| The low recovery ratio of the working face and the greater flammability of floating coal in the goaf have always been the main problems to study and solve in the top coal caving mining method.Based on inner-staggered roadway layout of stagger arrangement method used in 2#coal seam of Xishan Coal-electricity Group,it is studied and analyzed in this paper that how strata pressure behaviors influence spontaneous combustion in goaf when extracting easy-spontaneous combustion coal seams.In this paper,several methods are adopted in the research process as follows:theoretical analysis,formula deduction,physical analog simulation experiment in the laboratory,numerical simulation,field experiment including data collecting and evaluating and so on.The paper puts forward to influence the hulking coefficient,porosity and permeability of coal or rock for improving the recovery ratio and prevention of spontaneous combustion in goaf by using inner-staggered roadway layout of stagger arrangement method.Firstly,the paper introduced the importance of extracting thick coal seams and analyzed the two main problems during the mining process of fully-mechanized top coal caving mining method.Then spontaneous combustion in goaf including overseas and domestic research status and roadway layout of stagger arrangement method were processed and analyzed based on field situations of Zhencheng Mine.On the basis of analysis above,the main research contents,methods and technical route were established in the paper.1.Besides the analysis of the characteristics of spontaneous combustion zones in goaf in the full-mechanized caving mining,the paper also included the concept,basic parameters of sublevel caving goaf porous media,fissure formation theory and seepage law at the same time.It is found that:(1)It is widely accepted not only by the domestic researchers but the foreign ones that the zones of spontaneous combustion in goaf are divided into three parts:heat radiation zone,oxidization and heat accumulation zone and suffocative zone.However,as an important basis for the control of spontaneous combustion in goaf,the classification standard of self-ignition triband has not been formed a common standard in the present academic circles.(2)There are important concepts and parameters of the porous media in the goaf as follows:porosity,ratio surface,bending rate,solid particle size(particle size distribution),pore size,etc.And speed,ratio flow,permeability,saturation,capillary pressure and other flow basic characteristics parameters.(3)Then it was analyzed in the paper that three-dimensional spatial distribution characteristics of overburden rock porosity in goaf.The subsidence of instability of strata was calculated by using the elastic thin plate bending theory based on flexure.The paper deduced calculation formations on rock porosity of gob caving rock zone and abscission zone and tested the deduced the formula.Calculation Formulas are shown as follows:Main gas composition and seepage models in goaf were analyzed.The paper studied the relationship among air leakage parameters,including air leakage flow pattern,the porosity of caved coal or rock,air volume and air pressure gradient and seepage and so on.It is concluded that:? The wind flow rate of surrounding rock in the roadway forming porous medium in the fractured coal rock is generally less than 0.25 and the flow status conforms to the laminar flow of the linear Darcy law.? It is mainly to reduce the porosity and seepage coefficient of wind pressure gradient,flow rate and air leakage medium for the prevention of spontaneous combustion in goaf.? The main factors leading to air leakage wind pressure of caved coal or rock included float coal accumulation caused by oxidation temperature and form affected by wind pressure and thermal properties of the coal pillar stability which produced cracks caused by the side of leeway;? There is barely end top-coal and roadway top-coal in the adjacent working face with the roadway layout of stagger arrangement method,namely reducing the amount of float coal in mining process,which makes the thermal wind pressure decrease and is conducive to prevention of air leakage and spontaneous combustion in goaf.2.Through the theory of voussoir beam,and the key strata theory,the Kirchhoff's theory,plastic mechanics,Hooke's law and the rock caving angle,this paper studied the relationship between spontaneous combustion and surrounding rock vertical stress analysis in goaf.The main conclusions are as follows:(1)The paper several common mechanical models of basic strata were established based on different pillar sizes with roadway layout of stagger arrangement method(RLSA).The formula of the peak value distribution of the corresponding model and the calculation formula of the fracture interval were obtained respectively.Based on the analysis above and the calculation formula of bending moment distribution of RLSA method,vertical stress and plastic zone distribution around triangular pillar were analyzed.And the fixed calculation formula on plastic zone development depth around triangular pillar was shown as follows:The vertical stress calculation formula on the long side of triangular pillar in the replacing working face was obtained by analyzing the large and small structure and stability of the fault strata in RLSA method:Px= ?0h=?x? ·?0·tan?Pmax=?0Ltan?