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翻译原文 Numerical investigation of groundwater outbursts near faults in underground coal minesLi Lianchonga, , , , Yang Tianhongb, Liang Zhengzhaoa, Zhu Wanchengb, Tang Chunanaa School of Civil and Hydraulic Engineering, Dalian University of Technology, Dalian 116024, P.R. Chinab Center for Rock Instability and Seismicity Research, Northeastern University, Shenyang 110006, P.R. ChinaReceived 18 August 2010. Revised 13 December 2010. Accepted 13 December 2010. Available online 21 December 2010.AbstractPermeable geologic faults in the coal seam can cause intermittent production problems or unexpected amounts of groundwater outburst from the underlying aquifers. With the acknowledgment of the basic mechanism for groundwater outbursts, the groundwater outburst along the fault zones in coal mines are numerically investigated using RFPA, a numerical code based on FEM. The fracture initiation, propagation, and coalescence in the stressed strata and the seepage field evolution in the stress field are represented visually during the whole process of groundwater outburst. The numerically obtained damage evolution shows that the floor strata could be classified as three zones, i.e. mining induced fracture zone, intact zone and fault reactivation zone, in which the intact zone is the key part for resisting groundwater outburst and directly determines the effective thickness of water-resisting rock layer. With understanding of the evolution of stress field and seepage flow in floor strata, the groundwater outburst pathway is calibrated and the transformation of floor rock mass from water-resisting strata to outburst pathway is clearly illuminated. Moreover, it is shown that geometrical configuration, including inclination angle of faults and seam drop along faults, have an important influence on groundwater outburst. Finally, based on geological, hydrogeology survey and numerical results, the mechanism analysis of groundwater outburst in an engineering case is studied, which can provide significantly meaningful guides for the investigation on mechanism and prevention of groundwater outburst induced by faults in practice.Research HighlightsThis study provides supplementary information on the stress distribution and failure-induced stress re-distribution that cannot be observed directly in situ or in experiments, within the areas of floor rock mass with the influence of fault. This study gives an interpretation of the fracture initiation, propagation, and coalescence in the stressed strata and the seepage field evolution in the stress field during the whole process of groundwater outburst. The transformation of floor rock mass from water-resisting strata to outburst pathway is clearly illustrated with the fracture evolution. This study makes an assessment of safety regarding water-resisting floor rock mass containing a fault with different configuration, including inclination angle and seam drop. And concretely illustrate the influence of fault configuration on groundwater outburst by a case study.Gadget timed out while loadingKeywordsGroundwater outburst; Geological fault; Rock failure Process; Numerical simulation; Underground coalmines1. IntroductionWhen the seam floor is not strong enough to resist the groundwater with high pressure, the groundwater can break it and burst into the working areas in underground coal mines. This phenomenon is called groundwater outburst and can be a severe geological hazardous event if an unexpected amount of groundwater were to appear suddenly from the underlying aquifers through the fractured seam floor. This could cause grievous casualties and heavy economic losses for underground coal mines.It is of vital importance to know when, where, and how groundwater outbursts could develop during mining processes ( Donnelly, 2006, Wang and Park, 2003, Wu et al., 2004, Yang et al., 2007, Zhang and Shen, 2004, Zhang et al., 2009 and Zuo et al., 2009). Rock is a heterogeneous geological material which contains natural weakness at various scales. When rock is subjected to mechanical loading, these pre-existing weaknesses can close, open, extend or induce new fractures, which can in turn change the structure of the rock and alter its fluid flow properties ( Karacan et al., 2007, Oda et al., 2002, Schulze et al., 2001,Souley et al., 2001, Tang et al., 2002 and Wong et al., 1997). the mining conditions in coal deposits in tectonically stressed masses are characterized by a number of features that are manifestations of mine pressure. The distribution of stresses around a major fault zone that intersects the mine entry roadway is of considerable importance in determining the stability and safety of mining operations. When mining excavations are made, the re-distribution of the stress field leads to the initiation and growth of fractures, and potentially creates a highly permeable damage zone around these excavations. This damage zone creates a pathway for water flow, reduces effective stresses close to the excavation, which in turn may further extend and dilate fractures that comprise the damage zone. Groundwater outburst occurs when the water pressure is greater than the strength of the seam floor beneath the mining excavations. When a fault is developed in the sedimentary rock mass, as shown in Fig. 1b, the damage zone of floor rock mass and the potential fluid pathway of groundwater outburst are distinctly different from those in the case without fault. In general, fault zones have weaker strength than those areas unaffected byfaults, where the mining accidents often happen ( Cao et al., 2001, Islam and Shinjo, 2009a and Islam and Shinjo, 2009b). For the case with fault, the potential groundwater outburst