Abstract
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.
Highlights
Karst collapse is a type of karst dynamic geological action and phenomenon, where the rock and soil above the karst caves are deformed and destroyed under the action of natural or human factors, and collapse pits are formed on the ground
The risk of tunnel collapse increases with the increase in the size of the karst and the decrease in the distance between karst and tunnel [3]. us, not all karst caves pose a larger threat to the construction safety of the tunnel, and the safety of the project can be guaranteed by only taking the relevant treatment measures for the more dangerous karst caves instead of all karst caves. e crux of building a metro tunnel in karst areas lies in accurately detecting the karst distribution characteristics and estimating the stability of the surrounding rock in tunnels as affected by karst caves
Li et al reported a possible method to predict the top of a concealed karst cave based on displacement monitoring during tunnel construction. e numerical calculation was carried out to discuss the surrounding rock deformation affected by the top karst cave during tunnel excavation, and it was found that the simulation results were consistent with those predicted by the proposed method [3]
Summary
Karst collapse is a type of karst dynamic geological action and phenomenon, where the rock and soil above the karst caves are deformed and destroyed under the action of natural or human factors, and collapse pits (holes) are formed on the ground. E crux of building a metro tunnel in karst areas lies in accurately detecting the karst distribution characteristics and estimating the stability of the surrounding rock in tunnels as affected by karst caves. E numerical calculation was carried out to discuss the surrounding rock deformation affected by the top karst cave during tunnel excavation, and it was found that the simulation results were consistent with those predicted by the proposed method [3]. A comprehensive investigation, which detects the karst distribution characteristics and estimates the stability of the surrounding rock in the tunnel excavation process, is rarely reported. According to the detection results, the numerical calculation was performed to investigate the effect of different sizes and locations of karst caves on the displacement and stability of surrounding rock in tunnels using FLAC3D. Due to the relatively thick karst overburden and strong environmental interference such as electromagnetic, stray current, and vibration
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