| The prevention of roof accidents has always been the focus of coal mine disaster prevention.With the development of science and technology,coal mining technology of large mining height has gained wide application for the mining of thick coal seams.Increase of the height of the mining could increase the risk of rib spalling and roof falls,and tectonic fissures in the unfavorable geological bodies such as faults,subsided columns and fold,would develop again under the action of mining-induced stress in the process of mining,thus increasing the probability of occurrence of rib spalling and roof falls.At present,in term of the problems caused by unfavorable geological bodies,the passive emergency treatment is mainly applied to projects,that is,the chemical material grouting is used when the working face is advanced to the area affected by unfavorable geological body,roof falls occur.Its existing problems are high cost and hidden dangers.What's more,it also greatly affects the mining speed of the working face and seriously lowers the advantages of mining of the large mining height working face safely and efficiently.Pre-grouting in front of the working face can effectively solve the above problems.When using pre-grouting,the position of the geological body cannot be accurately located first;secondly,tectonic fissures are filled or extruded so that the amount of grouting is too small to meet the reinforcement requirements.Finally,there is a lack of grouting material and corresponding grouting reinforcement technology for grouting reinforcement of unfavorable geologic bodies.In this paper,on the basis of summarizing the unfavorable geologic bodies damage,detection technology and treatment technology,methods of numerical simulation,theoretical analysis and field measurement were employed to investigate the distribution law of the front abutment pressure and the development law of the cleat in coal body under the front abutment pressure to illustrate timeliness grouting reinforcement.At the same time,according to the demand for advanced pre-grouting of unfavorable geological bodies,grouting materials applied to grouting reinforcement of unfavorable geological bodies have been developed.Aiming at the particularity of grouting reinforcement in the front abutment pressure zone,timeliness grouting reinforcement technology was proposed.According to the specific location and scale of the unfavorable geological bodies obtained by the slot wave detection,the corresponding grouting reinforcement scheme was formulated,and the industrial test was carried out at 1307 panel in Zhaozhuang Mine.The grouting effect was tested by various evaluation methods.Through the research,the following conclusions were obtained:(1)The unfavorable geological bodies of the large mining height face were mainly faults,collapse columns,fold.The unfavorable geological bodies have structural cracks and would change continuity and stress state of the coal body,mining would cause the panel stress to redistribute.Under the effect,tectonic fissures in unfavorable geological bodies was forced to develop and reduce the carrying capacity of coal,therefore,rib spalling and roof falls occurred easily.The distribution law of the front abutment pressure of the 1307 large mining height working face in Zhaozhuang Mine was investigated in this paper by employing methods of theoretical analysis,numerical simulation and field measurement.The results showed that the peak position of the front abutment pressure was located at 8.3?13.2m in front of the working face,the peak stress was about 40 MPa,and the influence zone of the front abutment pressure was about 40?50m in front of the working face.(2)According to the distribution law of the front abutment pressure of the large mining height working face,the development law of coal body fissure under mining-induced stress was studied by numerical simulation and field measurement.The results showed that under the action of mining stress,a large number of fissures began to appear in the coal body 25?35m away from the working face,and the closer to the working face,the higher the degree of coal fissure development.According to the results of the water injection test,the front abutment pressure was divided into sharp growth zone,slow growth zone,and primary rock stabilization zone,and the ranges were 5?15 m,15?33 m,and 33?42 m,respectively.The experiment of loading on coal sample showed that when the coal sample reached about 80% of the peak strength of uniaxial compression stress,the sound velocity dropped sharply and the acoustic emission event increased sharply.The results of the axial compression and unloading confining pressure test under the action of mining-induced stress indicated that the crack began to develop when the axial pressure reached 20-25 MPa.(3)To meet the grouting material applied to the requirement of unfavorable geological bodies,high-strength and double liquid quick setting grouting material had been developed.The key of the material was strong permeability and fluidity.The strength of 24 h could reach more than 20 MPa,and the strength of 7 days could reach45 MPa.Double liquid material was composed of two components: the slurry did not solidify within 6h,and after mixing,the material would not flow in 1?5 minutes,later it hardened in 5?10 minutes,the stoning rate of the slurry could reach 100% at a water-cement ratio of 0.8?2:1,the strength could reach 8?15 MPa within 2h,and the final strength could reach 23 MPa.The material would solidify normally at 0? and its supreme strength could be attained when water temperature was 20 ?,curing temperature was 0?.