineering applications of the DPRs. The revelation of the failure mode of the cushion under a horizontal load can be used as the reference for the design and research of vibration isolation foundation in large-scale bridge.