# Reconstruction and Measurement of Irregular Karst Caves Using BLST along the Shield Metro Line
## 5 Case Study in Jinan Metro
### Detection of Complex Karst Caves and Three-Dimensional Data Acquisition  
The main idea of this exploration is to accurately locate the karst cave based on the drilling and comprehensive geophysical prospecting methods, and then use the BLST method to obtain the morphology and characteristic parameters of the karst cave. On the basis of the results of comprehensive geophysical prospecting and drilling, the key points and abnormally complex areas are highlighted. The cross-hole resistivity CT method, owing to its high resolution, has great advantages in the detection of complex water diversion channels. Therefore, this method was adopted for fine detection to obtain the accurate positions of karst caves. In view of the fact that there are a large number of karst caves, pipelines, and cracks along the metro tunnel, only the disaster sources were selected for accurate laser scanning, such as karst caves and cracks, which are large in scale and present serious challenges in the grouting process. Targeted drilling was carried out and a karst cave point cloud model was reconstructed. The three-dimensional laser scanning detection system consists of a microlaser probe, a directional rod, cable wires, and a point cloud data acquisition and processing system. The microlaser probe entered the interior of the cave through targeted drilling and scanned the cave quickly to construct the three-dimensional shape of the cave and then extract the corresponding characteristic parameters, including the volume and depth of the karst cave. The laser probe had a built-in 3D navigation module to measure the position and direction of the probe in the cave in real time, and accurately record the \(x\), \(y\), and \(z\) coordinates and azimuth of the probe. The probe, which integrates a camera and an infrared lighting system, is able to monitor the movement of itself in the borehole and eschew obstacles or depressions in time to avoid any damage. The fine detection of a karst cave mainly includes two parts: field detection and indoor data processing. First, the measurement and layout of boreholes should be carried out and arranged before laser scanning and detection, so as to fully know about the on-site detection environment. Then, the detection devices are assembled and debugged, confirming that the detection system works properly. The laser probe with cable wires is lowered along the pipe vertically to monitor the internal borehole environment in real time. When the probe reaches the predetermined cave position, the depth meter is read, the data is recorded, and the mechanical scanning and accuracy parameters are set; then, the microprobe begins to measure the point clouds, and the cavity scan system simultaneously collects the point cloud data. After the scanning task is completed, the probe moves up and the field collection work finishes. The detailed steps are shown in Figure 12.  
Based on the results of field drilling and comprehensive geophysical exploration, the borehole scanning method was used to locate and quantitatively detect karst caves at 6 borehole locations (17#, 20#, 21#, 23#, 24#, 25#). The detection results are as follows: a karst cave with a volume of 3.02 m\({}^{3}\) was measured at the borehole 17# location, at a depth of about 15.4 m; the detailed morphology and characteristic parameters of the karst cave are shown in Figure 12a, recorded as KC1. A cave with a volume of 3.89 m\({}^{3}\) was measured at location 20#, at a depth of about 11.8 m; the three-dimensional model of the cave is shown in Figure 12b, recorded as KC2. A karst cave with a volume of 3.66 m\({}^{3}\) was measured at borehole location 21#, at a depth of about 1 m; the three-dimensional model of the karst cave is shown in Figure 12c, recorded as KC3. A water-bearing cave with a volume of about 3.84 m\({}^{3}\) was measured at location 23#, at a depth of about 15.7 m; the three-dimensional model of the cave is shown in Figure 12d, recorded as KC4.