(4)The deformation and failure characteristics of coal samples under different loading conditions were studied by employing the method of loading coal samples by means of uniaxial compression,cyclic loading and shaft compression and unloading confining pressure.The results showed that the peak strength of coal samples was24MPa?28MPa under uniaxial loading.Due to the heterogeneity of coal samples,the stress-strain curve during cyclic loading showed a typical "sawtooth" shape with peak strength of 16MPa?37MPa.Coal samples,typical brittle materials,exhibited shear failure characteristics upon loading.The triaxial test results showed that the peak strength of the coal sample increased continuously with confining pressure increasing.When the confining pressure rose to a certain extent,the peak stress hardly changed.The results of the axial compression and unloading confining pressure test showed that the stress-strain curve was more complicated in the first stage of loading,but it was roughly concave.In the second stage,stress and strain were roughly linear.In the third stage,the axial pressure continued to increase with the confining pressure decreasing,and the coal sample began to break.(5)Experimental results of coal sample reinforcement body showed that the wave velocity decreased after loading,and it would reach the wave velocity of the original coal sample after grouting reinforcement,indicating that grouting reinforcement could efficiently backfill the cleat in coal to improve the integrity of coal samples.After grouting reinforcement,the overall bearing capacity of broken coal samples greatly increased,and the bearing capacity of the broken coal sample would be restored to a larger extent.the strength of grouting reinforcement body could attach 68%?85% of the strength of the original coal sample,The grouting reinforcement body experienced bulging failure during the loading process,and the plasticity of the brittle coal samples was greatly improved after grouting.The ratio of lateral strain to vertical strain became smaller,improving the overall stability.The results of timeless loading test of coal sample and reinforcement body showed that axial load and confining pressure had great influence on axial deformation of coal sample.Time effect increased axial displacement and lateral displacement of coal samples and reinforcement body to make the experiment occur easily.After 12 d loading,axial displacement was1.68 mm,the axial displacement of the direct loading was 2.44 mm,while the axial displacement of the 7d loading was 2.28 mm.The results was little different,indicating that the earls strength was good for stability of grouting reinforcement body.The curve of lateral deformation and vertical deformation showed that grouting reinforcement could lower the ratio to improve the overall stability of coal samples.(6)The mechanism of timeless grouting in front abutment pressure zone was illustrated from the grouting material,grouting timing and grouting process,respectively.Through researches,it was able to ensure that development degree of cracks in the 0-30 m area in front of the working face could meet the need of grouting reinforcement.The grouting principle was to focus on rib spalling.The timeless grouting reinforcement technical system mainly included: grouting equipment,grouting system and grouting technology.Double-liquid grouting pump and self-developed constant volume tank and high-speed pulping machine were adapted as grouting equipments.In the grouting process,the double-liquid quick-setting grouting material was applied to shallow hole grouting,and the surface crack was sealed by the low-pressure and slow-injection method,and the slurry leakage problem in the grouting process was solved at the same time.The single-liquid high-strength grouting material was applied to deep-hole grouting.The designed water-cement ratio was 0.6?0.8:1,the grouting pressure was set to 15?20 MPa,and the method of high-pressure fast-injection was adopted to increase the slurry diffusion radius.(7)In the 1307 large mining height working face of Zhaozhuang Mine,the industrial test was carried out,and 136 holes were arranged in the bad geological body.The grouting material was used for 805.12 t,and the average grouting volume was5.92 t per hole,the grouting amount of the largest single hole was 11.3t,and the grouting pressure was basically maintained at about 15 MPa.After the grouting,the rate of roof falls was reduced by 86%,which improved the condition of the rib spalling on the working face.The amount of chemical slurry used for emergency treatment reduced by 69.3%,and the working face advancement speed increased by73%.The borehole peeping showed that after grouting,the coal body cracks were filled with slurry,indicating that the grouting effect was better.Furthermore,it also showed that the time-dependent grouting reinforcement technology for unfavorable geological body in large height mining is creative and practical. |
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