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英文原文A Method for The Design of Longwall Gateroad Roof SupportW.LawrenceGeowork Engineering, Emerald, QLD, AustraliaAbstract:A longwall gateroad roof support design method for roadway development and panel extraction is demonstrated. It is a hybrid numerical and empirical method called gateroad roof support model(GRSM), where specification of roof support comes from charts or equations. GRSM defines suggested roof support densities by linking a rock-mass classification with an index of mining-induced stress, using a large empirical database of Bowen Basin mining experience. Inherent in the development of GRSM is a rock-mass classification scheme applicable to coal measure strata. Coal mine roof rating(CMRR) is an established and robust coal industry standard, while the geological strength index(GSI) may also be used to determine rock-mass geomechanical properties. An elastic three-dimensional numerical model was established to calculate an index of mining induced stress, for both roadway development and longwall retreat.Equations to calculate stress index derived from the numerical modeling have been developed. An industry standed method of quantifying roof support is adopted as a base template (GRSUP). The statistical analyses indicated that an improved quantification of installed support can be gained by simple modifications to the standard formulation of GRSUP. The position of the mathematically determined stable/failed boundary in the design charts can be changed depending on design criteria and specified risk.Key words:Coal mine;Roof control;Support;Design.1 IntroductionLongwall gateroad strata stability is essential to ensure uninterrupted production. In Central Queenslands Bowen Basin, immediate gateroad roof lithology varies from coal to weak interlaminated material, to strong almost massive sandstone, with localised areas of weak fault affected strata. It is usual for roof conditions within any one mine to vary significantly. Typically, longwall mines in the Bowen Basin have specified gateroad roof support based on past practice. Modifications to gateroad support are generally reactive, due to encountered difficult strata conditions, and less proactive. Current gateroad support design approaches have limitations, which have restricted their applicability and adoption as mine site design tools.A prototype for an improved gateroad support design methodology has been developed that is integrated and systematic, based on rock engineering principals, but requires engineering judgement and experience. There were several broad objec- tives for the design methodology. A consistent and unambiguous definition of strata conditions and behaviour was required. Gateroad roof support needed to be assessed and specified. The method had to provide design calculations and justification for compliance and statutory purposes, and could serve as a framework for a mine strata management system. Mine site support designers must be able to readily use the method to manage uncertainty and risk. The method must be able to be reviewed, modified and expanded.2 Current Roof Support Design Methods for Longwall GateroadsNumerous roof support design methods have been proposed over the years, but none havegained widespread acceptance by the coal mining industry. There are empirical databases, some proprietary, based on industry practice, which specify gateroad primary and secondary support densities, using a statistical approach. Analytical methods are not appropriate when rock-mass yield due to high mining induced stresses occurs, but may be applicable and adapted to low stress environments. The application of complex post-yield numerical modelling in the design process for excavation support is valid although contentious, and requires a more comprehensive justification and better industry understanding of its strength and limitations. The complete mathematical representation of rock-mass properties and behaviour is a complex issue, which is still outside the capability of current numerical modelling code.Engineers and mathematicians do not have the current capability to fully define rock-mass geomechanical properties and their mathematical representation. Elasticplastic numerical modelling is a useful tool if used appropriately. It is not exclusively correct or unique, or always superior to other available and accepted design techniques. These aspects have been recognised during recent collaborative Australian Coal Association Research Program research on longwall microseismics, where it was considered that current 3D numerical models lack sufficient validated constitutive relationships, and are forced to make compromises when dealing with complex rock-mass behaviour.Simplified