pathway is located in the region of adjacent to mining-induced fracture zone and fault zone. High permeability and dilation within thefault zone allows the upward migration of groundwater from the base of the fault, as local areas of confined aquifer. Moreover, further wetting of the fault zone, though not completely connected hydraulically with the aquifer, would decrease the mechanical strength of water-resisting strata ( Islam and Shinjo, 2009b and Wu et al., 2004). According to statistics, more than 80 percent mine water outburst accidents in underground coal mines are related to the faults, the rest may be related to the underlying minor faults in floor rock mass ( Li et al., 1996 and Wang and Miao, 2006;). The fault is an important water outburst channel. So it can conclude that the configuration of existing faults is the key factors determining the safety of coal mining with underlying aquifers. At present, the research on water outburst mechanism is mainly concentrated on either reactivation of fault or damage of floor strata ( Babiker and Gudmundsson, 2004, Barton et al., 1995, Caine et al., 1995, Evans et al., 1997, Gudmundsson et al., 2001 and McLellan et al., 2004). They did not involve the failure analysis of the key part in water-resisting rock layer associated with faults. Although previous theoretical studies and numerical investigations have contributed significantly to the scientific understanding of groundwater outbursts, crucial questions of when, where and how these events may develop during mining remain unanswered. Especially the fracture initiation, propagation and coalesce, together with the formation of groundwater outburst pathway adjacent to faults, in floor strata are not completely understood.This paper provides the results of the numerical investigations, conducted to provide insights into where, when, and how, groundwater outbursts may occur in underground coal mines subjected to the influence offaults, with particular reference to mining geometries at the Fengying Coal Mine (Jiaozuo Mine Group, central China). This is completed through explicit simulations of the evolving pathway of groundwater outburst.2. Brief introduction to the numerical methodIn this study, all the numerical simulations are conducted with RFPA2D (Rock Failure Process Analysis code). This code was originally developed by Tang (1997), based on FEM (finite element method) and improved at Mechsoft, China (RFPA User Manual, 2005). By introducing the heterogeneity of rock material properties into the model, RFPA can simulate nonlinear deformation of a quasi-brittle rock with an ideal elastic-brittle constitutive law for local material. On account of the heterogeneity of rock-like materials, the local mechanical parameters of the elements are assumed to follow Weibulls distribution, which is defined as follows ( Tang et al., 2000 and Wong et al., 2006):(1)where u is the parameter of the element (such as the Youngs modulus or strength); the scale parameter u0 is related to the average of element parameter and the homogeneity index m defines the shape of the distribution function and represents the degree of material homogeneity. A larger m implies a more homogeneous material and vice versa. A heterogeneous material can be numerically produced in a computer simulation for a model composed of many elements.This numerical method, based on discontinuum mechanics, seepage hydraulics, and damage mechanics, can be used to perform stress analysis, seepage analysis, failure analysis, and fluid-stress-damage (FSD) coupling analysis (Tang et al. 2002). Stress analysis is accomplished by FEM. Element damage is assessed by a Coulomb criterion with a tension cutoff, which is called the revised-Coulomb criterion (Brady and Brown 1992). An element is damaged in tension mode when its minimum principal stress (3) exceeds the tensile strength (t) of the element, that is 3 t. And an element is damaged in compression-shear mode when the shear stress satisfies the MohrCoulomb failure envelope : Herein, 1and 3 are the maximum and minimum principal stress, cis the uniaxial compressive strength, and is the internal friction angle. When an element is damaged, its rigidity is decreased by a large amount and its strength is reduced to the residual strength level.If an element in tensile damage mode is continuously under tensile stress, when the tensile strain increases and approaches the ultimate tensile strain, the damaged element will become completely cracked and its elastic modulus and strength decrease to approximately zero. Therefore, cracked elements can experience a large deformation. New faults or fractures (discontinuum) are formed through the coalescence of failed (damaged or cracked) neighboring elements (continuum). On the other hand, the rigidity of the element in the compression-shear mode is enhanced when it continues to be compressed to the ultimate compressive strain (UCS). In this way, crack closure is simulated (Yang et al., 2004).The fundamental assumption behind the model presented here is that the rock is fully saturated and the flow of the fluid (water) is governed by the Biots consolidation theory. Changes in permeability are accommodated by relating permeability magnitudes to effective stresses, and fracturing process. The complete set of mechanical and flow equations for steady behavior are defined as following.As isotropic conditions are considered for the hydraulic behavior at the elemental scale, according to the Darcys law of seepage flow in porous media, the following equation of the isothermal seepage flow in rock mass can be obtained.