Roof deformation of the Beishan Rock Carvings with negative Poisson's ratio anchor support under varied precipitation conditions

Beishan Rock Carvings in Chongqing, a renowned cultural heritage site in China, flourished during the Tang and Song dynasties and are often referred to as the "Stone Carving Art Museum of the Tang and Song Dynasties." Cave 168 is a key component of the Beishan Rock Carvings. At present, several through-going cracks have developed in the roof of Cave 168, severely compromising the structural stability of the grotto. The early internal steel plate supports have suffered severe corrosion and can no longer provide effective reinforcement. In addition, the presence of steel columns obstructs visitor access and negatively affects the viewing experience. A new reinforcement method is urgently needed. Therefore, studying the deformation patterns of the structure is of critical importance. This study analyzes the stratigraphic parameters and fracture distribution of Cave 168, considering key influencing factors such as rainfall, self-weight, and the overlying Quaternary soil. On-site monitoring and physical model experiments were conducted to evaluate the changes in roof crack width and displacement before and after reinforcement with negative Poisson's ratio (NPR) anchor cables. The results reveal that the roof of Cave 168 contains several through-going cracks and numerous microcracks, which serve as infiltration channels for surface water. These accelerate the softening of the mudstone and pose a significant threat to the cave's structural safety. During the experiment, the main change in the crack exhibited a "semi-arch-shaped" propagation pattern. In the first ten minutes, as the rock transitioned from dry to moist conditions, a slight crack closure was observed. As rainfall continued, crack propagation accelerated. After rainfall ceased, crack width remained stable over a short period. Under NPR anchor support, the influence of rainfall on roof settlement was effectively mitigated, ensuring the safety and stability of the roof. The NPR anchors successfully limited the roof settlement to within 0.3 mm and provided effective control over both total and differential settlement. These findings offer valuable insights into the application of NPR anchor cables in the conservation of grotto heritage structures.