Abstract: The connection between cutoff walls and geocomposite membranes is a critical impervious system widely used in cofferdam engineering, where the deformation compatibility at the joint directly governs the reliability of the impervious system. Focusing on a specific cofferdam project, this study designed two connection types: a conventional folded expansion joint and a U-shaped expansion joint internally filled with deformable foam. Four sets of centrifugal model tests are conducted to simulate the strain characteristics of the geocomposite membrane due to relative displacement between the cutoff wall and the membrane. The test results reveal: The strain in the geocomposite membrane decreases with increasing distance from the cutoff wall. For the folded expansion joint, anchored flush with the cutoff wall, the end restraint ("clamping effect") induces significant localized strain concentration in the membrane. In contrast, the U-shaped expansion joint can effectively mitigate localized strain concentration. Furthermore, the deformable foam filling at the transition zone between stiff and soft materials within the U-type joint helps reduce differential settlement between the cutoff wall and the cofferdam fill, thereby improving the localized stress state of the geocomposite membrane.