Discussion on safety of coal (rock) pillar in “Three Down” Mining Standards

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 (H_b); Furthermore, the concept of protection factor (B_s) 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 B_i=(2, 3, 4, 5, 6, 7), and the threshold for dividing the protection coefficient under the bedrock aquifer is B_i=4. Based on this, the risk of roof water damage is classified into three levels: water inrush zone (B_s≤0), hazardous zone (0<B_s<B_i), and safe zone (B_s≥B_i). In addition, drawing on the concept of water inrush coefficient of the bottom plate, the head pressure borne by the unit thickness of the protective layer is called the pressure coefficient of the protective layer (T=P/H_b). Through the analysis of the pressure coefficients under three types of coal water structure conditions, it is concluded that coal mining under the loose aquifer of the Quaternary system and the non coal bearing bedrock aquifer can not consider water pressure. The risk of water inrush caused by the increase of mining depth and water pressure under the coal bearing bedrock aquifer increases. Finally, it was analyzed that when there is a mud filled water barrier at the top of the bottom aquifer, the expression for the bottom waterproof safety coal (rock) column (h_a≥h₁+h₂+h₄) given in the “Three Down Mining Specification” is contradictory to the attached figure. The correct expression should be h_a≥h₁+h₂−h₄.