Hazard development mechanism and deformation estimation of water solution mining area

J. Cent. South Univ. Technol. (2006)06- 0738-05DOI: 10.1007/511771-006- -0024-6Hazard development mechanism and deformation estimation of watersolution mining areaHE Yue-guang(贺跃光)", LI Zhi- wei(李志伟)}, YANG Xiao-li(杨小礼户(1. School of Highway Engineering, Changsha University of Science and Technology. Changsha 410076, China;2. School of Info-Physics and Geomatics Engineering, Central South University, Changsha 410083, China;3. School of Civil and Architectural Engineering, Central South University, Changsha 410075, China)Abstract: Based on the hazard development mechanism, a water solution area is closely related to the supporting effect ofpressure- bearing water, the relaxing and collapsing effect of orebody interlayer, the collapsing effect of thawless material in orebody,filling effect caused by cubical expansibility of hydrate crystallization and uplifting effect of hard rock layer over cranny belt. Themovement and deformation of ground surface caused by underground water solution mining is believed to be much weaker than thatcaused by well lane mining, which can be predicted by the stochastic medium theory method. On the basis of analysis on theengineering practice of water solution mining, its corresponding parameters can be obtained from the in-site data of the belt waterand sand flling mining in engineering analog approach.Key words: water solution mining; hazard; ground surface deformation and movement; probability integral methodCLC number: TD803.36; TD173*4Document code: Aplate of the third mining group is the sub-segment of1 INTRODUCTIONreussite rock, and its thickness varies from 35.37 m to43.59 m. The rock properties are mainly mud and cloudWater solution mining-based mineral depositquality calcium Glauber's salt rock, and its upper part isbelongs to the ancient landlocked evaporation of salt lakemauve mud stone with a thickness from 37.65 m tomineral deposits. The salty mineral products of mining122.90 m. The upper part of bedrock is the fourth clayarea, such as verde salt, rock salt, glauberite, etc., mainlyand gravel rock with a thickness from 77.95 m to 138.55exist in the salty rock segment that is formed by the thirdm, and its lower hardpan is aquafer, which is partitionedsulphate sediment of group of new ditch. This rockby middle aquifuge. Their types are both ponsegment consists of mudstone， anhydrite-includedpressure-bearing aquafer, and the above glueing soilcalcium Glauber's salt rock, calcium Glauber's salt rocklayer is the water-blocking layer.of mudstone quality, verde salt and rock salt. AmongThe designing parameters of the third mining groupthem, the third mining group is a verde salt industrialin the No. I well field are as follows: upper boundary'smining group, which mainly includes verde salt anddepth is from 187.19 m to 264.12 m, average depth isglauberite. Verde salt is composite mineral layer of225.66 m, ore group thickness is from 10.46 m to 21.53multi-layer single-reussite. The stratum of mining aream, average group thickness is 17.81 m, including 6 to 11mainly includes puatermary system and palaeogenewater free Glauber's salt single-layers, single-layer'ssystem. The Shibu-Tanjiapu syncline organization 0thickness is from 0.13 m to 5.93 m, accumulatingmining area is a mineral-control one, and the reussitethickness is from 4.28 m to 14.44 m, the averagemineral deposit mainly lies in the southerm wing of thethickness is 9.53 m, mostlty water free Glauber's salt rocksyncline and its transition point. The F1o faultage locatedblocks，including mud and cloud quality calciumin southem part of the mine is the boundary of theGlauber's salt rock, and the average ore rate is 57%. Thereussite mineral deposit. The faultage is water-blocked,inserted gangue consists of mud and cloud qualityand it does not destroy the mineral deposit.calci中国煤化工re body includes 5 toThe adjoining rock and interlayer of the mineral10 sFom 0.12 m to 5.13m.layer are both mud-contained reussite rocks. The upperTheMYHC N M H Gn 67.76% to 79.83%,Reeived date: 2006 -03-18; Accepted date: 2006 05-05Foundation item: Projec(40404001) supported by the National Natural Science Foundation of ChinaCorresponding author: HE Yuc guang, PhD, Professor; Tel: +86-731-2309592; E- mail: hyg.csu@ tom.com空努数据.HE Yue-guang, et al: Hazard development mechanism and deformation estimation of water solution mining area739with an average of 70.47%. We-drilling hot water andgradual and strictly according to the production capacityheat dissolving oil mining method was adopted. The