Influence of magma intrusion on rock mechanics characteristics of coal seam roof in Liujiang Basin

There are two coalfields of CarboniferousPermian and Jurassic in Liujiang Basin.The basin experienced several periods of magmatic intrusion and eruption after coal formation,and the magmatic activities had transformed coal seams and their surrounding rocks.This study investigates the influence of magmatic activity on the mechanical characteristics of coal seam roof in the main carboniferous and Permian coalfields in order to provide rock mechanics parameters for the stability evaluation of coal seam roof and floor,mine design and equipment selection,and also provide reference for coalfields with the same or similar geological conditions.Firstly,the influence of magma intrusion and eruption mode on coal seam roof in the basin is analyzed.Then,according to the approximate location of the ancient crater and the structural characteristics of the basin,the exploration project will be deployed to carry out surface geological survey,core geological drilling,core fine geological logging and engineering geological logging in detail.Subsequently,11 boreholes on 4 main sections are selected,and the systematic rock mechanics and stratified sampling and test analysis of coal and rock are carried out on the rocks exposed by boreholes at different magmatic rock intrusion positions of coal seam 3 and its roof,and on the rocks of the same intrusion position and different lithology.Finally,the influence of magmatic rock intrusion on coal seam and roof rock mechanics in the basin is comparatively analyzed and comprehensively evaluated,and the engineering geological problems existing in mining and the problems that should be paid attention to in the selection of mining equipment are pointed out according to the evaluation results.The results show that the multiple eruption and intrusion of Jurassic multi-stage magma in the basin have altered the roof of coal seam to various degrees,forming five types of direct roof lithologic association:phagocytosis,invasion,recombination,primary strengthening and primary type.The roof lithology of coal seam 3 is dominated by diorite porphyrite intrusion-phagocytosis type,followed by intrusive roof,compound roof and primary roof.The comprehensive effects of magma intrusion,baking,filling and extrusion have strengthened the engineering geological characteristics of coal seam roof,which are shown as increasing density,decreasing porosity and water content,and enhancing key mechanical parameters such as compressive strength,shear strength and rock mass integrity.According to the drilling and mine roadway exposure,five magma phagocytosis zones have been preliminarily delineated in the basin,all of which are located in ancient volcanic vents or magma ejection channels.Along the phagocytosis zone,the roof type of coal seam gradually changes into intrusive,composite and primary zones,with a ringshaped distribution rule on the plane.In the mine design,the gobup area can be used as the ideal location for the key chamber of the mine.In the mining process,the gobup area is the key factor that determines whether the mine can normally be operated.The compound roof area is the area where roof accidents frequently occur.The roof in the intruded area has a hard roof hanging for a long time.It is necessary to adopt manual forced roof caving measures.It is the key area for the management of mine rock burst and the key problem for the safe production of the mine in the basin.The primary area is mediumprone to roof caving.In the selection of mine equipment,the strengthening effect of magma intrusion on the mechanical characteristics of coal and its roof rocks must be considered.In order to ensure the normal mining of the mine,the effective exploration and advanced pretreatment of igneous rock dirt bands,phagocytes and reformed hard coal in the working face must be carried out before mining.