Abstract: 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.

Key words: composite pile foundation, loading and unloading, lateral constraint pile, horizontal deformation rule