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1.
This paper aims at a numerical study of coupled thermal, hydrological and mechanical processes in the excavation disturbed zones (EDZ) around nuclear waste emplacement drifts in fractured crystalline rocks. The study was conducted for two model domains close to an emplacement tunnel; (1) a near-field domain and (2) a smaller wall-block domain. Goodman element and weak element were used to represent the fractures in the rock mass and the rock matrix was represented as elasto-visco-plastic material. Mohr–Coulomb criterion and a non-associated plastic flow rule were adopted to consider the viscoplastic deformation in the EDZ. A relation between volumetric strain and permeability was established. Using a self-developed EPCA2D code, the elastic, elasto-plastic and creep analyses to study the evolution of stress and deformations, as well as failure and permeability evolution in the EDZ were conducted. Results indicate a strong impact of fractures, plastic deformation and time effects on the behavior of EDZ especially the evolution of permeability around the drift.  相似文献   

2.
The study of the creation and evolution of the excavation disturbed zone (EDZ) in argillaceous rocks is a major issue for the safety of nuclear wastes underground repositories. In this context, the argillaceous Tournemire site has provided a unique opportunity to study the evolution of the EDZ with time thanks to the existence of three openings of different ages. A thorough characterization of the EDZ has been conducted by different means such as visual observation, analysis of samples extracted from drilled boreholes, EDZ permeability measurements, etc. On the basis of these measurements, a conceptual model of the EDZ initiation and propagation at the Tournemire site has been proposed. In order to validate this model, numerical simulations of increasing complexity have been carried out. In a first attempt, the response of the rock mass to the excavation phase, followed by seasonal cyclic variations of temperature and relative humidity inside the opening, has been simulated by means of a purely mechanical analysis, using a simple elastic material model. The EDZ has been estimated by post-processing the calculated stress states, using a Mohr–Coulomb failure criterion. The results obtained show that no EDZ could be predicted unless adopting a low cohesion value for the rock mass. Moreover, the deferred nature of the EDZ formation in Tournemire could not be reproduced. These limitations have then been suppressed by using a coupled viscoplastic-damaging mechanical model, the parameters of which have been identified from different laboratory experiments. With this model, a time evolution of the EDZ could be predicted, but the EDZ pattern could not match the one observed in situ. Finally, in view of the importance of the hydraulic couplings, unsaturated hydro-mechanical calculations have been carried out to investigate the effect of the numerous seasonal variations cycles and the resulting shrinkage.  相似文献   

3.
The mechanical behavior of clay shales is of great interest in many branches of geo-engineering, including nuclear waste disposal, underground excavations, and deep well drilling. Observations from test galleries (Mont Terri, Switzerland and Bure, France) in these materials have shown that the rock mass response near the excavation is associated with brittle failure processes combined with bedding parallel shearing. To investigate the brittle failure characteristics of the Opalinus Clay recovered from the Mont Terri Underground Research Laboratory, a series of 19 unconfined uniaxial compression tests were performed utilizing servo-controlled testing procedures. All specimens were tested at their natural water content with loading approximately normal to the bedding. Acoustic emission (AE) measurements were utilized to help quantify stress levels associated with crack initiation and propagation. The unconfined compression strength of the tested specimens averaged 6.9 MPa. The crack initiation threshold occurred at approximately 30% of the rupture stress based on analyzing both the acoustic emission measurements and the stress–strain behavior. The crack damage threshold showed large variability and occurred at approximately 70% of the rupture stress.  相似文献   

4.
Summary  During the crossing of brittle rock formations at the L?tschberg base tunnel, failure phenomena have been observed both at the tunnel face and at the walls. A detailed analysis has been undertaken to explain these behaviours, based on the recent developments of Canadian research on brittle failure mechanisms. At the tunnel walls, a very good agreement is found between the calculated and observed damage and between two prediction methods, i.e. a semi-empirical failure criterion and elastic calculations with the “brittle Hoek-Brown parameters.” Near the face, due to the 3D nature of the stress conditions, some limitations of these approaches have been highlighted, and the growth of wall failure has been analysed. This research allowed a better understanding of the brittle rock mass behaviour at the L?tschberg base tunnel and showed that brittle failure processes dominate the behaviour of deep, highly stressed excavations in massive to moderately jointed rock. It also illustrates where improvements to the adopted approaches are required.  相似文献   

