Pore pressure fluctuations of overlying aquifer during residual coal mining and water-soil stress co

onlineatwww.sclencedirect.comMININGScienceDirectSCIENCE ANDTECHNOLOGYELLSEVIERMining Science and Technology 19(2009)0648-0652www.elsevier.com/locate/jcumtPore pressure fluctuations of overlying aquifer during residualcoal mining and water-soil stress coupling analysisDONG Qing-hong"2, SUI Wang-hua", ZHANG Xiao-cui, MAO Zeng-min'aschool of Geology and Exploration Engineering, Xinjiang Universiry, Urumgi, Xinjiang 830046. Chinachool of Resources and Geosciences, China University of Mining Technology, Xuzhou, Jiangsu 221116, ChinaState Key Laboratory for Coal Resources and Mining Safety, China University of Mining Technology,Xuzhou, Jiangsu 221008, ChinaAbstract: Three test models and a simulation model were constructed based on the prevailing conditions of the Taiping coalmine inrder to analyze pore pressure fluctuations of an overlying aquifer during residual coal mining. As well, the relation between poreressure and soil stress was evaluated. The model tests show the vibrations of pore pressure and soil stress as a result of minictivities. The simulation model tells of the response characteristics of pore pressure after mining and its distribution in the sandquifer. The comparative analysis reveals that pore pressure and soil stress vibration are activated by unexpected events occurringin mines, such as collapsing roofs. An increased pore pressure zone always lies above the wall in front or behind the working faceof a mine. Both pore pressure and vertical stress result in increasing and decreasing processes during movements of the workingace of a mine. The vibration of pore pressure always precedes soil stress in the same area and ends with a sharp decline. Changespore pressure of sand aquifer are limited to the area of stress changes, Obvious changes are largely located in a very small framever the mining face.Keywords: pore pressure fluctuations; water-soil stress coupling analysis; residual coal mining1 Introductionthe accumulated experiences over the last 10 yearshas been summarized about residual coal mining,There are several mining areas in the east and northdjacent to weathered zones and under unconsoli-of China that have increased coal production by im- dated layers". Furthermore, preliminary understandproving the upper limit level under Quaternary aqui- ing of the factors for the evaluation of residual coalfers, but there is still a large number of leftover bits mining and quicksand inrush mechanism duringand pieces of coal as residual to be developed where mining, under unconsolidated layers, has been ac-the thickness of the overburden rock mass above the quired. Dong and Sui have observed vibrations ofcoal seams varies largely from 0-60 m in height. Due pore pressure and soil stress of unconsolidated sandto the shallow overburden rock strata, the overlying layers before and during water and sand inrush intoaquifer in the Quaternary above the coal deposits be- the workface in their model tests, although the inforcomes a serious impediment to mine safety which mation of vibrations can be a type of decision-makingcannot be determined from traditional experience andinformation to evaluateregulations. At the same time, aquifers in the uncon- sand inrush disasters". Regulation of water pressuresolidated layers of the Quatermary are important water fluctuations and changes in the state of soil stressresources for local production and society, requiringhave not been fully recognized.that ground water in the Quaternary must be effec.This is a percolation process and also a fluidtively protected during mining processes. Given this structure/solid interaction process. Since Darcy estab-condition, water resource protection and water proof- lished the linear rule of seepage in 1856, much pro-ng become two constraints in residual coal mining. It gress in soil rock seepage mechanics has been madeshould be pointed out that these are important eleThe first improvement in understanding the interac-ments of green techniques in coal mining". As well, tion b中国煤化工mlmsReceived 05 March 2009: accepted 10 June 2009CNMHGProject supported by Qing Lan Project of Jiangsu, ChinaCorrespondingauthorteL:+86-516-83591015:E-mailaddressdonggh@126.comDONG Qing-hong et alPore pressure fluctuations of overlying aquifer during residual coal mining andsidence was provided by Terzaghi, who establishedthe one-dimensional, elastic, saturated porous media 3 Soil and water pressure fluctuation testconsolidation model and the well-known formula foreffective stress! So far. the formula is still the basicAn ideal model for pore pressure and stress tests isprinciple for porous media and fluid interaction. Biot presented in Fig. 2. In the model, all formations areextended Terzlaghi's work to a three-dimensional horizontal for ease of assembly; the thickness of theconsolidation problem, which is an isotropic and ani- overburden rock mass is 20 m and that of the sandsotropic elastic consolidation theory of porous media aquifer 15 m. The mining thickness of each slice is 2under the condition