Soft-strong supporting mechanism of gob-side entry retaining in deep coal seams threatened by rockbu

International Journal of Mining Science and Technology 24(2014)805-810Contents lists available at Science DirectInternational Journal of Mining Science and TechnologyELSEVIERurnalhomepagewww.elsevier.com/locate/ijmstSoft-strong supporting mechanism of gob-side entry retaining in deepcoal seams threatened by rockburstNing Jianguo Wang Jun, Liu Xuesheng, Qian Kun, Sun BiCollege of Mining and Safety Engineering, Shandong University of Science and Technology, Qingdao 266590, ChinaARTICLE INFOABSTRACTArticle historWhen gob-side entry retaining is implemented in deep coal seams threatened by rockburst, the cementReceived 20 February 2014based supporting body beside roadway will bear greater roof pressure and strong impact load. Then theReceived in revised form 19 April 2014Accepted 17 June 2014supporting body may easily deform and fail because of its low strength in the early stage. This paperAvailable online 13 November 2014established the roadside support mechanical model of gob-side entry retaining. Based on this model,we proposed and used the soft-strong supporting body as roadside support in the gob-side entrystaining In the early stage of roof movement, the soft-strong supporting body has a better compressibility which can not only relieve roof pressure and strong impact load, but also reduce the supportingresistance and prevent the supporting body from being crushed. In the later stage, with the increase ofthe strength of the supporting body, it can better support the overlying roof. The numerical simulationSoft-strong supporting bodyresults and industrial test show that the soft-strong supporting body as roadside support can be betterapplied into the gob-side entry retaining in deep coal seams threatened by rockbursto 2014 Published by Elsevier B V on behalf of China University of Mining Technology1 Introductiongob-side entry retaining is implemented in deep coal seams withthe threat of rockburstIn order to retain the roadway for the next section working faceBased on the geological and mining conditions in Suncun coalafter the upper section working face has been mined gob-side mine of the Xinwen Mining Group this paper proposed soft-strongentry retaining uses different supporting schemes (filling bodies, supporting body as roadside support when gob-side entryrock refuse, single props, etc. )to support the lateral roof in the retaining was implemented. Firstly, the mechanical models ofgob behind the working face or in the roadway [1-5]. Gob-side cement-based support and soft-strong support were establishedentry retaining is usually implemented in the internal stress field in order to reveal the roadside supporting mechanism of gob-sideof abutment pressure around the working face, where it is less entry retaining in deep coal seams threatened by rockburst. Secinfluenced by ground pressure and easy to keep stability [6-8. ondly the supporting capacity of cement-based supporting bodyThe implementation of gob-side entry retaining not only has an and soft-strong supporting body was analyzed by using numericalportant role in improving mining rates, decreasing the cost of simulation. Finally, in order to verify the effects of these two kindstunneling and extending the service life of mines, but also of supporting schemes, an industrial test was carried out in theoptimizes the use of abandoned waste rock and reduces ground Suncun coal mineollution, which is consistent with the development of greenmining and mining science [9]When gob-side entry retaining is implemented in deep coal 2. Mechanical model of roadside support for gob-sideseams threatened by rockburst, the roadside support will bear retaining in deep coal seams threatened by rockbuy entrygreat roof pressure [10, 11 If cement-based supporting bodyAs shown in Fig. 1a, after the upper section working face has(using cement as the main cementing material)is used as roadside been mined, the immediate roof will cave and fall in the gob, andsupport, it easily deforms and fails under the roof pressure due tots lower strethe basic roof will break inside the coal wall and sag down whiletrength at the early stage (12-15 Therefore, it is urgent rotating. In Fig. 1, the height of the roadway is h, the thickness ofto further study how to perform the roadside support wheiimmediate roof is mz, the fracture length of basic roof is La andthe distance betwedarv of suinnorting body and coalCorresponding author. Tel :+86 15954841060wall is Lx. W中国煤化工 is usedE-mailaddress:njglxh@126.com(Ning).support for gob-siCNMHGhttp://dx.doiorg/10.1016/j.ijmst.2014.10.0122095-2686 2014 Published by Elsevier B V on behalf of China University of Mining TechnologyJ. Ning et al Intemational Journal of Mining Science and Technology 24 (2014)805-810with greater stiffness due to its smaller shrinkage. The mechanical increases rapidly with the increase of the amount of compressionmodel is