5.
Summary A series of numerical tests including both rock mechanics and fracture mechanics tests are conducted by the rock and tool (R–T2D) interaction code coupled with a heterogeneous masterial model to obtain the physical–mechanical properties and fracture toughness, as well as to simulate the crack initiation and propagation, and the fracture progressive process. The simulated results not only predict relatively accurate physical–mechanical parameters and fracture toughness, but also visually reproduce the fracture progressive process compared with the experimental and theoretical results. The detailed stress distribution and redistribution, crack nucleation and initiation, stable and unstable crack propagation, interaction and coalescence, and corresponding load–displacement curves can be proposed as benchmarks for experimental study and theoretical research on crack propagation. It is concluded that the heterogeneous material model is reasonable and the R–T2D code is stable, repeatable and a valuable numerical tool for research on the rock fracture process.  相似文献   

6.
硬脆性围岩在开挖完成后，其强度在高应力的影响下具有明显的时间效应，这导致围岩开挖损伤区的发展也呈现出与时间相关的特征。在岩石强度时效性演化模型的基础上，以锦屏二级水电站试验洞钻孔摄像、声波、变形监测等开挖损伤区实测结果为目标函数，采用正交设计方法、最小二乘支持向量机模型、粒子群优化算法等方法，建立了考虑时间效应的LSSVM-PSO智能反演分析方法，并以锦屏二级水电站试验洞为例，研究了开挖完成后的25 d里，围岩强度在高地应力条件下的时效性演化特征，进而获得这一时段内开挖损伤区扩展过程。研究结果表明：（1）高应力地区，隧洞开挖后，围岩损伤区的主要扩展方向受地应力控制，且最大扩展方向为最小主应力方向，且破坏区（破坏接近度FAI≥2）也集中于该方向； （2）开挖损伤区面积随时间近似呈S形曲线变化，表明开挖损伤区初始发展较为缓慢，随着时间推移呈现线性增加的趋势，最后又逐渐趋于稳定；（3）开挖后第3～10 d为开挖损伤区快速增长阶段。该研究成果对高应力地区硬脆性围岩开挖损伤区时效性演化研究具有指导意义。  相似文献   

7.
Summary  A micromechanics-based model, able to quantify the effect of various parameters on the complete stress–strain relationship, is described. The closed-form explicit expression for the complete stress–strain relationship of a rock material containing an echelon cracks arrangement subjected to compressive loading is obtained. The complete stress–strain relationship including the stages of linear elasticity, non-linear hardening and strain softening is established. The results show that the complete stress–strain relationship and the strength of rock with echelon cracks depend on the crack interface friction coefficient, the sliding crack spacing, the perpendicular distance between the two adjacent rows, the fracture toughness of rock material and orientation of the cracks. The present model is used to evaluate the complete stress–strain relationship and strength for crack-weakened rock at the underground cavern complex of the Ertan Hydroelectric Project. The predicted strength is in agreement with that obtained by the Hoek–Brown criterion. The numerical results obtained with the complete stress–strain relationship seem to be in good agreement with the measured values. Author’s address: Xiao-Ping Zhou, School of Civil Engineering, Chongqing University, 443002 Chongqing, P.R. China  相似文献   

8.
The brittle failure behavior of an over-consolidated clay shale (Opalinus Clay) in undrained rapid triaxial compression was studied. The confining stress levels were chosen to simulate the range of confining stresses relevant for underground excavations at the Mont Terri Underground Research Laboratory, and to investigate the transition from axial splitting failure to macroscopic shear failure. Micro-crack initiation was observed throughout the confining stress range utilized in this study at a differential stress of 2.1 MPa on average, which indicates that friction was not mobilized at this stage of brittle failure. The rupture stress was dependent on confinement indicating friction mobilization during the brittle failure process. With increasing confinement net volumetric strain decreased suggesting that dilation was suppressed, which is possibly related to a change in the failure mode. At confining stress levels ≤0.5 MPa specimen rupture was associated with axial splitting. With increasing confinement, transition to a macroscopic shearing mode was observed. Multi-stage triaxial tests consistently showed lower strengths than single-stage tests, demonstrating cumulative damage in the specimens. Both the Mohr–Coulomb and Hoek–Brown failure criteria could not satisfactorily fit the data over the entire confining stress range. A bi-linear or S-shaped failure criterion was found to satisfactorily fit the test data over the entire confinement range studied.  相似文献   

