張雙樓煤礦1.5Mta新井設(shè)計【含CAD圖紙+文檔】
張雙樓煤礦1.5Mta新井設(shè)計【含CAD圖紙+文檔】,含CAD圖紙+文檔,張雙樓,煤礦,mta,設(shè)計,cad,圖紙,文檔
專題關(guān)鍵層對采動應(yīng)力影響的研究不同關(guān)鍵層位置與厚度對采動應(yīng)力演化的數(shù)值模擬研究摘要:根據(jù)工作面實際情況,運用udec軟件建立工作面開采數(shù)值模擬模型,研究了不同關(guān)鍵層位置與厚度對工作面超前支承壓力與采空區(qū)后方垂直應(yīng)力演化的影響。結(jié)果表明,數(shù)值模擬得出關(guān)鍵層的影響與理論計算的結(jié)果基本一致,關(guān)鍵層與煤層垂距越大,工作面超前應(yīng)力峰值與應(yīng)力集中系數(shù)的增加越平緩;關(guān)鍵層厚度越大,工作面超前應(yīng)力峰值與應(yīng)力集中系數(shù)的增加越劇烈,可為采場巖層控制提供依據(jù)。關(guān)鍵詞:關(guān)鍵層位置;關(guān)鍵層厚度;應(yīng)力演化;數(shù)值模擬目前關(guān)于采場超前支承壓力的研究,國內(nèi)外不少專家學者提出了許多研究方法,并取得很多研究成果。開采后的上覆巖層所形成的結(jié)構(gòu),由“煤壁-已冒落的矸石”支撐體系來支撐,只是在下位巖層中才可能由“煤壁-工作面支架-采空區(qū)已冒落矸石”支撐體系支撐。又由于上覆巖層的結(jié)構(gòu)大部分是半拱式的結(jié)構(gòu),因此煤壁一端幾乎支承著回采工作面空間上方懸露巖層(指由于離層而導致懸露)的大部分重量1,工作面前方采場應(yīng)力進行重新分布,形成“橫三區(qū)”( 卸壓區(qū)、應(yīng)力集中區(qū)和原始應(yīng)力區(qū))2。由德國學者v.??撕蚏.舊特采爾特及蘇聯(lián)學者F許普魯特提出的壓力拱理論,較好的解釋了工作面圍巖支承壓力的存在,較好的說明了工作面支架上的壓力遠小于上覆巖層重量的原因3。根據(jù) Winkler 假設(shè),基礎(chǔ)的反力與地基的沉陷成正比。采場上覆巖層的撓曲下沉決定支承壓力的分布,利用帶狀載荷在半無限彈性體中傳播的彈性理論,便可求得復(fù)合關(guān)鍵層條件下工作面前方煤體支承壓力4。但以上研究大多是將上覆巖層簡化為均布載荷而進行的研究,還很少有學者對不同關(guān)鍵層條件下的采場超前及采空區(qū)后方的支承壓力及應(yīng)力峰值位置進行系統(tǒng)研究。筆者采用數(shù)值模擬的研究手段,對不同關(guān)鍵層位置及不同關(guān)鍵層厚度條件下的采場超前及采空區(qū)后方的支承壓力以及應(yīng)力峰值位置進行了分析。1.工作面概況煤層厚度為3m,底板為軟巖,厚度為12m,關(guān)鍵層之下的煤層頂板為軟巖,上覆巖層總厚度為200m,煤體沿推進方向的長度為600m。2.模型建立對工作面實際情況進行適當簡化,運用udec軟件進行模擬分析,模擬不同關(guān)鍵層位置與厚度條件下隨著工作面推進距離的增加,采空區(qū)后方垂直應(yīng)力及工作面前方超前應(yīng)力的演化情況。模型概況:以煤體起始位置與煤層頂板的交點為坐標原點,推進方向為x軸正向,垂直向上指向地表為y軸正向。關(guān)鍵層與煤層的間距以6倍、8倍、10倍、12倍、15倍煤層厚度(即18m、24m、30m、36m、45m)進行討論,此時關(guān)鍵層厚度為20m保持不變。關(guān)鍵層與煤層間距為6倍煤層厚度模型示意圖如圖2-1所示。 關(guān)鍵層厚度以4倍、6倍、8倍、10倍、12倍煤層厚度(即12m、18m、24m、30m、36m)進行討論,此時關(guān)鍵層與煤層間距為15m保持不變。關(guān)鍵層厚度為4倍煤層厚度模型示意圖如圖2-2所示。圍巖物理力學性質(zhì)參照實際巖體力學特性和參數(shù)確定。節(jié)理特性考慮采動影響,圍巖本構(gòu)關(guān)系采用摩爾-庫侖模型。巖層的塊度依據(jù)巖層厚度和采動巖體特點進行劃分。