關(guān)于地下開采中斷層附近突水的數(shù)值研究外文文獻(xiàn)翻譯

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翻譯原文 Numerical investigation of groundwater outbursts near faults in underground coal mines Li Lianchonga, , , , Yang Tianhongb, Liang Zhengzhaoa, Zhu Wanchengb, Tang Chunana a School of Civil and Hydraulic Engineering, Dalian University of Technology, Dalian 116024, P.R. China b Center for Rock Instability and Seismicity Research, Northeastern University, Shenyang 110006, P.R. China Received 18 August 2010. Revised 13 December 2010. Accepted 13 December 2010. Available online 21 December 2010. Abstract Permeable 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 Highlights ?This 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 loading Keywords Groundwater outburst; Geological fault; Rock failure Process; Numerical simulation; Underground coalmines 1. Introduction When 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 method In 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 (σ3) exceeds the tensile strength (σt) of the element, that is σ3 ≥ σt. And an element is damaged in compression-shear mode when the shear stress satisfies the Mohr–Coulomb failure envelope : Herein, σ1and σ3 are the maximum and minimum principal stress, σcis the uniaxial compressive strength, and φ 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, α = Biots coefficient and εv = 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) (λ+G)μj,ji+Gμi,jj+fi+(αp),i=0 where λ = 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[?β(σii/3?p)] where the k0 = initial permeability of rock element, β = coupling coefficient, and σii/3 = average total stress. In this stage, it is assumed that 0 < α < 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, μl is the viscosity coefficient of the fluid (water), and g is the acceleration due to gravity. In this stage, it is assumed that α = 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 description Faults 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 α. 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, φ 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 simulation 4.1. Numerical model Based 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 (σv) 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 fault Table 1. Physico-m