## 1 Introduction  
Coal resources are widely distributed in China [1]. The varying occurrence conditions of coal seams across different regions have resulted in complex and changeable roof and floor conditions. Among the roof conditions of coal mines in China, soft rock roofs account for a relatively high proportion [2]. As the rock of soft roof has poor mechanical properties, it can easily loosen and break under mining stress, making the roadway unstable and the roof uncontrollable [3, 4]. With the increase of mining depth, deep stresses have caused severe deformation in the roof and floor and severe contraction in the sides during the service of the roadway [5]. At the same time, major roadway deformation has also led to capacity reduction or even failure of the support system [6, 7]. Therefore, it is an urgent task to control the surrounding rock of roadways with soft rock roof. The safety of mining roadways is the top priority of coal mine safety efforts. At present, most mines in China use big pillars to protect their mining roadways [8]. This places the mining roadway in a high stress area [9]. The stress environment is complex and harsh, and the surrounding rock is severely damaged, especially under soft rock roofs [10, 11]. Figure 1[12] shows an example of a soft rock roof roadway failure. In terms of soft rock roof support schemes, some studies have found that the combined supports from rock bolts and shotcrete are the most feasible method [13; 14; 15]. Pillarless mining is one of the effective ways to solve the problem of surrounding rock control for gob-side mining roadways with a soft rock roof. It also increases the coal recovery rate and reduces the waste of coal resources [16]. As a mining method, gob-side roadway forming technique allows for pillarless mining between working faces. As known by the classic mine pressure theory [17], on the one hand, in pillarless gob-side entry retaining mode, there is no stress concentration; on the other hand, the solid coal side has been damaged by compression, and the stress peak is deep inside the coal wall. The stress distribution is shown in Figure 2. Therefore, the mine pressure at the location of the roadway is at a low level and changes steadily. By reinforcing the roadway surrounding rock and building roadside support, it can bear the pressure of the roof and maintain a good roadway forming effect. This greatly reduces the dynamic hazards of the roadway and effectively increases the service life of the mine [18; 19]. Researchers around the world have carried out a lot of research on pillarless mining by gob-side roadway forming, especially in traditional mining countries such as Britain, the former Soviet Union and Germany, having made great breakthroughs in both theory and field application [20; 21; 22]. In the 1930s, some coal mines in the former Soviet Union began to carry out pillarless mining experiments by reserving no pillars and building masonry filling zones. In the 1950s, Britain, German and Poland reported gob-side roadway forming research and experiment. In the 1970s, Britain developed high-water quick-setting materials and supporting pumping and filling systems [23]. German coal mines adopted a retreating method for working faces and traditional along-solid seam excavation for section haulage roadways. They used gypsum as the roadside filling support, which was finally kept as the gob-side forming roadway and used as the return airway of the next section. They also developed a gob-side roadway forming process system consisting of three major systems: excavation, pumping, and recovery [24]. Figure 1: Deformation of soft rock roof [12]. (**a**) Severe shrinkage; (**b**) Severe deformation; (**c**) Roadway damaged. At present, pillarless mining is basically popularized in foreign countries by implementing a gob-side roadway forming bulkhead with an inorganic-organic hybrid structure together with a steel structure skeleton [25].