9.
This paper describes current knowledge about the nature of and potential for thermo–hydro–mechanical–chemical modelling of the excavation damaged zone (EDZ) around the excavations for an underground radioactive waste repository. In the first part of the paper, the disturbances associated with excavation are explained, together with reviews of Workshops that have been held on the subject. In the second part of the paper, the results of a DECOVALEX [DEmonstration of COupled models and their VALidation against EXperiment: research funded by an international consortium of radioactive waste regulators and implementers (http://www.decovalex.com)] research programme on modelling the EDZ are presented. Four research teams used four different models to simulate the complete stress–strain curve for Avro granite from the Swedish Äspö Hard Rock Laboratory. Subsequent research extended the work to computer simulation of the evolution of the repository using a ‘wall-block model’ and a ‘near-field model’. This included assessing the evolution of stress, failure and permeability and time-dependent effects during repository evolution. As discussed, all the computer models are well suited to sensitivity studies for evaluating the influence of their respective supporting parameters on the complete stress–strain curve for rock and for modelling the EDZ.  相似文献   

10.
Surface electrical and electromagnetic surveys were conducted on top of a solid waste facility in Unguwan Dosa, Kaduna State, Northwest Nigeria. The aim of the geophysical survey was to detect vertical and subvertical fractures that may provide pathways for groundwater and contaminant transport. Results from the 2D electrical resistivity imaging showed vertical and subvertical contacts overlain by 6–10 m thick overburden. Quantitative interpretation of the VLF-EM data correlates well with the results of the 2D resistivity imaging delineating the vertical and subvertical contacts as good and weak conductors (fractures zones) with resistivity values of 40–220 and 300–420 Ω m, respectively. Azimuthal Schlumberger VES measurements yield apparent anisotropy values ranging from 1.01 to 1.47 for electrode spacings of 1–45 m with the highest value recorded at spacing of 2 m. However, azimuthal variations at large spacings (30–45 m) showed no fracture anisotropy due possibly to the array’s low sensitivity to anisotropy at these spacings. The result of the study showed that pollutants in the leachate can reach and contaminate the groundwater. Therefore, urgency for leachate treatment at this site is recommended to prevent contamination of groundwater.  相似文献   

11.
Kinmen Island is a small, tectonically stable, granitic island that has been suffering from a scarcity of fresh water resources due to excessive annual evapotranspiration over annual precipitation. Recent studies further indicate that shallow (0–70 m) sedimentary aquifers, the major sources of groundwater supply, have already been over-exploited. Therefore, this preliminary study is to investigate the existence of exploitable water resources that can balance the shortage of fresh water on this island. Site characterization data are obtained from island-wide geophysical surveys as well as small-scale tests performed in a study area formed by three deep (maximum depth to 560 m) vertical boreholes installed in mid-east Kinmen northeast to Taiwu Mountain. Vertical fracture frequency data indicate that the rock body is fractured with a spatially correlated pattern, from which three major fracture zones (depths 0–70, 330–360, and below 450 m) can be identified. Geologic investigations indicate that the deepest fracture zone is caused by the large-scale, steeply dipping Taiwushan fault. This fault may have caused a laterally extensive low-resistivity zone, a potential fractured aquifer, near Taiwu Mountain. The middle fracture zone is induced by the Taiwushan fault and intersects the fault approximately 21 m southeast of the study area below a depth of 350 m. Slug testing results yield fracture transmissivity varying from 4.8 × 10−7 to 2.2 × 10−4 m2/s. Cross-hole tests have confirmed that hydraulic connectivity of the deeper rock body is controlled by the Taiwushan fault and the middle fracture zone. This connectivity may extend vertically to the sedimentary aquifers through high-angle joint sets. Despite the presence of a flow barrier formed by doleritic dike at about 300 m depth, the existence of fresh as well as meteoric water in the deeper rock body manifests that certain flow paths must exist through which the deeper fractured aquifers can be connected to the upper rock body. Therefore, groundwater stored within the Taiwushan fault and the associated low-resistivity zone can be considered as additional fresh water resources for future exploitation.  相似文献   