of small deformation of solids m. according to our ideal model, a similar experiand non-compressibleAn equivalent con- mental model was established with a scale of 1: 100tinuum model and a fracture network model have, for Its similarity has been discussed by Sui and Dong innow, become two basic models for studying the role 20084. Fig 3 is the monitoring system of our modelof soil and water coupling1-2. Tests in laboratoria (a-h are the sensor locations and numbers ) Part 3 isand computer simulations are two of the most impor- the sand aquifer including pore pressure sensors a-gtant means to reveal the process of seepage and wa- and a load sensor h above it. One section of a softter/soil interactionwater bag is packaged by unconsolidated sandBased on the geological situation of the Taiping particular particle size to simulate the confined aqui-oal mine in Jining, China, three test models and a fer and being loaded by a stability water head as thesimulation model are used to reveal the pore pressure border condition on each side. The monitoring cablefluctuations of overlying aquifers during residual coal will continue transmitting signals generated by sen-mining and water-soil stress coupling processessors to the computer at a frequency of 40 Hz2 Geological condition and modelYanzhou mining area of Shandong, China, has been燃密mining a residual coal seam for more than ten yearFig. I shows the main geological and hydrogeologicalOverburden rock massstructure of the coal deposit. The height of the ovburden rock mass is not more than 60 m and the water head above the buried erosion surface is more than15 m. Because of mining adjacent to a weatheredFig. 2 Ideal model for pore pressure and stress testzone, the thin, low intensity overburden rock mass ismostly occupied by overburden failure from coalmining. Large displacements and strain will occurduring the mining process. This kind of geological/hydrogeological structure has already led to waterinrush or quicksand hazards. It has been pointed outthat the pore pressure and stress in the sand aquiferFig 3 Monitoring systemare potential sources of information1 Coal seam No. 3: 2. Overburden rock mass; 3. Sand aquifer of Quater-situation of roof water above the buried erosion sur-ary:4. Other unconsolidated layers of Quatemary; 5. Monitoring cableface in the Quatermary systemThe model is remolded as a 2d plane strain struc-ture under the vertical load of gravity and horizontaldisplacement constraints on both sides. Three graingroups, with particle diameters of 0. 1-0.5 mm, 0.5-2mm and 2-5 mm, corresponding to test models wIw2 and w3, are used to represent different sand aquifers. Curves in Fig. 4 are pore pressure(sensor b)and vertical stress(sensor h) fluctuations generatedduring the mining process (Time)of model WI andw2 before water or sand rushes into the workface■ Coal seam N3口 AquiMonitoring shows that, if the sensor is located oversAndstonethe中国煤化工: pore pressure ofn mining to the leftWeathered zone口 Sand aquifer of QuatemarCNMH Ge pressure wouldgracll ue upset vation point in theFig 1 Geological modelfront of the workface is far away from the front wallof the mining face, no obvious changes inMining Science and Technologyo1. 19 No5sure appear. Consequently, while the workface nears ward trend after first increasing. The initial breakage,to the location below the sensor point, the pore pres- which is caused by first slice mining, always resultssure increases instantaneously. When mining passes in greater amplitude fluctuations in vertical stressthis sensor point, the pore pressure reveals a rapid The second mining slice caused a small number ofdecline but retains a high water head until the end of increases in pressure but these eventually droppedHowever, the third mining slice caused only slightThis process tells us that various positions in the changes or none at all. As a result of mining, thereupper confined aquifer refer to sensor points at thewas a great stress release area above the goaf in themining face and that different changes of pore pres- profiles, which included the measuring pointssure will appear. This kind of variation in pore presBesides the relationship between pore pres-sure originates mostly from mining and the relative sure-mining and soil stress-mining, there is a roughposition of sensor points relative to the mining face. correlation between pore pressure and soil stress as aStable downward changes in water pressure only oc- function of time(or mining). But a coupling relation-ur after closing of the working faceship could not be easily derived and stated precisely,Simultaneously, the vertical stress in the sand aq. because of test errors, interference, the discretionaryuifer is correspondingly volatile. when the measuring location of points and the scale of the test model. Apoint is located on top of the mining face, the vertical simulation model is another way to find some cluesstress increases and when mining passes through the about regularity to answer this questionobservation point, the vertical stress shows a downVertical stressVertical stressIPore pressuTime(*10s)(a) Model