Study on numerical simulation to water lowering in an open coal mine digging

Global Geology , X 2 ):199-203( 2006 )Article ID : 1673-9736( 2006 )2-0199-05Study on numerical simulation to water loweringin an open coal mine diggingYanjun ZHANG' , Fenglong ZHANG2 , Fuli QF and Fuquan NI1. College of Constructiom Engineering , Jilin University , Changchun 130026 , China2. Institute of 904 Hxdrogeology and Engineering Geology Prospecting , Heilongjiang Province ，Shuangchengbao150100 , Heilongjing，China3. Sichuan Agricultural Unirversity , Yaan 625014 , Sichuan , ChinaAbstract: In view of the situation of excavation of open coal mine for the underground water disaster , weshould carry out simulation studies for the numerical value of the water lowering project and improve the accu-racy and the level of the water lowering project. On the basis of the hydrological geological conditions of certainopen mine digging , a more reasonable seepage numerical model was built according to MODFLOW. It wassimulated in advance that the process of the confined water level descending with the time , and combining withthe actual observations to test the correctness of the model. The calculation showed that the results coincidedwell with the results of actual measurement. Based on this , different water lowering numerical simulationswere built for the open coal mine digging. It could be simulated and forecast that the changes of the groundwa-ter level in drainage process within and outside the mine pit , and it was quantitatively assessed that the possiblewater lowering result of the opencast water drainage process , which provide an important basis for the actualwater lowering project and the possible project disposal.Key words :open coal mine water lowering ; model building ; visual MODFLOW ; numerical simulationcess ,and so on. But these methods hadn' t been fur-Introductionther studied and widely applied. Currently , the deepTo dry out the groundwater was one of the keywell drainage process is mainly adopted in our coun-problems in the open coal mine of our country. Attry. The coal mine hydrology geology condition irpresent，most of the mines adopted water loweringmost parts of our country was complicated , and theprocess in our country , such as deep well drainagework on it was extremely important in the ground-process in Lingquan mine , laneway drainage processwater Iowering desion of nnen coal mine digging ,in Yuanbaoshan mine , and seepage gathering and中国煤化工production safety of anpumping process in west of Fushun open coal mine，opeMHCNMHGand so on. In addition , there were many otherAt present ,during the water lowering design ofmethods such as big well horizontal shedding pro-open coal mine , it was mainly based on the watercess，vacuum well drainage process , open channellowering depth ，the hydrology geology conditiondrainage pr3C驱城and infiltration pipe drainage pro-( groundwater , recharge form，etc. ) ,and the types200YJ. Zhang ,F.L. Zhang ,F.L. Qi ,et al.of the well to choose the appropriate seepage analyticvestigate the rule of the groundwater movement.formulas. The Dupuit formula was most commonlyThere were four parts in the paper. Firstly , theused and brought into the code of our country ,principle of MODFLOW was generally introduced.which was Technical code for groundwater loweringSecondly ,it was described that the building of nu-engineering in building and municipal JG]/T111-98.merical model. Thirdly , the model was testified ac-The formula fitted for the water lowering design oncording to the observation data and was used to thesingle well or group wells under the circumstancegroundwater flow simulation. In the end ,it came tothat the aquifer structure and the boundary conditionthe conclusion.was simple. But it was difficult to calculate the com-1 Theory of visual MODFLOWplicated boundary condition , to analyze the changesof underwater seepage state such as the process ofIt could be simulated with Visual M( )DFLOWwater level descending , and simulation hydrologythat the stationary flow and transient flow changesgeology unit. Furthermore , how to ensure some hy-in phreatic water , confined water , and water- resist-drology geology parameters was another difficulty .ing layer. MODFLOW also provided lots of methodsThe biggest advantage of numerical simulation con-to solve finite difference formula of groundwater , forsists in the ability to accurately describe heteroge-example ,SIP ,SOR ,PCG2 ,and SSOR ,and so on.neous ,vary-thickness and complicated hydrology ge-During the process of MODFLOW solving , Stressology condition.Period conception was introduced , which was divid-In recent years , for the study of the under-ed the whole time into several Stress Period whichground flow simulation , it was mainly concentratedcould also