The characteristics of Karst Collapse Pillar Wall Angle and its significance in Xishan Coalfield

With the extension of mining to the deep, karst collapse pillar has become one of the most hidden geological factors causing disaster in coal mining in North China. Pillar wall Angle is one of the important indexes to describe the morphological characteristics of collapsed pillars. Based on 265 karst collapse pillars cut by fully mechanized mining in upper and lower coal seams in Xishan coal field as data sources, the geological information contained in the wall corner of karst collapse pillars in Xishan coal field was excavated through zoning, statistics, function construction and geological analogy, and the following understanding was obtained: (1) The average pillar wall Angle of the collapse pillar in Xishan Coalfield is 82.49°, and the pillar wall Angle of over 50% of the collapse pillar is between 85 and 90°. The average pillar wall Angle increases gradually from northwest to southeast, from 82.05° to 87.57°. (2) The wall Angle of the collapse pillar in Xishan coal field is inversely proportional to the area of the collapse pillar revealed by No. 8 coal seam. According to the relationship between the wall Angle of the collapse pillar and the area of the collapse pillar, the collapse pillar can be divided into: full collapse pillar (90° ~ 85°), sub-full collapse pillar (85° ~ 81°) and inadequate collapse pillar (< 81°); The corresponding subsidence pillar area of No. 2 coal seam is less than 556 m², between 556 and 1700 m², and greater than 1700 m², and the equivalent radius is less than 13.3m, 13.3 m-23.3m, and greater than 23.3m. The corresponding collapse pillar area of No. 8 coal seam is less than 1250m², between 1250m² and 2750m², and greater than 2750m². The radius is less than 20m, between 20 and 30m, and greater than 30m. The fully collapsed collapse pillar is an important type of water-conducting collapse pillar in Xishan coal field, which generally has the top cavity and the top cavity cavity, and the pillar structure is relatively loose. (3) The rainfall results of the geological historical period show that the karst collapse pillar in the Xishan coalfield with "north pillar and south phase" may have formed in the Paleo-Oligocene period with hot and humid climate. The strong karst process absorbed atmospheric CO₂ and locked it in the sedimentary area, which may be an important reason for the sharp reduction of global atmospheric CO₂ content.