Key Structural Body Theory Of Coal And Gas Outburst

The mechanism of coal and gas outburst is an important scientific problem that people pay attention to continually in the field of coal mine safety.It has been more than 180 years since the first coal and gas outburst occurred at Isaac Coal Mine located of Ruarey Coal Field,France in 1834.During this period,a large number of domestic and foreign scholars and engineers have conducted various of researches and experimentations on the coal and gas outburst,and a variety of hypotheses,predictions,prevention methods was proposed.However,as a special dynamic disaster of coal and rock system,coal and gas outbursts still occur frequently,and its prevention remains a worldwide difficult and troubling problem.Focusing on the special geological structure and environmental characteristics of coal and gas outburst location,this thesis combines the mechanism of coal and gas outburst with field engineering conditions,and innovatively proposes the disaster-causing theory of coal and gas outburst based on the key structural body model.By combining laboratory experiments,theoretical analysis,numerical/physical simulation and field outburst cases analysis,four key scientific and technological problems are explored.Firstly,"what kind of special geological structure environment is needed for the preparation and start-up of coal and gas outburst and what are the special features of outburst coal?" are studied.Secondly,this thesis researched the coupling effect of mining stress and gas pressure around mining face and their combined disaster-causing mechanism.Thirdly,the author attempts to combine the study of coal and gas outburst mechanism with the field engineering structure so as to make the mechanism more practical.Finally,the thesis clearly points out the main control factors of coal and gas outburst,proposes a unified coal and gas outburst initiation criteria and specifies the objects of prediction and control in the prevention and control of coal and gas outburst.Main research contents and achievements in this thesis are listed as follows:(1)Referring to the idea of "Tectonic-level-control rule of gas occurrence",distribution and geological environment of coal and gas outburst mining area,and the stress field and geological structure around the outburst spot are studied.Result indicates that some certain zones,including plate margin tectonic zone,intraplate orogenic zone,deep structure steep changing zone,deep active fault zone,nappe structural zone and strong deformation zone,are key factors determining the distribution of coal and gas outburst mining area.Moreover,stress field in coal and gas outburst mining area belongs to geodynamic field generally,which is significantly influenced by geological structure or tectonic movement.The contribution of geological tectonic movement to coal and gas outburst is mainly due to the formation of structural coal body,which creates the high stress environment and the geological tectonic environment conducive to gas storage and the outburst occurrence.(2)There is no significant difference in the industrial analysis results,adsorption constant a and initial velocity of gas emission between outburst and non-outburst coal seams.In other words,there is no critical value that can clearly distinguish whether there is outburst danger in coal seams.The adsorption constant a and initial velocity of gas emission ?p show a trend of firstly decreasing and then increasing with the increase of coal metamorphism;The Qin Yueping formula(Qt=AB?t/(1+B?t))has obvious advantages in describing the time-varying law of desorption amount of tectonic coal samples.The higher the degree of coal fragmentation or pulverization,the higher the initial velocity of gas emission can be found.The acoustic emission characteristics and mechanical behavior of briquette specimens which can reflect the properties of in situ outburst coal are largely different from original coal specimens in the process of deformation and failure,and briquette samples show better characteristics of ideal plastic.The whole stress-strain process of raw coal samples,in chronological sequence,includes elastic stage,strain hardening stage,strain softening stage and residual strength stage.However,the whole stress-strain process of briquette samples includes elastic stage,strain hardening stage,ideal plastic stage,strain softening stage,and residual strength stage(The residual strength stage is not obvious when the uniaxial load or confining pressure is low).(3)Based on the dual-porosity model of coal body,the gas-solid coupling controlling equations of coal body with double-porosity structure are established under mining disturbance.The interaction mechanism between mining pressure field and gas pressure field in front of working face is studied with COMSOL Mulphysics software.According to the results,in soft and hypotonic coal seams,it is easier to accumulate high-energy gas because of its low strength(having solid material foundation for initiating outburst)and low permeability.The low permeability areas and pressure relief failure areas caused by mining pressure concentration can function as the "barrier wall”,which hinders gas emission and energy release of deep soft outburst coal body,and form the "mining genesis" abnormal geological structure.Disturbance conditions such as blasting,deep cutting and roof and floor breakage,may cause instantaneous instability and destruction of the "barrier wall" in front of the working face.The soft coal with high-energy gas in the deep part will expose suddenly,which results in uncontrollable release of elastic potential energy and gas internal energy in coal and rock mass,eventually lead to coal and gas outburst accidents.(4)There are abnormal areas of in-situ stress near the"natural genesis" geological structure anomaly areas such as soft and hard coal variation zone,coal seam thickness variation zone,fold structure belt and fault structure belt.These abnormal concentration of in-situ stress will lead to the higher elastic potential energy of coal and rock mass in this area.Meanwhile,there are permeability reduction areas near the geological structure anomaly areas,which trap gas significantly and make the coal mass in these areas accumulate a large number of high-energy gas.When the mining face is close to these anomaly areas,the abnormal concentration of mining pressure will furtherly reduce the permeability in front of the mining face,which hinders the gas escaping from the deep coal mass in front of the mining face to the mining space,and maintains a higher gas pressure gradient of the coal mass in front of the mining face.There are both abnormal accumulation areas of high elastic energy and high gas internal energy in front of the mining face.The uncontrollable release of elastic energy and gas internal energy in coal and rock mass may occur when disturbed by dynamic load such as blasting,deep cutting,roof and floor breakage in coal mine,resulting in coal and gas outburst accidents.(5)The key structural body model describing the characteristics of engineering structural environment of the coal and gas outburst location is proposed.The key structural body theory of coal and gas outburst based on the key structural body model is established.Results show that the key structural body is a necessary condition for the initiation of coal and gas outburst.The typical coal and gas outburst can be divided into two types from the perspective of the initiation mechanism of coal and gas outburst,the delayed outburst under quasi-static load and the instantaneous outburst under dynamic load based on the key structural body model.The process of coal and gas outburst has four stages:preparation,startup,development and termination.The preparation stage of outburst starts when the geological structural movement rebuilds coal properties.The triggering stage is manifested in the abrupt destabilization of structure-2,which is subordinate to the preparation stage.Outburst initiation is the abrupting point when structural 1 is exposed and instable.Whether the coal and gas outburst launchs or not depends on the mechanical condition and the energy condition of structure-1,after the abrupting destabilization of structure-2 will trigge outburst.The mechanical criteria and energy criteria of outburst triggering and initiation are established based on the key structural body model.(6)Based on the key structural body theory of coal and gas outburst,the mechanism of synergistic outburst prevention for energy dissipation by unloading and transformation of coal and rock medium attribute in deep mining is revealed.New understandings have been gained from the advanced detection of outburst dangerous areas,the prediction and early warning of outburst danger and the prevention of outburst disasters.It mainly includes:engineering practice of the field outburst prevention should focus on the overall idea of "detecting key structural body-eliminating key structural body-preventing and controlling key structural body"and pay attention to the development of precise detection technology and equipment for key structural body in front of mining face.Outburst prediction work should pay more attention to the measurement and application of gas pressure which is the direct power of outburst initiation.The early warning work should focus on the continuous real-time monitoring of the precursor information of instability and catastrophe for"structure 2" in key structural body.The outburst prevention should focus on the triggering and initiation.Eliminating the conditions of outburst triggering and initiation are two different and effective ways of outburst prevention,and the fundamental measure is to change the force and energy environment of "structure 1".