Experimental Study on Roof Structure Characteristics and Its Failure Pattern in Coal Roadway

Jun.2004Journal of China University of mining &. TechnologyVol 14 No. 1Experimental Study on Roof StructureCharacteristics and ts failurePattern in Coal RoadwayXUEYa-domg(薛亚东), KANg Tian-he(康天合)2, HUANG Hong-ei(黄宏伟Geotechnical Engineering, Tongji UrTaiyuan University of Tech, Taiyuan, Shanxi 030024, ChinaAbstractd on the investigation and statistics of logs of 211 bole holes and strata data from 79 roadways in 13coal mines located in Xishan, Jincheng, Lu'an, Fenxi, and Huozhou in China, the roadways'roof structures wereclassified as multi-thin-layer, thin-thick combined layer, integrated thick layer, thick-coal layer, and cracked layeraccording to the geometric features and spatial strength distribution of surrounding rock. Then eight sub-categorieswere defined as different situations. And seven simulation modeling tests were carried out. The strata structures ofthese models were different from each other. At last, the relationship between roof structure and its failure patternKey words: mining roadway; structure of roof stability; similitude testCLC number: TD 326 Document code:a Article ID: 1006-1266(2004)01-00-42-05choose a suitable theory to explain1 Introductionwaydeformation mechanism and to design an effectiveThe most coal roadways are excavatedeaker rock mass (such as coal. mud rock, shalePusuba(1990) pointed out, therock and etc. Their surrounding rock has large surrounding rock mass structure is the basis tonumbers of distinct beddings and well-developed analyze the cause, features, and magnitude of largeractures in most situations. On the other hand, deformation of tunnel during excavation. It plays ao dynamic big role in the whole procedure. Song(1996)putloading due to mining. Therefore, their forward the concept of "inner and outer stressdeformation and failure laws are different from fields "to analyze and control coal drift stresses.those roadways located in hard rock. Mining This concept is also related to the structuralengineers, geology engineers and researchers have characteristics of surrounding rock. The boldone a great amount of work trying to reveal the supporting mechanism was also developed from thedeformation and failure mechanism of costudying of strata structural characteristicsroadways, and to understand the relationship Although it is essential to analyze roadwaysbetween deformation and supporting schemes fully. deformation and supporting mechanism, due to theHowever, since the physical and mechanical difficulties of investigation on surrounding rock inproperties of surrounding rock are very complicatedH中国煤化工 Actural characteristics ofand the stratigraphy of rock mass variesCNMHentdramatically from site to site, it is very difficult toIn this paper, the authors endeavored toReceived date: 2003-07-08Foundation item: Youth Natural Science Foundation of Shanxi prov. (20021021)BiograplFxxf-dong (1971-), male, from Shanxi province. Ph. D. associate profeengaged in the research on rock mechanicsXUE Ya-dong et alExperimental Study on roof Structure Characteristics and43investigate geology data from 19 coalmines in rock geometry is referred to the spatial arrangementShanxi province, and carried out several simulation of strata with different thickness, while themodeling tests in laboratory. Based on the study, distribution of mechanics properties refers to thefive categories of structural characteristics and eight distribution of strata strength. Based on the datasub-categories are proposed and the deformation and from 21 coal seams of 13 coalmines in Xishanfailure patterns of laneways are studied. The results Jincheng, Luan, Fenxi and Huozhou, totally 211can be used to design perfect supporting system for boreholes and 79 coal roadways were investigatedcoal drifts and others which are located in weaker The roof structures of coal roadways are classified asrock massfive categories: multi-thin-layer, thin-thickcompound layer, integrated thick layer, thick coal2 Roof Structural Characteristicsseam and cracked layer, according to the geometryThe coal roadway roof structure involves of rooftigatedsurrounding rock geometry and spatial distribution roadways falling into the first three categories isof mechanical properties. At here, the surrounding listed in Table 1Table 1 Summary of roof strata properties and thicknessStratigraphyCategoryMudLimestonee thick1.451.050.951.5712.12LowerThin-thickComponent/%76.47Average thickness/mthick layerComponent/%tegratedAverage thickness29thick laveComponent21,7010,8420,48From Table I we can see that the roof strata listed in Table 2. Generally speaking, the roof rockare mainly mudstone, sandy stone, limestone, and mass has low strength index and high deformationdstparameters of the samples from these strata areTable 2 Physical and mechanical properties of roof strataStratumDensity/(kg.m-3) Unconfined CompressiveTensile StrengthModulus ofPoisson's ratioStrength/MPaMPaSandstone24905,104680Mudstone230029.212,9264760.36Sandy mudstone31.243917Combine the aspects from both geometry and can be classified as eight sub-categories, which aredistribution of strength, the structure of roof strata listed in Table 3Table 3 The classes and characteristics of roof strataategoryGeometryStrength properties中国煤化工 bacing less than0.3MIlin-IaCNMHGwak conjunctionThin soft rock withthick hard rockOne or several thin layers above (single layerhin-thick layerhickness<1.o m, total thickness<2.0 m)Thin hard rock withthick layer abovethick soft rock万方数Journal of China University of Mining & TechnologyVol 14 No. 1Continued from Table 3Sub-categoryGeometryStrength propertiesDescriptionHard rockIntegrate thick rock mass(3.0 m) above coal seam:tegrate thick layerfracture spacing>1 m: Integration Index >0.75Mid-hard coal seamThick coal laverSoft coal seamThick coal seam above the tunnel roof; thickness>>3.0 mfractured layerSoft rock massTunnel is in failure zone.770mm×230mm×480mm. Hydraulic pressure3 Simulation Modeling Testsservocontrol system is used for loading. Nicon3. 