模型中的力學參數(shù)性質(zhì)見表1與表2。表1塊體力學參數(shù)表 塊體力學參數(shù)巖層容重d/kN.m-3體積模量 K/Gpa剪切模量 G/Gpa摩擦角 f/(o)粘結(jié)力 C/Mpa抗拉強度 t/Mpa軟 巖24343031關(guān)鍵層243040403010煤 層13342020.5表2節(jié)理力學參數(shù)節(jié)理力學參數(shù)巖層法向剛度jkn/Gpa切向剛度jks/Gpa粘結(jié)力jc/Mpa摩擦角jf/(o)抗拉強度 jt/Mpa軟 巖622100關(guān)鍵層302015300煤 層6211003.數(shù)值模擬過程與結(jié)果分析用生成空區(qū)域來模擬工作面采煤,用強度較弱的松散介質(zhì)模擬充填采空區(qū),工作面開切眼位置為x=150m處,模擬過程中,工作面每推進10m,布置測線y=-1.5m,記錄并保存該測線上每隔5m的點出的垂直應(yīng)力,直到推進距離達到300m為止,因為此時已達到充分采動,繼續(xù)推進,采場應(yīng)力分布與峰值情況變化不大。將udec保存的數(shù)據(jù)調(diào)取出來輸入至excel,通過處理可得到隨工作面推進采空區(qū)后方垂直應(yīng)力與超前應(yīng)力的詳細變化曲線。下面進行詳細說明。3.1不同關(guān)鍵層位置關(guān)鍵層與煤層距離為6倍采高 圖3-1 關(guān)鍵層與煤層間距為6倍采高時應(yīng)力變化曲線關(guān)鍵層與煤層距離為8倍采高 圖3-2 關(guān)鍵層與煤層間距為8倍采高時應(yīng)力變化曲線關(guān)鍵層與煤層距離為10倍采高 圖3-3 關(guān)鍵層與煤層間距為10倍采高時應(yīng)力變化曲線關(guān)鍵層與煤層距離為12倍采高 圖3-4 關(guān)鍵層與煤層間距為12倍采高時應(yīng)力變化曲線關(guān)鍵層與煤層距離為15倍采高 圖3-5 關(guān)鍵層與煤層間距為15倍采高時應(yīng)力變化曲線綜合分析圖3-1圖3-5,可以得出如下規(guī)律:(1) 不同關(guān)鍵層位置條件下,工作面超前支承壓力峰值均隨著工作面推進而增加。(2) 關(guān)鍵層距離煤層越遠,工作面超前支承壓力峰值增加得越平緩,即更容易達到充分采動。(3) 關(guān)鍵層距離煤層越遠,對應(yīng)測點位置的工作面超前支承壓力峰值越小。如關(guān)鍵層與煤層間距為6倍采高時工作面超前支承壓力峰值在449m處達到34MPa,關(guān)鍵層與煤層間距為10倍采高時工作面超前支承壓力峰值在449m處為29.26MPa,關(guān)鍵層與煤層間距為15倍采高時工作面超前支承壓力峰值在449m處為28.35MPa。3.2不同關(guān)鍵層厚度關(guān)鍵層厚度為4倍采高 圖3-6 關(guān)鍵層厚度為4倍采高時應(yīng)力變化曲線關(guān)鍵層厚度為6倍采高 圖3-7 關(guān)鍵層厚度為6倍采高時應(yīng)力變化曲線關(guān)鍵層厚度為8倍采高 圖3-8 關(guān)鍵層厚度為8倍采高時應(yīng)力變化曲線關(guān)鍵層厚度為10倍采高 圖3-9 關(guān)鍵層厚度為10倍采高時應(yīng)力變化曲線關(guān)鍵層厚度為12倍采高 圖3-10 關(guān)鍵層厚度為12倍采高時應(yīng)力變化曲線綜合分析圖3-6圖3-10,可以得出如下規(guī)律:(1) 不同關(guān)鍵層厚度條件下,工作面超前支承壓力峰值均隨工作面推進而不斷增加。(2) 關(guān)鍵層厚度越大,對應(yīng)測點位置的工作面超前支承壓力峰值越大;當關(guān)鍵層厚度達到一定程度后,其對工作面的影響會穩(wěn)定。如關(guān)鍵層厚度為4倍采高時工作面超前支承壓力峰值在449m處為29.48MPa,關(guān)鍵層厚度為6倍采高時工作面超前支承壓力峰值在449m處為31.59MPa,關(guān)鍵層厚度為10倍采高時工作面超前支承壓力峰值在449m處達到34.36MPa,關(guān)鍵層厚度為12倍采高時工作面超前支承壓力峰值在449m處達到32.47MPa。