张双楼煤矿1.5Mta新井设计【含CAD图纸+文档】
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专题关键层对采动应力影响的研究不同关键层位置与厚度对采动应力演化的数值模拟研究摘要:根据工作面实际情况,运用udec软件建立工作面开采数值模拟模型,研究了不同关键层位置与厚度对工作面超前支承压力与采空区后方垂直应力演化的影响。结果表明,数值模拟得出关键层的影响与理论计算的结果基本一致,关键层与煤层垂距越大,工作面超前应力峰值与应力集中系数的增加越平缓;关键层厚度越大,工作面超前应力峰值与应力集中系数的增加越剧烈,可为采场岩层控制提供依据。关键词:关键层位置;关键层厚度;应力演化;数值模拟目前关于采场超前支承压力的研究,国内外不少专家学者提出了许多研究方法,并取得很多研究成果。开采后的上覆岩层所形成的结构,由“煤壁-已冒落的矸石”支撑体系来支撑,只是在下位岩层中才可能由“煤壁-工作面支架-采空区已冒落矸石”支撑体系支撑。又由于上覆岩层的结构大部分是半拱式的结构,因此煤壁一端几乎支承着回采工作面空间上方悬露岩层(指由于离层而导致悬露)的大部分重量1,工作面前方采场应力进行重新分布,形成“横三区”( 卸压区、应力集中区和原始应力区)2。由德国学者v.海克和R.旧特采尔特及苏联学者F许普鲁特提出的压力拱理论,较好的解释了工作面围岩支承压力的存在,较好的说明了工作面支架上的压力远小于上覆岩层重量的原因3。根据 Winkler 假设,基础的反力与地基的沉陷成正比。采场上覆岩层的挠曲下沉决定支承压力的分布,利用带状载荷在半无限弹性体中传播的弹性理论,便可求得复合关键层条件下工作面前方煤体支承压力4。但以上研究大多是将上覆岩层简化为均布载荷而进行的研究,还很少有学者对不同关键层条件下的采场超前及采空区后方的支承压力及应力峰值位置进行系统研究。笔者采用数值模拟的研究手段,对不同关键层位置及不同关键层厚度条件下的采场超前及采空区后方的支承压力以及应力峰值位置进行了分析。1.工作面概况煤层厚度为3m,底板为软岩,厚度为12m,关键层之下的煤层顶板为软岩,上覆岩层总厚度为200m,煤体沿推进方向的长度为600m。2.模型建立对工作面实际情况进行适当简化,运用udec软件进行模拟分析,模拟不同关键层位置与厚度条件下随着工作面推进距离的增加,采空区后方垂直应力及工作面前方超前应力的演化情况。模型概况:以煤体起始位置与煤层顶板的交点为坐标原点,推进方向为x轴正向,垂直向上指向地表为y轴正向。关键层与煤层的间距以6倍、8倍、10倍、12倍、15倍煤层厚度(即18m、24m、30m、36m、45m)进行讨论,此时关键层厚度为20m保持不变。关键层与煤层间距为6倍煤层厚度模型示意图如图2-1所示。 关键层厚度以4倍、6倍、8倍、10倍、12倍煤层厚度(即12m、18m、24m、30m、36m)进行讨论,此时关键层与煤层间距为15m保持不变。关键层厚度为4倍煤层厚度模型示意图如图2-2所示。围岩物理力学性质参照实际岩体力学特性和参数确定。节理特性考虑采动影响,围岩本构关系采用摩尔-库仑模型。岩层的块度依据岩层厚度和采动岩体特点进行划分。模型中的力学参数性质见表1与表2。表1块体力学参数表 块体力学参数岩层容重d/kN.m-3体积模量 K/Gpa剪切模量 G/Gpa摩擦角 f/(o)粘结力 C/Mpa抗拉强度 t/Mpa软 岩24343031关键层243040403010煤 层13342020.5表2节理力学参数节理力学参数岩层法向刚度jkn/Gpa切向刚度jks/Gpa粘结力jc/Mpa摩擦角jf/(o)抗拉强度 jt/Mpa软 岩622100关键层302015300煤 层6211003.数值模拟过程与结果分析用生成空区域来模拟工作面采煤,用强度较弱的松散介质模拟充填采空区,工作面开切眼位置为x=150m处,模拟过程中,工作面每推进10m,布置测线y=-1.5m,记录并保存该测线上每隔5m的点出的垂直应力,直到推进距离达到300m为止,因为此时已达到充分采动,继续推进,采场应力分布与峰值情况变化不大。将udec保存的数据调取出来输入至excel,通过处理可得到随工作面推进采空区后方垂直应力与超前应力的详细变化曲线。下面进行详细说明。3.1不同关键层位置关键层与煤层距离为6倍采高 图3-1 关键层与煤层间距为6倍采高时应力变化曲线关键层与煤层距离为8倍采高 图3-2 关键层与煤层间距为8倍采高时应力变化曲线关键层与煤层距离为10倍采高 图3-3 关键层与煤层间距为10倍采高时应力变化曲线关键层与煤层距离为12倍采高 图3-4 关键层与煤层间距为12倍采高时应力变化曲线关键层与煤层距离为15倍采高 图3-5 关键层与煤层间距为15倍采高时应力变化曲线综合分析图3-1图3-5,可以得出如下规律:(1) 不同关键层位置条件下,工作面超前支承压力峰值均随着工作面推进而增加。(2) 关键层距离煤层越远,工作面超前支承压力峰值增加得越平缓,即更容易达到充分采动。(3) 关键层距离煤层越远,对应测点位置的工作面超前支承压力峰值越小。如关键层与煤层间距为6倍采高时工作面超前支承压力峰值在449m处达到34MPa,关键层与煤层间距为10倍采高时工作面超前支承压力峰值在449m处为29.26MPa,关键层与煤层间距为15倍采高时工作面超前支承压力峰值在449m处为28.35MPa。3.2不同关键层厚度关键层厚度为4倍采高 图3-6 关键层厚度为4倍采高时应力变化曲线关键层厚度为6倍采高 图3-7 关键层厚度为6倍采高时应力变化曲线关键层厚度为8倍采高 图3-8 关键层厚度为8倍采高时应力变化曲线关键层厚度为10倍采高 图3-9 关键层厚度为10倍采高时应力变化曲线关键层厚度为12倍采高 图3-10 关键层厚度为12倍采高时应力变化曲线综合分析图3-6图3-10,可以得出如下规律:(1) 不同关键层厚度条件下,工作面超前支承压力峰值均随工作面推进而不断增加。(2) 关键层厚度越大,对应测点位置的工作面超前支承压力峰值越大;当关键层厚度达到一定程度后,其对工作面的影响会稳定。如关键层厚度为4倍采高时工作面超前支承压力峰值在449m处为29.48MPa,关键层厚度为6倍采高时工作面超前支承压力峰值在449m处为31.59MPa,关键层厚度为10倍采高时工作面超前支承压力峰值在449m处达到34.36MPa,关键层厚度为12倍采高时工作面超前支承压力峰值在449m处达到32.47MPa。4.结论通过以上研究可以发现,关键层对采动应力演化影响明显。关键层距离煤层越远,工作面超前支承压力峰值越小,即关键层对采动应力影响越小。关键层厚度越大,工作面超前支承压力峰值越大,即关键层对采动应力影响越显著;当关键层厚度达到一定程度时,其对采动应力的影响趋于稳定。参考文献1 钱鸣高,石平五.矿山压力与岩层控制M.徐州:中国矿业大学出版社,2003.11.2 李守国.采场应力变化模拟分析J.煤矿安全,2011,1003-496X 10-0135-04.3 马其华. 长壁采场覆岩“O”型空间结构及相关矿山压力研究.山东科技大学,2005.4 潘宏宇.复合关键层下采场压力及煤层瓦斯渗流耦合规律研究.西安科技大学,2009.5 王建树,刘军,曹广远,黄炳香.双突软煤层大采高综采面支承压力分布规律研究 J.煤炭工程,2010,( 2):4042.6 张正斌.工作面推进过程中支承压力的发展规律研究J.山东煤炭科技, 2010,( 4):1861877 于海勇.综采开采的基础理论. 北京:煤炭工业出版社,19958 王省身.矿井灾害防治理论与技术. 徐州:中国矿业大学出版社,19899 . 中国煤炭建设协会.煤炭工业矿井设计规范. 北京:中国计划出版社,200510 岑传鸿、窦林名.采场顶板控制与监测技术. 徐州:中国矿业大学出版社,200411 蒋国安、吕家立.采矿工程英语. 徐州:中国矿业大学出版社,199812 李位民.特大型现代化矿井建设与工程实践. 