(2)where k = permeability, p = pore pressure, S = Biot coefficient, = Biots coefficient and v = volumetric strain.The equations of equilibrium and the strain-displacement relations can be expressed as:(3)(4)where fi = component of body force and ui = component of displacement in the i-direction. The governing equations for mathematical model of an isotropic linear poroelastic medium deformation considering the fluid pore pressure can be expressed as:(5)(+G)j,ji+Gi,jj+fi+(p),i=0where = Lames constant, G = shear modulus.In the mathematical model, rock permeability can decrease or increase with deforming and fracturing process. The varying law of the permeability for elements in the presented code is illustrated as the following.Based on this observation, the stress is directly associated with the changes of permeability of rock and some permeability-stress relationships have been established ( Louis, 1974, Tang et al., 2002 and Zhang et al., 2000). In the stage of elastic state, rock permeability decreases when the rock compacts, and increases when the rock extends. The permeability variation for an intact rock element in elastic state can be described as ( David et al., 2001 and Louis, 1974):(6)ke=k0exp(ii/3p)where the k0 = initial permeability of rock element, = coupling coefficient, and ii/3 = average total stress. In this stage, it is assumed that 0 1.In the fracturing stage, the permeability undoubtedly increases as fractures initiate and propagate. This is one of the important concerns in the model. In the post-peak stage, dramatic change in rock permeability can be expected as a result of generation of numerous micro fractures. In order to apply appropriate post-peak hydraulic characteristics, the use of a strain-based formulation for permeability variation is more suitable (Susan et al., 2003 and Yuan and Harrison, 2005). In RFPA, the hydraulic conductivity for a damaged rock element is expressed as:(7)where V is the change of volume of the element, l is the viscosity coefficient of the fluid (water), and g is the acceleration due to gravity. In this stage, it is assumed that = 1.The model is finely discretized to accommodate local variations of material heterogeneity. During simulation, the model is loaded in a quasi-static fashion. At each loading increment, the seepage and stress equations are solved and the coupling analysis is performed. The stress field is then examined, and those elements that are stressed beyond the pre-defined strength threshold levels are assumed to be irreversibly damaged. The stiffness and strength of the damaged elements are reduced, and permeabilities are accordingly increased. The model, with associated new parameters, is then re-analyzed. The next load increment is added only when there are no more elements strained beyond the strength-threshold corresponding to the equilibrium stress field and a compatible strain field. The model iterates to follow the evolution of failure along a stress path, and in pseudo-time. The evolving state variables (stress, strain, fluid pressure) and material properties (modulus, permeability) overprinted on the initially heterogeneous field of strength and modulus, may be visualized to follow the progress of the outburst process.3. Problem descriptionFaults are found nearly everywhere in the upper crust and may act as major channel for concentrated fluid flow. Because groundwater outburst is mainly through the normal fault, the main objective of this paper is to study the normal fault. As shown in Fig. 1, the inclination angle of the fault is . In coal seam, when the abutment pressure of the coal seam floor reaches or exceeds the critical strength of the floor strata, the damage may occur in a certain range of rock mass of the working face floor, resulting in brittle fracture, so-called zero level fracture (Wang and Park, 2003). Accordingly, gravitation and mining induced stress concentrations are the basic factors for the occurrence of the zero level fracture, while the maximum depth of the fracture depends on the width of the plastic zone near the mining face, as well as the frictional angle of rock strata in the floor. By employing plastic slip-line theorem of punching a load on an infinite continuum, the maximum depth of fracture in floor strata is derived as follows (Wang and Park, 2003):(8) Fig. 1. Sketch of key pathway for groundwater outburst.In the above formula, is the internal friction angle of floor rock mass.xa is the length of yield zone in coalseam, it could be gained by practical measure in-situ.When mining excavations are made, the re-distribution of the stress field leads to the reactivation of faults, and potentially further enhances the permeability of fault zone. Hence a potential pathway for groundwater outburst is created between fault and the damage zone in floor strata. In addition there is abundant evidence that the passage of fluids in faulted areas is episodic and linked to the variation of inclination angle (a) and seam drop (d) related to the fault. Fault affects mine groundwater outburst in three aspects: (1) water diversion and storage function of fault; (2) the fault shortens the distance between coal bed and correlative aquifer; (3) the fault decreases the strength of rock masses.Thus, both inclination angle of fault and seam drop along fault play an important role in groundwater outburst, in which the inclination angle directly determines effective thickness of water-resisting rock mass Tand the seam drop determines the distance between mining-induced fracture zone and aquifer, d1. The effective thickness of water-resisting rock mass T is the shortest and the most critical way for groundwater outburst through fault. If the aquifer water only breaks this zone, water outburst through fault will be easily formed. So the authors focus on the research of the instability of the regional rock mass, using