established as shown in Fig. 1bHowever, when soft-strong supporting body is used as roadsideig 1b shows, in the mechanical model, the deformation of the support, its stiffness is smaller than that of the cement-based supcement-based supporting body is As as same as the given roof porting body and its support resistance increases more slowly withconvergence, the stiffness of the cement-based supporting body the increase of the amount of compression. The stiffness curvesand coal seam are K, and K3, respectively then the compatibility cement-based supporting body and soft-strong supporting bodyequation of deformation is as follows:are shown in Fig. 3KIThe lateral roof of working face has a great of impact energ(1) roadside supporting body when gob-side entry retaining is(2) implemented in deep coal mining. So, according to conservation(3) of energy, the energy balance equation can be given by,G=F△Swhere AP is the support resistance increment of the supporting where G is the impact energy of the overlying roof on the supportbody(kN): As the amount of compression of the supporting body(m): K, the stiffness of the supporting body(kN/m): P the support ing body(kJ); and F the roof pressure on the supporting body(kN)resistance of the supporting body(kN): Po the setting load of theTherefore, the roof pressure on the supporting body is obtainedsupporting body(kN); Ka the expansion coefficient of gangue inAs shown in Fig 2a, when the supporting scheme for gob-side FsGthe goafentry retaining uses soft-strong supporting body that contains acombination of the filling bodies A and B, the filling body a is From Eq.(9 ), it should be noted that, with the increase ofexpandable soft material with greater compressibility, and its deformation of the supporting body, the roof pressure on thestrength can gradually increase with itself process of compression support gradually decreases. At this moment, the relationshipdeformation. However, the filling body b is cement-basedbetween the defeformation of the support and the roof pressure onsupporting material with the characteristics of smaller amount ofthe support is depicted in Fig. 4.mpression and high strength. In brief, the total amount of comFig 5 is the combination of the figs. 3 and 4. It can be seen frompression of the soft-strong supporting body is the sum of those of Fig. 5, when the cement-based supporting body is used as roadsidefilling bodies A and B. When filling bodies a and B can be seen as support, the stiffness curve of cement-based supporting body andtwo kinds of springs with different stiffness, and the roof pressure the roof pressure curve intersect at point A. At this point, theapplied on filling body A is equal to that of filling body B, then the amount of compression of the supporting body is AS, and roofcompatibility equation of deformation is as followspressure is Fa during the early period of lateral roof movement. Itmeans, while the amount of cosion of the supporting body(4) is smaller, the roof pressure is larger, then the body easily deforms△P=K1×△S3=K2×△S2(5) and damages under the roof pressure because of the lower early(6) strength of cement-based supporting body, and the gob-side entryThe total stiffness K of soft-strong supporting bodyHowever, when soft-strong supporting body is used as roadside△PK1×K2upport, the stiffness curve of soft -strong supporting body and the△SK1+K2(7) roof pressure curve intersect at point B. At this point, the amount ofcompression of the supporting body is ASb and the roof pressure iswhere As is the total amount of compression of the soft-strong Fb. It should be noted that, while the amount of compression of thesupporting body (m): AS, the amount of compression of filling body soft-strong supporting body is larger, the roof pressure is evenB(m): AS2 the amount of compression of filling body a(m): K the smaller, compared with cement-based supporting body. Since filltotal stiffness of the soft-strong supporting body(kN/ m): KI the ing body a has excellent compression deformability and the largestiffness of filling body b(kN m); and K, the stiffness of filling body amount of compression, it makes a major contribution to theA(kN/m).deformation of the total supporting body(asb), which is helpfuSince all stiffness of filling bodies a and B is positive, from Eq. to relieve the roof pressure and reduce the impact load generated(7), it should be noted that the total stiffness of soft-strong by strong roof movement. It is also helpful to improve the strengthsupporting body is smaller than K, and K2of filling body B. Furthermore during the later period of lateral roofFrom Eqs. (1)and (7), it can be obtained that, when the cement- movement, the improvement of strength of filling body b canbased supporting body is used as roadside support, the stiffness of support the overlying roof. The support resistance of the total fill-the support(Ki) is comparatively large and its support resistance ing body is mainly from the filling body B. Thus, it can be seen that,Basic roof女强Coal seam中国煤化工CNMHGFig. 