# Reconstruction and Measurement of Irregular Karst Caves Using BLST along the Shield Metro Line
## 4 Fine Measurement and Reconstruction of Complex Kast Caves  
### Borehole Laser Scanning Technology (BLST) Automatic Laser Scanning System in Kast Caves  
At present, laser scanning technology has been widely used in tunnels and underground engineering projects, such as in the acquisition of rock mass structural plane information, the deformation of roadway surrounding rock, the establishment of rock mass models, and even for noncontact monitoring. Most of the above applications use vertical laser scanning technology in large operating spaces, however karst caves are usually concealed. Even if the location and depth of karst caves are obtained by drilling and comprehensive geophysical prospecting, the vertical laser scanner cannot enter the interior of the cave to assess the parameters due to instrument size limitation. Thus, it is necessary to use a laser detection system that can enter inside the cave through a narrow drilling channel. The C-ALS is the first laser automatic scanning drilling device developed by MDL (UK), which consists of a laser ranging module, three-dimensional servo mechanical control system, data transmission system, and data processing system. Its greatest advantage is that the laser probe is miniaturized with a diameter of only 5 cm, which can be used to enter the karst cave through narrow boreholes and carry out three-dimensional scanning of rock mass surfaces. The main structure of the C-ALS is shown in Figure 8.  
Figure 8: Borehole laser scanning technology (BLST) method: (**a**) laser ranging module; (**b**) integrated detector; (**c**) processing software; (**d**) the whole system.  
The drilling laser scanning system is based on the laser ranging principle to obtain the point cloud coordinate data. The probe has a 3D navigation system that can read the depth of the probe in real time, so as to obtain the exact location of the karst cave. Through the three-dimensional mechanical servo rotation system, the three-dimensional scanning state of the laser ranging probe are controlled, and the three-dimensional point cloud coordinates of the inner wall of the cave are measured. Finally, the point cloud model is encapsulated to form a real cave model by using point cloud processing software to realize the precise exploration of the location, shape, and volume of the karst caves. The operation process is as follows: (a) Targeted drilling. Based on the results of drilling and comprehensive geophysical exploration, the location of the targeted drilling is determined and the drilling is carried out to provide the downward exploration channel for the laser probe. (b) The laser probe is lowered inside the cave, and the laser emission module is adjusted to prepare the cave point cloud for detection. (c) Point cloud scanning. The laser ranging module emits and receives the laser, and the corresponding point cloud coordinates are calculated according to the three-dimensional coordinates and angles. (d) Point cloud model processing. Through the C-ALS software, the point cloud model is obtained and then encapsulated to form a real cave model, in order to obtain the cave size, shape, volume, and other parameters, as shown in Figure 9.