wellof mining well， the mining method is actually agroups are connected through the initial-stage troughs.room-and-pillar mining method, which belongs toThe three wells that have a well distance of 40 m arepartially mining method. The prediction of groundregarded as one group, and the well group arrangessettlement of such kind of mine can be made by adoptingdistance and interval are both 80 m. The designing rateprobability integral method to modify settlementof re-mining is 20.38%, single well production capacitycoefficients. In such a case, the flling effects of interlayis 1.82X 10* t per year, and mine production capacity isrelaxing and collapsing and the expansion of waterless5X 10* t per year, while the actual production capacity isCaSO4 on water- crystalized CaSO4 must be considered2X 10> t per year at present.in choosing parameters.2 HAZARD DEVELOPMENT MECHA-2.3 Effect of splint theoryNISMWhen the splint theory is applied to analyze theground surface movement, the mechanism of groundWater solution mining will form undergroundsurface settlement can be explained as follows:thesolution cavity, and alter the stress balance ofdevelopment of the inbreak and rupture of the upperunderground rock and soil body. When the undergroundplate over the cell evacuated area are quite insufficientsolution cavity develops to a higher degree, it may causebecause celling mining area is small, and some rockmovement of upper rock and soil body and destroy thelayer over the rupture belt lies in a kind of stabilitystability of ground surface. However, the certainresuling from the uplifting force. The pressure fromproperties of water solution cavity may prevent theitself and the rock and soil layers over it (until grounddestruction of ground surface to some extent' 4. Thesurface) will shift to the place that is around rock bodies,hazard development mechanism of water solution miningand is burdened jointly by the rock body over the mininglies in the following aspects.pole, the boundary mineral body and possible upliftingforce from liquid. This alters the stress state of underling2.1 Uplifting effectrock body, and the demission of side stress and theThree mining wells form a group, and they areenhancement of vertical stress make the rock bodydistributed along the trend of mineral deposit. Thedeform compressively. The compression deformationdesigned trend length of solution cavity is 120 m, and thewill lead to the settlement of the covering rock body, andmining width along inclination is about 40 m. Thethe ground surface settlement is the joint and cumulatinghorizontal projection area of solution cavity is aboutcontribution of covering rock bodies' settlement.4 800 m', and the direct upper plate coefficient (0) rangesfrom 3 to 6. Due to the uplifting effct of3 GROUND SETTLEMENT AND MININGpressure-bearing water, the probability that the span ofDAMAGE ESTIMATIONsolution cavity and width of mining pole can make theupper plate stability relatively great.3.1 Theoretical background of estimationThe estimations of ground settlement and mining2.2 Filling Effectdamage of water solution mining area mainly refer to theThe designed recovery ratio of the mine is 20.38%.mining-related ground movement and deformation andIn the solution process of verde salt, due to the flligits impacts on ground surface protective objects. With theeffect from the relaxation of the interlayer, the relaxationknowledge of mining mechanism and properties ofand collapsing of the insolubles in the mineral body, andcovering rock bodies, the deformation can be estimatedthe volume inflation when glauberite is caught by waterbased on the probability integral method3-5!.and its remains, the actual rclaxing materials are flld3.1.1 Setlementaround the mining pole and the pressure around iincreases, which contributes to the stability of the miningW(x,y)=- [SWomexp[ -号(x-s)2 })s.Wmaxpole to some extent. According to relevant experimentmeasurement of solution cavity, the inflation rate is9% Waexexpl-气(v-9g)2]dq .()J-qogenerally 1.05-1.40. The calculation results show thatwhen the salt ore rate reaches 40% , and the salt depositwher中国煤化工nal potential groundgrade is 70%, the volume of the mining residue in thesettleng radius of groundsolution cavity can reach 80% of the total volume of thesurfa:MH.CNMHG. direction of mineralsolution cavity after such expansion.trend; 2go is horizontal projection of the mining widthAs for the current mining method, if the mining isalong. y direction of the mineral inclination.包SpringerJ. Cent. South. Univ. Tech, 2006, 13(6)3.1.2 Horizontal movement along x and y directionKx=-xEx,y)，K,=-e,(x,y) (11)br. Wwmexro.. 