elastic numerical methods have merit and are certainly applicable for more massive sedimentary rock-masses. An assessment of their applicability to weaker, laminated clastic rock-masses is required. Hybrid numerical and empirical methods have been developed for the geotechnical design of undercut and production level drifts of block caving mines.3 Geotechnical Roof Classification of Longwall GateroadsTwo classification schemes were considered appropriate. Firstly, the coal mine roof rating(CMRR), which is an established coal industry standard. Secondly, the Geological Strength Index, GSI with strength parameters included. A recent publication has contended that GSI estimates of rock-mass strength should not be used for coal mine roof problems, where the geometrical scale of the problem is similar to discontinuity spacing. A distinction needs to be made between the GSI classification and the related HoekBrown failure criterion. This scale effect and situations where the failure criterion should not be used have been discussed. However, this does not mean that a classification of the rock-mass cannot be made. Indeed, this scale issue is a problem inherent in any rock-mass classification scheme, not just GSI, and for any failure criterion. For example, some mines appropriately use unconfined compressive strength (UCS) as an index or failure criterion, but UCS is also scale dependent and has the same limitations.Within the support design methodology, the rock-mass classification schemes will link mining-induced stresses (or stress index) and required installed roof support. Therefore, the classifications should be independent of environmental and geometrical factors, such as mining induced stresses and excavation orientation and size. A rock-mass classification scheme must also provide rock-mass geomechanical properties to enable the calculation of mining induced stresses. It is anticipated that CMRR will be the principal classification scheme used. However, the single rock-mass classification scheme that is best suited is the GSI derived global rock-mass strength. For numerical or analytical models, HoekBrown failure criterion parameters, modulus of deformation and rock-mass strength can be estimated from GSI. Direct utilisation of either CMRR or GSI is included within the design methodology.4 An index of mining induced stressAn index of mining induced stress in the gateroad roof at a location of interest is required. The three-dimensional (3D) stress distribution about a longwall panel including goaf reconsolidation, and the continuous stress redistribution that occurs during panel retreat, is a complex and difficult phenomena to quantify. One approach would be to construct a full elasticplastic, 3D numerical model. This approach would have limitations to a verified, unique and readily achieved calculation of stress, for several reasons. Generalised model roadway and goaf geometry may not always match the actual geometry. Generalised model roof lithology may not always match the actual lithology and variations. The roof/seam/floor interaction is a complex system and is difficult to model accurately. Rock-mass geomechanical properties, in particular post-yield cannot be fully defined. The geomechanical properties of the goaf, extent and behaviour of strata fracturing and caving, and goaf stress reconsolidation are largely unknown. The model may take many days to complete just a single scenario. While calculated mining induced stress from a detailed elasticplastic, 3D numerical model may be an appropriate parameter, there is little justification to improved accuracy compared to other methods. An alternative approach is to calculate mining induced stress from elastic 3D numerical models. Calculated mining induced stress in the immediate gateroad roof just outbye of the face-line may not be accurate if rock-mass yield occurs, but as an index of stress, it may be appropriate. An important criterion of its suitability would be how reasonable its relative variation is with changes in input parameters. A significant advantage is that it could be readily calculated for variable scenarios and would be within the range of capability of more geotechnical engineers. Maximum elastic tangential stress in the roof of a modelled gateroad could be considered a better indicator of rock-mass failure than the residual post-yield stress. Undoubtedly, significant rock-mass failure and subsequent stress redistribution do occur, which are not reflected in an elastic model. In the immediate roof of the gateroad, these failures are initiated at a critical mining induced stress. The stress index is a reasonable and appropriate measure of this critical stress, even if it may not agree in absolute magnitude after stress redistribution occurs. For