1. Mechanical model of cement-based support for gob-side entry retainingJ. Ning et al / International Journal of Mining Science and Technology 24(2014)805-810(a) Structure model(b) Mechanical modelFig. 2. Mechanical model of soft-strong support for gob-side entry retainingbody is not better. Because the surrounding rock will fall whenthe displacement of surrounding rock reaches a certain value, likepoint C, that is called the caving point, as shown in Fig. 5. So, the3. Numeric simulation of roadside support in deep gob-sideSoft-strong"supportentry retainingCompression of the support(m)3. 1. ModelingFig 3. Stiffness curves of roadside support.According to the geological and mining conditions of No 2213working face in Suncun coal mine of Xinwen Mining Group, a basicnumerical model was built by using FLAC3D [14 15. No 2213working face uses strike longwall mining technology. It is about1200 m in depth, 1200 m in strike length, and 160 m in trendlength. The average thickness of coal seam is 3.0 m. The coal seamdip angle is 8. The fractured length of lateral basic roof is 22.5 m,F=(△Sand the thickness of immediate roof is 6 m. The expansion coefficient of gangue in the goaf is 1.35. The mechanical parameters ofcoal and rock mass are shown in table 1. the cross-section of theroadway is excavated as a rectangle with 4.0 m in width andDeformation of the support(m)3.0 m in height. The roof is reinforced by bolts and anchor cablesThe bolts are made of high strength alloy steel with the tensileFig 4 Curve between roof pressure and the deformation of the roadside support. strength of 200 KN, 20 mm in diameter and 2.4 m in length.Thespacing and row of bolts on the roof is 0. 8 m x 0.8 m The anchorcable is made of steel strand with the strength of 353 kN, the diam-eter of 17. 8 mm, and the length of 6.0 m One anchor cable wasCement-based supportinstalled at the middle of the roof every 5 m along the advancingdirection. The spacing and row of bolts on the side is 0.9 m x 0.8mThe numerical model is generated in three dimensions and itsSoft-strong supportsize is 80 m x 64 m x 70 m, corresponding to x, y, z direction,respectively. The in-situ stress is reproduced by setting initialconditions lateral boundaries of the model are fixed to eliminatehorizontal displacement, and the bottom is fixed to eliminatevertical displacement. The top of the model is applied a verticalstress which is equal to the gravity of overlying stratum (?HFig. 5. Curves between roof pressure and compression of the support with different and the horizontal stress coefficient on the lateral sides is set as0. 8. The constitutive relation is mohr-Coulomb modelTwo roadside support schemes are designed. The first one useswhen gob-side entry retaining is implemented in deep coal seams a cement-based supporting body with 3 m in height and 2 m inith the threat of rockburst, the roadside support should have a width as roadside support. The second one uses a soft-strongcertain compression deformability in order to relieve the roof supporting body with 2 m in width as roadside support that conpressure and strong impact load generated by roof subsidence sists of the expandable soft material with the height of 0.3 m andduring the early period of lateral roof movement. During the later the cement-based supporting body with the height of 2.7 m. Theperiod, the roadside support should have a certain bearing capacity cement-based body is composed of cement, coal gauge and waterin order to support the overlying roof. Consequently, soft-strong with the proportisupporting body has those characteristics that are suitable for strength and the中国煤化工 hown in Tabl2roadside support when gob-side entry retaining is implemented The elastic modulCNMHG soft material arein deep coal seams threatened by rockburst.270 MPa and 0.24cly. Il uc llulnicl ical model, the softIt should be noted that, during the early period of lateral roof material uses elastic model, while the coal, rock and cement-basedmovement, the greater amount of compression of the supporting body uses Mohr plastic model. The goaf is seen as a null unit in theJ. Ning et al Intermational Journal of Mining Science and Technology 24(2014)805-810Table 1Mechanical parameters of coal and rock mass.theloThickness(m) Bulk modulus(GPa) Shear modulus(GPa) Internal friction angle() Cohesion(MPa) Tensile stFine-grained sandstone 3.013.074.03,01ne10.24Lime mudstone1.59.366.81766244Sandy mudstone10.95Fine-grained sandstone 10.0442Lime mudstone13.644.225.54.5412.53394.158.2Uniaxial compressive strength of cement-based supporting bodytrength(MPa)12.8Age(dayStrength(MPa)27027.828.50.81.732032.032.0■Nonenear- tension-H shear-p tension-ptension-n shear-p tension-pFig. 6. Plastic zone distributing of supporting body with the advancing distance of(a) Advancing 8 mShear-/ shear-p百皇hear-l] tension-m shear-p tension-p10Tension-m shear-p tension-p(b) Advancing 24 mFig. 7. Stress curve of supporting body with the advancing distance of 8 mnumerical simulation. According to the field monitoring, thetension-l shear-p tension-pdistance between the lateral basic roof fracture location and coalall is 10 m in order to reflect the influence of immediate roofand basic roof on the supporting body after basic roof fractured,structural units of interface are set in the corresponding positi(c) Advancing 52 mof the numerical model [16]Fig 8, Plastic zone of roadside supporting bodThe simulation processes are as follows. Firstly, the modelreaches initial balance under the weight of the overlying strata.Secondly the roadway is excavated and supported after releasing 3. 