###### Abstract  
The Yuanjue Grottos are representative of the sandstone grottos in southwest China. The structural stability of their wallrocks has suffered a certain degree of damage, resulting from weathering cavities, which seriously threatens the long-term preservation of these cultural relics. This study utilizes 3D numerical simulation to assess the impact of weathering cavity depth on the structural stability of the grottos. The results reveal that the presence of cavities has led to significant wallrock deformation and settlement, particularly in the front side of the entrance and baseplate area. As the cavity depth increases, the deformation effect intensifies. This study also characterizes the relationship between stress deformation and the distribution of weathered fissures, indicating that joint areas are more susceptible to spalling. Displacement monitoring indicates that the baseplate has experienced uplift, followed by settlement as the cavity depth increases, suggesting an initial phase of elastic deformation followed by plastic and brittle deformation stages, which is consistent with the numerical simulation results. Notably, the baseplate's settlement trend was found to accelerate when the cavity depth exceeded 4 m, with significant changes being observed when the depth exceeded 5 m. Therefore, the depth of the weathering cavity beneath the entrance of the Yuanjue Grottos should be controlled within 5 m, in order to ensure their long-term stability. This research contributes to the understanding of the mechanisms affecting the structural stability of grottos and provides valuable insights for conservation efforts. Y 2024 2024 329  
After 850 years, due to the long-term effects of various natural forces, a large number of fissures have formed due to weathering and denudation [3]. A great deal of cavities have formed, along with weathering, as the fissures have gradually expanded [4; 5]. The maximum width of the weathering cavities beneath the baseplate of the grottos reaches 3.5 m, and the maximum depth has reached 10 m, resulting in gradual changes in the stability of the surrounding rock and roof of the Yuanjue Grottos. With the expansion of the weathering cavities beneath the Yuanjue Grottos, the side of the entrance faces the risk of hanging and instability [6]. In order to prevent the instability and damage caused by the weathering cavities beneath the grottos, ancient people used strip stone support for these cavities. However, some of the supporting stones have been fractured, and others now only play a filling role [7]. Due to fissures having formed in the left rear of the roof, the roof of the Yuanjue Grottos is, at present, close to the ultimate stable state [8]. Therefore, it is urgent to analyze the stability of the grottos, as well as study the development trend of the deformation, in order to preserve the cultural relics of the grottos for a long period of time. At present, commonly used methods for stability analyses of grottos include the engineering analogy method [9; 10], the block balance theory method [11], the analytical method [12], the numerical simulation method [13; 14; 15; 16; 17], the physical model test method [18; 19], and the nonlinear analysis method [20; 21]. The engineering analogy method relies heavily on experiential judgment, and it can overlook critical factors due to its subjective nature [10]. Conversely, the block equilibrium theory excels in assessing simple block geometries, but it can be less precise when considering complex or irregular forms [11]. Analytical methods are particularly effective for caverns with a regular configuration, yet they encounter limitations when analyzing rock masses under high stress, especially in anisotropic conditions [12]. Physical model testing is costly, and its accuracy is subject to the scale and materials selected [18; 19]. Finally, nonlinear analysis, while sophisticated and requiring specialized expertise, entails significant computational costs [20; 21].