## 1 Introduction
Although the shape and size of the backfill sample adopted in the current research on the strength of the waste rock cemented backfill are different, it is found that the strength of the waste rock cemented backfill is mainly affected by the mix ratio of waste rock and cemented material (cement), and the strength of the backfill sample shows the law of first increasing and then decreasing with the increase of the proportion of waste rock [35; 36]. In order to further reveal the reasons for the change in the strength of the waste rock cemented backfill, some scholars used CT scanning imaging technology to record the whole process of the mechanical failure of the waste rock cemented backfill sample [37; 38]. In terms of waste rock gradation, relevant studies have shown that the mechanical properties of Talbot cemented filling with a Talbot grading index between 0.4-0.6 are the best [39]. [PERSON] presented a comparative laboratory strength test of three-point and four-point wooden rib models. The study found that filling three- and four-point ribs with gangue increases their maximum load several times compared to empty cribs [40]. The above research not only successfully solved the problem of preparation and transportation of high-concentration filling slurry, but also improved the strength of the stope backfill, and also reasonably analyzed the stress condition of the backfill to ensure the stability of the stope backfill. Therefore, the safety of stope operators is fully guaranteed, and at the same time, the storage pressure of the tailings pond is effectively relieved, and the potential safety hazards of the goat are eliminated. However, the development of filling theory not only provides security, but also brings significant economic benefits. Not only can the filling method improve the recovery rate of ore, so as to produce good economic benefits, but in addition, as the slurry concentration increases, the amount of cement used is reduced, which further reduces the cost of filling [28]. In addition, during the construction of the filling system, the construction cost of the filling system is effectively reduced by using low-cost filling equipment to build various filling systems [18]. In the practice of filling, in recent decades, a variety of green filling cementitious materials have been developed using pozzolanic solid wastes, such as slag, steel slag, fly ash, calcined coal gangue, and red mud, which significantly reduces the cost of filling cementitious materials [21]. In the field of filler slurry transport, the researchers have improved the filling slurry using foaming technology. This innovative measure has succeeded in reducing the transport resistance of the filling slurry, thus effectively reducing conveying costs [41]. In specific filling operations, the filling method reduces costs by combining backfills of different strengths. For example, a layered filling strategy is used, with a low-cement, low-cost, low-strength filling material used for the bottom layer, and a high-strength filling material for the upper layer to meet production requirements. The application of the filling method not only brings significant economic benefits to mine production, but also has a huge economic contribution in terms of corporate taxation. The Environmental Protection Tax Law of the People's Republic of China stipulates that environmental protection tax (15 RMB/t) will be levied on tailings discharge from January 2018. On 4 September 2023, the Ministry of Finance issued a notice that, in order to encourage the intensive mining and utilization of coal resources, the resource tax on the coal replaced by fill mining will be reduced by 50% from 1 September 2023 to 31 December 2027 [42; 43; 44; 45]. And the introduction of these policies has, to some extent, promoted the use of the filling method in Chinese mines. There are a large number of domestic and international examples of the transition from the shrinkage method to the efficient, environmentally friendly, safe and economical filling method.