一(x+s0)2U,(x)=_r -.^[exp3.2 Results of ground movement estimationWmaxr23.2.1 Mining poles and upper plate being stableexp-n(x-s0)些f+9)exp(-xE)d2 (2)Three mining wells form a group, and they aredistributed along the trend of mineral deposit. The trendlength of solution cavity is 120 m, and the mining widthalong inclination is about 40 m. The medium is hard tobr_. Wmax- π(y+q)2U,(x,y)="[exp 2soft along upper plate directly, which is generally withafcoefficient 0) ranging from 3 to 6, and its stability mustexp-n()-902_." (*+)exp(-z2)dx (3)be analyzed along the mineral trend direction based on(x-so)the experiments of rock body mechanics. If the width ofmining poles can make the upper plate stable, the effectswhere b is coefficient of horizontal movement.3.1.3 Horizontal deformation Ex, Ey, shear deformation 2zof mining on ground surface farmland and buildings arepermissible or acceptable.and the maximal horizontal deformation Emax3.2.2 Mining poles and upper plate being unstableE(,)=zxb"e(2+50 expFr (+501')-In the report of geologic reconnissance, based onthe probability integral method and in-site production-exp[-nrexp( -x2 )dhnd designing data and under the assumption of waterVht N(y-gq)solution mining, the cumulated mining thickness h (six(4)layers in total, i.e, from the sixth to the eleventh layer) iscalculated as follows:,()=rW{2+9o expFn 0+9)9-r- r的=言家*kch, =7.81 (m)(12)6台y-90xp[π C-90)**exp-Z})d2where k; is single-layer mineral rate; C; is weighted grader"(x-)of single layer; hq is thickness of the third mining group.According to the ore layer's inclining angle(5)a=5*42', and in reference to the parameters of otherYy(x,)=-Wmaxh(exD- . (x+so)”_mines with the similar rock properties, it is assumed that:ndsubsidence coefficient n:=0.867; horizontal movementcoefficient b=3.03; influence propagating angle ofexpFr (-8023}expEr9+9goj-mining θ=86°7"; transition point offset S= -0.153H.Under the above parameters and conditions, it can becalculated that: Si=- 38.43 m; Sz- -34.49 m; S=- -36.73 m;(y-9o)4main influencing radius ni=- -82.89 m, r2=- -74.93 m,exp[-π(6)r=- -79.22 m. Based on the above derivation andEqns.(1)-(8), the potential deformation of the mine canEεmx(x,y)=z[εx +e, +(Ex-e,)2 +咧]2be estimated as follows: maximal settlement Wmax=6 279(7mm; maximal inclination imax=-84.94 mm/m, imx1=3.1.4 Inclination of ground surface point T:(x,y) along x81.18 mm/m, imxz -90.46 mm/m; maximal curvaturesdirection，T(x,y) along y direction and Tj(x,y) alongkmx=1.07X103 m~, kmax1=0.98X103 m"', kmax2=1.22X10~ m~'; maximal horizontal movement Umax=specific direction2221 mm; and maximal horizontal deformationT,(x,y)==U.(x,y)(8)Emx- 42.6 1 mm/m, Emx1-40.72 mm/ m,Emax2" =45.37 mm/m.b1These values are however much larger than those of thepermissible deformation of IV type building, and thusT,(x,y)=jU,(x,y)(9devia中国煤化工avily. .25 rYHCNMHGIdsveradiuisofTa(x,)=(T2 +r;)"2auuHuLIIL uuu uic uesigning departmentwhere Tmax(x,y) is the maximum of Tfx,y).re-estimates the deformation as follows: maximal3.1.5 Curvature of ground surface point Kx and K,setlement Wmax=4 970 mm; maximal inclination imx=包SpringerHE Yue-guang, et al: Hazard development mechanism and deformation estimation of water solution mining area60.24 mm/m; maximal curvature kmx= 0.73X10~ m';100h-+5.1(13)maximal horizontal movement Umx=l 640 mm; maximalHr= 5.ln+5.2horizontal deformation Emax- -30.32 mm/m. Unfortunately,0Hq=-100h -+4.0(14)5.ln+5.0 .these values are again far exceeding the limits of variousbuilding protective levels, and therefore deviate from thewhere Hp is the maximal altitude of water flowactual conditions, too.Based on the aforementioned mechanism, thefracturation belt; h is the cumulated thickness of minelayers; n is the number of mine layers.authors believe that with the currently designed miningThe calculation results show that the maximalmethod, if the mining strictly follows the plan and isaltitude of fracturation belt is H=27.3 m, which isunder the production capacity of mining wells, thesmaller than the mining depth. Thus, even roof cavingmining method is actually a room and pole type, whichhappens, it will not affect the upper water covered layers.belongs to partially mining method. The estimation ofAs the thickness of covering system loose andground stlement of such kind of mine can be made byflabbiness layers is large, when roof caving occurs to aadopting the method of probability integral method tocertain location, a suspended plate may be formed, andmodified settlement coefficients. Essentially, becausethe ground movement may