numerical methods to study the failure condition of the regional rock mass. Unlike static stress analysis approaches in which the fractures have to be inserted in the model, the applied numerical methods can model the complete fracturing process. This fracture modelling technique can provide valuable insight regarding groundwater outburst processes that are impossible to observe on site and difficult to consider using static stress analysis approaches.The main objectives of this study are as follows:1.Provide supplementary information on the stress distribution and failure-induced stress re-distribution that cannot be observed directly in-situ, in floor rock mass with the influence of fault.2.Give an interpretation of the fracture initiation, propagation, and coalescence in the stressed strata and the seepage field evolution in the stress field during the whole process of groundwater outburst. Then illustrate transformation of floor rock mass from water-resisting strata to outburst pathway.3.Make an assessment of safety regarding water-resisting floor rock mass containing a fault with different configuration. And concretely illustrate the influence of fault configuration on groundwater outburst by a case study.4. Model setup and numerical simulation4.1. Numerical modelBased on the knowledge of fault configuration, physico-mechanical complexity of faults, and the mechanism of groundwater inrush induced by faults, in this section numerical tests with RFPA are conducted to investigate the initiation of fractures, reactivation of faults and formation of groundwater outburst pathway with mechanical model for rock mass with faults in coal mining above confined aquifer. A conceptual mechanics model was constructed with consideration of the effects of the structural planes offaults (Fig. 2). The model is discretized into a mesh that contains 360 240 = 86,400 elements with geometry of 240 m 160 m. The water pressure in aquifer is 3 MPa. A compressive vertical stress (v) of 5.0 MPa is imposed on the top boundary to represent the ground stress induced by overburden strata. Normal displacements are constrained on the right, left side and the bottom boundary. Plain strain is assumed for all calculations. The mechanical parameters employed in our modelling are listed in Table 1.Fig. 2. Geometry and loading conditions for groundwater outburst model with faultTable 1. Physico-mechanical parameters employed in the simulationsRock layerYoungs modulusE0/GPaCompressive strengthc0/MPaTensile strengtht0/MPaInternal cohesive angle /()Poissons ratio Density/(kgm-3)Hydraulic conductivityk0/(md-1)Fault1.020.1270.402300100.0Aquifer10.0151.5380.252500100.0Coalseam1.270.1300.3015000.1Water-resisting rock (floor strata)5.0121.2350.2525000.1In order to investigate the effect of fault configuration on groundwater outburst, eight different cases, while keeping other parameters constant, are simulated to illustrate the relative importance of fault configuration. These eight cases are listed in Table 2.Table 2. Simulation cases studied in the paperConfiguration of faultValueConditionInclination angle a/( )30seam drop d = 20 m45seam drop d = 20 m60seam drop d = 20 m75seam drop d = 20 mSeam drop d/m10inclination angle a = 45 30inclination angle a = 45 40inclination angle a = 45 50inclination angle a = 45 4.2. Strength reduction rule of element and determination of safety factorAs an alternative approach, the fundamental principle of strength reduction has been widely employed in FEM to conduct failure analysis problem related to geological or rock engineering ( Griffiths, 1982, Griffiths and Kidger, 1995, Li et al., 2006, Zheng et al., 2006 and Zheng et al., 2008). In RFPA, the fundamental principle of strength reduction has been incorporated into the constitutive model of element. In the computing for strength reduction analysis, the strength 0(including c0 andt0) addressed in the constitutive model above linearly degraded according to the following equation:(9)where Fstrial is the trial safety factor and 0trial is the trial strength of element. The trial strength 0trial is employed in RFPA to investigate the strength of the geological medium (in this case, the rock masses).In this study, the bearing capacity of the water-resisting rock layer (referring to floor stratum between coalseam and aquifer) is examined. Stability simulation in RFPA is run with the trial strength 0trial until the critical pathway for groundwater outburst in floor strata is determined. Simultaneously, the correspondingFstrial is the safety factor Fs of the floor strata. For the detailed introduction to the models and verifications, the readers can refer to previous numerical simulations ( Li et al., 2006, Tang et al., 2002 and Yang et al., 2007).4.3. Numerical simulation of groundwater outburst processBefore reporting the results of the simulations in progressive fracture modelings, the stress distribution that governs the models behaviour was first focused on. One of the most widely used techniques to visualize stress fields is the technique of photoelasticity. Photoelasticity provides the contours of difference in principal stresses. Fig. 3 is the distribution of stresses around the working area. As the stability of floor strata rock adjacent to fault zone and working face is the main concern, we just show a small portion of the model in a big box. The fringes in general appear as broad bands, the thickness of the fringe is indicative of the gradient of the stress variable. The fringes are very broad when the gradient is small, and vice versa. The zone of high density of fringes indicates a zone of stress concentration. Thus, a mere qualitative observation of the fringes can yield a wealth of useful information about stress