An Investigation into the Crack-Arresting Effect and Mechanism of a Rigid-Flex Coupled System at the Rammed-Earth Infill-Host Interface of Basal Undercut Zones in Rammed Earthen Sites

Differential cracking at heterogeneous soil interfaces caused by environmental fluctuations poses a major challenge in the conservation of earthen architectural heritage. To address this, a novel rigid-flex coupled tie-bar system is proposed. This study investigates its crack-arresting effectiveness, underlying mechanism, and synergistic evolution in mitigating interfacial cracking between the repair infill and host structure under varying temperature and humidity. Using an undercut section of a rammed-earth heritage wall as a case study, the system was implemented on-site to bridge the rammed-earth infill and host structure. Axial strains in the rigid and flexible tie-bar sections and the evolution of interfacial cracks were continuously monitored under environmental variations. Results show that the rigid — flex coupled tie-bar system effectively suppresses interfacial cracking, improving crack-arresting efficiency by approximately 36%. Its core mechanism “flexible introduction — rigid dispersion” enables the flexible section to serve as the primary load transfer and deformation coordination zone, while the rigid section acts as a stable anchorage and stress dispersion zone. The two sections operate synergistically, further enhanced by crack deflection and dispersion, the dowel effect, and the confinement effect. The tie bars exhibit distinct creep behaviors and continuous strain accumulation under different environmental conditions, with their stress evolution highly synchronized with the progression of interfacial damage. Based on these findings, a force-transfer model for the anchored interface under the synergistic evolution of the rigid-flex tie-bar structure is established. This study provides new theoretical insights and technical guidance for the protection and reinforcement of earthen architectural sites under environmental change.