12.
The Opalinus Clay (OPA) is an argillaceous rock formation selected to host a deep geologic repository for high-level nuclear waste in Switzerland. It has been shown that the excavation damaged zone (EDZ) in this formation is heavily affected by the anisotropic mechanical response of the material related to the presence of bedding planes. In this context, the purpose of this study is twofold: (i) to illustrate the new developments that have been introduced into the combined finite-discrete element method (FEM/DEM) to model layered materials and (ii) to demonstrate the effectiveness of this new modelling approach in simulating the short-term mechanical response of OPA at the laboratory-scale. A transversely isotropic elastic constitutive law is implemented to account for the anisotropic elastic modulus, while a procedure to incorporate a distribution of preferentially oriented defects is devised to capture the anisotropic strength. Laboratory results of indirect tensile tests and uniaxial compression tests are used to calibrate the numerical model. Emergent strength and deformation properties, together with the simulated damage mechanisms, are shown to be in strong agreement with experimental observations. Subsequently, the calibrated model is validated by investigating the effect of confinement and the influence of the loading angle with respect to the specimen anisotropy. Simulated fracture patterns are discussed in the context of the theory of brittle rock failure and analyzed with reference to the EDZ formation mechanisms observed at the Mont Terri Underground Research Laboratory.  相似文献   

13.
The Hoek-Brown failure criterion constants m and s are equivalent rock friction and cohesion parameters, respectively. On the laboratory scale, m depends on the rock type and texture (grain size), while s = 1 for all rocks. On the field scale, m is a function of rock type, texture, and rock mass quality (geological strength index, GSI), while s is simply a function of rock mass quality. The brittle Hoek-Brown damage initiation criterion (m-zero criterion) is a modification to the conventional Hoek-Brown failure criterion with m = 0 and s = 0.11. The m-zero damage initiation criterion has been shown to better predict depths of failure in excavations in some moderate to massive (GSI ≥ 75) rock masses, but over predicts depths of failure in other rock types. It is now recognized that the Hoek-Brown brittle parameter (s) is not the same for all hard, strong, brittle, moderate to massive rock masses, but depends on the rock type. However, there are no guidelines for its determination for specific rock types. This paper presents a semi-empirical procedure for the determination of rock-type specific brittle Hoek-Brown parameter s from the rock texture, mineralogical composition, and microstructure. The paper also differentiates between brittle and tenuous rocks. It is shown that, while the use of the term ‘brittle’ is appropriate for rock mechanical excavation and mode of failure in weak rocks with limited deformability, it is inappropriate for use in explaining the difference in resistance to stress-induced damage in different rock types, and can cause confusion. The terms ‘tenacity/toughness’ are introduced to describe rock resistance to stress-induced damage in excavation performance assessment, and a rock tenacity/toughness rating system is presented.  相似文献   

14.
Liao  Jianxing  Gou  Yang  Feng  Wentao  Mehmood  Faisal  Xie  Yachen  Hou  Zhengmeng 《Acta Geotechnica》2020,15(2):279-295

