| Gob-side entry retaining(GER)as the ultimate goal for gateway using,the retained effect depend on the geological conditions themselves and support quality of surrounding rock in the early stage.In this thesis,base on the different complex geological conditions of gateways in “Sichuan coal group” coalmines,and taking the field investigation,theoretical analysis,similar materials simulation and numerical simulation as the means,it evaluated the adaptation of GER,and found that the geological factors of dip angle and roof integrity had the most influence to GER adaptability.Then,under specific geology,the technology of cutting roof and retained gateway(CRRG)in flat seam and close distance coal seam group,the key technology for GER in steep coal seam,and the failure mechanism for poor integrity gateway roof were researched.Moreover,the mechanics models for roof control were established.Finally,the failure characteristics of surrounding rock and supporting technology of GER gateway were systematically analyzed in earlier stage.1)Bult adaptation grading system for GER.Six geological factors,including coal seam dip angle,mining height,cover depth,thickness influence coefficient of immediate roof,lithology of immediate roof and roof integrity,were determined to evaluate the adaptation,and their weight were analysed using analytic hierarchy process,indicating that the coal seam dip angle and roof integrity were the most important factor to the adaptation of GER.Moreover,five adaptive grades were classified and gob side support strategy for entry of each grade was provided according to statistical information of 145 gateways and field experience.Finally,fuzzy comprehensive evaluation was used to weight adaptive grade index for 143 gateways in “Sichuan coal group” coalmines,and the results were generally recognized by the relevant experts and engineers.2)Proposed the "bearing structure" model with caving roof for CRRG.Taking the geological conditions of flat seam and close distance coal seam group in Baijiao coalmine as the background,the thesis carried out similar material simulation experiment for 3 gateways CRRG.It found that gateway surrounding rock appeared obvious stress unload phenomenon after the cut roof caving;Moreover,when there was a certain cut line deviation angle to vertical plane,the cut height was greater than the mining height,and the caved roof did not broke,the retained process for 3 gateways showed that caved roof formed "bearing structure" balance model.Then carried on the force analysis for the model and solved the function relation for gateway roof balance control force with cut height and cut line deviation angle.3)Proposed the key technology for GER in steep coal seam.After the analysis for roof caved structure,filling characteristic,dynamic effect and roof stress redistribution characteristic,it showed that GER in steep coal seams had more advantages than that in flat and gently inclined coal seam.However,naturally filling as the only retained method in steep coal seam,and the key technology was to effectively block waste rock on gateway side,and the invented method of rock blocking device and grouting reinforcement can well against the waste rock impact force and reinforce the waste rock and soft floor.4)Researched the soft roof failure mechanism and supporting method for GER.After investigating the soft roof failure characteristics for GER in field,it found that the soft rock mass at the working face end will break with large area,and the integrity and stability will further deterioration by the actions of the lateral stress unloading loose,tensile,and shear stresses in the low layer caused by uneven support and no coupling support,repeatedly load and unload,and dynamic disturbances.Moreover,the roof rock mass failed in shear mode along the inside of man-made constructed wall in the stability zone of the retained gateway,and finally the equation of the bolt ultimate equilibrium tension force,a function of the seam dip angle,gateway width,soft roof thickness,and bolt installation angle,was established to discuss the support countermeasures for the soft roof.5)Discussed the gateway broken zone cross section features for GER in early stage.Though the 55 gateways broken zone test in surrounding rock with GPR,it found that the broken zone had large range,and the broken widths in the roof and high side wall were relatively large compared to those in the low side wall.Then,Combined with the tested broken widths and plastic zones in the 3DEC model,each gateway EBZ cross-section scope was determined.It was found that the EBZ distribution shapes were basically elliptical.The long axis was along the seam inclination direction,the short axis was along the vertical direction of the rock layer,presenting asymmetric failure formation.Also,the elliptical shape had little relationship with the gateway cross-section shape and driving method.6)Discussed the initial load-bearing characteristics for surrounding rock broken zone after basic support.After FLAC3 D numerical simulation analysis,it found that the pretension force had lower diffusion sphere in the anchored rock zone,and the main diffusion sphere was in the bolt free rock zone.Moreover,further analysis showed that the initial bearing zone thickness of combined arch structure in larger broken zone can be expressed by the bolt free length,when using high mechanic property bolt and steel with high pretension force,and clearly put forward the bolt length selection rule for driving period basic support under the combined arch supporting theory. |
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