Pillar wall angles of karst collapse pillars in the Xishan coalfield: Characteristics and implications

Significance  As coal mining expands into deep parts, karst collapse pillars have become one of the most concealed disaster-causing geological factors in coal mining from mines in North China. The pillar wall angle of a collapse pillar serves as an important index used to describe the morphology of the collapse pillar.

Methods and Results  With 265 karst collapse pillars cut by the fully mechanized mining of the upper and lower coal seams in the Xishan coalfield as the data source, this study extracted the geological information contained in the pillar wall angles of karst collapse pillars in the Xishan coalfield through zoning, making statistics, function construction, and geological analogy, obtaining the following insights: (1) The collapse pillars in the Xishan coalfield exhibit an average pillar wall angle of 82.49°, with over 50% of them manifesting pillar wall angles ranging between 85° and 90°. Their average pillar wall angles increase gradually from 82.05° to 87.57° from northwest to southeast. (2) The wall angles of collapse pillars in the Xishan coalfield are inversely proportional to the areas of the collapse pillars revealed by the No. 8 coal seam. Based on this relationship, the collapse pillars can be categorized into completely collapsed pillars (90°‒85°), less completely collapsed pillars (85°‒81°), and incompletely collapsed pillars (< 81°). Correspondingly, collapse pillars in the No. 2 coal seam exhibit areas of < 556 m², 556‒1 700 m², and > 1 700 m² and equivalent radii of < 13.3 m, 13.3-23.3 m, and > 23.3 m. Furthermore, collapse pillars in the No. 8 coal seam have areas of < 1 250 m², 1250‒2750 m², and > 2 750 m² and equivalent radii of < 20 m, 20‒30 m, and > 30 m. The completely collapsed pillars generally exhibit cavities on their tops and loose structures, which can connect fracture water in the roofs to water in cavities on pillar tops and confined Ordovician limestone water in the floors. Therefore, completely collapsed pillars are significant hydraulically conductive collapse pillars in the Xishan coalfield.

Conclusions The analogy with rainfall in the geological history reveals that the karst collapse pillars in the Xishan coalfield, exhibiting the characteristics of karst sinkholes and caves in southern China, might have been formed during the Paleo-Oligocene with hot humid climates. During this period, CO₂ in the atmosphere was absorbed due to intense karstification and was then locked in sedimentary areas. This might be an important reason for the sharp decrease in global CO₂ concentration in the atmosphere.