mining induced stresses from an elastic 3D numerical model to be a reasonable representation, several issues influencing the stress distribution must be considered, which include strata fracturing and caving and goaf reconsolidation. For bulking-controlled caving, empirical relationships are used to predict the height of caving (goaf) and fracturing :Hc =Hf =100hc1h+ c2100hc4h+ c5+ c3+ c6(m)(1)(m)(2)Where is the caving(goaf) height above top of extracted horizon, is the thickness of the fractured zone above top of caving zone, h is extraction thickness, and , , , , and are coefficients depending on lithology(Table1).Table1Coefficients for average height of caving zone17.LithologyCompressive strength(MPa)Coefficientsc1c2 (m)c3 (m)c4c5 (m)c6 (m)Strong and hard402.1162.51.228.9Medium strong20-404.7192.21.63.65.6Soft and weak206.2321.53.154Weathered-7631.2583Goaf stressstrain behaviour can be been defined (Eq. (3), based on earlier work, as follows:1 ( m ) =E0(MPa)(3)where, and are the vertical goaf strain and stress, respectively, is the initial tangent modulus, and is the maximum possible strain of the bulked goaf material. The initial bulking factor, BF, defines as follows: = BF 1mBF(4)The initial tangent modulus, , can be defined as a function of the compressive strength of rock pieces, and the bulking factor, BF:10.39 1.042E0 = CBF7.7(MPa)(5)The FLAC3D double-yield constitutive model is used to simulate a strain-stiffening material with irreversible compaction, i.e. volumetric yield, in addition to shear and tensile failure. Upper-bound tangential bulk and shear moduli are specified, with the incremental tangent and shear moduli evolving as plastic volumetric strain takes place. In addition to the shear and tensile strength criteria, a volumetric yield surface or cap has to be defined. The cap surface, defined by the cap pressure, , is related to the plastic volume strain, . The cap pressure, , is not the goaf vertical stress, . The relationship between cap pressure and plastic volume strain is derived from an iterative FLAC2D compression test model, using a one element, 1m1m, grid. Loading was simulated by applying a velocity to the top of the element, which has confined sides and base. The constitutive equation was derived from the iterative results by a Microsoft Excel Solver regression analysis, assuming a linear function. Goaf deformation and material strength parameters are defined as follows(Table2).Table2FLAC3D goaf reconsolidation parameters.Upper bound tangent modulus230MPa Poissons ratio0.3Density1.7gm/ccCohesion0.001MPaFriction angle25Dilation2Tensile strength0MPa0.4811.99e3c + 7.83e3 pc + 0.449BFTable3FLAC3D numerical model geometrical, geomechanical and geotechnical para- meters.ParameterRangeRoadway hight2-3.4mRoadway width4.8-6.5mLongwall panel width200-300mPillar width15-45mDepth60-330mImmediate roof UCS8-62MPaRatio ofin situhorizontalto vertical stressRock-mass stiffness dependent. Ranges from 1.2(coal) to 2.0(competent rock) for the major principal stressRock-mass stiffnessE = 1 Dci 10(GSI10) 40m2 100Rock-mass Poissons ratio0.25 for stone,0.30 for coalThere are many theories on goaf reconsolidation, based on sound principles. Results from the various formulations do vary significantly. Which, if any, are correct is unknown, as goaf stresses have not been measured. For no other reasons than it is well described, and includes more of the parameters perceived to be important, the goaf stressstrain behaviour as definedFig. 1. Typical 3D model geometryhorizontal section taken from the top of seam.Fig. 2. Example of FLAC3D model output.is utilised in the calculation of a stress index. The elastic FLAC3D numerical model simulates a single two-heading longwall. Roof and floor strata are composite, uniform continuum. Strong contact is assumed between the coal seam and roof and floor. No discontinuities were modelled. Pillars will always be stable, which means that the actual pillar design must be appropriate and pillars adequately sized for the strata conditions. Some rock-mass geomechanical properties may be derived from the geological strength index.5 Characterisation of installed roof supportA standard measure of the intensity of installed support, widely used within the Industry is GRSUP (ground support rating), given by 4GRSUP= LbNbCb + LbNtCt14.6Sbw 14.6Stw(6)Where is the thickness of the bolted horizon defined by roof bolts (m), is the average number of roof-bolts in each bolt row, is the ultimate tensile strength of roof-bolts(kN), is the spacing between roof-bolt rows(m), is the average number of cables in each cable row, is the ultimate tensile strength of cables (kN), is the spacing between cable rows(m), w is the roadway width(m), and 14.6 is a constant that is needed to convert