be divided into several time steps. Theon three-dimension modeling development , the cal-MODFLOW applied FDM to simulate the ground-culation method on flow velocity and streamline，thewater in the water layer. In the process of solvinggeneralization on the heterogeneous parameter andprocess，MODFLOW adopted isometric or non-iso-the optimization of complicated data. To simulate re-metric orthogonal rectangular parallelepiped grad ingional seepage flow was common , while it was sel-space.dom to find the simulate design on open coal mineMODFLOW was a three- dimension finite dif-deep well. It was only found in surrounding descen-ference groundwater flow model , according to thedent calculation around the foundation pit combiningfollowing basic equation :with MODFLOW( Xu et al. ,2004 ). And MOD-FLOW was a good software with Finite Difference[Kxn, ]g[Kwo, ]+Method( FDM )on groundwater numerical simulate ,h1w=s。(1)which was a worldwide used software in numerical[K.可2.simulation of groundwater ( Marler et al. , 2003 ;In the formula: Kxx，Ksv , Kzz are the hy-Wuetal.,2001;Moench,2004;Zhangetal.，draulic conductivitv which follows the direction of2004 ).hea;YH中国煤化Iaxis(LT-1). His theWater lowering process of a certain open coalc N M H Gf unit volume throughmine was simulated with MODFLOW in the study ,the homogeneous and isotropic soil medium in thenumerical simulation was tried to use as a tool to testcondition of nonequilibrium , which is the source andhydrology geology parameter in the study field , tosink of the groundwater( T-1 ). Ss indicates thepredict the瓦熬据of groundwater flow ,and to in-storage cofficient of the porous medium(L-1 ). TStudy on numerical simulation to water lowering in an open coal mine digging201is the time.2 Foundation of the field numerical! Srfiocesoil(Xmodel2.1 General situations) 30Sndandganed8;This water lowering area was located in Hulun-540 ;Mutsoe,beier plain and in the east of Inner Mongolia Au-: Cailtonomous Region. The study area was about 900 m900 m ,and the elevation of which was about 605.0Fig.1 Discretization of soil horizonm. Since the whole area was smooth , the rechargecame from precipitation , and the drainage was main-calculation of the water drawdown were conductivi-ly from evaporation , manual exploitation ,or seepagety( horizontally Kx Ky and vertically Kz ), Storageto supply the downside coal fracture water through(S), which were key hydraulic parameters in theclerestory". According to the in site survey data ，process. Referring to the provided date , the value ofthe layer of the open coal mine area was from up todifferent regions was assigned initially ，seeing todown as : clay , impervious layer , the thickness oftable 1.which was around 6~ 15 m ; the sand and gravel lay-er , confined aquifer ，the thickness of which wasTable 1 Conductivity and storage of pumping areaaround 5~40 m , there were many small pores withKx KyKzstratalithologicgood water permeability ; mudstone layer , which(m/d) (m/d)( =0.1Kx)_ ( 1/m)1surface layer8.64 8.64).8640.000 01was about 4~ 15 m thick ; impervious layer ; coallayer with thickness of about 7~ 20 m. The mainsand and gravel 42 ~ 10642 ~ 1064.2- 10.60.1~aquifer of the mining area was sand and gravel layer.soil and gravel 0.000 10.000 10.0001 0.000 012.2 Foundation of hydrology geology numerical0.01 -coal layer1~5 1-5 0.1-0.50.000 01_The study area could be abstracted as a rectan-gle , the area of which was 900 m 900 m with depth2.4 Boundary conditionof 80 m , the ground elevation was considered as 605To simplifty the data input ,and make the phys-m. ,and the model was divided into 20 m 20 m unit .ical concept more clearly , stress period concept wascell with a total of 1978. In the vertical direction ,itintroduced to MODFLOW for time , and during dif-could generally be divided into four strata accordingferent stress period ，different intensity was set forto the lithology of the layers , as shown in Fig. 1.different external sources and sink. To make theThe total water lowering area was divided into 7912model actually reflect the real process , the east andgrid. The surface patterns of study area were shownthe, west boundary should be controlled by the long-by the referenced relief map , and the subsurface lay-中国煤化工ith 124 stress period;er thickness was valued by the vary sections individ-east IYHCN MH Glled by the long- termually.observation wells G24 with 94 stress period ; north2.3Value of hydrogeological parameters in theborder was controlled by long term observation wellG2 with 93 stress periods ，these were all headGeneaF数循e main parameters related with theboundaries with many stress periods ; the bottom202Y.J. Zhang,F.L. Zhang,F.L. Qi ,et al.boundary of the calculated area( Z= 520 m ) was ap-Most relative difference value of the calculateproximately