Porosity Distribution Law of Overlying Strata in the Goaf of the Adjacent Working Face: From the Perspective of Section Coal Pillar Types

Abstract

The porosity distribution law of overlying strata in the goaf has an important guiding role in distinguishing hidden disaster-causing factors in the goaf, such as the gas enrichment area and spontaneous combustion area. Existing research is concentrated on the overlying strata in the goaf of a single working face (GSWF), and the porosity distribution law of overlying strata in the goaf of an adjacent working face (GAWF) must be different from that in the GSWF. By selecting Longshan Coal Mine as an engineering background and applying theoretical analysis, numerical simulation and formula-fitting methods, the porosity distribution law of overlying strata in the GAWF was obtained for different section coal pillar types. The results demonstrate that (1) according to the supporting effect of different sections of coal pillar widths on overlying strata, the GAWF can be divided into three types: goaf of an adjacent working face with small-section coal pillar width type (GFST), goaf of an adjacent working face with moderate-section coal pillar width type (GFMT), and goaf of an adjacent working face with large-section coal pillar width type (GFLT). (2) In the goaf of a working face, the offset distance from the maximum porosity value area of each overlying rock stratum to the middle of the rock stratum is positively correlated with the distance between the overlying strata and the coal seam floor. In the area affected by the section coal pillar (ASCP), the porosity of each overlying rock stratum increases with an increase in the section coal pillar width, but is still smaller than its own initial porosity, and its increase rate continuously decreases. (3) From the coal seam floor upward, the porosity spatial form distribution of overlying strata in the GFST and GFMT is described as follows: partial “dustpan” shape–unilateral “concave-convex peak” combined shape. The porosity spatial form distribution of overlying strata in the GFLT is described as follows: “dustpan” shape–“concave-convex peak” combined shape-“Λ” shape.

Coevolution mechanism and branch of pillar‐overburden fissures in shallow coal seam mining

“…The width b m of the fracture zone of the section coal pillar can be obtained by substituting each parameter into Formula (23)  (24) According to the mechanical calculation model, the lagging breaking step rock beam structure of the subkey stratum and the hinged rock beam structure in the main key stratum are mainly carried by the section coal pillar fracture zone b m. The load in this area can therefore be simplified. The calculation expressions of the crack zone of the section coal pillar can be obtained by substituting Formulas (22) and (24) into Formulas (20…”

“…From the above analysis, the support resistance R required for the mechanical structure of the coal pillaroverburden rock (or fissure) to maintain stability during the mining process of the working face can be calculated simultaneously through Equations ( 19), (24), and (25). However, due to the cumbersome calculation, there is little traceability here.…”

“…Concurrently, O–X breakage and collapse of the working face with the coal pillar as the boundary lead to the stress concentration phenomenon of a triangular area of the working face 20–22 . In addition to the influence of coal pillar support, surface fissures (cracks) are mainly controlled by the fracture structure of the upper key stratum of the coal pillar 23,24 . When a step rock beam structure is formed in the fractured rock strata, the surface fissures are mainly step dislocation accompanied by a small number of horizontal dislocation cracks 25,26 .…”

“…[20][21][22] In addition to the influence of coal pillar support, surface fissures (cracks) are mainly controlled by the fracture structure of the upper key stratum of the coal pillar. 23,24 When a step rock beam structure is formed in the fractured rock strata, the surface fissures are mainly step dislocation accompanied by a small number of horizontal dislocation cracks. 25,26 When a hinged rock beam structure is formed in the broken rock stratum on the upper part of the coal pillar, surface fissures (cracks) are mainly horizontal dislocation cracks, and a small amount of step dislocation fissures form at the edge of the fissures.…”

Disaster-Causing Mechanism of Hidden Disaster-Causing Factors of Major and Extraordinarily Serious Gas Explosion Accidents in Coal Mine Goafs

“…The irregularly shaped collapsed rock mass is subject to compaction degree differences, resulting in numerous hole and crack structures in the goaf. This structure not only provides air leakage channels but also provides a suitable space for gas accumulation [4][5][6][7]. Under the action of air flow in these air leakage channels, notable spontaneous combustion of residual coal in the goaf can occur [8][9][10].…”