# Fine Exploration and Control of Subway Crossing Karst Area
## 1 Introduction  
The distribution area of karst in China is 3.65 million square kilometers, accounting for more than one-third of the territory. Karst is most developed in the Southern provinces of Guizhou, Hunan, Jiangxi, Sichuan, Yunnan, and Hubei, and in the Northern provinces of Hebei, Shandong, and Liaoning. Additionally, many karst development areas are distributed in metro construction cities, such as Jinan Metro, which needs to pass through hard rock water-rich caves [1], Wuhan Metro, which needs to pass through \"honeycomb caves\", Changsha Metro, which needs to pass through complex underwater caves, and so on. In the soluble strata, limestone may contain limestone fragments and fissures locally (Figure 1). The existence of limestone fissures provides water storage conditions and transportation channels for groundwater [2]. Caves of different sizes, shapes, and buried depths are formed due to the dissolution and erosion of groundwater [3]. If these caves are not accurately identified, the following risks are likely to occur:  
1. Shield machine pitch, jamming or water inrush (Figure 2);
2. Karst cave collapse above the tunnel leading to the surface collapse;
3. During the operation period, vehicle vibration causing surface collapse or karst cave collapse under the segment, which leads to risks in train operation;
4. The existence of karst water leading to the continuous development of karst caves, which threatens the long-term operation of the metro.  
In the aspect of karst detection, a large number of geophysical methods (including electrical, electromagnetic, elastic wave, and microgravity) have emerged [4; 5; 6; 7; 8; 9; 10; 11; 12; 13]. However, the geological conditions in the karst area are complex, and it is difficult to accurately investigate the underground conditions by a single geophysical method [14; 15]. With the advancement of geophysical methods and interpretation techniques, great achievements have been made in karst exploration [16]. More successful detection cases demonstrate that comprehensive detection method can effectively reduce the uncertainty and reduce the error interpretation results when exploring the underground karst structure. However, geophysical prospecting methods can only locate karst caves, which is difficult to quantify. Borehole laser scanning technology is a method to get the point cloud data of the three-dimensional surface of the inner wall of the cavity by extending the probe with integrated laser scanning function into the cavity. Its principle is the same as that of the standing laser scanner, but its greatest advantage is the miniaturization of the laser probe, which can adapt to various narrow channels and space and get data deep into the rock mass [17; 18]. At present, drilling laser technology is mainly used in the goal of coal mine, but it is not combined with geophysical prospecting methods, which leads to the problem of blind drilling. Therefore, this paper synthetically determines the karst development area by means of various geophysical prospecting. Targeted drilling and quantitative exploration of karst cave by drilling laser are carried out.  
Figure 1: Dissolution limestone and limestone fragments. (**a**) dissolution limestone; (**b**) limestone fragments.  
Figure 2: Water inrush from shield tunnel. (**a**) front of the tunnel; (**b**) inside the tunnel.