## 3 Overview of the Backfilling Method  
The backfilling method originated in the 1950s, which was initially a dry filling of waste rock for waste treatment and then went through the processes of tailing sand cementing filling, paste filling, waste rock cementing filling, full tailing sand cementing filling, etc. Its specific development history and representative mines in each period are shown in Figure 3 below. This method supports the surrounding rock through the filling body to achieve the effect of controlling ground pressure, reducing ore loss and surface deformation, etc. In addition, compared with the sublevel caving method, the backfilling method has the following obvious advantages:  
1. Reducing the ore dilution rate and loss rate: By using filling materials to fill the mining goal, the crumbling and movement of the ore can be effectively controlled and the loss of ore can be reduced. At the same time, the filling material can support the ore body, reduce the deformation and rupture of the ore body, improve the recovery rate of the ore, and reduce the ore dilution rate [32]. 2. Better stope stability: the filler supports and protects the stope, reducing roof subsidence and perimeter rock deformation [32; 33]. 3. Adaptation to complex ore body conditions: for irregular ore bodies or high-grade ore bodies, the backfilling method has stronger adaptability [34]. 4. Reducing the accumulation of tailing ponds: The use of the filling method can reduce the accumulation of tailings by filling solid wastes, such as waste rock and tailing sand, produced during the mining process into the underground mining area instead of discharging them directly to the surface or tailing ponds. In addition to the above advantages, after years of development, the backfilling method also presents diversified development trends. At present, the mainstream development trends of the backfilling method in China include, but are not limited to, the following aspects: intelligence and automation [35], filling material innovation [36], comprehensive filling technology, numerical simulation and optimization, and so on [23; 37], as shown in Figure 4.  
Compared with the initial extensive backfilling method, in China, there are already clear requirements regarding the filling materials that can be used in underground mining (such as tailings, fly ash, cement, blast furnace slag, gangue, etc.) and their physical and chemical properties. First, in terms of chemical properties, the pH value of the leachate of filling materials needs to be within the range of 6 to 9. Second, in terms of physical properties, the general requirement for organic matter content is less than 2% (except for coal gangue). In addition, in order to make filling materials meet the construction requirements of green mines, relevant departments have also issued relevant laws and regulations. Figure 3: The development history of filling mining materials in China. In 2017, the \"Implementation Opinions on Accelerating the Construction of Green Mines\" was issued, with the goal of \"basically reversing the overall image of the traditional crude and wasteful mining industry and forming a new model and a new pattern of mining development that meets the requirements of ecological civilization construction\". Coupled with the fact that in 2018, the Chinese government issued the \"Green Mining Construction Norms for the Nonferrous Metals Industry\", the first national green mine construction industry norms were formed, further promoting the green mine construction process in China. Both of the above policies emphasize the importance of green mine construction, including the adoption of environmentally friendly and efficient mining technology to reduce damage and pollution to the environment, and the backfilling method, as a kind of green mining technology, has advantages that other mining parties do not have in terms of environmental protection, resource utilization, and safe production.