<p>The cut-off wall and core wall are critical components for seepage control in earth-rock dams. The interfacial normal interaction behavior between these elements can significantly influence the overall stability of such dams. However, current research on the normal interfacial contact mechanical behavior between cut-off walls and core walls is limited, leaving the contact mechanism during cut-off wall penetration into core walls poorly understood. To address this knowledge gap, this study proposed a novel finite element simulation approach of cut-off wall penetration into core walls considering the nonlinear normal contact model. The contact nodal force and contact stiffness matrix are determined with the nonlinear normal contact model and incorporated into the load vector and global stiffness matrix of finite element, respectively. Solution of displacement and stress is realized with a global equilibrium iteration under the combined effects of contact forces, external loads, and boundary constraints. The proposed approach was subsequently implemented into the finite element analysis to simulate the penetration process of the cut-off wall into the clay core wall. Simultaneously, physical model tests, systematically investigating the relationship between cut-off wall-end stress and penetration displacement using a custom-designed apparatus on clay specimens, were conducted to verify the effectiveness of proposed simulation approach. Experimental results demonstrate a significant nonlinear relationship between the wall-end stress and penetration displacement during the penetration process. The numerical simulations yielded results consistent with those obtained from the physical model tests. This research elucidates the mechanical behavior of cut-off wall penetration into clay core walls, provides a theoretical foundation for analyzing interfacial interactions, and offers valuable insights for the stability assessment of cut-off walls in practical engineering applications.</p>