(2)Combining with the analysis of rock pressure behaviors characteristics to influence air leakage mechanism of different mining stages in RLSA,it is suggested that the porosity around triangluar pillar slope and its value has shrunk in RLSA method when compared with traditional sub-level caving and slicing mining.Through the Darcy's law,air leakage volume decreased in the corresponding area under the same wind pressure gradient condition.However,the roof of air-intake roadway in the replacing working face needs the grouting method for the control of spontaneous combustion.(3)The triangular pillar in RLSA method increased the recovery ratio of the working face and reduced the production quantity of float coal causing spontaneous combustion in goaf.the porosity value around caving zone in goaf was lower than that around other side of goaf and corresponding position in traditional top-coal caving mining,which lead to a new distribution of porosity in caving zone,namely an asymmetric distribution way:(4)Through the principle of fuzzy mathematics and analytic hierarchy process(AHP),spontaneous combustion risk fuzzy evaluation model was established for RLSA method.Then this paper calculated the spontaneous combustion index and it is shown that the index value decreased from the traditional caving mining of 0.56(as part of Danger Level I)to RLSA method of 0.493(as part of Danger Level II),which is conducive to the prevention of spontaneous combustion in goaf.3.Based on the engineering background for this paper,physical simulation experiment was performed in the lab to analyze the characteristics of the caving upper strata in mining process and the influence of porosity and permeability.The comparison was made in the paper that the stability of coal pillar between RLSA method and traditional top-coal caving mining including the influence on air leakage.The main conclusions are as follows:(1)For RLSA method,roadways in the replacing mining face were located in the goaf beside the first mining face.And it was rocks that caved in the goaf which lead to much less loose coal accumulation.Furthermore,the air leakage caused by thermal wind pressure was weakened which is conducive to reducing the possibility of spontaneous combustion in roadway.(2)When extraction in the replacing face for RLSA stepped into the stage of periodic caving mining,the stability of triangluar pillar was improved as a result of located under the large structure beside the air-return roadway of former working face,which is conducive to the prevention of air leakage.But the roof of replacing face was formed by caved rocks causing more severe air leakage than in the traditional caving mining.The grouting method should be adopted to air-intake lane side in goaf.Even though the more severe air leakage situation,the chances of spontaneous combustion were still decreased due to the slope section of pillar without top-coal.(3)From the analysis of a comparison between RLSA with pillar and traditional top-coal caving mining,it was obtained that the stability of coal pillar for RLSA was better than that for the traditional way.RLSA method effectively reduced crack generation and its development in pillar which was conducive to prevention of spontaneous combustion caused by different wind pressure between the two sides of coal pillar.(4)Based on the analysis of the stress data from stress strain gauge in the physical simulation experiment,vertical stress around the slope of triangular pillar in goaf for RLSA was greater than that in the traditional top-coal caving method while the permeability was lower in the corresponding area.The theory was verified that the porosity distribution for the caved zone in goaf was an unsymmetrical one.4.The software ANSYSR18.0 was used combined with the engineering background in the numerical simulation.The situation about vertical stress and plastic zone development for three different kinds of roadway layout in RLSA and one traditional way was studied and analyzed including the influence on the hulking coefficient of coal and rock,the porosity,permeability and other spontaneous combustion factors.The main conclusions are as follows:(1)Roadway layout for RLSA method reduced the loss of coal and increased the recovery ratio considering the prevention effect of spontaneous combustion.With the same size of pillar,RLSA was a better choice in terms of the prevention of air leakage.Through the analysis of four kinds of solutions,it is concluded that the control effect of spontaneous combustion in goaf are as follows:inner-staggered roadway layout>inner-staggered roadway layout without pillar>RLSA with 5m pillar>traditionalsub-level caving with 5 m pillar for similar easy-spontaneous combustion conditions.