## 1 Introduction  
The caving method for coal mining may result in ecological damage [1, 2], such as surface subsidence [3], water loss [4] and building damage [5]. Previous studies have shown that mining technologies that can protect the ecology mainly include hydraulic sand backfill [6], high-water backfill [7, 8], paste backfill [9, 10, 11] and solid backfill [12, 13]. The first three are in the form of slurry to pump the filling material into the goat, which have been widely used in countries such as Germany [14], Russia [15], Turkey [16, 17], Poland [18], and India [19, 20, 21]. The latter is to fill massive gauge and loess into goat and squeeze them to support overlying strata, a method mainly used in China at present. The basic principle of solid backfill is shown in Figure 1. The transportation route of the solid backfilling material is as follows [22]: gauge dump and loess dump \(\rightarrow\) vertical feeding shaft \(\rightarrow\) surge bin \(\rightarrow\) goat (the blue route in Figure 1a). The scraper conveyor at the back of the backfilling hydraulic support (Figure 1b) in the working face puts the filling material into the goat, and the tamping mechanism at the back of the backfilling hydraulic support (Figure 1b) squeezes the filling material. The higher the squeezing degree, the higher the backfilling rate [23, 24], and the better the effect of supporting the overlying strata [5, 25].  
Previous studies have shown that backfilling hydraulic support is the core equipment of solid backfill [26, 27]. On the one hand, enough support resistance can support the overlying strata and maintain the space of goat [28]. On the other hand, the backfilling rate depends on the force of the tamping mechanism. Therefore, an efficient design and manufacturing process, as well as good operation performance, is the guarantee of backfilling. The research shows that at present, the design method of backfill hydraulic support is mainly redesigned according to the weight of rock stratum, and there are few cases of transforming other hydraulic support into backfilling hydraulic support; so, the design and manufacturing cycle of backfilling hydraulic support is longer [29]. The main method to test the operation performance of backfilling hydraulic support is a physical test [30], as shown in Figure 2; the object of physical test is the prototype that has been manufactured, and its test cost is high and the test process is slow. Some scholars also use simulation to test the strength of shield hydraulic support [31; 32], mainly simulating the strength of key components, such as columns and bases. Additionally, some scholars use the digital twin theory to test the virtual hydraulic support [33; 34], but it is difficult to test the compressive strength of the hydraulic supports.  
Figure 1: Basic principle of solid backfill mining: (**a**) transportation route; (**b**) basic principle of backfilling hydraulic support.  
Through the above analysis, the following problems in the backfilling hydraulic support can be found: long design cycle and high manufacturing cost; the inspection method is mainly from the perspective of strength, and seldom considers the comprehensive influence of structure and strength. Therefore, this paper proposed the design method of transforming top-coal cavity hydraulic support into backfilling hydraulic support. The paper calculated the bearing capacity of the top-coal caving hydraulic support and the backfilling hydraulic support theoretically. ABAQUS (6.14) was used to simulate the strength of the reformed backfilling hydraulic support, Creo was used to perform a simulation check of the structure. In addition, this paper demonstrates the feasibility of the reformation with field applications. The results of the study contribute to the efficient design and performance evaluation of the backfilling hydraulic support.