1 Tests schemesDTM-53le displacement measure instrument wasSeven schemes of simulation modeling test were used for measuring surface displacements. Thecarried out to study the deformation and failugeometric similarity ratio ar= 25, while densitypatterns with different categories of roadway rooftystructure (sub-category 1 to 7). The simulated 3. 2 Results of the testsunconfined compressive strength is 25 MPa for softWith loading step, the displacement andock, 70 MPa for hard rock, 15 MPa for soft coafractures begin to appear, then developing and atand 20 MPa for mid-hardlast failure come forth Some models, deformationThe three dimensions for simulated models are procedure was illustrated in Fig. 1(a)Multi-thin-layer(b) Thin soft rock with thick hard roo(c) Integrated thick hard roof(d) Integrated thick soft roofFig. 1 The deformation and fracture development patterns of simulation models3 3 Discussion of the tests resultst中国煤化工 resulted the failure o3. 3. 1 Multi-thin-layerHCNMHcent of loads further IntoWith the increase of load on the model, tensile the slteWanls, iiauluies ueveloped quickly due tozones occurred in both sidewalls of the drift. which the failure of sidewalls, the span of roof increasedresulted vertical fractures; lower layers subsided and the subsidence of roof also increased, so did theerswhichsed spacing between thin layers; eventually the roofdiscontinuities between layers the fractures close to failed, which formed a failure archXUE Ya-dong et alExperimental Study on roof Structure Characteristics and3.3. 2 Thin-thick-layer compounddeformation and failure patterns. For the sidewallsIn the case of soft thin layer with hard thick the main mechanism of failure involves tensile andlayer, with the increase of loads, roof subsided shearing fractures. In some of the models, shearingshearing fractures appeared and developed at the failure occurred first in sidewalls, and then tensilebottom of two sidewalls and finally caused the failures occurred, which is very harmful to thefailure of sidewalls the subsidence of roof tore the stability of drifts. For the roof, the main failurethin layer from the thick layer; furthermore, tensile mechanism includes shearing, tension and alsozones appeared deep in the sidewalls; finally failure layer-tearing. The patterns of failure include blockof roof occurredfailure, multi-layer parabolic arc failure, tensionIn the case of hard thin layer and soft thick failure, falling arc, and massive subsidence failure.layer, the subsidence of roofmuch less. The To avoid tensile stresses. which is fatal to driftfailure of roof was caused by shearing failure in two stability, external support should be appliedIn addition, from the tests we also know that3.3.3 Integrated thick layerthe first failure zones always appear in the twoIn the case of hard roof, with the increase of sidewalls. This will help us to understand theloads, fractures appeared in two sidewalls; roof importance of keeping the integration of sidewallsstarted to subside and the fractures in the sidewalls and enforcing support for them, especially for softtarted to connect at the bottom of two sidewalls. rock mass tunnels. From the view offailure occurred in the angle of 59 degrees to the bolts are effective for supporting the sidewalls. Thehorizontal plane(generally the friction angle of coal design of bolts should take consideration of the angletwo sides of the roof, fractures developed, and tension failure pattern, bolts should be settledperp3.3 4 Analysis and discussionof bolts is also an issue to consider in practice.In the case of soft roof the failure of sidewalls4 Conclusionswas mainly caused by tensile fractures. Thesubsidence finally resulted into the fail1) According to geometry features andThe height of the failure arc is equivalent to the mechanics properties'spatial distribution, the roofe tunneof coal drifts can be classified as five categories, i3. 3. 5 Thick coal layere., multi-thin-layer, thin-thick-layer compoundIn the case of hard coal, with the increase of integrated thick layer, thick coal layer, and crackedloads, fractures occurred in both roof and sidewalls layer, and also eight sub-categories. The simulationand oriented almost vertically; with the gradually modeling tests showed that the patterns foronnecting of the tensile fractures, a falling arc deformation and failure are quite different betweenformed and failure occurred in the two sides of roof different categories.because of high stresses; the failuthe two2) The simulation tests also showed that thesidewalls cause the formation of a failure arc in the failure of coal drift always starts from sidewalls, andfinally incurs the failure of roof and the whole driftIn the case of soft coal layer, fractures orientedat an angle of 67 degrees to the horizontal plane in thTYH中国煤化工 sm of sidewalls,fromCNMHGS, involves shearing andthe two sidewalls: the roof subsided and then ths could be a very effective supportsidewalls failed a parabolic arc formed in the roof method to stabilize the sidewalls. Parameters forand then the roof failedolts should be chosen according to different failurThe片数把 w that different structures ofmechanismsroadway roof rock will result to quite different4) Although the shape and dimensions of46Journal of China University of Mining & TechnologyVol 14 No. 1tunnels could affect the deformation and failure for us to understand more about drifts in coal andpatterns, which are not involved in this paper, the helpful for supporting system designonclusions drawn from the study are still importantReferences1]Y普斯图巴.影响巷道稳定性的因素[M].巷道支护技术译文组.北京:煤炭工业出版社2]宋振骐,赵经彻陈立良,中国回采巷道矿山压力控制研究现状及方向[A].三峡库区地质环境国际学术讨论会论文集[C].北京:煤炭工业出版社,19963]薛亚东.回采巷道围岩结构破坏规律及锚杄支护的研究[].太原:太原理工大学采矿工程系,1996.[4]李先伟.岩块力学性质[M].北京:煤炭工业出版社,19835]蒋金泉,韩继胜,石永奎.巷道围岩结构稳定性与控制设计[M].北京:煤炭工业出版社,1999中国煤化工CNMHG