considered as a boundary of flow sur-water level was under 10% , which was really aface , that is , there were no groundwater inflows andsmall one seeing to the big calculating area of 1 km2outflows.and the calculation error could satisfy the three-di-3 Modeling test and results analysismension compute request completely. It was obviousthat the compute model was quite accurate and theon simulationmodel accorded with the practice.To identify the model , it should be consideredUnder the calculate boundary and hydrology ge-that the conductivity K and the storage of theology in the model which had been built , and aimingaquifer , and the effect of the boundary condition.at different situation adopted in the digging of theAfter the foundation of the model , based on the fieldopen coal mine , simulation and calculation was donepumping test and the local construction experience ，on the study water lowering area. Current instancethe parameters were adjusted as 90 m/d conductivi-was to find the influence on groundwater depth byty ,and 0.002( 1/m )Storage in the pumping area.adding 5 pumping wells. Simulate the instance underBased on the existing precipitation and observationthe calculation path shown above. According withdate in the study area ,47 confined wells and five ob-the practice ，the groundwater drawdown hadservation wells were selected for the simulation. Us-dropped to 20 m after 320 days work by 47 drainageing WHI algorithm to solve the problem , the resultswells. Keeping the pump quantity unchanged ，andfrom simulation calculations were shown in Fig. 2 ,added 5 drainage wells , the drawdown could totallywhich was as following : groundwater head hadreach 30 m and the biggest drawdown can be 568 m.dropped about 20 m after 165 days of pumping andThe descent filler simulate result on 410h day wasthe 47. pumping wells dry out ,that is , the calculatedshown as Fig. 3 , and the ground flow distributing onwater head was generally similar to the water head of320h day was shown as Fig. 4.observation wells. Thereinto , the error between cal-culation and observation in G2 well was0.3 m. Theobservation value was sharply increased around the75*h day , which was initially considered as the result那of the pump stopped nearby.32Head vs. TimeG1 (Ohserved) .. -61 (Calculnted)G2 (Observed)团)▲G2(Calculated)G24 (observed)阳G24(cnlculaled)21中国煤化工53a 50: 50HIMYHC N M H G0M day in B sub-situation0.2750.27100. 27150.27In situation adding 5 pumping wells in the wa-T ime[dnys]ter lowering area situation of no-added wells in someFig.2 Fitting results between the groundwater level mon-it耐亦数据the simulationwater lowering area.Study on numerical simulation to water lowering in an open coal mine digging203drology geology numerical model accorded with thefield.(3 ) Though MODFLOW was a simple soft-517642ware ,it can numerical simulate different water low-5176100ering instance and the possible groundwater flow dis-175600tribution easily , which had important practical sig-nificance in actual operation in project.17200Above all , the modeling method in the paper517400was feasible to numerical simulate groundwaterdrainage process in open coal mine digging , and cal-culated result was conformity with the actual condi-1400tions. It had not only widened the application scope517:2800of groundwater numerical simulation ，but also en-472800 17:200 47:600 471000 474100 471800 475200 1751040 475800hanced the ability to solution actual problems.Fig.4 Groundwater flow distributing map on 320" dayReferences4 ConclusionMoench A F.2004. Importance of the vadose zone in analysesof unconfined aquifer tests. Groundruater ,42( 2 ) :223-( 1 ) The MODFLOW program was reasonable233.structured, easy to understand and operate , withMarlerJ ,Ge S M.2003. The Perme ability of the Elkhornsimple discrete methods , diversified solution meth-Fault Zone , South Park , Colorado. Groundwater ,41ods , besides the wide use to seepage calculation , wa-(3)223-233.ter resource assessment , and solute migration ,it alsoWuQ, Yin Z M, Wu X. 2001. Groundwater study in thehas great use-value in the water lowering works.Weerselo area ，Overijssl ，the Netherlands. Bejing :( 2 ) Much parameter adjusting work was donePetroleum Industry Press ,47-74.to finalize the parameter value of the gravel in waterXu Y D, Tong L H. 2004. Forecasting of ground subside inlowering process [ Conductivity 90 m/d , storagecertain foundation pit pumping with Modflow. Hyclrolo-gy and Engineering Geology (6):96-98.0.002( 1/m )], and the error was very small be-ZhangZZ, Wu Q.2004. Foundation on coupled model oftween the calculation and the observation data of theriver water and groundwater and its application. Journalfive observation wells , which indicated that the hy-of Liaoning Technical University 23( 4 )49-452.中国煤化工MHCNMHG