| China's coal resource reserves are rich,which have wide distribution areas,complicated and various occurrence conditions.In central and eastern china,coal mines have poor geological conditions,mining depth is deepened ceaselessly and tends to 1000 m or above gradually.In western area,coal seam has the characteristics of shallow buried depth,large thickness and simple mining conditions,meanwhile,open pit mining,fully-mechanized mining with large mining height and top coal caving are widely used.However,underground coal mines occupy a large majority,open pit coal mines are just 2% in our country.To exploit underground coal resources,tens of thousands meters of roadway or chamber is excavated annually,and most of them are coal roadways.A great quantity of roadways with coal roof will appear when full-mechanized caving mining is applicated,even there will be full seam roadways in coal mines if special thick seam mining or steeply inclined thick seam horizontal slice mining is used.In order to overcome the shortcomings of arch roadway and increase productivity,cross-section of actual mining roadway mostly is rectangular.Due to mining intensity and yields of mines are greately improved,the equipment needed become larger and larger.Roadway maintenance have become a problem restricting safe and efficient mining.Accordingly,it is critically important to control stability of coal roadway surrounding rock,which is a research focus in this field all along,nevertheless,the starting point of previous research mainly is roof or floor,rib is not the research emphasis generally.However,roadway is a whole consisting of roof,rib and floor,once there exist deformation and failure in rib,the whole roadway will appear instability,rib breakage will be obviously increased if the roadway cross-section is larger,furthermore,roadway excavation is an unloading process of surrounding rock.Therefore,it is very valuable to explore rib breakage mechanism of large cross-section coal roadway by selecting typical engineering background,basing on unloading opinion and applying multifold measures in theory and engineering.In a setting of practical problems such as rib breakage and spalling of large crosssection coal roadway during excavation in Hongqinghe Coal Mine.Taking these methods of site investigation and testing,laboratory experiments,theoretical research,numerical experiments as well as engineering practice.Comprehensively using the knowledge of mining science,mathematics,mechanics and computer science.Topic of this dissertation was systematically researched in both depth and extent,and the main results and conclusions were as follows:(1)Rib breakage of large cross-section coal roadway was very serious in Hongqinghe Coal Mine,spalling in shoulder,middle part and bottom were the three principal failure forms,and the failure in rib shoulder was the most common.Preliminary analysis and guess indicated that the main reasons for rib breakage might involve in-situ stress level,joint fissure of 3-1 coal,size and shape of roadway cross-section,unreasonable support coupled with other factors.Roof rock was stable,but its strength was relatively low;strength of 3-1 coal was fluctuant.The maximum horizontal principle stress lay between 3.76 MPa and 28.68 MPa,the minimum horizontal principle stress was among 2.79 MPa and 23.42 MPa,and direction of the maximum horizontal principle stress was about NE.The difference between the maximum and minimum principle stress was small and could keep relative steady with the increase of depth.(2)Through uniaxial compression test,conventional triaxial compression test and confining pressure reduction tests with different confining pressures of roadway rib coal in laboratory,its deformation and failure characteristics under different stress paths were obtained.Coal sample deformation would be increased with the raise of deviatoric stress,comparing with loading,the dilatation would be more obvious during unloading with the same deviatoric stress.The coal sample axial strain was the same as radial strain in conventional triaxial compression test,while the former was just half of the latter under unloading.During conventional triaxial compression test,with the raise of confining pressure,the coal sample deformation would be changed from brittleness to ductility,but there only existed brittle failure under unloading.Change of confining pressure had significant influence on the strength of coal sample,though the peak strength was obviously lower under unloading.Coal sample failure form was single in uniaxial compression and conventional triaxial compression test,while it was a compound failure lying between of them during unloading,in this condition,shear failure was the chief form,meanwhile,splitting failure also might appear.Coal sample would be destroyed more easily under unloading stress path.(3)Basing on the data from laboratory test,the concept of damage factor characterized by relative deformation modulus and confining pressure difference ratio were proposed,the damage and fracture of coal under unloading with different confining pressures could be quantitatively evaluated.By summarizing the merits,demerits and applicability of 5 classical rock mass strength criteria,fitting the experimental data with various curves,applicable strength criterion and Mohr strength envelope curve of coal under unloading were selected.The vertical stress within a certain range of rib coal would be increased for excavation,while the horizontal stress was decreased gradually.Influence scope and degree were aggravated with the raise of roadway height,rib coal breakage caused by the increase of difference between the vertical and horizontal stress would appear.The mechanical model used to analyze rib breakage mechanism of coal roadway was established based on unloading stress,and its rationality was validated by a case study.(4)By taking into account of theoretical research,site investigation and previous achievements,it was concluded that rib breakage of coal roadway might be caused by buried depth,roadway height,in-situ stress states,internal friction angle and cohesion of coal,and these factors were grouped around three themes: design,geological conditions and mechanical parameters.The influence of every factor on the maximum rib horizontal displacement was simulated through FLAC3 D,regression results of correlation coefficient(R)showed that: all factors had significant effects on research object.Internal friction angle,cohesion and support strength were controllable factors,while roadway height,side pressure coefficient and buried depth belonged to uncontrollable factors.Three controllable factors were analyzed by response surface methodology,and the results indicated that: regression model of the maximum rib horizontal displacement was second order response surface.Every single influence factor and their interaction had effect on the maximum horizontal displacement.(5)By comprehensively considering all research conclusions of this dissertation and related reinforcement technology on coal face failure,control technology consisting of reducing rib coal stress difference,improving coal property and promoting excavation level was presented to prevent and control coal roadway rib breakage.Typical UDEC numerical model of large cross-section coal roadway was established to study fracture field and deformation failure characteristics of rib coal,meanwhile,the influence of different roadway heights,excavation technologies,coal mechanical properties and support strength on fracture,deformation and failure of rib coal was analyzed.Results showed that with the raise of roadway height,rib coal fracture development height,depth and scope increased;fracture distribution gradually changed from narrow and sparse state to vast and dense condition;failure degree and rib convergence aggravated,and rib breakage appeared at upper more easily.From the rib coal fracture development,deformation and failure characteristics after excavation unloading,excavating roadway at twice was better than full section excavation,however,as for roadway surrounding rock,excavating from top to bottom had different control effects relative to excavating from left to right,discrepant excavation technologies should be adopted under various surrounding rock conditions.Rib coal fracture development,deformation and failure degree would be decreased to some extent with the increase of coal internal friction angle or cohesion,while this effect was limited.Increasing support strength of roadway surrounding rock could not only improve rib coal stress condition,reduce stress difference,but also enhance coal strength within certain range.Rib coal fracture development,deformation and failure extent after excavation unloading diminished significantly with the enhancement of support strength,however,there existed a critical support strength,once beyond this critical value,control effect on rib breakage was no longer apparent.(6)In order to control rib breakage of large cross-section coal roadway for excavation unloading,the concept of successive excavation,flexible surface protecting,primary and strong support was established,meanwhile,concrete control technology for 3-1101 working face belt transportation roadway of Hongqinghe Coal Mine was proposed and conducted.Mine pressure monitoring results in the industrial test showed that the whole stability of coal roadway surrounding rock was better,rib breakage finally was effectively controlled. |
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