(2)Different vertical stress peak values appeared at different locations in goaf for inner-staggered RLSA,specifics are as follows:vertical stress in central part of goaf>that in lower part(the slope)of goaf>that in upper part of goaf.Then the conclusion from physical simulation experiment in the Fourth Chapter and Section 3.7 in Third Chapter was testified that the hulking coefficient,porosity and permeability changed around triangular pillar area due to the gradually decreased mining height in RLSA.(3)Based on results of Scheme 2 in the numerical simulation,it is suggested that the deformation and displacement of the surrounding rock inair-intake entry are roughly the same.On the other hand,the deformation and pressure in air-return roadway(the slope section)are smaller which makes the excavation and maintenance easier.It is demonstrated that the inner-staggered roadway layout for RLSA weakens the phenomenon of roadway roof caving and effectively avoids high risk areas of ignition in roadways.5.Via fluid software FLUENT,seepage field,three-dimensional goaf numerical simulation on stress field and concentration field for RLSA method was performed,including the relative static pressure,velocity vector distribution,component concentration and spontaneous combustion oxidation zone range.The numerical simulation was based on actual engineering conditions and the new changes of the spatial distribution of porosity for RLSA method.The main conclusions are as follows:(1)The current in stope flows into the air-intake roadway,mainly through the working face and the supporting part,then finally into the air-return roadway.The amount of leakage flowed into the goaf is related to the porosity and permeability of porous media in goaf,the pressure in goaf,the gas gushing and the wind velocity aside air-return roadway.This leakage circular flow mainly exists in the shallow area near the working face and at the height of one roadway in the direction of Z plane.Furthermore,the flow velocity in other space is small and the local flow field is affected by the gas concentration field.(2)Since the U-shaped ventilation was the "One source One remittance"situation,the wind flow at the top was also flowing to the working face,then flowed out from the air-return roadway.(3)In the corner of the working face,there is less static pressure and the negative pressure zone was formed.The gas of each component in the goaf was inclined to gather easily in the corner of the adjacent working face.(4)The distribution of gas in the goaf shows high CH4 concentration in the deep zone of goaf and low CH4 concentration in goaf aside the working face.The gas distrbution in the goaf is generally asymmetrical and CH4 concentration is low at air-intake roadway side while high concentration appears aside air-return roadway.(5)The concentration of CH4,O2,CO and CO2 were affected by fresh air leakage at air-intake side and each concentration of them was low.But the concentration was gradually increasing in the direction of the deep goaf.Moreover,the concentration of O2,CO and CO2 were high at the side of air-intake in goaf aside the lower corner of the working face while the concentration of CO and CO2 was much lower than that of O2.(6)Considering that porosity and permeability of porous medium at trianglular pillar side(air-return side)in goaf were smaller than that at the other side(air-intake side)in goaf,combined with the influence of U-shaped ventilation was more obvious in shallow part than that in deep part of goaf,it is shown that the concentration distribution of each gas(CH4,O2,CO and CO2)in goaf is an asymmetric one,namely all gas concentration is higher at air-return side(trianglular pillar side)than that at other side of goaf.Moreover,this asymmetry distribution characteristics decreases gradually along the direction of deep goaf.(7)The distribution of CH4,O2,CO and CO2 in the height of the Z plane is shown as follows:On the lower plane level,each gas concentration is low at the shallow part of goaf and accumulates towards air-return side;While on the higher plane level,each gas concentration is high and affected by Buoyancy effect.Each gas tends to accumulate easily at the air-return side in goaf.