北京:煤炭工业出版社,200113 综采设备管理手册编委会.综采设备管理手册. 北京:煤炭工业出版社,199414 中国煤矿安全监察局.煤矿安全规程. 北京:煤炭工业出版社,200615 朱真才、韩振铎.采掘机械与液压传动. 徐州:中国矿业大学出版社,200516 洪晓华.矿井运输提升. 徐州:中国矿业大学出版社,200517 中国统配煤矿总公司物资供应局.煤炭工业设备手册. 徐州:中国矿业大学出版社,199218 章玉华.技术经济学. 徐州:中国矿业大学出版社,199519 郑西贵、李学华.采矿AutoCAD2006入门与提高. 徐州:中国矿业大学出版社,200520 王德明.矿井通风与安全. 徐州:中国矿业大学出版社,200721 杨孟达.煤矿地质学. 北京:煤炭工业出版社,200022 刘刚.井巷工程.徐州:中国矿业大学出版社,200523 中国煤炭建设协会.煤炭建设井巷工程概算定额(2007基价).北京:煤炭工业出版社,200824 林在康、李希海.采矿工程专业毕业设计手册. 徐州:中国矿业大学出版社,2008XXX大学毕业论文任务书学院 矿业工程学院 专业年级 采矿工程 学生姓名 任务下达日期: 20XX年 1月 8日毕业论文日期: 20XX年 3月 12日至 20XX年 6月 9日毕业论文题目: 张双楼煤矿1.5Mt/a新井设计毕业论文专题题目: 关键层对采动应力影响的研究毕业论文主要内容和要求:院长签字: 指导教师签字:英文原文Finite element analysis of three-way roadway junctions in longwall miningR.N. Singh, I. Porter, J. HematianFaculty of Engineering, Uniersity of Wollongong, Northfields Avenue, Wollongong, NSW 2522, AustraliaAbstract:This paper presents a three-dimensional finite element analysis of three-way roadway intersections in longwall mining, and assesses the stable/unstable behaviour of three-way intersections under a range of loading conditions. Loads were applied to the model by means of uniform stresses on the internal free faces. This method of loading the model from the inside helped to reduce its size and to eliminate the boundary effects. Stress concentrations and displacement results on the mid-height of the pillars, roof and floor strata adjacent to the three-way intersections and cut-throughs were calculated.Based on this study, guidelines for designing the support system for three-way intersections are suggested. The results were validated by a case study of a three-way intersection in an underground coal mine in the southern coal fields of the Sydney Basin. Keywords:underground coal mining; gate roadway; intersections; stability; finite element method1. IntroductionA trend exists in Australia for installing high productivity longwall faces producing 3.04.0 milliontonne raw coal per annum per face. The mainconcern for the success of the high-production longwallfaces is to achieve high rates of developmentand to maintain stability of access roadways andtheir intersections during the life span of the face.Intersections are formed when the pillars betweenthe two roadways are intersected by driving a crosscut. Roadway intersections in underground mines areparticularly susceptible to ground control problemsdue to inherently wide roof spans used and the difficulty in installing roof supports promptly inhighly mechanised headings. Stresses induced duringintersection formation may result in high incidenceof roof and rib failures. Despite many investigationsinto the stability of gate roadways intersectionsadverse conditionssuch as high horizontal stress and unsteady state ofabutment pressure from moving longwall faces maycause instability of gate roadway intersections.For example in 1985; major strata control problems inthe main gate of no. 6 longwall panel at WestcliffColliery resulted in roof fall, which stopped coalproduction for a period of 6 weeks. Similarly, a rooffailure incident at Pacific Colliery caused the longwallequipment to be buried resulting in stoppage ofthe longwall operations for a period of 3 months.Thus, unprecedented stratacontrol problems may have significant effects onoverall production from high-productivity longwallsystems even over a short duration.This paper containsan investigation