| As China's coal mining focus shifts to the western regions with good resource endowments and mining conditions,deep mining in this region has become an inevitable trend.Based on the consideration of engineering factors,the height of coal roadway is generally less than the mining height of working face,causing the roadway with the coal-rock composite roof to become more and more common in Western China,especially in Ordos area.Due to the low strength and well-developed joints of the deep coal seam,the coal seam is broken and deformed seriously,the top coal is easy to be separated from the immediate roof,and the coal wall is prone to large-scale splitting and damage,which brings great difficulties to the maintenance and control of the roadway.At the same time,the rapid development of coal mining equipment in the western region has comprehensively promoted the progress of fully mechanized mining technology,while the development of the corresponding fully mechanized excavation technology is relatively lagging,and the continuity of mining and excavation is highly tight,which once again increased the difficulty of controlling coal roadways.Therefore,the difficulty of controlling the roadway with coal-rock composite roof and the low excavation efficiency have been plagued by the safe and efficient production of coal mines in western regions.It is of great significance to study the deformation failure mechanism and the efficient control technology of coal-rock composite roof in deep roadway,which is of great significance to solve the problem of surrounding rock control and rapid excavation restriction.Taking the deep roadway of the coal-rock composite roof in Hulusu Coal Mine as the engineering background,aiming at the scientific problems of poor roadway safety and low support efficiency,this paper adopts the research methods of field measurement,laboratory test,numeral calculation,theoretical analysis,similar simulation,material development and field test to analyze the gradual collapse law of coal-rock composite roof,reveal the thick layer cross-boundary anchoring mechanism of coal and rock composite roof,clarify the bearing and failure mechanism of thick layer anchorage system of composite roof,and innovate the flexible structure of crossboundary long anchorage.The main achievements of this paper are as below:(1)The deformation and failure characteristics of the roadway with coal-rock composite roof are revealed.Through field test and analysis,the maximum horizontal principal stress is as high as 22.33 MPa,and the pores and fractures of the coal seam and the composite roof are very developed,especially the coal seam is distributed with a large number of horizontal and vertical interlaced micro-fractures,resulting in the compressive strength of the coal body and the composite roof being only 10.8 MPa and32.1 MPa,which is the internal cause of the separation failure of the roadway with coalrock composite roof.The roadway span is 5.4 m,the initial anchoring force of the bolt is only 26 k N,and the anchoring depth of the bolt is 2.1 m,which cannot contain the initial deformation and subsequent continuous deformation of the surrounding rock of the roadway.This is the external reason for the deformation and instability of the roadway with coal and rock composite roof.(2)The difference in mechanical properties of coal and rock composite samples and the energy dissipation process are clarified.According to laboratory experiments,as the height of the coal sample increases,the greater the strain increase area of the combined sample,the greater the possibility of local strain mutation,and the smaller the mechanical parameters of the sample.The process of energy dissipation shows that the energy evolution is mainly elastic strain energy,accounting for 81%-98.3% of the total energy.When the energy exceeds the key node of peak strength,the elastic strain energy of coal sample is released rapidly,which promotes the rock sample to sprout fractures at the interface,and causes the expansion and penetration of fractures,resulting in the tensile and shear failure of composite samples.It is further analyzed that the elastic deformation energy released by roadway excavation is the main factor for the deformation and fracture development of shallow top coal.Timely strong support can make the micro fractures close again,curb the increase of energy consumption and restore the relative energy balance of surrounding rock of roadway.(3)The gradual failure rule of coal-rock composite roof in the process of stress release was discovered.According to the discrete element simulation analysis,with the gradual release of stress,the deformation of the composite roof gradually increases in stages.The top coal breaks first,then causes the immediate roof to deform and rupture,and finally collapses in a "triangular" shape.It is the main factor that induces the overall deformation and gradual instability of the surrounding rock.It is pointed out that the key to control the progressive failure of coal and rock composite roof is to control the fracture propagation and penetration of top coal.At the same time,it is clarified that the deformation of surrounding rock and the number of roof fractures have a strong positive correlation with the thickness of coal seam,and the thickness of top coal increases the control difficulty of roadway.(4)The principle of thick layer cross-boundary anchoring of coal and rock composite roof is analyzed.According to the simulation calculation and analysis,the increase of bolt length fundamentally changes the roof deformation mode,from largescale "triangle" fracture type settlement to small-scale "arc" type uniform settlement,and at the same time,it reduces the fracture expansion range,which is widely distributed inside and outside the bolt anchorage area,and then to the deepest distribution in the bolt end area,and finally only exists in the shallow part of the bolt anchorage area.It is revealed that the damage zone at the end of the bolt moves up and gradually weakens with the increase of the bolt length.(5)The thick layer anchoring system of roof is developed and the cross-boundary long anchoring technology is proposed.According to the theoretical analysis,it is the connotation of thick layer anchoring system that the long bolt can realize the continuous stress transfer in the horizontal and vertical directions of the roof,which has the characteristics of high bending rigidity,low degree of cranny and high efficiency of bolt support.It is verified that the number of tension fractures can be effectively controlled by the strong surface protection under the cross-boundary anchorage of thick layer,which reduces the proportion from 34.9% to 20.5%.The continuous transmission of roof stress can relieve the pressure on the coal side,and the two-way optimization of roof slope control is conducive to the long-term stability of the roadway.(6)The bearing mechanism of thick layer anchorage of coal and rock composite roof is determined.According to the analysis of similar simulation,the high-prestressed flexible long bolt constructs a high-strength and high-rigidity roof thick layer anchoring structure,which fully mobilizes the surrounding rock deeper in the roof to participate in the load bearing,reduces the roof stress release range,and improves the antideformation ability of the roadway.It is revealed that the higher the initial pretightening force of the bolt,the more sensitive the bolt response will be,and the more timely the supporting effect on the surrounding rock will be,thereby inhibiting the expansion of fractures.The impact dynamic load experiment shows that the thin-layer anchored structure of the roof is instantly washed out by the strong dynamic load,showing an overall "knife-cut" type of damage,while the thick layer anchorage structure has strong impact resistance characteristics,and the roadway side is first washed out,the roof to "fan-shaped" overall subsidence,the integrity of surrounding rock is effectively maintained,ensuring the safety of coal roadway.(7)The flexible bolt which is not limited by roadway height and can be installed quickly by rotation is developed.The parameters of locking sleeve which affect the mechanical properties of flexible bolt are determined through the experimental analysis under various working conditions.The peak force of the bolt is more than 330 k N and the elongation rate is 5%.The bolt has good bearing capacity and ductility.The mechanical characteristics and failure mechanism of flexible bolt under long-term load and cyclic load are revealed,and the reliability of flexible bolt in special underground environment such as different drenching environment and different installation angle is verified,and it has been popularized and applied in three kinds of complex roadway conditions.(8)The engineering verification of thick-layer anchoring systems was carried out in the two typical roadways with coal-rock composite roof of Hulusu Coal Mine and Menkeqing Coal Mine,and the roadway driving speed increased by 60%,in particular,Menkeqing Coal Mine has set a record of 1040 m per month for single lane,single row and single cycle roadway with large cross-section coal and rock composite roof.At the same time,the roadway control effect is significantly improved,the roof fractures are reduced to less than 0.8 m,and the coal bank deformation is also fundamentally improved,which provides a powerful reference for the popularization and application of roadways under similar conditions.This dissertation totally have 159 figures,28 tables,and 175 references,respectively. |
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