Failure Mechanism And Flexible Reinforcement Technology Of Coal Face In Thick-coal Seam

In view of the actual of coal face failure and roof caving influencing production in thick-coal mining. The coal face failure forms were summarized through field investigation, combined with the test of coal sample intensity, the factors influencing coal face failure were obtained. According to the factors influencing coal face stability, constructing mechanical model coal face failure on “roof-coal face-support” and analyzing the sensitivity of coal face stability factors. Having developed a test platform which comprehensively considered coa l face pressure, support resistance and coal face height and conducted similar simulation experiment of coal face stability in large-cutting- height working face using this test platform; Using UDEC2 D software to simulate the deformation and damage of coal face under different influencing factors. Combining the results of theoretical analysis, similar simulation, numerical simulation, the deformation and failure mechanism of coal face in thick coal seam were systemly analysed and summarized. In view of the characteristics of coal face failure, the full length anchorage flexible supporting technology of "manila+grouting" was proposed. The mechanic property of manila is analyzed by using the drawing experiment, and having developed the strength experiment of grouting of broken coal samples to study the strength characteristics. The mechanism of coordinate deformation between manila and coal face is revealed by building the constitutive model of "Manila- serous-coal". The research results are as follows.(1)The Coal face failure mainly has five external manifestations: the upper failure, lower failure, top and bottom damage, and the overall damage. The inner forms are shown as shear failure and tensile failure, of which shear failure is dominant and tensile fracture is seldom appear, what’s more, the smaller coal strength is, the more possibility the shear failure occures.(2)Adopting point load instrument which is easy to carry, simple to operate and widely used have conducted the strength of irregular shape coal samples. The rebound hammer can be directly used to test the strength of coal face in situ, which is helpful for guiding production because of it is closer to actual working condition. Therefore, the rebound hammer and point load instrument can be used to test the coal rock mass strength of coal face.(3)The factors influencing coal face failure are roof pressure on coal face, confining pressure and coal body strength. And the roof pressure on coal face is relevant to mining height, supporting capacity and face flipper protecting, mining angle, advancing speed. The confining pressure is associated with mining depth of working face and the strength is mainly affected by the cohesion and internal friction angle of coal body.(4)The roof pressure on coal face is the main reason casuing coal face failure. The higher mining height is, the smaller the roof pressure on coal face is, thus, the more easily coal face failure is. Increasing cohesion and internal angle of coal mass can improve the stability factor of coal face, but effect of increasing internal angle is small support capacity, and decreasing roof load of coal face are the key to improve coal face stability. The role of face flipper protecting in preventing broken coal body from sliding down is large, but it is small in prevent plastic zone from further enlargement.(5) The larger is mining height, the smaller is the critical pressure casuing coal face failure and the worse is the coal face stability. The more is the coal strength, the larger is roof pressure coal face can stand, thus, the better is the coal face stability. The cracks occure in the upper of coal face first, then with the development, expansion and penetration of cracks, the upper coal face is failure. And the main form is shear failure. With the increase of coal cohesion, the layered tensile fracture occurs in the middle and upper of coal face first, but the final failure is shear failure. The x-displacement of coal face is larger before coal face is failure in soft coal seam, thus, coal face has the characteristics of large deformation.(6) The larger is mining height, the wider is the plastic zone. The larger is the support capacity, the smaller is the roof pressure on coal face, thus, the more difficult is coal face failure.But it is limited for support to control coal face failure.With the increase of coal cohesion and internal angle, the deformation and failure area of coal face become smaller gradually, but the effect of coal internal angle is not big. The larger is the mining angle, the worse is the coal face deformation and failure. The deformation and failure area of coal failure is larger during periodic pressure and initial pressure, increasing advancing speed can relieve the failure degree of coal face, but it can’t control radically coal face failure. The X-displacement is very large before coal face occures failure. Coal face have the characteristics of large deformation, and exposed gradually which is obvlously affected by occurrent characteristics.(7) The key point of the coal face control is to prevent larger coal body falling down, achieving safe and efficient mining of working face. In view of the low elongation, small supporting strength, high cost, poor shear capacity, short cycle progress existing in the traditional coal face reinforcement technique, a flexible coal face reinforcement technique is proposed. Flexible reinforcement mechanism of coal face is that coir rope which has certain rigidity and elongation is better adhered to the coal body by grouting, forming a full length anchorage into the coal face. At the same time, flexible material can adapt to the large deformation caused by woking face mining, so as to prevent broken coal body falling down.(8)The tensions of coir rope whose diameters are 14 mm,18mm,21 mm,24mm are 34 MPa,36.7MPa,32.5MPa,61.3MPa, the values of which are larger than tensile stress casuing coal face failure. The average elongation were 8.3%, 10.7%, 9.6% and 15.3%, and the value of them are much greater than that of the cable and anchor, which can substitute anchor bolt and anchor cable in controlling coal face failure when using the grouting reinforcement technique.(9) The mechanism of grouting for controlling coal face failure is increasing correspondly shear strength of coal face by increasing cohesion of coal, there exists a reasonable proportion of slurry making uniaxial compressive strength, residual strength, Strain, shear strength, cohesion force of coal samples reach the maximum; Coir ropes adhered into coal body have improved the shear strength of grouting coal samples; And when the coal samples are failure, the rope is not cut. At the same time, coal samples exhibited higher shear strength with the increase of load force.(10)The slurry of flexible reinforcement technology of “manila+grouting” can densely filling the coal body in plastic and fracture zone, improving the integrity of coal face, making fissured coal body restore to the approximate elastic state, so as to eliminate of effect of plastic deformation. According to site observation, the total deformation rate of coal face is 8% ～ 12%, which is smaller than 15% of the manila elongation. Therefore, manila can adapt to the characteristics of large deformation of coal face, and sufficient tensile strength of it can ensure the broken coal body can’t fall down.(11) The selection of rope diameter and grouting hole position are keys of coal wall damage prevention using “rope+grouting” flexible reinforcement technology. Different rope diameters have different damage prevention effect and there is a rational rope diameter which could maximize the coal wall damage prevention effect. With different position of the grouting hole, there could be a huge difference among grouting effects. The best position of the grouting hole locates at the initial damage point of coal wall, nearly 60%～70% of the mining height.(12) Coal face flexible reinforcement schemes and parameters in normal section, structural section and sections which are influenced by fault and bedding were determined through the optimization of drill hole diameter, rope diameter and grouting quantity. Combined with increasing initial support force, increasing support working resistance and strengthening working face management, flexible reinforcement technology has a significantly coal wall damage prevention effect in kinds of working face such as bedding coal seam working face with 6.3m large mining height, inclined upward mining caving face and inclined broken roof working face. Greatly shorten the time of handling accidents like coal wall damage and roof- fall. Meanwhile, coal mining machine operating rate increased significantly.