1. College of Mining Engineering,Taiyuan University of Technology,Taiyuan　 030024,China; 2. Shanxi Coking Coal Group Co. ,Ltd. ,Taiyuan　 030024, China

The morphological and fabric characteristics of Karst collapse pillars seriously influence the interpretation of the geophysical data,mining process,and the prediction of hydrogeology situations in North China coalfield. The field observation on the surface and in the roadway,overlay comparison,and statistical analysis were used to study the mor- phology characteristics of the collapse pillars in the Xishan coalfield including its height,span,pillar wall angle,and composition. The diameters of most pillars in mining roadways are 30-50 m,the heights of pillars from the top of Ordo- vician limestone are uneven,from 50 m to 350 m,and the pillar wall angles are 82°-85°. The ‘ pot-bottom’ structure of stacked rocks can be seen on the soft rock layer which covers the top of the pillars. The structure and composition characteristic of collapse pillars is divided into three parts. In the top of the pillars,it is observed that the development of caves,high water content as well as secondary minerals crystals. In the middle of the pillars,the pillar body is highly dense and dry. Fragment rocks with slickenside surface or scratch structure exhibit inlaid structure. And the gap of large blocks is filled by fabrics as debris flow. In the bottom of the pillars,the sorting changes better,the particle size decreases,and fragments connect closely when compared with other parts of the pillars. The diameters of the irregular edges fragment rocks are 20-30 cm on average ( maximum to 3 m,minimum to 1 cm). Closing to the margin of the collapse pillars,the dip direction of the coal seam incline to the pillars,the coal quality clearly become lower,and many fractures inclined to the pillars appear. The maximum falling distance of the blocks in pillars is 40-200 m. In the destruction process of the North China Craton,the evolution process of the structure and geomorphology in different ar- eas is different,which leads to the different evolution process of the Karst water system in the subsurface system,and further leads to the different formation time,distribution density,morphology and fabric characteristics of pillars. The distribution of the pillars is in the path of the Karst underground water which formed in the weak layers. The evolution of multi-terms of Karst underground water could lead to the overlaid of the pillars which will cause the activation of the previous pillars. The weathering of the fabrics in pillars is the critical signal of the activation which is one of the key factors of water transportation in pillars. Finding and comparing the root of Karst collapse pillars will provide some in- sights into detecting and distinguishing the younger and elder pillars when using geophysical methods.

ZHAO Jingui,GUO Mintai,LI Wensheng. Morphological and fabric characteristics of Karst collapse pillars in Xishan coalfield[J]. Journal of China Coal Society,2020,45(7):2389-2398.