distribution. The redistribution of mining-induced stresses can cause important deformation inside and around the mine openings. Even though that the rock mass in fault zone is much softer than those surrounding the fault, the fault zone acts as a barrier for the excavation-induced stress to transfer, which induces evident stress concentration within the region adjacent to the fault. Then the reactivation of fault and associated damage around the fault zone is likely to happen.Fig. 3. Numerically obtained stress distribution in the model.To make detailed observations of the fracture pattern, the development and formation of critical pathway for groundwater outburst, the damage evolution of water-resisting rock layer (floor strata) have been numerically obtained in the numerical modeling. The results for case with inclination angle is a = 45 and seam dropd = 20 m . Results show that the damage mode in floor strata is influenced by strata pressure as well as hydraulic pressure in aquifer. In front of the mining face, the floor strata are subjected to a compressive state under the so called abutment pressure. Near the mining face and inside the mined area, floor strata are in a transition state from shear compression to extension. During the transient process, the floor strata will inevitably produce shear and tensile fractures due to bending. The unloading effect induces the expanding of floor strata. Fractures could then open and develop downwards. Concerning the fracture conditions, the floor strata could be classified as three zones in the vertical direction, i.e. (i) mining induced fracture zone, (ii) intact zone and (iii) fault reactivation zone induce by coupled action of mining and hydraulic pressure . Owing to the existence of fault, the mining induced fractures zone in left is obviously wider and deeper than that in right. Even so, the fractures in faultreactivation zone disconnected with those in mining induced fracture zone. On the whole, the water-resisting rock layer in intact zone remains stable. With the strength reduction, the damage in the mining induced fractured zone expands towards the fault reactivation zone. Meanwhile the reactivation of the fault is more active and newly generated damage continuously appears around the fault. Gradually, the damage develops, connects to each other and a critical failure plane is formed in the water-resisting rock layer . Fig. 4 is the corresponding groundwater outburst process illustrated with shear stress and fluid flow rate. In the figures of stress field, the dark elements represent the nucleated flaw. Fractures form by connection of flaws. The shading intensity indicates the relative magnitude of the maximum shear stress within the elements. Numerical results show that the shear stress is concentrated on the two sides of the working face. The concentration area (brighter) extends downwards over 35 times the mining height. With the fracturing process, it can be seen that the macro-fractures result in evident stress concentration around the fracture tips (Fig. 4a-2). Even so, the stress concentration within the region adjacent to the fault is still intensive (Fig. 4a-3). As soon as the pathway for groundwater outburst forms, the intensity of the stress concentration for the floor seems to be released with mining distance. It appears that a stress shadow area (grayer), due to stress release of damaged rock mass, is located at the adjacent region of floor (Fig. 4a-4). It is noted that the strong interaction between the isolated fractures in this high-stress field makes the fractures propagate in an unstable manner. The variation in failure mode is highly sensitive to the local disorder feature of the rock mass. As a result the critical pathway is not straight but flexural, and the fracture surface is rough. In reality there are two types of failure: high-stress failure and low-strength failure, for different materials. In a homogeneous material, failure begins at the high-stress site, whereas in heterogeneous material, e.g. rock, failure may start at the weaker locations because of the presence of pores, micro-fractures, grain boundaries, etc. This is the reason why Fairhurst (1964) introduced the notion of “stress severity”, which represents the ratio of the theoretical stress at the moment of failure to the stress that would theoretically be necessary for failure at any given point. Heterogeneity is the main reason for the failure that occurs in locations where the stress is not necessarily the greatest.Fig. 4. Numerically obtained groundwater outburst process.Fig. 4b shows the computed seepage velocity field with arrows indicating the velocity vector before and after groundwater outburst. While the intact zone in floor strata is stable, the seepage velocity is very low, as shown in Fig. 4b-1. Once the scattered damage adjacent to the fault reactivation zone links up with the mining-induced fracturing zone, the seepage velocity reaches a quite high value instantaneously. It is shown that the newly generated fractures in the water-resisting rock layer have created a groundwater flow network. Studies reveal that the network consists of two main hydrogeological structures, namely a core and a connected fracture zone ( Babiker and Gudmundsson, 2004, Gudmundsson et al., 2001 and Islam and Shinjo, 2009b). The core, consisting mainly of cataclastic rocks, has a low permeability. In contrast, the connected fracture zone, consisting mainly of fractures of various sizes, has a higher permeability than the core. As well as the trend of the connected fracture zone is generally parallel to the hydraulic gradient. The core acts as a low-permeability barrier to flow, while the connected fracture zone acts as an important