WI(b)ModelFig 4 Pore pressure(sensor b)and vertical stress(sensor h)I. The I"slice of mining: 2. The 2 slice of mining: 3. One pass cutting coal mining in sub area4 Simulation of water stresscame 0.35 m/d. After that, dynamic changes of porepressure and effective stress were calculated from aAccording to the geological and hydrogeological number of mining steps. Fig. 5 shows the two stepconditions of workface 8304 in the Taiping coal mine results of pore pressure-location and pore presand to remain consistent with the test models, we de- sure-time at each sensor point. The times indicated assigned a plane strain model for water-soil coupling I and 2 represent the end of mining in areas A and Bsimulation, corresponding to the structural test modIt can be seen that pore pressure of the sand aquiferels as shown in Fig. 2. Given that the thickness of the shows a downward trend above the mining area A oroverlying strata above the coal seam is 30 m and the B. At the same time, an accumulation of pore presfirst slice no more than 2 m thick, the water head sure appeared at the sides of the mined-out area. Butabove the bottom of the Quaternary system is 40 m,Pore pressure(kPa)imposed by a"pore pressure "load on the sand aqui0100kPfer. Moreover, self-stress is loaded onto the model bygravity. Assuming that the permeability coefficientduring the mining process remained constant, thepermeability coefficient of the roof and floor forma-tions was set at near zero. The coefficient of the sandaquifer was obtained by tracer experiments between中国煤化工three wells and two underground water points. Porepressure and self-stress balance were achieved byCN MHGO 200 220previous iterations before mining simulation, inFig. 5 Relationship between soil and waterwhich the value of the permeability coefficient beessure-time/positionDONG Qing-hong et alPore pressure fluctuations of overlying aquifer during residual coal mining andit does not show symmetry at the center of the work simulation, are mostly controlled by the assumptionface at time step 1 or 2. The pore pressure changes of the principle of effective stress, a similar trend ofunderwent a number time steps, while the peak of the soil stress and pore pressure has been observed bothpore pressure accumulation only existed for a short in model tests and in our simulations. In summary,timenumerical simulation explained the phenomenaobserved in our experiments. Stress and pore pressure5 Coupling analysis of soil- water pressure in the sand aquifer above the mined-out area wereconsistent in both research methods. Changes inThe diagrams of Fig. 6 show pore pressure,indicators, which can be tested, just appear in the areaeffective stress and vertical stress in the area of the above the side of the working face. But it also showssand aquifer at model time 1+4. The dissipated area that the results of numerical simulation are notof pore pressure, located just above the mining area, entirely consistent with the characteristics from ourcan be clearly seen in Fig 6a, as well as a significant experiment. It seems that the time of accumulationfall in effective stress and vertical stress in the same and dissipation of pressure/stress is shorter in theregion of Fig. 6b and 6c. This shows that the total simulation model than in the model tests. Anotherstress has decreased in the area. Additionally, above difference is that the zone of horizontal pressure/both sides of the mined-out areapressure and stress changes was too narrow in the simulationeffective/vertical stress increasedsame time. model, but could not be fully observed in model testsAlthough characteristics of thesebtained by because of the sensor layout■11.009080.70.60.50403MPa2s-E8R45535.5 16MPaFig. 6 Pore pressure, effective stress and vertical stress in calculation plane6 Conclusions3)Changes in pore pressure of sand aquifers duringmining are limited to the scope of changes in stress.1)Cyclical changes of water pressure in sand Obvious changes occur mainly in a very smallaquifers over mining areas are basic characteristics, framework over the mining facewhere each cycle of vibration is activated by miningOur research has tried to discover the relationshipevents such as collapsing roofs. Dramatic downward between pore pressure and skeleton stress of sanepore pressure occurs instantaneously only after aquifers in residual coal mining under shallowcollapsing roofs and overburden deformation. Walls bedrock conditions. A major shortcoming of the studyin front or behind a mining face always lie under is that it did not consider the conversion of permea-areas of increased water pressurebility. This part of the research is a work in progress2)Both pore pressure and vertical stresses result inincreasing and decreasing processes during theReferencesmovement of a working face in mines But vibration中国煤化工of pore pressure always precedes stress in the same [I]technique in coalarea and ends in a sharp decline. Furthermore, theCNMHGersiry of Miningamplitude of pore pressure is significantly higher than (2) Sui W H, Dong QH. Cai G T. 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