4.結(jié)論通過以上研究可以發(fā)現(xiàn),關(guān)鍵層對采動應(yīng)力演化影響明顯。關(guān)鍵層距離煤層越遠,工作面超前支承壓力峰值越小,即關(guān)鍵層對采動應(yīng)力影響越小。關(guān)鍵層厚度越大,工作面超前支承壓力峰值越大,即關(guān)鍵層對采動應(yīng)力影響越顯著;當關(guān)鍵層厚度達到一定程度時,其對采動應(yīng)力的影響趨于穩(wěn)定。參考文獻1 錢鳴高,石平五.礦山壓力與巖層控制M.徐州:中國礦業(yè)大學出版社,2003.11.2 李守國.采場應(yīng)力變化模擬分析J.煤礦安全,2011,1003-496X 10-0135-04.3 馬其華. 長壁采場覆巖“O”型空間結(jié)構(gòu)及相關(guān)礦山壓力研究.山東科技大學,2005.4 潘宏宇.復(fù)合關(guān)鍵層下采場壓力及煤層瓦斯?jié)B流耦合規(guī)律研究.西安科技大學,2009.5 王建樹,劉軍,曹廣遠,黃炳香.雙突軟煤層大采高綜采面支承壓力分布規(guī)律研究 J.煤炭工程,2010,( 2):4042.6 張正斌.工作面推進過程中支承壓力的發(fā)展規(guī)律研究J.山東煤炭科技, 2010,( 4):1861877 于海勇.綜采開采的基礎(chǔ)理論. 北京:煤炭工業(yè)出版社,19958 王省身.礦井災(zāi)害防治理論與技術(shù). 徐州:中國礦業(yè)大學出版社,19899 . 中國煤炭建設(shè)協(xié)會.煤炭工業(yè)礦井設(shè)計規(guī)范. 北京:中國計劃出版社,200510 岑傳鴻、竇林名.采場頂板控制與監(jiān)測技術(shù). 徐州:中國礦業(yè)大學出版社,200411 蔣國安、呂家立.采礦工程英語. 徐州:中國礦業(yè)大學出版社,199812 李位民.特大型現(xiàn)代化礦井建設(shè)與工程實踐. 北京:煤炭工業(yè)出版社,200113 綜采設(shè)備管理手冊編委會.綜采設(shè)備管理手冊. 北京:煤炭工業(yè)出版社,199414 中國煤礦安全監(jiān)察局.煤礦安全規(guī)程. 北京:煤炭工業(yè)出版社,200615 朱真才、韓振鐸.采掘機械與液壓傳動. 徐州:中國礦業(yè)大學出版社,200516 洪曉華.礦井運輸提升. 徐州:中國礦業(yè)大學出版社,200517 中國統(tǒng)配煤礦總公司物資供應(yīng)局.煤炭工業(yè)設(shè)備手冊. 徐州:中國礦業(yè)大學出版社,199218 章玉華.技術(shù)經(jīng)濟學. 徐州:中國礦業(yè)大學出版社,199519 鄭西貴、李學華.采礦AutoCAD2006入門與提高. 徐州:中國礦業(yè)大學出版社,200520 王德明.礦井通風與安全. 徐州:中國礦業(yè)大學出版社,200721 楊孟達.煤礦地質(zhì)學. 北京:煤炭工業(yè)出版社,200022 劉剛.井巷工程.徐州:中國礦業(yè)大學出版社,200523 中國煤炭建設(shè)協(xié)會.煤炭建設(shè)井巷工程概算定額(2007基價).北京:煤炭工業(yè)出版社,200824 林在康、李希海.采礦工程專業(yè)畢業(yè)設(shè)計手冊. 徐州:中國礦業(yè)大學出版社,2008XXX大學畢業(yè)論文任務(wù)書學院 礦業(yè)工程學院 專業(yè)年級 采礦工程 學生姓名 任務(wù)下達日期: 20XX年 1月 8日畢業(yè)論文日期: 20XX年 3月 12日至 20XX年 6月 9日畢業(yè)論文題目: 張雙樓煤礦1.5Mt/a新井設(shè)計畢業(yè)論文專題題目: 關(guān)鍵層對采動應(yīng)力影響的研究畢業(yè)論文主要內(nèi)容和要求:院長簽字: 指導教師簽字: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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