2. Analysis of simulation resultsthe stress. Thirdly, the working face advances 4 m each day, thesupporting body is filled behind every 4 m of working faceWhen the cement-baseorting hodv is ised as roadsideadvance, and the strength of the filled cement-based supporting support, and the adv,中国煤化工 ce is up to8mbody changing to the next stage of strength is shown in Table 2. the plastFinally the model reaches stress balance at every 4 m of working curve of the supportiCNMHGhen soft-strongface advance, and the stress and displacement of the roadway-side supporting body is used, the plastic zone and stress curve of thesupport are monitored.supporting body are shown in Figs. 8 and 9, respectivelyJ. Ning et al /International Journal of Mining Science and Technology 24(2014)805-810In conclusion, during the early period, great pressdestroy the supporting body in the first scheme with cement-basedsupporting body. However, the soft-strong supporting body haslow strength and great compressibility that can release the roofpressure in the early stage and protect the supporting body frorbeing crushed. The strength of the bottom cement-based bodyincreases with time and becomes high enough to support the roofel4. Field observationFig 9. Stress curve of cement-based body in soft-strong supporting bodyTo validate the supporting effect of soft-strong supportingbody, the industrial test was performed in the No 2213 roadwayof the Suncun coal mine(the geological and mining conditionsare provided in Section 3. 1). The experimental roadway with thel城length of 300 m was divided into two 150 m length parts. One ofthem used cement-based body as supporting material, the otherused soft-strong body as supporting material with the height ofGoaf发签及Basic poin3 m. The expandable soft material in the soft-strong supportingbody was Uroica No. 1. According to the eq 3), the roof convergence is 240 mm. Because the compression ratio of soft materialis 80%, the height of the soft material is 300 mm and the uniaxialcompressive strength is 3 MPa after two hours. The strong materialof the supporting body was a cement-based body (its proportion issic measuring points of roadwashown in Section 3. 1 ) Its strength could reach about 20 MPa afterroof stability [11According to Figs. 6 and 7, when the working face advances 8 m,The decussation monitoring method was used to measure thethe supporting scheme with the cement-based supporting body surface displacement of the roadway with the advancing of thefails. All the stress area on the cement-based supporting body is working face, including roof-to-floor convergence and two-sideplastic and the largest stress is 18.42 MPadisplacement. Three monitoring stations were set at 10 mAccording to Fig 8, when advancing distance of working face is intervals, and steel tacks of 200 mm length were fixed in the sidesup to 8 m, the second supporting scheme with the soft-strong roof and floor of the roadway at each station as basic points, assupporting body, the small part of the supporting body at the side shown in Fig. 10. Measuring guns and poles were used to meaof the gob is into plastic zone. When the advancing is 24 m, part of the deformation of the surrounding rock of the roadway, and thene supporting body is into plastic zone. When the advancing is horizontal deformation and vertical deformation that are shown52 m, all the supporting body is into plastic zone. According to in Fig. 11Fig. 9, the stress of the bottom cement-based body in the softccording to Fig. 11, the deformation of surrounding rockstrong supporting body increases slowly at first and quickly later, supported by the cement-based supporting body is very low whenand the largest stress is 26. 