Abstract: The ancient underground caverns in Longyou Grottoes are immovable cultural relics under the key protection in China, with a history of thousands of years, but their rock mass strength shows a trend of deterioration. In the recent 20 years, many field investigations found that cracks in the surrounding rock of the roof of No.3 cavern developed rapidly, and local blocks fell off at the thin roof rock. In order to further analyze the evolution and expansion of cracks in the roof of No.3 cave in Longyou Grottoes, high-precision three-dimensional point cloud data of the cave contour were obtained by using three-dimensional laser scanning technology. Also highprecision geometric and geological models were established combined with rock mass structure information. The development process of cracks in the roof of No. 3 cave in Longyou Grottoes was numerically simulated and analyzed by using the strength reduction method. The research results show that, based on high-precision 3D laser scanning point cloud data combined with a geological survey, the 3D geological model of Longyou Cave 3 is constructed. The results of numerical simulation on this high-precision grid model can accurately reflect the evolution and expansion law of the roof crack of Cave 3. The cracks in the roof of No.3 cave in Longyou Grottoes were mainly generated around the entrance and rock column, and the expansion form is mainly annular. With the increase in the strength reduction
coefficient of rock mass, the tensile stress concentration area of the roof surrounding rock is distributed at the tip of existing cracks and in the middle of multiple cracks, and the longitudinal expansion of tensile cracks mainly appear in the outermost surrounding rock of the tunnel roof. Variations of the displacement field of the roof rock body show the characteristics of spatial heterogeneity. The maximum displacement in the fracture concentration area on the northern side of No. 3-1 rock pillar is 6mm, while the maximum displacement in the fracture concentration area on the northern side of No.3-3 rock pillar is 12.14 mm. This study obtained the distribution, shape and development trend of cracks in the roof of Longyou Grottoes No.3 for the first time, which is generally consistent with the field survey data. Therefore, this study can provide a reference for the mechanical analysis and prediction of the surrounding rock of the cave and the long-term protection of the grottoes.

Key words: Longyou grottoes, 3D geological model, crack evolution in cavern roof, strength reduction method