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This paper explores the design of safe coal (rock) pillars and the control effect of water pressure on roof water inrush in the “Code for Coal Pillar Retention and Coal Mining in Buildings, Water Bodies, Railways, and Main Tunnels”. Firstly, by analyzing the header text of Appendix 4–3 of the standard, it is pointed out that the relevant regulations on the thickness of the protective layer only apply to loose aquifers (including surface water bodies), and it is not clear how to determine the thickness of the protective layer for coal mining under bedrock aquifers; Furthermore, taking both mudstone and cohesive soil layers as bridges with water blocking function, it is inferred that coal mining under the bedrock aquifer can be carried out according to the condition that the cumulative thickness of cohesive soil layer under the loose layer is greater than the mining thickness. Following the principle of “high not low”, it is advisable to uniformly take four times the mining thickness as the protective layer under the bedrock aquifer; Since the applicable conditions of the specification are that the thickness of a single layer should not exceed 3.0 m, it is difficult to understand the provision in Appendix 4–3 that “the thickness of the loose layer is less than the thickness of the mining”, and it is recommended to delete it. Secondly, based on the scientific connotation of the term “protection”, the “protective layer” is redefined, which means that the waterproof rock layers between the top interface of the water conducting fracture zone and the bottom interface of the aquifer all have a “protection” function and should be collectively referred to as the protective layer (Hb); Furthermore, the concept of protection factor (Bs) was proposed, which refers to the ratio of the thickness of the protective layer to the thickness of a single layer of mining; The threshold for dividing the protection coefficient under the loose aquifer is Bi=(2, 3, 4, 5, 6, 7), and the threshold for dividing the protection coefficient under the bedrock aquifer is Bi=4. Based on this, the risk of roof water damage is classified into three levels: water inrush zone (Bs≤0), hazardous zone (0

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 (Hb); 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 (Bs) is proposed, which is the ratio of the thickness of the protective layer to the mining height (Bs=Hb/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 Bi=(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 Bi=(0,4). According to the protection coefficient and zoning threshold, the evaluation area can be divided into water inrush zone (Bs≤0), danger zone (0

The first and currently only Jurassic coal seam mining floor Baotashan sandstone water inrush and well flooding accident occurred in the New Shanghai No. 1 Coal Mine in Shanghai Miao Mining Area, Inner Mongolia. This paper researched the water inrush risk assessment method and prevention and control technology of the Baotashan sandstone in the Jurassic coal seam mining floor, which is highly water-rich. Firstly, it is clear that the water inrush source of the "11.25" accident is the Baotashan sandstone aquifer in the lower part of the Jurassic Yan'an formation, and the water barrier between the coal seam and the floor Baotashan sandstone has the characteristics of low strength, low softening coefficient and poor cementation, so it belongs to geological soft rock. Secondly, a water inrush evaluation system was established using “aquifer thickness, aquifer water richness, aquifer permeability, aquifer water pressure and the effective thickness of the 18 coal floor aquifer” as the main indicators, and the water inrush of the Baotashan sandstone aquifer was the dangerous zones are divided into three types: safety zone (32.21%), threat zone (37.53%) and dangerous zone (30.26%), and the accuracy of the evaluation model was verified through the “11.25” water inrush accident. Finally, combined with the engineering geological characteristics of the floor aquifer, the method of “fault grouting to form a local restricted boundary+group hole water release” was used to prevent and control Baotashan sandstone water inrush. It was set that “Water drainage begins until drilling site #1 exits work, the 2#, 3#, and 4# drilling fields continue the water drainage work until 3# exits work, and the 2# and 4# drilling sites continue to drain water until the model is replenished and balanced.” According to the specification, the maximum curtain permeability coefficient after grouting is 2×10−2 m/d, When the curtain permeability coefficient after fault grouting is 2×10−2 − 2×10−8 m/d, the drainage volume is 4.02×10−4 m3 − 3.478×10−4 m3. The proportion of safe areas in the mining area is 43.34%−67.37% respectively. Combining the grouting cost, drained water volume and safe area, when the curtain permeability coefficient is not greater than 2×10−4 m/d, can effectively ensure the safety and technical and economic benefits of the mine. The combined technology of fault grouting local restriction boundary+water drainage can effectively prevent and control sandstone water damage in Baotashan. The research work in this paper is of great significance and engineering practice value for the prevention and control of floor water in the Jurassic Coalfield development in western China.

According to some hot spots such as exploration before excavation，difficulties like drainage mining under special conditions and contradictory while operating procedures in recent coal mining water prevention projects, confusions brought to production practice and safety supervision are analyzed, some opinions are also put forward. Firstly, the connation of “doubt will be prospected” is discussed in this paper. It is assured that “doubt will be explored” comes after “prediction and forecast”. The “prediction and forecast” are key factors to problems like types of water hazards, water pressure and positions of water, based on what water exploration and release can be designed. From the historical background and realistic conditions, the analysis shows that the exploration before excavation proposed in [2011] No.70 document formulated by Shanxi Province is implemented under specific conditions, with strong pertinence (ancient wells and cellar water). The type of water damage is clear, the space position of water pressure source is roughly known, and the design conditions of drilling hole are available. Therefore, design conditions of water exploration drillings are acquired and “exploration before excavation” is reasonable. However, the scope of “exploration before excavation” cannot be extended infinitely to other fields, otherwise it will be aimless, causing bumps in design, execution and supervision. Secondly, degradation of water on soft rocks is analyzed. Degradations like crumble, rock mud, expansion and floor heave can happen on complicated soft rocks which possess weak cementation, low altitude, high expansion and strong watery, limiting production efficiency. The purpose of pre-dredging weak aquifer under soft rock condition is to improve the production conditions and to increase production efficiency. It is suggested that the regulations should make principles constraints on such drainage holes, so as to facilitate enterprises to formulate construction technical measures in combination with the actual situations. Otherwise, there will be a difference in understanding between government regulators and enterprises. Finally, according to the definition in the procedure, the water-conducting fracture zone does not include the caving zone (above the caving zone); nevertheless, the empirical formula in the procedure and the attached drawings show that the water-conducting fracture zone includes the caving zone, which has inconsistency between the front and the back. For this case, two alternative revision suggestions are advanced. Through the analysis of this paper, some fuzzy concepts are clarified, which is convenient for the production unit to accurately implement the rules and regulations, conducive to the precise supervision of safety supervision department. Besides, dilemma between the supervision department and the enterprise can be avoided.