# A LiDAR-Based Backfill Monitoring System
## 4 Experiment
### Simulation Environment of Backfill  
For tunnel paste backfill simulation, CARLA is not only necessary to edit the dynamic properties of the backfill body to simulate the backfill process, but also to build different lengths and shapes of tunnels to improve the generalization ability of the backfill monitoring model. The flexible environment editing capability in CARLA can meet the above requirements. The simulation environment of backfill is designed correspond to the on-site backfill scenario described in Section 4.1.  
In CARLA, three different shapes (straight, slightly curvy, and curvy) of tunnels are chosen from the 3D model of an underground mine, the dip angle of each tunnel is \(5^{\circ}\), and the selected filling length is about 170 m (Figure 8). The simulation represents an idealized scenario where the tunnel is not supported by any structures such as rock bolts or reinforcing mesh, and there are no backfill dams controlling the backfill process. First, retaining walls are placed at the bottom of the tunnels, and the tunnels are mapped by SLAM technology to obtain the initial point cloud data. Secondly, a new retaining wall is built at the entrance of each tunnel (Figure 9a), and a space on the top of the retaining wall is reserved for placing the LiDAR sensor(Figure 9b). A LiDAR sensor is set up that meets the basic parameters of the Livox Mid-40 LiDAR, and the measuring ranges of 100 m and 200 m are designed, respectively, to simulate application scenarios in various lengths of tunnels. Finally, three cuboids are placed beneath tunnels and edited to rise continuously (Figure 9a), simulating the process of filling the tunnel with slurry. Two cubes are placed in the surrounding rocks of each tunnel to construct correspondences for the registration algorithm. The diagram of LiDAR scans at three different times and the point cloud of the unfilled tunnel are shown in Figure 10. Obviously, the only change in the scanning area is the red slurry top surface, due to the fixed position of the LiDAR placement. By processing each LiDAR scan, the backfill progress of different periods can be calculated.  
Figure 7: System workflow.  
To improve the generalization ability of the filling monitoring model, different simulation environments are designed. Parameters of simulation environments are presented in Table 2. According to the mechanism of backfill progress computation and the experiment results, results are not affected by different dip angles. Therefore, only tunnels with a 5\({}^{\circ}\) dip angle are adopted.  
\begin{table}
\begin{tabular}{l l l} \hline \hline
**Simulation Environment** & **Curvature** & **LiDAR Range** \\ \hline
1\# & Straight & 100 m \\
2\# & Slightly curvy & 100 m \\
3\# & Curvy & 100 m \\
4\# & Straight & 200 m \\
5\# & Slightly curvy & 200 m \\
6\# & Curvy & 200 m \\ \hline \hline \end{tabular}
\end{table}
Table 2: Simulation environments’ parameters.  
Figure 8: The overview map of the underground mine simulation environment: (**a**) lateral view; (**b**) top view. In the CARLA simulation environment, cuboids and retaining walls are used to restrict the LiDAR laser beams. Therefore, the size can be larger than in real-world conditions.