of the application of a three-dimensionalfinite element method to calculate stressesand displacement around three-way roadway intersections.The effects of individual parameters such as depth of cover, the ratio of horizontal to verticalstress (K) and the width of opening on the stability of the three-way intersections are examined. Theresults are compared with the field observations at anunderground coal mine in the southern coal field ofthe Sydney Basin.2. Stability analysis of three-way intersections using three-dimensional finite element modelsThe procedure used in the stability analysis of thethree-way intersections comprised of defining themechanical properties of the rocks surrounding theintersection, the geometry of the intersection and thevirgin state of stress. The stresses and displacements induced around the intersections were computed usinga three-dimensional finite element method. Ifunstable conditions existed, either the design of supportsystem was changed or the geometry of thestructure was modified.Important input data forthese models were vertical stress and the ratio ofhorizontal to vertical stress K for a given lithologyand dimensions of the roadway intersection (see Fig.1).Assuming symmetrical conditions around a threewayintersection, only half of the structure wasmodelled using eight-node solid elements comprisinga total of 7190 elements and the 11 597 grid points.The computer running time was 17 h using around 1Gb of memory. The rock mass properties assigned tothe intersection model are presented in Table 1.The loads were applied to the Fig. 1. Plan and section of the finite element three-dimensional intersection modelTable 1 Rock properties assigned to three-way intersection modelsRock typeThickness /mE /GPaMedium grain sandstone4.010.00.20Fine sandstone and mudstone3.06.00.25Coarse sandstone and shale2.03.00.20Top coal1.03.50.3Coal3.03.50.3Mudstone1.08.00.25Coarse sandstone4.012.50.2Medium grain sandstone5.010.00.2model by means ofuniform pressures on the internal free faces. Thistechnique of applying load from the inside helped to reduce the size of the model and to eliminate boundaryeffects. For all the loading configurations depictedin Table 2, a linear solution method was used.Table 2 Loading conditions applied to the three-dimensional modelLoading configurations / MPa / MPa / MPa 1 10.0 0.0 0.0 2 10.0 10.0 10.0 3 10.0 10.0 20.0 4 10.0 20.0 10.0 Fig. 2 Vertical stress concentration at mid-height of the intersection.(a) Kx=1, Ky=1; (b) Kx=1, Ky=2.Preliminary computer analysis was carried out tocompute the induced vertical stress distributionthroughout the three-dimensional model for a litho-staticcondition. In order to gain better understandingon the behaviour of the structure, the vertical stressconcentration on various horizontal and verticalplanes was shown for different loading conditions byplotting stress concentration contour lines for variousratios of induced virgin stresses. These results are discussed in the subsequent sections.3. Pillar behaviour at three-way intersectionFig. 