低周反复荷载下沉箱-垫层-桩复合基础承载性能试验

沉箱垫层桩复合基础具有良好的承载性能以及隔震性能，并且施工简单,工程造价相对低廉，是一种适用于强震区域软弱土体地基的新型桥梁深水基础形式。为了研究沉箱-垫层-桩复合基础的水平承载性能及抗震性能，设计了复合基础试验模型，进行了4组复合基础的低周反复荷载试验研究，分析了复合基础的破坏形态，选取垫层厚度、竖向荷载、地基形式为变量，研究了复合基础整体的滞回性能、骨架曲线、刚度退化、强度退化、耗能能力等，并通过在沉箱顶部设置位移计、桩身贴设应变片等测试方法，单独分析了试验过程中沉箱的沉降状态及下部桩体的受力状态。试验结果表明:复合地基的使用可有效提高基础的抗震性能；在相同的循环位移下，复合基础的水平承载力、强度、刚度随着垫层厚度的增加及竖向荷载的减小而降低；采用复合地基的基础单圈耗能能力更强，累积耗能更高；垫层厚度的增加对沉箱沉降及倾角控制影响并不明显；竖向力的减小会导致沉箱出现的倾角更大；复合地基会使沉箱的沉降更为均匀，沉降速率更慢，但对于倾覆控制影响不大；复合地基中的群桩以前排桩承担的水平荷载最多，中排桩次之，后排桩最少，中桩承担的水平荷载略高于边桩；桩身最大弯矩位置在40%~50%桩长处；垫层的存在有效地减小了桩体分担的水平荷载。

Experimental Study on Bearing Performance of Caisson-cushion- piles Composite Foundation Under Low Cyclic Loading

As a new type of deep-water foundation suitable for soft ground in seismic areas, the caisson-cushion-piles composite foundation has numerous advantages, such as high bearing capacity, excellent seismic resistance, simple construction, and low engineering cost. To investigate the bearing capacity and seismic behavior of the caisson-cushion-piles composite foundation, a model of the composite foundation was designed, and low-cyclic-loading tests were carried out on four composite foundations with different cushion thicknesses, vertical loadings, and foundation bases. The failure mode, hysteretic loops, energy dissipation, rigidity degeneration, strength degeneration, settlement and inclination of the caisson, and bending moment of the piles of the composite foundation were analyzed. The test results indicate that the composite foundation effectively improved the seismic performance of the specimen. Under the same cyclic displacement, the horizontal bearing capacity, rigidity, and strength of the foundation decrease with an increase in cushion thickness and a decrease in vertical loading. More energy is dissipated by the use of a pile composite foundation. The increase in cushion thickness has no notable effect on the caisson settlement and inclination. The inclination of the caisson increases with a decrease in vertical loading. The settlement of the caisson is more uniform and the rate is slower with the application of the composite foundation, but the inclination is not markedly affected. The middle piles endure more lateral loads compared with the side piles in the composite foundation. The front-row piles in the composite foundation endure the most lateral load, followed by the middle-row piles, and it is the least in the back-row piles. The maximum pile-bending moment occurs mainly at depths from 0.4 to 0.5 times the pile length from the pile head. The cushion significantly reduces the bending moment of the piles under cyclic loading.