Although hydraulic fracturing has been massively studied and applied as a key technique to enhance the gas production from tight formations, some problems and uncertainties exist to accurately predict and analyze the fracture behavior in complex reservoirs, especially in the naturally fractured reservoirs like shale reservoirs. This paper presents a full 3D numerical model (FLAC3D) to study hydraulic fracturing behavior under the impact of preexisting orthogonal natural fractures. In this numerical model, the hydraulic fracture propagation direction is assumed perpendicular to the minimum principal stress and activated only by tensile failure, whereas the preexisting natural fractures can be activated by tensile or shear failure or a combination of them, and only tensile failure can open the natural fracture as well. The newly developed model was used to study the impact of preexisting orthogonal natural fractures on hydraulic fracturing behavior, based on a multistage hydraulic fracturing operation in a naturally fractured reservoir from the Barnett Shale formation, northwest of Texas in USA. In this multistage operation, two more representative stages, i.e., stage 1 with a relatively large horizontal stress anisotropy of 3.3 MPa and stage 4 with a comparatively small one of 1.3 MPa, were selected to conduct the simulation. Based on the numerical results, one can observe that the interaction between hydraulic and natural fracture is driven mainly by induced stress around fracture tip. Besides, the horizontal stress anisotropy plays a key role in opening the natural fracture. Thus, no significant opened fracture is activated on natural fracture in stage 1, while in stage 4 an opened fracture invades to about 90 m into the first natural fracture. Conversely, the hydraulic fracture length in stage 1 is much longer than in stage 4, as some fluid volume is stored in the opened natural fracture in stage 4. In this work, the shear failure on natural fractures is treated as the main factor for inducing the seismic events. And the simulated seismic events, i.e., shear failure on natural fractures, are very comparable with the measured seismic events.

  相似文献   

15.
Late Alpine fissures and fractures in amphibolite-facies basement gneisses at Arvigo (Val Calanca, Swiss Alps) show distinct cm-sized reaction selvages parallel to the fracture walls that composed of subgreenschist facies assemblages produced by the interaction of water present in the fracture porosity with the old high-grade gneiss assemblages. The process of selvage or reaction-vein formation occurred in the brittle deformation regime and at temperatures characteristic of, first the prehnite-pumpellyite facies and then later of the zeolite facies. The vein formation occurred during uplift and cooling at very late stages of the Alpine orogeny. The reaction veins are composed of a selvage of altered gneiss on both sides of the central fracture and a central zone with fissure minerals that have been growing in the open fracture pore space. The central zone of the Arvigo veins contains an early assemblage with epidote, prehnite and chlorite and a late succession sequence of various species of zeolite. The veins of the Arvigo quarry are convincing evidence that fracture fluids in gneiss and granite have the potential to precipitate Ca–zeolite. This is an important find because many fluids recovered from deep continental drill holes and from geothermal energy exploration are found to be oversaturated in respect to a number of Ca–zeolite species. Vein formation during late uplift and cooling of the Alps occurred at continuously decreasing T and at hydrostatic pressure: (1) coexisting prehnite/epidote records temperatures of 330–380°C, (2) chlorite formation at temperature of 333 ± 32°C and (3) formation of zeolites <250°C. In the selvages the prime reaction is the replacement of plagioclase by albite along a sharp reaction front that separates the selvage from unaltered gneiss. In addition to albitisation, chloritisation of biotite is the second important reaction in the alteration process. The reactions release components for the formation of Ca–Al silicates. The water–rock interaction is associated with a depletion of Al, Si, Ca, Fe and K in the altered wall rock. The overall reaction is associated with an increase in porosity of up to 14.2 ± 2.2% in the selvage zone (altered wall rock), caused by the volume decrease during albitisation and the removal of biotite. The propagation of the sharp reaction front through the gneiss matrix occurred via a dissolution-reprecipitation mechanism. Zeolite formation is tied to the plagioclase alteration reaction in the rock matrix, which releases components for zeolite formation to a CO2-poor aqueous liquid.  相似文献   

