The existence of karst caves poses a large threat to safe tunnel construction in a karst area. This paper presents a synthetic method to evaluate the collapse risk before subway tunnel construction with Yang-Jian interval tunnel as a case study. The crosshole seismic Computed Tomography (CT) integrated with Geological Drilling (Geo-D) was first applied to accurately delineate the karst location and its scale. Then, 483 groups of seismic wave CT images were recorded, and 524 karst cave anomalies were found. The height of karst caves in the study area is 1–20 m and mainly concentrated at approximately 5 m. The vertical distance between the karst cave and the tunnel is mainly within 15 m. According to the detection results, a series of numerical models were built and calculated using FLAC3D to investigate the effect of different sizes and locations of karst caves on the displacement and stability of the surrounding rock in tunnels. Afterwards, based on the simulation results, the disturbance degree evaluation index was established to quantitatively evaluate the risk level of karst caves. The evaluation results indicate that the buried depth of the karst cave greatly affects the disturbance degree. No treatment is required for the deeply buried karst cave that is more than 7 m from the tunnel. When the distance between the cave and the tunnel is less than 7 m, there is a critical size of the cave. Karst caves that are larger than that critical value must be filled with a single slurry or binary slurry before tunnel construction to eliminate the risk of tunnel collapse. This study can be used to provide a more efficient and economical program for metro tunnel construction above a karst cave.

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To explore the influence of bedding structure on mechanical properties and failure mechanism of anisotropic rocks, six stratified sandstone specimens with different bedding orientations were tested by uniaxial static and cyclic loading. Acoustic emission (AE) and digital image correlation (DIC) were exploited to analyze the mechanism of crack initiation and propagation. The results show that with the increase of the bedding orientation, the peak strength of the stratified sandstone decreases first and then increases, behaving an obvious anisotropic characteristic. The AE characteristic-stress-strain curve can be divided into four stages: crack closure stage, linear elastic deformation stage, stable crack growth stage, and unstable crack growth stage. Moreover, as the bedding orientation increases from 30° to 90°, the crack closure stage gradually lengthens, while the linear elastic deformation stage first shortens and then lengthens. Under uniaxial static compression, due to the presence of bedding surface, the rock surface has a relative slip trend as a result of the uneven distribution of strain on the left and right parts, and the strain field boundary is almost consistent with the bedding orientation. Furthermore, cyclic loading can increase the rock stiffness, and the stain field boundary of 60° stratified sandstone sample in each cycle is slightly different in the cyclic loading.