Research On Mechanical Properties Of Cementing Casing Of Surface Boreholes Using Gas Drainage In Mining Subsidence Area

Relieved and gob gas drainage using surface boreholes is an effective method of gas control. However, due to the effect of mining activities, when the working face pushed through the surface boreholes, they are often prone to damage and lose their extraction efficiency. This paper adopts some methods, such as theoretical analysis, laboratory test and numerical simulation combined with field application. We designed and developed an experimental platform, which was used to study shear failure of borehole casing. We respectively studied the change of the number of borehole casing layer and double-layer borehole casing within different filling materials, which have influence on the mechanical properties of borehole casing. By combining with the method of numerical simulation, we know the rule of shear failure of surface borehole casing. Research achievements have been carried on the practical application in Yuwu Coal Mine in Shanxi Province.Through theoretical analysis and using the FLAC3 D calculate mining process of coal seam, we get the movement distortion rules of overlying rock of gob:(1) In the process of coal mining, nonuniform movement and deformation of overlying rock have serious damage on surface borehole casing. While their damage mainly has the following two failure modes: tensile failure mode and shear failure mode.(2) Along with working face is constantly moving forward, tensile failure and shear failure area are increasing, which is the most concentrated damage area from the roof within the scope of 23.2 to 48.2 m, even at the highest of 90.4 m also appears more serious damage. And the area is mainly close to the intake airway and return airway within 0 ~ 30 m.Through study the change of the number of borehole casing layer and double-layer borehole casing within different filling materials, which have influence on the mechanical properties of borehole casing. We reached the following conclusions:(1) Service life and maximum displacement deformation of double-layer borehole casing are respectively 2.5 times and 2.33 times that of single-layer borehole casing, which indicate that using double-layer borehole casing can improve its shear performance and prolong its service life.(2) The maximum load force and the shear stress that double-layer borehole casing can withstand are associated with the filler materials: When the hardness of the filler materials are greater, the maximum load force and the shear stress are bigger. When the load force of double-layer borehole casing is very big(It surpassed its maximum load force that it can withstand.), the hardness of the filler materials are smaller, its service life is longer.(3) From the view of engineering practice, filling cement grout into double-layer borehole casing can not only improve its non-deformability and service life, but also have some following advantages: convenient to operate, low cost, and so on.Research achievements have been carried on the practical application in Yuwu Coal Mine in Shanxi Province. The effect of gas drainage using surface boreholes is remarkable: Amount of gas drainage from 3# surface boreholes are about 2 times that of 1# surface boreholes and 2# surface boreholes, which its gas drainage time is the longest. So we reach the conclusion: Filling cement grout into double-layer borehole casing can improve its non-deformability and resist damage significantly.