*7.1. Height-Restricted Mining*

The Yushuwan Coal Mine, located in the south of the study area is shown in Figure 18a. It has five minable coal seams and Seam 2−<sup>2</sup> is uppermost and 11 m thick. The bedrock thickness varies from 85–142 m and the impermeable soil thickness is around 73–96 m. The mine is categorized into sand-soil-bedrock stratigraphic structure. According to Equation (6), the MAMH was calculated and found between 7–9 m, which is smaller than the actual seam thickness. This indicates that WCCM is needed. Early suggested in Section 6.1, height-restricted mining could be adopted at this mine. Eventually, the actual mining height for the Yushuwan Coal Mine was determined to be 5.5 m. More than five longwall panels have been successfully extracted now with no mining induced WCFZ reaching the overlying Salawusu Formation, demonstrating the achievement of WCCM and local water resource is effectively protected [51].

#### *7.2. Backfill Mining*

The main seam being mined at the Yuyang Coal Mine is between 2 m and 4 m thick. The overlying bedrock is 60 m to 310 m thick, and the impermeable soil thickness varies between 15 m and 120 m. Though this mine is within Yuyang District, Yulin City, its local geology is similar to that of sand-soil-bedrock stratigraphic structure of Yu-Shen mining area due to their geographic proximity (Figure 18b). The MAMH for this mine was calculated using Equation (6) to vary from 3 to 16 m. A comparison with the actual coal thickness suggests that WCCM method needs to be implemented in some parts of mine. The Yuyang coal mine takes eolian sand from the surface as a major component of the paste-like backfill material in order to reduce the backfill cost. At shortwall face 2301, about 5.2 <sup>×</sup> 104 m3 of backfill material has been injected into 50 branch roadways, through with the overburden movement and failure were effectively controlled. According to measurements conducted after mining,

the surface subsidence decreased by over 50% and the water table declined by less than 1.61 m on average, implying good water protection effect has been reached [52,53].

#### *7.3. Narrow Strip Mining*

The Yubujie Coal Mine is a small mine (annual production: 0.3 Mt/a) located at the southern boundary of the Yu-Shen mining area (Figure 18d). Seam 2<sup>−</sup>2, the uppermost minable seam, is 4–6 m thick; the overlying bedrock is 90–100 m thick and impermeable soil is 20–90 m thick. The mine geology is categorized into soil-bedrock stratigraphic structure. Using Equation (9), the MAMH was estimated to vary between 4–6 m, indicating that WCCM method is needed in some parts of this mine. Given the small size of this mine (with annual production below 0.9 Mt/a), narrow strip mining was used. The coal strips mined are 12 m wide and the coal pillars left unmined are 8 m wide. This has been used at face 300 and other faces. Field observations demonstrated that the coal pillars were stable and the roof was general intact. The decline of water level in the Salawusu Formation was less than 0.3 m, and the water table remains within 5 m below surface, demonstrating the effective protection of the local ecological water environment [7,54–56].

#### **8. Discussion**