16.
Stability Analysis and the Stabilisation of Flexural Toppling Failure   总被引：1，自引：0，他引：1  
Flexural toppling is a mode of failure that may occur in a wide range of layered rock strata in both rock slopes and large underground excavations. Whenever rock mass is composed of a set of parallel discontinuities dipping steeply against the excavated face plane, the rock mass will have the potential of flexural toppling failure as well. In such cases, the rock mass behaves like inclined superimposed cantilever beams that bend under their own weight while transferring the load to the underlying strata. If the bending stress exceeds the rock column’s tensile strength, flexural toppling failure will be initiated. Since the rock columns are “statically indeterminate,” thus, their factors of safety may not be determined solely by equations of equilibrium. The paper describes an analytical model with a sequence of inclined superimposed cantilever rock columns with a potential of flexural topping failure. The model is based on the principle of compatibility equations and leads to a new method by which the magnitudes and points of application of intercolumn forces are determined. On the basis of the proposed model, a safety factor for each rock column can be computed independently. Hence, every rock column will have a unique factor of safety. The least factor of safety that exists in any rock column is selected as the rock mass representative safety factor based on which simple equations are proposed for a conservative rock mass stability analysis and design. As a result, some new relations are established in order to design the length, cross-sectional area and pattern of fully grouted rock bolts for the stabilisation of such rock mass. Finally, the newly proposed equations are compared with the results of existing experimental flexural toppling failure models (base friction and centrifuge tests) for further verification.  相似文献   

17.
A discrete element model is proposed to examine rock strength and failure. The model is implemented by UDEC which is developed for this purpose. The material is represented as a collection of irregular-sized deformable particles interacting at their cohesive boundaries. The interface between two adjacent particles is viewed as a flexible contact whose stress–displacement law is assumed to control the material fracture and fragmentation process. To reproduce rock anisotropy, an innovative orthotropic cohesive law is developed for contact which allows the interfacial shear and tensile behaviours to be different from each other. The model is applied to a crystallized igneous rock and the individual and interactional effects of the microstructural parameters on the material compressive and tensile failure response are examined. A new methodical calibration process is also established. It is shown that the model successfully reproduces the rock mechanical behaviour quantitatively and qualitatively. Ultimately, the model is used to understand how and under what circumstances micro-tensile and micro-shear cracking mechanisms control the material failure at different loading paths.  相似文献   

18.
This simulation study shows how widely different model approaches can be adapted to model the evolution of the excavation disturbed zone (EDZ) around a heated nuclear waste emplacement drift in fractured rock. The study includes modeling of coupled thermal-hydrological-mechanical (THM) processes, with simplified consideration of chemical coupling in terms of time-dependent strength degradation or subcritical crack growth. The different model approaches applied in this study include boundary element, finite element, finite difference, particle mechanics, and elasto-plastic cellular automata methods. The simulation results indicate that thermally induced differential stresses near the top of the emplacement drift may cause progressive failure and permeability changes during the first 100 years (i.e., after emplacement and drift closure). Moreover, the results indicate that time-dependent mechanical changes may play only a small role during the first 100 years of increasing temperature and thermal stress, whereas such time-dependency is insignificant after peak temperature, because of decreasing thermal stress.  相似文献   

19.
In this paper, a quantitative procedure for evaluating the gravitational stress field in a hard rock massif is presented, using only a geological hammer and compass. Using the orientation of the fracture planes and their statistical distribution, a method for calculating of the tendency of the fractures to reactivate under gravitational load is proposed, based on Coulomb failure criterion. The method is applicable for assessing the most stable layout of the underground excavations and for evaluating the geometry of the stress field at a point during the initial stage of rock failure.  相似文献   

20.
层状岩体的非均质性及各向异性导致其破裂方式及规律与均质岩体有显著不同。对层状岩体分别进行不同方式的单轴、双轴、三轴试验, 分析应力-应变曲线特征; 再利用ANSYS有限元软件进行数值模拟, 观察应力、应变在岩体上的分布, 通过曲线和图件的对比分析, 并结合岩石破裂理论, 总结不同应力状态下层状岩体的破裂方式、顺序及规律; 最后以富台地区为例, 对分析结果进行验证。研究结果表明, 不同受力方式对层状岩体破裂的影响体现在施加的载荷及约束与层面的方位。当应力方向与岩层面平行时, 强度大的石灰岩岩体发生集中应力, 首先破裂; 而应力与岩层面垂直时, 强度小的泥岩岩体首先破裂。岩石试验、数值模拟结果以及实例均成功验证了这个规律。   相似文献   

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