Developmental Regularit And Reduction Control Technology Of Mining-induced Ground Fissures Of Shallow Burial Coal Seam In Mountainous Area Of Southwestern China

Significantly different from the geological environment of coal seams and the development regularity of mining-induced ground fissures for the shallow burial coal seam distribution area in Northwestern China,the shallow burial coal seam in mountainous area of Southwestern China has its unique characteristics of close-distance coal seam groups,repeated mining,peak cluster landform and "thin surface soil layer and thick bedrock".Therefore,the mining-induced ground fissures have their own development regularity.Taking the mining-induced ground fissures for shallow burial coal seam in the mountainous area of Southwestern China as the research object,taking the development regularity of mining-induced ground fissures and its reduction control as the breakthrough point,taking field measurement,laboratory experiment,numerical simulation and theoretical analysis as the research method,fully utilizing the theoretical knowledge of mining science,mining subsidence,elastic-plastic mechanics,geo-mechanics and so on,the dynamic and static development regularity,the development mechanism and the reduction control method of mining-induced ground fissures are systematically studied.The main research results are as follows:(1)Through statistical analysis of coal seam occurrence characteristics,stratum characteristics,geomorphological characteristics and surface soil characteristics of typical shallow burial coal seam mines in mountainous area of Southwestern China such as Faer Coal Mine,Hongfa Coal Mine and Xinglin Coal Mine,the classification system of shallow burial coal seam occurrence in mountainous areas is established,it provides specific reference and basis for guiding the classification of shallow burial coal seam occurrence conditions.Five prominent characteristics of shallow burial coal seam in mountainous area of Southwestern China are summarized,which are close-distance coal seam group,repeated mining,peak cluster landform,rock-soil combination structure of "thin surface soil and thick bedrock" and stratum lithology of"upper hard and lower soft".(2)The roof structure is divided into four types:thin direct roof and basic roof,medium thick direct roof and basic roof,layered roof and single thick hard roof.The intrinsic relationship between the width of ground fissures and roof structure is revealed.The mining-indcued ground fissures of shallow burial coal seam in mountainous area are divided into three types:expanded-type,sliding-type and stepped-type.Based on the site survey of the development location of mining-induced ground fissures,the optimum development location of ground fissures and their"divisional characteristics" are clarified.(3)Compared with the distribution area of shallow burial coal seam in Northwestern China,there are two significant differences in the development characteristics of ground fissures for shallow burial coal seam in mountainous area of Southwestern China:1.Significantly different from the inverted "C" pattern of ground fissures in Northwestern China,the spatial distribution of ground fissures for shallow burial coal seam in mountainous area of Southwestern China is significantly affected by the change of geomorphology,and the distribution pattern of ground fissures has no obvious "C" type.2.It is obviously different from the rule that the extension direction of ground fissures is roughly parallel to the inclination of working face for shallow burial coal seams in Northwestern China.The extension direction of ground fissures for shallow burial coal seams in mountainous area of Southwestern China is different,and is roughly parallel to or obliquely intersected with the striking direction of contour,but not parallel to the inclination of working face.(4)Dynamic development regularity of mining-induced ground fissures was revealed.The newly developed ground fissures develop at a certain distance ahead of the advancing position of the working face,and the advance distance is positively correlated with the slope of the mountain and the depth of the coal seam.The development scale of ground fissures presents dynamic law with the advancing of the working face.When the advancing position of the working face changes from lagging behind to ahead,its development width presents a process:increases slowly,increases rapidly,decreases slowly,and tends to be stable,while its fall presents a process:increases slowly,increases rapidly,and tends to be stable.The dynamic extension process of the growth length of ground fissures can be divided into three stages:slow growth stage,rapid growth stage and stable stage.When the development width and the fall of ground fissures tend to be stable,the dynamic development of ground fissures mainly shows the dynamic extension of development length.(5)The development characteristics of mining-induced ground fissures during single-layer mining and repeated mining are discussed in detail.The development scale of ground fissures for single-layer mining is not continuously increased under influence of repeated mining,but is a dynamic process of "closure→expansion→stability" based on the original development scale.The development of ground fissures during repeated mining can be divided into five stages,i.e.the continuous expansion stage on the basis of single-layer mining,the gradual closure stage under influence of repeated mining,the secondary development stage under influence of repeated mining,the gradual expansion stage under influence of repeated mining and the stable development stage under influence of repeated mining.The development width,fall and depth of ground fissures are not increased under influence of repeated mining,but mainly by a certain development scale in a mining stage.(6)The specific characteristics for the influence of mining height,slope gradient and slope fluctuation on the development form and scale of mining-induced ground fissures are clarified.The response characteristics of surface subsidence and overburden movement to the development of ground fissures are expounded.The development position of ground fissures is closely related to the area where horizontal tension deformation occurs.Significantly different from shallow burial coal seam mining in Northwestern China,there is a distinct "strata pressure appearing partition"for shallow burial coal seam mining in mountainous area of Southwestern China.(7)The critical value of deformation and failure of surface soil with unequal surface gradient is obtained,and the development criterion of ground fissures is obtained.A rectangular block structure model on one side of ground fissures is established and its boundary shear stress value is solved.The intrinsic relationship between the development of ground fissures and the activity of slope body is clarified.The development position of ground fissures is closely related to the fault line of slope body.The structural model of slope body is established and the criteria for its rotation and sliding instability are obtained.Sufficient conditions for the formation of stepped-type and sliding-type ground fissures are obtained,that is,when the fracture form of thick and hard roof is "stepped rock beam" and the active position of slope is"sliding instability",the development type of ground fissures is mostly stepped-type;when the fracture form of thick and hard roof is "masonry beam" and the active position of slope is "rotary instability",the development type of ground fissures is mostly sliding-type.(8)According to the practical experience of field engineering,five principles of ground fissure reduction control of shallow burial coal seam in mountainous area are summarized,that is,the principle of differentiation of divisional treatment,the principle of weakening surface movement and deformation,the principle of scientific deployment of mining system,the principle of dynamic ecological restoration and the principle of integration of safety and economy.This paper summarizes eight ways of working face layout and concludes that the four layout modes of working face,i.e.outward staggered configuration(inward staggered configuration),inward stacked configuration for coal pillar spacing,outward stacked configuration for coal pillar spacing are the best choice for mining layout of coal mine.The mining damage zones of different working face deployment modes are divided into zones A(overlapping area),B(inward staggered area),C(outward staggered area),D(boundary alignment area)and E(coal pillar area).This paper puts forward a cooperative treatment technology combining underground "solid material backfilled and strip mining" and ground fissure "differential treatment".Practical results show that this technology effectively reduces the degree of surface subsidence and the development of ground fissures,conforms to the scientific connotation of "green mining".