from the original NIOSH equation, which was in Imperial units, to SI units.Roof-bolt variables included in the installed support parameter are length, spacing within and between rows, installed density, and effective density, i.e. Cable variables included in the installed support parameter are ultimate tensile strength, capacity of end anchorage, e.g. barrel and wedge arrangement, row spacing, installed density, installed over conveyor belt structure, grouted, chemically encapsulated or point-anchored, and pretension. The statistical analysis of the database was used to propose modifications to parameters such as N (average number), C (ultimate tensile strength) and S (spacing).6 Design methodology6.1 IntroductionThe design methodology is tailored for roadway development and longwall gateroads. Roof support for bord and pillar first workings can also be assessed. Evaluating roof support using GRSM incorporates several design steps. An initial roof characterisation or classification is required, followed by a calculation of a stress index. Suggested minimum GRSUP is then determined. Finally, primary and secondary roof support patterns are proposed, also considering the influence of factors not assessed by GRSM.6.2 Rock-mass characterizationA classification is required for the immediate 2m of roof, and if a longwall retreat assessment is required, the 4m section above that. Typically, it would be expected that most practitioners would calculate CMRR. Alternatively, the GSI global rock-mass strength may be calculated. Similarly, the intact rock strength should not be overestimated, particularly when using a geophysical correlation.6.3 Stress indexTo effectively use GRSM it is important to be able to quickly and accurately calculate a stress index, without having to resort to a FLAC 3D numerical model. Equations to calculate stress index have been developed for two situations; roadway development and longwall retreat. A series of Microsoft Excel Solver analyses were conducted to define equations that could replicate this elastic numerical modelling calculation of stress index. It is recognised that there may be situations where the calculated stress index could be varied. At this stage in the development of GRSM no guidance can be offered about any adjustments. Intersections, both for roadway development and longwall retreat, have different mining-induced stress compared to roadways. Longwall start-up, before regular caving occurs, and major weighting events along the longwall face may have higher abutment stress. As a longwall approaches intersections, there may be an increase in mining-induced stress.Table4Stress index equation for roadway developmentinput parameters and constants.ParametersConstantsCMRRGSIa-7.66-7.43Immediate 2 m roof ( or GSI global rock-mass strength )0.0330.0088Roadway or excavation height (m)0.2270.227Roadway or excavation width (m)0.00130.0041Depth of cover (m)0.006770.00681Solid or rib-to-rib pillar width (m)-0.0013-0.0013Ratio of in situ horizontal stress to vertical stress for0.7670.772immediate 2 m roof;whereis the angle between the roadway orientation and0.2800.282the in situ major principal horizontal stress. Only use apositive number between 0and 180Table5Stress index equation for longwall retreatinput parameters and constants.ParametersConstantsCMRRGSIc-22.40-28.10Immediate 2 m roof ( or GSI global rock-mass0.7970.168strength )Upper 4 m roof ( or GSI global rock-mass-1.064-0.150strength )Roadway or excavation height (m)0.8170.794Roadway or excavation width (m)-0.406-0.215Depth of cover (m)0.01080.0101Solid or rib-to-rib pillar width (m)-0.0129-0.0098Longwall panel width,rib-to-rib (m)0.000210.00057Ratio of in situ horizontal stress to vertical0.6860.690stress for imm. 2 m roofRatio of in situ horizontal stress to vertical1.5530.953stress for upper 4 m roof;whereis the angle between the gateroad0.6740.637orientation looking inbye and the in situ major principal horizontal stress. Clockwise is positive. The is taken as 20 . For the angle ,only use a positive number between 0 and 1801 12 21 12 2There are 197 roadway development data points and 78 longwall retreat datapoints. The proposed roadway development stress index calculation is given by Eq.(7), with the input parameters and constants defined in Table4. The proposed longwall retreat stress index calculation is given by Eq.(8), with the input parameters and constants defined in Table5. Both Eqs. (7) and (8) have a correlation coefficient () of 0.99.SIDEV= a+ (bx + b x + ) + (bx + b x + )2MPa(7)SILR= c+ (d y + d y + ) + (d y + d y + )2MPa(8)1 12 21 12 2where, is the stress index for roadway development, is the stress index for longwall retreat, , are independent input parameters,
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