conduit for flow. Pressured groundwater clearly bypasses the core and selects the connected fracture zone as shown in Fig. 4b-4.Failure trajectories indicate that mining-induced fracture in floor strata is not sufficient to generate water inflow into the mine. However, due to the stress re-distribution and mechanical strength degradation in floor strata induced by wetting in fault zone, the traditional damaged mode of floor strata is changed. The addition of a coalescent damage zone in floor strata provides a primary fluid pathway, then water fluid is focused into the pathway along the fault zone and is enhanced into the working face in mine. These results support many researchers explanations to the fluid flow associated with faults ( Babiker and Gudmundsson, 2004,Karacan et al., 2008, Islam et al., 2009 and McLellan et al., 2004).The investigation above shows that the formation of the path for groundwater outburst is an evolutive process of damage in floor strata. Numerical results provide an opportunity to investigate the effect of faultas well as fracture initiation, propagation and coalescence on the stress re-distribution. It provides a better understanding of the formation of the pathway for groundwater outburst.Based on the above investigations, it is clear that, in addition to the groundwater pressure, fault zones play an important role in groundwater outburst. The configuration, including the inclination angle of fault and seam drop along fault, need to be considered in the sensitivity analyses of the fault zone, because they are the key factors determining the critical pathway for groundwater outburst. With different fault configuration listed in Table 1, the bearing capacity of floor strata is numerically investigated and the corresponding safety factors for all cases are presented in Fig. 5. For the cases with a constant seam drop (Fig. 5a), the steeper the inclination angle is, the higher the safety factor is. For the case with a = 30, the safety factor is close to 1.0, because in this condition the effective thickness of water-resisting rock layer is too short to that thefault is nearly linked up with the mining-induced fracture zone. While for the cases with a constant inclination angle (Fig. 5b), the greater the seam drop, the lower the safety factor is. It is shown that the bearing capacity of floor strata is more sensitive to the inclination angle than to the seam drop. Since the inclination angle directly determines the effective water-resisting thickness (T shown in Fig. 1) in floor strata, while the seam drop mainly affects the ascending distance of pressured water.Fig.5. Numerically obtained bearing capacity of the water-resisting rock layer (floor stratum) with differentfault configuration.4.4. A case study from Fengying Coal MineJiaozuo coal mine field, is located in Henan Province, central China, covers an area of 1300 km2, with an annual production of 350 million tones. The thickness of the principal coal seam No. 2 (2#coal seam) is between 6 and 9 m. From the top to the bottom there exist four large rock formations, as shown in Fig. 10: the Quaternary System (Q), the Permian System (P) containing coal seam; the Carboniferous System containing the limestone aquifers L8 and L2; and the Ordovician System (O2) containing a large limestone aquifer with the thickness of 400 m. So far the groundwater outbursts more than several hundred times have been recorded in this area, in which the maximum fluid flow rate reached about 6300 m3/h. These accidents caused grievous casualties and heavy economic losses. Faults, fractures and karst are well developed in this area. The faults cut the rock layers and produced fault zones in the rock systems. Therefore the aquifers L8, L2 and O2 are the serious threat to the mining safety in 2# coal seam.Fig. 6. Geological condition and schematic diagram of working face No.1301 in Fengying Coal Mine.Fengying Coal Mine is located in Jiaozuo coal mine field. A typical groundwater outburst induced by faulthad happened in No.1301 working face in this mine. No. 1301 working face is located in the upper of Xiazhuang fault. Base on incomplete geology exploration, the inclination angle of thefault is presumed to be about 70. However the exact inclination angle of the fault is only about 5055. Due to the mistake in estimating the Xiazhuang fault configuration, in mining process the pressured groundwater in aquifer L8, L2 and O2 instantaneously outburst into No.1301 working face along the fault. The production level of Fengying Coal Mine was fully submerged in a few hours, which caused grievous casualties and heavy economic losses.In order to investigate the mechanism of groundwater outburst in No.1301 working face, a numerical model with a geometry of 300 m 200 m in size is employed . In the numerical simulation, a fluid pressure of 2.0 MPa, as measured in situ, is applied to the rock layer of L8, L2 and O2. The other mechanical parameters are the same as those listed in Table 1. Fig. 7 is the numerical results of groundwater outburst in No.1301 working face. In these figures the gray level represents the magnitude of the shear stress. These graphical visualizations illustrate clearly the initiation, propagation, and the coalescence of fractures comprising the outburst path. Although the thickness of the rock layers of the seam floor between the coalbed and the aquifer is rather large, the faultswhich cut those layers break the continuity and decrease the strength of the rock layers. The size of designed safety coal pillar in mining is 37 m and the effective length of water-resisting pathway is 40 m, as shown in Fig. 6. Due to the variation of inclination angle of the fault and water penetration into the faultzone from the