2 indicates vertical stress concentration at themid-height of the pillar for various loading configurations.For the litho-static stress condition at K=Kx=Ky=1, the stress concentration at the midheight of the pillar has a symmetrical pattern (see Fig. 2a). The stress concentration zone on the ribside of the intersection has a width of 2.5 m, equal tohalf the roadway span. The maximum stress concentrationis about 1.4 times the virgin stress for theloading configuration Ky 1 for a limited zone at the corner of the pillar.When Ky 1, the vertical stress pattern at the mid-height of the pillar is no longer symmetrical;thestress is more pronounced along the roadway perpendicularto the direction of maximum horizontal stress(see Fig. 2b).No tensile zone along the rib side wasdetected. The maximum stress concentration zone islocated close to the edge of the pillar and extends along the roadway perpendicular to the major horizontal stress.4. Roof behaviour at three-way intersectionFig. 3 Vertical stress distribution over a plane 1.5 m above the roof lineThe vertical stress distribution on a plane 1.5-mabove the roof line is shown in Fig. 3, which indicatesthat the stress is 0.8 over the edge of pillar increasing to 1.0 at a distance of 6 m within the edge of the pillar. The stress distribution lines abovethe individual roadways show the contour lines atintervals of 0.2.This stress distribution pattern indicates a semi-dome shaped destressed zone overthe three-way intersection. When the ratio of horizontalto vertical stress, Kx or Ky increases, the stress contour line 0.2 moves towards the centre of the roadway while 1.0 line moves further into the pillar indicating that the height of the semi-domeshaped destressed zone becomes shallower in thefield of high horizontal stress.Fig. 4 Vertical stress distribution on a vertical plane at the mid-span of the main roadwayWhen KxKy , as shown in Fig. 3b, the stresspattern varies over the individual roadways and 0.2partly disappears in the roadway perpendicular to themajor horizontal stress. In this case, the boundary ofthe roof fall in this roadway will be controlled by thestress contour lines of 0.4 .However, the rate of changes in stress distribution across the roof line ofthe roadway parallel to high horizontal stress is moresignificant. The height of the roof fall in the roadwayintersection might be evaluated by using appropriatedestressed contour lines on the vertical plane at themid-span of the main roadways and the cut-throughs,respectively, as presented in Figs. 4 and 5. Thejustification of using 0.4 contour line to delineate the boundary of roof fall is presented in a subsequentsection. Fig. 5 Vertical stress concentration on the vertical plane at the mid-span of the cut-through.Fig. 5 also indicates that the radius of influence ofthe intersection over the individual roadways with respect to the stress distribution in the roof is estimatedto be one span from the centre of the intersection. Fig. 6 shows the vertical displacement on the roofline under various loading configurations at the roadwayintersection. The maximum sag occurs at the centre of the intersection and its maximum value is12 mm. It can also be seen that the roadway parallelto the major horizontal stress will show more roofsag than the roadway perpendicular to the horizontalstress.Fig. 6. Roof sag in millimetres on roof line at a three-way intersection.Fig. 7 Floor heave at the floor line at a three-way intersection at =10 MPa.Behaviour of the floor at the T-junction of athree-way intersection is given in Fig. 7 on the floorline for loading configuration Kx=1 and Ky=2. The floor lift patterns are similar to that of the roofsag except that the amount of the maximum floorheave is much less than the corresponding value forsag. 