Development of overlying strata collapse and water-conducting fractured zone in shallow coal seams mining

Shallow underground mining of coal seams has significant impacts on the overlying rock formations, not only exacerbating surface subsidence but also potentially leading to surface and groundwater loss, thereby affecting the development and safety of the ecological environment. To further understand the collapse law and fracture distribution characteristics of overlying strata during the shallow coal seam mining process, a study was conducted on the 2#, 3#, 5#, and 6# coal seams of the Shizuishan No. 2 Mine in Ningxia. Similar material simulation tests, numerical simulations, and empirical formula calculations were employed to analyze the development of water-bearing fracture zones and the collapse characteristic of the overlying strata under single and multiple mining operations. The results indicate that: (1) During the mining of shallow and closely spaced coal seams, the gob-side entry retention time of the upper coal seam is greater than that of the lower coal seam. (2) When mining a single coal seam, the collapse of the overlying strata occurs in the form of “hinged structure” and “step structure”. As to the mining of two or more coal seams, the stability of the “hinged structure” decreases significantly and the collapse structure mainly stabilizes as a “step structure” above the goaf. (3) During the initial mining, a “trapezoidal” fracture zone is formed, while during the secondary mining, an “M-shaped” fracture zone is formed, and during multiple mining operations, two “isosceles trapezoidal” fracture zones are formed. (4) The development height of the water-bearing fracture zone shows a steady increase or slow change during the initial mining. While during repetitive mining, the development height of the water-bearing fracture zone shows a rapid increase to steady growth. (5) The values obtained from the similar material simulation tests and numerical simulations are similar to the measured values, and all of them comply with the regulations for coal mine water control. These results can provide a basis for the efficient mining of coal seam groups in similar mining areas.