decrease. Thus, the groundcell mining area is small, the upliting influence fromsurface farmland and buildings are expected to lie withinpressure-bearing water in mining field is made andpermissible protection extent(10-16].interlayer relaxation happens, along with the flling ffectfrom volume inflation when waterless CaSO4 in mining3.4 Fact of ground surface in mining areaarea absorbs water and becomes crystalized CaSO4, theThe 6- -3" well has stopped work for 90-1 80 d, anddevelopment of the inbreak and rupture of the upperthen it will be abandoned. There is a mud-brickplate over the cell evacuated area are quite insufficient,structural house located at the east of well head, 40 mand some rock layer over the rupture belt lies in a kind ofaway, which is over the corrosion line of mining area. Nosuspended stability state. The pressure from itself and theevidence has been found to prove that the house isrock and soil layers over it (until ground surface) willdamaged, but if there is any deformation destructionshift to places around rock bodies, and is burdenedcaused by ground surface deformation on this kind house,jointly by the rock body over the mining pole and theit is very easy to be found. Furthermore, from the aspectsboundary mineral body. This alters the stress state ofof time and space, if roof is collapsed completely, theunderling rock body, and the demission of side stress andmovement and deformation of ground surface andthe cnhancement of vertical stress make the rock bodybuildings in this area should be presented. From the factdeform compressively. The compression deformationthat the irrigation works and ground surface around thewell are in good condition, it can be drawn that thewill lead to the settlement of the covering rock body, andimmediate roof of mining area is stable, and from thedue to the cumulated effects, the settlement of coveringdata given by the mines, the largest thickness of dividedrock body increases along with the elevation, and reacheslayer is 21 m when the immediate roof is mud-containedthe maximum at ground surface.reussite rocks of cavity at some local area, but the layerAs there are no complete monitoring data of aat the top of immediate roof still maintains thesimilar mine, according to the method of engineeringuncollapsed state of crack and bend, whose thickness isanalogism, the mean sttlement cofficient of mines withover 10 m. So the facts prove that the theory put forwardbelt water and sand flling mining method is adopted i.e,in this paper is right and believable.n1=0.02. Then, the estimated maximal settlementWmax=-958 mm; maximal inclination inmx=12.048 mm/m;4 CONCLUSIONSmaximal curvature kmex=1.46 X 107 m ; maximalhorizontal movement Umax :328 mm; and maximal1) In combination with the underground Glauber'shorizontal deformation Emx-6.064 mm/m. Thus, a part ofsalt ore mining and geological condition, due to thethe values exceed the permissible values of protectionuplifing force of pressure-carry water in mining area, theobjects of ground surface movement, while the actualfilling effects mine interlayers relaxing and collapsing,movement and deformation must be monitored byhe indissoluble minerals' collapsing, and the volumeground surface movement monitoring networkl6-9form中国煤化工layer in fracturation3.3 Calculating altitude of water flow fracturationbelt,HCNMHG.belt21 1 sulct rgualus 15 Lancul and over-mining isThe empirical formulae are chosen as follows:controlled, no significant ground surface deformation or包Springer742J. Cent. South. Univ. Tech, 2006, 13(6)geological hazard related to buildings and farmlandChinese Well Ore Salt, 1991 (4): 17-20(in Chinese)8] ZhANG Zhi-ping. Simply analyse water and dissolve and exploit thedestroying will happen.infuencthat subsides to the earth's surfacc of arcea of Hengyang[]}3) Due to the deficiency in mechanism knowledge,Chemical Mine Technology, 1994, 23(1): 55- -57.(in Chinese)and because the estimated cofficients are from9] ZHU Jian-jun, HE Yuc-guang. Theory and method of deformationmeasurement[M]. Changsha: Central South University Press, 2004.engineering analogism, periodical monitoring must be(in Chinese)launched and strengthened to master the deformation[10] HE Yue guang. LIU Bao-chen. Stochastic medium and engineeringstyle, and to collect more data for raising the recoveryexample of ground surface movements due to foundationrate and for avoiding geological hazard.excavation[J]. The Chinese Journal of Nonferrous Metals, 2004,14(3): 489- 493.(in Chinese)[11] HE Yue guang, YAN Rong-gui, ZENG Zhuo- qiao. 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