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.