4 MPa when the working face advances the working face advances 15 m, and increases rapidly when the52 m. Therefore, with the comparison of two supporting schemes, advancing more than 15 m. the supporting body fails whenhe compressibility and the uniaxial compressive strength of the advancing is 40 m. During the early period of lateral roof move-cement-based supporting body are small during the early period ment due to the low compressive strength of the cement-basedof lateral roof movement, and it can easily fail. However, when supporting body and the small compressibility the cement-basedusing soft-strong supporting body in the second supporting supporting body will bear great roof pressure and strong impactscheme, the upper soft material has great compressibility that load in the deep coal seam threatened by rockburst. So, theallows the roof to sink during the early period, release some roof supporting body is easily destroyed, which may lead to supportingpressure and decrease the resistance of the supporting body. scheme failure. However, the deformation of the surrounding rocks uring the later period, the soft material cannot be compressed supported by the soft-strong supporting body is bigger when thelymore and the strength of the bottom cement-based supporting advancing is between 0 and20 m. Because the supporting bodybody is big enough to support the roof, which will realize the has great compressibility that can release the roof pressure in thestability of roadway.early stage. Then the strength of the supporting body increasesSoft-strong support中国煤化工Longwall face advanced distance (m)CNMHG(a) No. 1 monitoring stationoring station810J. Ning et al /International Journal of Mining Science and Technology 24(2014 )805-810with the advance of the working face and the deformation of the Natural Science Foundation of China(no. 51344009), the researchsurrounding rock remains steady after the advancing is up to Award Fund for Outstanding Young Scientists of Shandong60 m, which means the soft-strong supporting body can well adapt Province(No. BS2012NJ007). the Ground Pressure and Stratathe movement of roof in deep gob-side entry retaining threatened Control Innovative Team Fund of SDUST(No. 2010KYTD105 ) andby rockburst.the Natural Science Foundation of Shandong Province (NozR2012EEZ002)5 ConclusionsReferences(1)Because of the great surrounding rock pressure in deepgob-side entry retaining threatenedockburst. the[1] Bai JB, Zhou HQ, Hou C]. Development of support technology beside roadway inning for next sublevel.J China Univ Min Technoroadside supporting body should have the characteristicscompressibility to release the impact load in the early[2 Miao XX, Qian MG. Research onmining of coal resources in China:stage of lateral roof movement, and great strength to bearent status and future prospects. J Min Saf Eng 2009: 26(1): 1-143 Ma Z, Gong P, FanJ Coupling mechanism of roof and supporting wall in gobthe great impact load from the overlying strata and supporthanized mining with gangue backfilling. Minthe roof in the later stageSci Technol2011;21(6):829-33(2)The supporting scheme with soft-strong supporting body as 14] Zhang N, Yuan L Han C Stability glide er 4): 593-9roadsidert Ised for the deep gob-side entry[5 Li HM. Control design of roof rocks for gob-side entry. Chin J Rock Mech Engretaining threatened by rockburst. It is composed of two2000;19(5):651-4parts. The upper supporting body is soft material that has [6] Hua XZ, Ma JL, Xu TJ. Anchor reinforced support and parameter optimizationlow strength and great compressibility that can release theimpact pressure and protect the supporting body from being [71 Gao K Liu ZZ, Liu J. Application of deep borehole blasting to gob-side entrycrushed. The bottom part of the supporting body is aretaining forced roof caving in hard andnd roof deep well. Chin J RockMech Eng2013;32(8):1587-94.cement-based filling body, and its strength is low in the 18) Han C, Zhang N, Yao y. Transfer bearing mechanism of thick composite roof inearly stage and increase to high enough to support the roofgob-side entry retaining Rock Soil Mech 2013: 34(1): 318-2in the later staHua XZ. Development status and improved proposals on gob-side entry(3)The calculation method for the height of upper expandable (10) Kang HP. Niu DL, Zhang Z Deformation characteristics of surrounding rock andsoft material of the soft-strong supporting body is proposedof god-side entryThe industrial test and numerical simulation show thatretaining in deep co.Chin rock Mech En29(10):1977-87using the soft-strong support as roadside support could[11] Ning ]G, Ma PF, Liurting mechanism ofpporting "besideway maintained along the goaf under hard rocks. Min Saf Engsuitable for the laws of roof movement when gob-side entry2013:30(3):369-74staining is implemented in deep coal seams threatened by [ 12] Wang MS, Wang M, Du HL. Gateway retained technology along goaf of coalrockburst113] Sun CD, Zhang DS. Wang XF Large-size test on creep characteristics of higl114] Chen Y, Bai JB, Zhu TL Mechanisms of roadside support in gob-side entryretaining and its application. Rock Soil Mech 2012: 33(5 ) 115 Xu G, Jing H, Ding S Evolution rules of stress and displacement for narrow coalThis research is supported by the National Basic ResearchProgram of China(No. 2010CB226805), the Taishan Scholar [16] Wu Q, Wang M, Wu x Investigations of groundwater bursting into coal mineConstruction Project of Shandong Province, China, the Nationalseam floors from fault zones. Intj Rock Mech Min Sci 2004: 41(4): 557-71中国煤化工CNMHG