Research On The Evolution Law Of Surrounding Rock Cracks In The Protected Layer Mining Tunnel During Close-range Coal Seam Group Minin

There are a large number of primary fractures in coal and rock mass.When the stress of coal and rock mass is lower than its ultimate failure strength,the internal fractures of rock mass will expand,and secondary fractures will accompany it.The stability of coal and rock mass will be greatly affected when the primary fractures and secondary fractures are interconnected.During the downward mining of close coal seams,the gob side entry of the lower protected layer is disturbed by the mining of the upper protected coal seam,and the primary fracture expands and evolves,which greatly affects the stability of the overburden of the gob side entry.Based on the engineering background of the minable coal seams C6 and C9 in Sheng’an coal mine,this paper concludes that the key factors affecting the overburden of the goaf side entry in C9 coal seam are the interval and the size of the roadway protection pillar through theoretical analysis.3DEC simulation software is used to study the fracture law of the underlying overburden at different interval and the fracture evolution law of the surrounding rock of the goaf side entry in the protected layer under different pillar widths,Further,the distribution characteristics and evolution law of roadway fracture are verified by similar simulation experiments and on-site borehole peeping method.The main conclusions are as follows:(1)According to the elasto-plastic theory,the parameters such as the depth of floor failure after the mining of the upper protective coal seam are obtained,and the classification basis for distinguishing the overburden structure is obtained.Based on the relative position between the damage depth of the floor of the upper protective coal seam and the direct roof between the upper and lower coal seams,the interlayer overburden of the upper and lower coal seams can be divided into four categories: no basic roof between layers,partial damage basic roof between layers,complete damage basic roof between layers and complete basic roof between layers.There is a significant difference between the complete interlayer damage and the interlayer no basic roof overburden fracture law and the single coal seam mining,and there is no significant difference between the partial interlayer damage basic roof and the interlayer complete basic roof overburden fracture law and the single coal seam mining.(2)By analyzing the fracture position of the basic roof;There are four basic forms of basic roof fracture location;The fault is above the solid coal body,above the roadway,above the goaf and above the roadway.The basic roof fracture has the least influence on the roadway above the goaf.(3)There is no obvious difference between the overburden breaking law and that of a single coal seam when the protected coal seam is mined under the downward mining of a close coal seam group and the basic roof between the layers is complete.When the interlayer rock structure is no basic roof structure or completely damaged basic roof structure,the fracture law and fracture development law of the overlying strata are significantly different from that of a single coal seam.When the interlayer structure is a structure without basic roof: the initial pressure step of the lower protected coal seam is less than 30 m,the periodic pressure is about 10 m,and the interlayer fissures are the most dense before the initial pressure of the lower protected coal seam.When the interlayer structure is a completely damaged basic roof structure:the initial pressure step of the lower protected coal seam is less than 40 m,and the periodic pressure is about 10 m.When the interlayer structure is a complete basic roof structure: the initial pressure step of the lower protected coal seam is less than 40 m,and the periodic pressure is about 20 m.(4)The failure mode of surrounding rock of gob side entry is mostly shear failure,and the cracks are mainly distributed in the surface of surrounding rock.As the coal pillar size becomes smaller,the surrounding rock fissures become more dense as a whole,and the fissures expand outward evenly.The height of the roof fissures is about 3.5m,and the side fissures are about 3M.Based on comprehensive consideration,the coal pillar for roadway protection is determined to be 13 m.(5)Taking the short-range coal seam group composed of C6 and C9 in Sheng’an coal mine as the background,the experiment was carried out in the working face of C9 coal seam.After the 10903 working face of C6 coal seam was mined out,13 m coal pillar was reserved to drive 10905 return air roadway.After the verification of borehole camera,there were sparse cracks at about 3.5m of the roof,about 3M of the coal pillar slope and about 3M of the solid coal slope,and the coal rock mass at the roof close to the mined out floor of C6 coal seam was seriously broken.The roadway fractures are mostly concentrated in shallow fractures,which still have a certain supporting capacity,and the safety of 10905 working face can be ensured with the help of corresponding support.