5. Case history of three-way intersectionsAn investigation into the mechanism of instabilityat roadway intersections was carried out at tail gatesof a longwall panel in an underground coal mine inthe southern coal fields of the Sydney Basin. Thefield measurements included roof sag, floor heaveand rib deformation monitoring ahead and behind thelongwall face. The overall objective of this studywas to validate the results of three-dimensional finiteelement modelling of the three-way junction by comparingthe results with the field measurements.5.1. Site location and the description of the site-specific ModelFig. 8 presents the details of the longwall panel, gate roadways and intersections at the site beinginvestigated. The panels were 200-m wide and 2000-m long with a double entry gate roadway system.Each roadway was 5 m wide, 3 m high, with 5540 m pillars centre-to-centre. The height of extractionvaried between 2.4 and 2.6 m.The actual sites ofmonitoring were 35, 36 and 9 intersections of 24longwalls tail gate and 35 and 36 cut-throughs. Thevertical stress at the site was 10 MPa at the depth of420 m, the major horizontal stress 25 MPa orientedparallel to the gate roadways and the minor horizontalstress 10 MPa at an orthogonal direction to thetail gates.Fig. 8. General plan view of the site of investigation.Fig. 9 Lithology at the site of investigation at 9 cut-through (A).and 36 cut-through (B).Fig. 9 illustrates the lithology profiles of the stratacolumn together with their thickness.The mechanicalproperties of the strata units are shown in Table3. Based on the above information, a number ofthree-dimensional finite element models were constructedand analysed to simulate the existing conditionsaround the sites of investigation. Both inducedstresses and displacements around the roadways andintersections were computed for each site of investigation. The results of the finite element analyses arepresented together with the values obtained from thefield displacement measurements.A series of roof, rib and floor extensometers were installed at and in between 35, 36 and 9 cut-throughsahead of 23 longwall panel. The objective of thisstudy was to determine the pattern of deformationaround the area of investigation and provide a measureof ground control. The extensometers site andlocation for principle modes of failure are also presentedin Fig. 8. The roof sag measurements have been carried outat different locations and compared with values predicted by the finite element model.In all cases, Table 3 Mechanical properties of rock at the site of investigationRock type Tensile strength / MPaUCS / MpaFriction factor NE / Gpa y Medium sandstone 450490.2Broken shale 1.5102.810.28Shale and sandstone 3303.350.25Coal 1.515360.3Shaleqsandstoneqclay 3.5353.560.23Mediumqcoarse sandstone 4.2534120.2Fig. 10 Roof sag measured and predicted values at no. 9 cross-cutThe differences between the measured and predicted values are very small. Fig. 10 indicates typical results of roof sag measurements, together with the predictedvalues of displacements at 9 cut-through, before andafter the longwall face has passed through the monitoringsite.Monitoring continued when the longwallface approached and passed 9 cut-through andreached the end of