复合桩基上堆载可能引起侧向约束桩侧移，导致侧向约束桩和复合桩基失稳破坏。采用室内模型试验研究复合桩基重复加、卸载过程中侧向约束桩水平变位规律。结果表明：（1）复合桩基上加载时，侧向约束桩的桩身侧移沿深度先增大、后减小、存在峰值，峰值随加载增大而增大，出现在距离地面0.4倍地面以下桩长处，峰值位移与桩顶（或地面）处位移的比例系数和发生侧移的桩身长度随荷载增大而增大；（2）复合桩基上重复加、卸载时，侧向约束桩的桩顶侧移随桩间距增大而增大，弹性变形则变小。中桩侧移比边桩大。桩顶侧移增长率随重复加载次数增加而减小。若荷载超过首次加载的最大荷载，侧向复合地基上加载-桩水平位移曲线将回到首次加载曲线的延长线，具有记忆效应。第 次卸载曲线与第i次卸载曲线线型相似且相互平行。每次卸载的初、中期，荷载的减小不影响桩顶侧移，只有卸载到最后1～2级时，侧移才开始减小，直到永久塑性变形。侧向约束桩的顶部水平位移回弹曲线特征与岩石或土在垂直加、卸载作用下的回弹曲线相似。

When the composite pile foundation is loaded, the laterally constraint pile will displace horizontally, and composite foundation with lateral constraint and vertical strengthen pile will become instable and failed. The horizontal deformation of the laterally constraint pile during the reloading and repetitive unloading of the composite pile foundation was studied by experiment. The results show that: (1) During loading, the horizontal deformation of lateral constraint pile increases first and then decreases, showing a peak along the depth. The peak value increases with the increase of loading. Position of the peak value is at the depth of 0.4 times pile length from the ground. With the increase of loading, the proportional coefficient between the peak displacement and the displacement of pile top or ground increases, the length of pile moving laterally also increases. (2) During reloading and repetitive unloading, with the increase of the pile spacing, the horizontal displacement of lateral constraint pile top increases, the elastic deformation of pile decreases. The lateral displacement of middle pile is larger than that of side pile. With the increase of reloading times, the increase rate of lateral displacement for pile top decreases. When the loading becomes larger than the maximum of the first loading, the loading and lateral displacement curve will merge into extension line of the first curve, showing memory effect. The (i+1)-th and the i-th curves between unloading and lateral displacement are parallel to each other. In the beginning and the medium term of unloading, the lateral displacement of pile remains constant, and begins to diminish only in the last 1-2 stages, and the permanent plastic deformation occurs finally. The horizontal rebound curve of pile top has a shape similar to the vertical rebound curve for rock or soil subjected to vertical loading and unloading.