Study on evaluation technology of coal seam roof water hazard risk with protection coefficient

Regarding the issue of determining the thickness of the protective layer for waterproof and safe coal (rock) pillars in mining near water bodies, Appendix 4-3 of the “Code for Retaining and Mining Coal Pillars in Buildings, Water Bodies, Railways, and Main Tunnels” (hereinafter referred to as the “Three Underground Mining Code”) is only applicable to coal mining under “loose aquifers”. As for how to determine the thickness of the protective layer for coal mining under bedrock aquifers and mining under goaf water bodies, it is not involved; The main content of commonly used roof water hazard risk assessment techniques and methods includes water abundance evaluation, conductivity evaluation, and water filling strength evaluation. When the water abundance of the aquifer is known, only “conductivity” can be evaluated, and the evaluation method can be further simplified. Based on the above two points, this article attempts to construct a simple criterion similar to the coefficient of water inrush from the bottom plate to evaluate the risk of roof water damage, starting from the “Three Underground Mining Standards”. Firstly, based on the scientific connotation of the term “protection”, it is proposed that the impermeable rock layers between the top interface of the water conducting fracture zone and the overlying aquifer have protective functions and should be collectively referred to as the protective layer (H_b); Based on the practice of selecting a certain multiple of mining height (A) for the thickness of the protective layer in the “Three Lower Mining Standards”, the concept of protection coefficient (B_s) is proposed, which is the ratio of the thickness of the protective layer to the mining height (B_s=H_b/A). Secondly, based on the provisions of Appendix 4-3 of the “Three Underground Mining Specification” regarding the thickness of the protective layer for waterproof coal pillars, the threshold value of the protection coefficient zoning for coal mining under loose aquifers is proposed to be B_i=(0, 2、3、4、5、6、7); The coal bearing strata under the bedrock aquifer are generally composed of sand and mudstone interbedded sedimentary formations, where mudstone has a water blocking function (equivalent to the cohesive soil layer under the loose layer), and the total thickness of mudstone is generally greater than the cumulative mining thickness. The thickness of the protective layer can be selected according to the condition of “the thickness of the cohesive soil layer at the bottom of the loose layer is greater than the cumulative mining thickness” in Appendix 4-3 of the “Three Underground Mining Specifications” and selected according to the maximum value, Therefore, the threshold value of the protection coefficient zoning for evaluating the bedrock aquifer is B_i=(0,4). According to the protection coefficient and zoning threshold, the evaluation area can be divided into water inrush zone (B_s≤0), danger zone (0<B_s<B_i), and safety zone (B_s≥B_i). When there are multiple aquifers above the coal seam, separate evaluations should be conducted. Finally, an example is given to illustrate the process and method of applying protection coefficient to evaluate the risk of roof water damage. It is pointed out that when the water content of the aquifer is medium or above, the connotation of “water inrush zone” and “dangerous zone” focuses on safety and is usually reserved as waterproof safety coal pillars; When the aquifer has weak water abundance or is economically drained, the “water inrush zone” and “dangerous zone” are mainly used to guide the design of drainage engineering.