the panel. Readings were regularlytaken over a period of 45 days, but for the sakeof simplicity, only the initial and final readings areshown.It can be noted that the difference betweenthe initial and final readings was very little. Therefore, it can be concluded that the time dependentdeformation of the roof was very little. In addition,visual examinations indicated that good roof conditionsprevailed throughout the investigation without displaying any strata softening and roof deterioration.Comparing the results of deformation at 9cut-through before and after longwall no. 23 passedthe site, it can be seen that tailgate behaviour issignificantly affected by the retreat of the adjacent face.5.2. Rib behaviourFig. 11 Rib displacement, measured and predicted values between 35 and 36 cut-throughs.The results of rib extensometers and those predicted by the finite element analysis are presented in Fig. 11. The results indicate a timedependent deformation of 0.4 mm dayy1. As thetime dependent behaviour of the coal seam could notbe modelled in the finite element analysis of thestructure, the predicted values are only the totaldeformation after complete relaxation and thereforeless than the measured values.The important aspect of the chain pillar between35 and 36 cut-throughs is the nature of the ribmovement. The extensometer readings indicate thatthe softening has occurred to a depth of 5 m. This isin contrast to rib behaviour observed at 9 cut-throughwhere the deformation into the pillar rapidly abatesfrom the rib line.5.3. Floor behaviourThe floor extensometers results and the displacementvalues predicted by the finite element method at 35 cut-through are presented in Fig. 12. The plotindicates that although the deformation initiated 5 mbelow the floor surface, the majority of deformationtook place between 1.0 and 1.5 m into the floor.Thus, floor heave takes place in the broken shale andthe laminated shale units as referred to in the lithologyprofile presented in Fig. 9. Although the shaleunit is surrounded by the laminated sandstone/shale,it can generate an uplift stress in the immediate floorwhen failure occurs within it. The significant floorheave at 9 cut-through is mainly attributed to thehigh horizontal stress and the side abutment stress ofthe longwall face.Fig. 12 Measured and predicted floor heave between 35 cutthroughsIt has been previously demonstrated in an earlierpublication by the present authors that roadways parallel to the major horizontalstress, where K (Kx or Ky) 1, will have greaterfloor heave and roof sag when compared to road-ways parallel to minor horizontal stress.6. Guidelines for designing the support system at three-way intersectionsThe results of investigation of three-way intersectionsshowed that the maximum vertical stress at themid-height of the coal seam occurs at the corner ofthe pillar and increases with the roadway width andthe depth below surface. The destressed zone overthe pillar extends along the roadway perpendicular to the major horizontal stress. A uniform pattern ofhorizontal dowels in conjunction with wire meshwould be necessary to ensure the integrity of thepillars. A minimum dowel length equal to 50% ofthe entry width at 1.0-m spacing is suggested. Thispattern should be implemented on the edge of thepillar extending along the roadways for a distanceequal to one roadway span. The