underlying aquifer, the distance between the coalbed and the top of the groundwater along thefault zone that connected with the limestone aquifer was much reduced. The real length of reserved safety pillar and water-resisting pathway are merely about 19 m and 29 m respectively.Fig. 7. Numerically obtained critical pathway for groundwater outburst in working face No.1301 of FengyingCoal Mine (The results are illustrated with the evolution of stress field.).Fig. 8 shows the computed seepage velocity field with arrows indicating the velocity vector before and after groundwater outburst. The longer the arrow is, the higher the flow velocity is. It can be seen that the fluid flow rate is in conformity with the permeability of rock strata. Before groundwater outburst, the water penetrates through the rock strata very slowly due to the water-resisting action of the rock layer between coal seam and aquifer, although the pressured groundwater has been promoted to a certain height along the fault. The seepage velocity increases dramatically as soon as the key pathway for groundwater outburst is formed completely. These numerical results confirm the results of in-situ observation of strata failure and fluid flow (Guan, 2005). The RFPA modelling provides a realistic approach to study rock failure mechanism in groundwater outburst.Fig. 8. Numerically obtained groundwater outburst process (The results are illustrated with the evolution of water fluid flow velocity in strata.).5. ConclusionsTo better understand the origin and evolution of pathway for groundwater outburst driven by coupled hydro-mechanical load, a series of numerical simulations were conducted, using RFPA2D (Rock Failure Process Analysis code) in which the progressive evolution of a fracture set is modelled, thus providing direct observations of pathway formation. The following conclusions can be drawn.The fault zone acts as a barrier for the excavation-induced stress to transfer, which induces evident stress concentration within the region adjacent to the fault and accelerates the reactivation of fault. Concerning the damage conditions, the floor strata could be classified into three zones, i.e. mining induced fracture zone, intact zone and fault reactivation zone, in which the intact zone is the key part for resisting groundwater outburst and directly determines the effective thickness of water-resisting strata rock. The variation in flow rate caused merely by mining induced fracture zone is limited. Once the scattered damage adjacent to the fault reactivation zone linked up with the mining-induced fracturing zone, the seepage velocity rate will reach a quite high value instantaneously, which indicates the fractures coalesce and a critical outburst pathway is formed in the water-resisting rock layer.The results of the failure trajectories indicate that the onefold mining-induced reactivation of the faults or the onefold mining induced fracture is less likely to generate heavy water inflow into the mine as long as the effective thickness of water-resisting rock layer is big enough and the strength is high enough. However, due to the stress re-distribution with different fault configuration and mechanical strength degradation in floor strata induced by wetting in fault zone, usually the traditional damaged mode of floor strata is changed, then the ascending of pressured water could occur along the fault zones and enhance water inflow into the mine. It is shown that the bearing capacity of floor strata is more sensitive to the inclination angle of fault than to the seam drop along fault. Besides fault zones could greatly reduce the strength of the floor strata due to the wetting effect and the production of the weakness zones, fault zones may significantly decrease the thickness of water-resisting rock layer, due to the variation of inclination angle of fault and the seam drop along fault, and greatly reduce resistibility to groundwater outburst. Therefore, attention needs to be paid to those fault characteristics.Based on the above investigation, it is clear that, in addition to fault zones, the groundwater pressure play an important role in groundwater outburst. However, since the water pressure is relatively easier to measure and its measurement error would not be significant, its uncertainty is not a major concern in numerical simulations.Although the simulations here are two-dimensional and many of the conclusions that are given here may be obtained in situ, or may even be common sense, the reproduction of these phenomena in a numerical simulation is significant. To the best knowledge of the authors, no convenient experimental method has been available for obtaining damage evolution, the stress and seepage flow field during the progressive failure process until now. In the numerical simulation, the fracture initiation, propagation, and coalescence in the stressed strata and the seepage field evolution in the stress field are represented visually during the whole process of groundwater outburst. The phenomenological approach provides supplementary information on the stress distribution and failure-induced stress redistribution, and shows in great detail the propagation of the fracture zone and the interaction of the fractures with the structure of fault in coal seam. Essentially, it is difficult to validate a mechanics model set up for simulations of groundwater bursting based on site data. This is because groundwater outburst may not happen before forecasting it, and, generally, it is not permissible to create a groundwater outburst. Furthermore, even though some records on groundwater outburst are available, it could be time and cost-consuming to obtain the site geological and mechanics parameters that are needed for model validations. The successful reproduction of observed failure phenomena in situ with a numerical method implies that our understanding of groundwater outburst has reached a reasonable level, which in turn will help us to make further progress in this field.AcknowledgementsThe study presented in this paper was jointly supported by grants from the China National Natural Science Foundation (Grant Nos. 50909013, 50820125405 and 50804006), the National Basic Research Program of China (Grant No.2007CB209404) and the Specialized Research Fund for the Doctoral Program of Higher Education (SRFDP) of China (Grant No. 20090041120024).References翻译 关于地下开采中断层附近突水的数值研究李连崇 ,杨天鸿,梁正召 , 李连崇 ,朱万成,唐春安土木水利学院,大连理工大学,中国大连116024围岩失稳和地震研究中心,东北大学,中国沈阳1100062010年12月21日摘要煤层中有渗透性的(有突水危险的)断层有可能会引起煤矿的不连续生产(生产的间断)或者是大量的地下水从下部岩层中突然涌出。