Dynamic Characteristic Of Hard Roof Fracture In Extra-thick Coal Seam And Its Application On The Control Of Roadway Surrounding Rock

Shuonan is a newly constructed mining area characterized by both thick coal seam and complicate geological condition. As there are several hard roofs which are difficult to collapse above the coal seam, the working face and gob-side entry appear strong dynamic characteristics during the mining period, which threatens the safe and highly-efficient production seriously. Based on the regional tentonic characteristics, stratum occurrence conditions and mining technical requirements of Majialiang mine, the dissertation studied systematically the characteristics of strata behavior and dynamic loading mechanism of fully mechanized top-coal caving of extra-thick coal seam by using comprehensive research method consisting of on-site survey, theoretical analysis, numerical simulation, physical similar simulation test and on-site monitoring. Then it was put forward that the weakening control principles and technical methods based on the hydraulic fracturing techniques and anti-dynamic loading support system.(1) Influenced by geologic structure and mining techniques, many strong characteristics of strata behavior, e.g. high pressure at the working face, coal wall spalling, hydraulic support damage, large deformation of gob-side entry, supporting structures broken, frequently appeared. Moreover, the strata behavior were also companied by other features, such as lack of initial hydraulic support resistance, large step and large strength of roof weighting, large dynamic coefficient, switch between big/small periodic weighting, large influencing range of abutment pressure, and periodical dynamic deformation characteristics of gob-side entry.(2) Having obtained the caving mode and caving step of double-hard roof in extra-thick coal seam, the study pointed out that the rupture instability of double-hard roof would generate “cantilever beam” and “masonry beam”in below and upper layers respectively. Then the study revealed that the main factor inducing strong strata behaviors at working face was the rupture instability of bottom double-hard roof in extra-thick coal seam.(3) Based on the determination of the main key strata position of Majialiang coal mine, the study established the dynamic load model for the rupture of hard roof and revealed the relationship between dynamic strength of working face and gob-side entry and that of hard roof. Then the mechanism of dynamic load characteristics in working face and gob-side entry and the main controlling strata were determined respectively.(4) The study analyzed the influences of the property and direction of dynamic stress wave on the stability of roadway surrounding rock. Then it was put forward that the superimposed effects of dynamic stress brought about tensile stress in the heading wave side and back wave side of roadway, which caused the accumulated damage and strong deformation. The dynamic response characteristics of roadway surrounding rock under supported/unsupported and static/dynamic conditions have obtained through numerical simulation.(5) By studying the effects of different hydraulic fracture modes, the study put forward the hydraulic fracture weakening control technology for hard roof, and determined the reasonable construction technology and key parameters. The study also optimized the roadway layout mode and parameters, and developed a anti-dynamic support system and related designing method by using high strength S-shaped bolt. The above controlling technology has applied into the field engineering successfully.