rest of the pillars inthe individual roadways should be reinforced if necessaryaccording to single roadway conditions. The four potential modes of failure should betaken into account when designing the optimum roofbolt pattern at three-way intersections.The first zoneof instability may manifest itself as a semi-domeshaped failure over the T-intersection. One side ofthe zone is parallel to the left of the main roadwaywith the base being a semi-circle. When KxKythe base of the zone will have a different length ineach roadway, with the longer length perpendicularto the major principle horizontal stress. Although theproperties of the roof strata have significant effect onthe stability of the roof, the stress contour lines 0.1and 0.3 have been used to define the boundary of the roof failure zone above the three-way intersectionfor Kx or Ky 2, respectively However, observations at two field sites in theSouthern Coalfield and Hunter Valley have indicatedthat the height of roof falls is generally governed bythe regional stresses, in particular, the ratio of horizontalto vertical stress, the width of the openingsand mechanical properties of the overlying strata.Previous observations at site 2 in Hunter ValleyCoalfield by one of the authors have indicated thatthe height of roof falls matched very well with thearea under stress contours of 0.3. Therefore, in order to be on a conservative side, a stress contour of 0.4was adopted as a criterion for roof fall height in the present study.The second mode of failure is due to shearingalong the bedding planes, which occurs when theshear stress exceeds the frictional strength of thebedding planes.The most probable location for slidingof bedding planes occurs closer to the rib sidethan to the roadway centre. The required length ofthe fully grouted bolts depends upon the cohesion, the coefficient of internal friction and the location ofthe bedding planes. Thus, a general roof bolt pattern cannot be devised for all conditions and an accurateanalysis of site-specific models based on the accuratefield data is necessary.The third potential mode of failure is gutteringalong and over the rib sides and corners of theintersection. This is more likely to happen when thehorizontal stress is greater than the vertical stress.Inclined roof bolts passing through this zone andanchored over the pillars are a possible solution.The fourth mode of failure is controlled by thepresence of a geological feature in the intersection.When a major geologically weak zone is present inthe area of the intersection, the roof instability ishighly influenced by the structural feature. In thatcase, the prediction of the roof fall would be governedby the orientation and inclination of the geologicalfeature, internal angle of friction and dimensionsof the intersection. In that case, a specialsupport measure will be required to ensure stability.Fig. 13 Zones of instability at three-way intersectionsThe three-way intersections cause specific stratadisturbances in the vicinity and can be convenientlydivided into two zones as indicated in Fig. 13. Inregion I, which is outside the zone of intersection, the roof condition is the same as in main roadways.Therefore, roof support is carried out based on theprocedure for individual roadways. Region II, whichis within the roadwa
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