随着对突水事故原理的了解的增加,煤矿开采中的地下水随着断层带涌出这一情况在用一个基于FEM.上的系统 RFPA来做数值调查。井下突水过程中受力岩层断面的破裂、发展和合并以及应力场中渗透区的变化将被以一种可视的形式表现出来。通过数值模拟获得的破损演化过程显示了底板岩层将被划分为三个区域:破裂区、未破裂区和断层恢复区,其中未破裂区是抵抗地下水涌出和直接决定隔水层的有效厚度的关键部分。了解了底板岩层承压区的发展和渗流区的流动后,地下水涌出的途径和隔水层用于地下水涌出的岩石预留量就非常明确了。此外,它以几何结构的形式表示出来,包含了断层的倾角和断盘面,在预示地下水涌出方面有重要意义。最后,基于工程案例中地质学、水文地质调查和数值模拟结果,工程案例中地下水涌出的机理分析已被研究,它在突水的机理上的数值调查方面提供了重要的引导作用,并在实践中有效的阻止断层附近的地下水涌出。调研的重要意义研究弥补了实地测量和实验中不能直接测的的应力分布和采动引起的应力重新分布,包括地板中断层影响区岩石预留区域。这项研究阐明井下突水过程中受力岩层破裂、发展和合并以及应力场中渗透区的变化并以一种可视的形式表现出来。这一研究评估了关于含有不形态的断层处阻隔地下水涌出的岩石量安全性,包含了断层的倾角和断盘面,在预示地下水涌出方面有重要意义。并通过实例直观的说明了断层处地下水涌出的影响。关键词地下水涌出 地质断层 岩石失效过程 数值模拟 地下开采1简介地下开采中,当底板岩层不能有效的阻挡高压地下水时,地下水就会冲破它并涌入工作空间。这种现象叫做突水,地下开采中大量地下水突然从含水层中冲破底板岩层涌出会引起严重的地质事故,并会造成惨痛的人员伤亡和严重的经济损失。预知地下水何时何地以何种方式突出至关重要岩石是一种不均匀的含有一些弱面的地质材料当岩石承受机械加载的力后,原来的弱面会闭合、张开、延伸或是产生出新的能改变改变地下水流动的结构的裂缝,结构松弛的受压岩体的矿床的开采条件是以一系列矿山压力显现的现象为特征的。这种沿贯穿水平大巷的主断层影响区的应力分布是决定煤矿生产稳定和安全的决定因素。煤矿在受到采动影响后,应力的重新分布会导致断裂的形成和加剧,并会在采掘区的周围生成高危区域。这种破坏生成了地下水涌出的通道,并减少了掘进头的有效应力。采掘中,如果地下水的压力超过了底板岩层对它的阻力,就会发生突水。有断层的区域有明显的不同。一般来说,断层区的强度要比那些没有被断层破坏区的弱,是矿难的频发地带。在断层的案例中,有突水危险的地方存在于破碎区和断层区。断层区的高渗透和高膨胀性促使地下水经原来的阻水层处的断层层下部上涌。而且,断层区的进一步。湿润虽没有直接压裂含水层,但会会对其造成机械破坏。根据统计,超过80%的井下突水事故与断层相关,其余的可能与地板岩层中的潜在小断层有关。断层是突水的主要通道。所以可以推断已有断层的形态是决定井下含水层是否安全的主要因素。目前,突水机理的研究主要集中在断层的活动和底板岩层的破坏。它们没有涉及对阻水层和断层联合起来的关键部分的失败的分析。即使先前的理论研究和数值分析已对我们科学的认识突水有了很大的贡献,但没有回答何时、何地以及以何种形式发生等这些关键问题。特别是破裂的开始、延伸和闭合连同靠近底板断层的地下水涌出路线的形成都没有完全解释。这篇论文提供了数值模拟的结果,并对地采中断层附近的矿井水何时、何地、以何种方式涌出做了深入的研究,特别提及冯营矿(位于中国中部的焦作煤田(煤层群)。这个通过模拟地下水的发展路径完成的2 数值思路的简介研究中所有的数值模拟是用RFPA2D(岩石失效过程分析法)导入的,这种方法的创始人是唐(1997年)基于 FEM(有限元法)并在Mechsoft中改进。通过讲岩石的不均匀性引入模型,RFPA可以进行一个外表易碎的岩石和一个当地的地质材料性能下理想的弹碎性岩石进行一个非线性的模拟。 (1)当u是参数(如杨氏模量或强度),标尺参数u0是与平均数有关和均匀性指标m定义了分散函数的形状并代表了材料均匀性的程度。一个更大的m隐含了一个更均匀的材料,反之亦然。一个均匀材料能够通过电脑模拟生成一个有许多岩块组成的模型。这个基于不连续力学、渗透水力学和损伤力学的数值方法可以用于演示压力分析、渗透分析、失效分析和液体损伤压力联合分析(FSD)。压力分析已通过 FEM完成。岩块的损害程度已被校正库伦准则以一库伦张力为单位的评估(Brady and Brown 1992),当张力模型中岩块的最小主应力(3)超出岩块的应力强度 (t) ,即3 t。在压缩剪切方式中,当剪应力满足莫尔库伦滑落包络线时,在此处,1和3是最大和最小枝力, 是内摩擦角。当岩块遭到破坏时,它的刚性将大量减少,它的强度也将减少到残余强度水平。如果张力破坏模型中一个岩块连续的处于拉应力下,当拉伸变形增加至接近极限,被破坏的岩块将破裂,它的弹性模量和强度减少至近似为0。所以,破裂的元件会经过一个大的变形。通过合并或者破碎形成新的断层或者羽状结构。另一方面,压缩剪切模型中当元件被连续压缩至极限压缩形变时,它的刚性会增加。用这种方法来模拟破裂闭合。模型代表的基本假定背后是岩石完全渗透,并且流体将被合并理论所支配。与相关的渗透性相关的有效应力的大小会调节渗透性和羽状结构的变化。一套完整的机械和流动方程的稳定行为的定义如下: 由于各向同性的条件被认为是数值范围内的水力行为,根据多空介质中的渗流的达西定律得到下面的岩石中的等温渗流方程式。 (2)当k = 渗透率,p = 孔隙压力,S = 比奥数, =比奥系数 ,v =体积应变。平衡方程和应变位移公式可被表示为 (3) (4)当fi = 单位体积力,ui =是在i方向上单位位移。考虑到流体孔隙压力多孔形变的各向同性曲线数学模型的控制方程可表达为: (+G)j,ji+Gi,jj+fi+(p),i=0 (5)其中 =拉姆常数,G =剪切模量 在这个数学模型中,岩石渗透率会随着破坏或是闭合的过程变小或是增大。代码中岩块不同的规则图解如下:基于这项观察,压力与岩石渗透性的变化直接相关,并建立起渗透压关系。在弹性状态阶段,岩石的渗透性会在岩石的结构紧凑时下降,松散时上升,一个弹性状态下完整无损的岩块的渗透率的变化可表示为( David et al., 2001 and Louis, 1974): ke=k0exp(ii/3p) (6)其中k0=岩块的初渗透率,, =粘着系数,ii/3 = 平均压力,在这个阶段,假定为0 1。在破裂阶段,当形成和延伸时,渗透性无疑是增加的。这是模型中很重要的一个关系。在峰后阶段,岩石渗透性的显著变化可被认为是微裂缝的再生的结果。为解释适当的水压峰后特征,用应变疲劳来说明渗透率的异常更加合适。损坏岩块的水力传导系数表示为: (7)当V 是岩块体积变化,l 是流体的黏度系数。g 是重力加速度。这一阶段,假定 = 1。该模型是适应当地不同材料的异质性很好的离散函数。在模拟中,模型以准静态的方式加载。每次载荷的增加,解渗透和应力方程并且执行耦合分析。如此可验证应力场,那些岩块将以超过预先设置极限水平的加压至不可逆转的破坏。受损岩块的刚度和强度降低,渗透率对应增加。然后重新分析新参数产生的模型。在只有没有其他的元素超出极限强度相应的均衡应力场和一个恒定的应力场时进行下一次加载。迭代模型遵循应力变化相应的失效过程。可能是不断变化的状态变量(应力,应变,流体压力)和材料特性(弹性模量,渗透性)相应的的强度和模量的初步异构领域,可视化的爆发过程中遵循的进展。3 问题的描述断层在上部地壳中随处可见,并会成为集中的地下水流动的主要渠道。由于地下水的爆发主要是通过正断层,本文的主要目标是研究正断层。如图1所示,断层倾角为。煤层中,煤层底板支承压力达到或超过底板岩层的临界强度时,在一定范围内的工作面底板岩体可能会出现破坏坏,产生脆性断裂,即零级破坏(王和帕克,2003年)。因此,重力和采矿引起的应力集中是零级破坏发生的基本因素,而断裂的最大深度取决于采工作面附近的塑性区的宽度,以及底板的岩层摩擦角。通过采用无限连续冲压负载的塑料滑移线定理,底板地层最大深度的断裂推导如下(王和帕克,2003年): 图1 地下水突出的关键途径图。在上述公式中, 为岩体的内摩擦角。Xa是在煤层的弯曲下沉的长度,它可以通过一些实际措施在现场获得。采矿发掘时,应力场的重新分布导致断层恢复活性,并可能进一步提高断层带的渗透性。因此地下水突出的一个潜在途径是建立在底板岩层之间的断层和破裂区。此外,有大量证据表明,断陷区的流体的通道是与倾角(a)和与断层有关的煤层下降(d)的变化联系起来的。断层在三个方面影响矿井地下水突出: (1)导水及断层存储功能; (2)断层缩短煤层和相关含水层之间的距离; (3)断层降低了岩体的强度。因此,断层倾角和煤层沿断层下降在地下水突出中发挥了重要作用,其中倾角直接决定阻水岩体T有效厚度,地层下降决定采矿引起的破裂带和含水层之间的距离d1。隔水岩体的有效厚度T是地下水通过断层突出的最短最危险的路径。如果含水层只有突破此区域,将容易形成断层突水通过。所以作者集中于区域岩体失稳的研究,并利用数值方法来研究区域岩体的失效条件,和静态应力分析的过程不同,应用数值方法可以模拟完整的压裂过程。这种破裂模型的建立技可以对地下水爆发过程提供有价值的的解释,那些不能或者很难在现场观察的可以考虑使用静态应力分析方法。这项研究的主要目标如下:1、对于不能直接观察到底板岩体与断层的影响的情况提供补充应力分布和断层引起的应力重新分布的资料。2、给地下水突出的全过程中应力区和渗流区的发展中破裂开始、传播、闭合一个解释。然后阐明底板岩层从阻水区域到地下水突出的变化。3、做一个关于含有断层并且结构不同的底板岩体阻水安全性进行评估。并具体说明断层结构的不同对于地下水突出的影响的案例研究。4.模型的建立和数值模拟4.1 数值模型基于断层结构、断层裂区域物理机理的复杂性的认识和断层附近的地下水突出的机理,在这个部分里用RFPA作的数值试验进行调查破裂的开始、断层活性的恢复和承压含水层以上的煤矿断层岩体力学模型中形成的地下水突出途径。在考虑断层结构面 (图2)的影响下,构建了一个概念力学模型。该模型离散成一个网状,其中包含360 240 = 86,400 个部分,几何上240米160米。在含水层的水压力为3兆帕。顶端边界上施加一个垂直压应力5.0兆帕( V )代表地面覆岩层的应力。正常的位移被限制在右侧、左侧和底部边界。所有的计算中队平面应变进行假设。在我们的模型采用的力学参数列于表1 。图2 含有断层的地下水突出模型的几何形状和载荷条件表1 模拟中的物理力学参数断层的形态值备注倾角 a/( )30岩层下沉量 d = 20 m45岩层下沉量 d = 20 m60岩层下沉量 d = 20 m75岩层下沉量 d = 20 m岩层下沉量 d/m10倾角 a = 45 30倾角 a = 45 40倾角 a = 45 50倾角 a = 45 为了研究底线谁突出中断层结构形态的影响,在保持其他因素不变的情况下,用模拟来说明断层结构的重要性。表2列出了这8个案例。 表2 模拟案例研究的文件岩层杨氏模量E0/GPa压力c0/MPa拉力t0/MPa内部凝聚力角/()泊松比 密度/(kgm-3)水压的传导率k0/(md-1)断层1.020.1270.402300100.0含水10.0151.5380.252500100.0煤层1.270.1300.3015000.1隔水层 (底板)5.0121.2350.2525000.14.2岩块强度减少的规则和安全系数的测定作为一个替代方法,强度降低的基本原则已被广泛采用有限元法进行地质或岩土工程有关的断层分析问题( 格里菲斯, 198
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