Stability estimation of ancient cavern chambers with sloping walls and fishtail-shaped pillars in Longyou Grottoes: a case study of Chamber No. 1 at Niuchang site

The Niuchang ancient cavern is a rare red sandstone cavern in southern China, located within the Longyou Grottoes in Zhejiang Province, China. Featuring a unique inclined roof structure combined with fishtail-shaped support pillars, it has maintained stability for millennia, embodying valuable ancient engineering wisdom. To investigate the mechanical behavior of this inclined roof and fishtail-shaped pillar system, this study first conducted a detailed investigation and analysis of severe deformation and failure phenomena in the cavern based on engineering geological conditions. Subsequently, using FLAC 3D software, three-dimensional models of the ancient cavern complex, one with fishtail-shaped pillars and another with hypothetical rectangular-section pillar, were constructed to simulate displacement and stress fields under self-weight conditions. Key findings indicate: (1) Long-term weathering (particularly biological weathering) and the excessively thin roof composed of gently inclined red beds are identified as the primary causes of significant deformation and failure in the Niuchang cavern; (2) Compared to a hypothetical “flat roof” cavern model, the inclined roof exhibits smaller roof displacement and experiences lower triaxial stress states in all three principal directions. (3) Compared to rectangular-section pillars, the fishtail-shaped pillars effectively reduce settlement and mitigate stress concentrations within the cavern complex, demonstrating the inherent stability and mechanical superiority of their original design. Conclusively, this study confirms that the ancient builders’ structural design, combining an inclined roof with fishtail-shaped pillars, provides enhanced long-term stability for the cavern complex compared to conventional flat roof structures with rectangular pillars.