(8)In this paper,the distribution range of the oxidation spontaneous combustion zone for the inner-staggered RLSA method was obtained by the intersection area between the region of air leakage wind speed VL<0.004 m/s and oxygen mass fraction CO2>10.6%.Base on this particular engineering background in the paper,the distribution of the oxidation spontaneous combustion zone was asymmetric along the inclination direction in goaf.Besides,the width of the oxidation zone at air-intake side was greater than that at air-return side(the slope section side).6.Finally,the paper analyzed roadway layout,recovery ratio in working face and control measures on the prevention of air leakage and spontaneous combustion combined with application situation of RLSA method to this particular engineering background.Pressure on hydraulic supports and running status of them were studied.Dynamic observation and data collection of spontaneous combustion index gases in goaf were performed and main conclusions were obtained from the processed data as follows:(1)Top coal at the end of working face and above roadways was extracted via inner-staggered roadway layout of RLSA method.Besides the coal pillar,the recovery ratio was improved more than 10 percentage and significant economic benefits were achieved.Without coal above roadways and loose coal at the end of working face,the prevention of spontaneous combustion between working faces,end of working faces and top of roadways were realized.This paper put forward to prevent spontaneous combustion through improving recovery ratio in working face and this particular project not only increased the recovery ratio but prevented spontaneous combustion with less loose coal in inflammable coal seams based on the change of roadway arrangement.(2)Through mining pressure behaviors in the field,it is found that working resistance of hydraulic support in central part of working face>that in the lower part(the slope region)>that in upper part of working face and all working resistance of hydraulic support was within the safe rating value.This field result verified that the porosity distribution of caved zone in goaf presented an asymmetrical spatial distribution instead of the conventional symmetric "shovel" shape,namely the porosity value along triangular pillar side was smaller than that along other side in goaf.(3)Based on multi-gas component observation experiments in goaf for the fully-mechanized caving method,the paper analyzed the data of gas and temperature in goaf from field observation and the corresponding three-dimensional contours and contour map were obtained.This paper provides data support for further study on the gas distribution law of concealed space,the distribution of temperature field and spontaneous combustion in goaf for RLSA method.(4)In the three-dimensional roadway layout system for RLSA method,there is no more loose coal at the end of working face and in adjacent goaf but little amount loss of triangular pillar,which has made the prevention of combustion problem better with fundamental improvement.The comprehensive method was formed to prevent spontaneous combustion in goaf based on loose coal elimination.Meanwhile,there have been no air leakage ignition or spontaneous combustion problems in the coal seam during the actual application of RLSA method for years.This paper has made several innovations as follows:1.Analysis of three-dimensional spatial porosity distribution of caved rock in goaf and calculation formulas deduction of the porosity in caved zone and abscission zone and the formulas were tested:2.Based on the traditional coordinate system and the location of origin point,it is proposed to define the coordinates of left side of roadway as the length of pillar for introducing the concept of "inner-staggered roadway layout".It complemented the existing theory on underground pressure partition.And it is also beneficial to calculate the slope length of the mining area,which is more accurate than inner-staggered with one roadway length and inner-staggered half roadway length in slicing mining method.3.Through theoretical mechanics,plastic mechanics and rock mechanics theory,the peak value of bending moment and distribution of fracture interval under several basic roof bracing conditions were obtained.Furthermore,the calculation formula about the maximum development depth of the plastic zone around the triangular pillar was fixed as follows:The vertical stress calculation formula on the long side of triangular pillar in the replacing working face was obtained by analyzing the large and small structure and stability of the fault strata in RLSA method:Px=?0h=?x? · ?n ·tan?pmax=?0Lan?4.Based on the analysis of mining pressure behaviors characteristics for RLSA method in certain chapters of the paper,it is proposed that the porosity value of caved zone in goaf at the slope section side of the triangular pillar is less than that at the other side in goaf for traditional top-coal caving mining.The porosity value presents a new asymmetric distribution in goaf,which is testified through physical simulation experiment,numerical simulation,field observation and other methods in the paper.It complements the research on porosity change in goaf for the inner-staggered roadway layout in RLSA method and provides the reference to a better and more comprehensive development of' spontaneous combustion control. |
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