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ABSTRACT: This paper describes the development of a 3-dimensional Discrete Fracture Network (DFN) approach for simulation and evaluation of hydraulic fracturing in low permeability fractured rock in the FracMan® reservoir analysis tool. The approach is based on an empirical algorithm approximating the effect of natural fractures and in-situ stress on hydraulic fracture propagation. The algorithm distributes frac-fluid between the propagating hydraulic fracture and pre-existing natural fractures to predict both the geometry of the hydraulic fracture, and the reactivation of the natural fracture network. The technique is demonstrated by comparison against ELFEN® geo-mechanical simulations, and by comparison of simulated and observed microseismic responses. 1. INTRODUCTION Hydraulic fracturing is increasingly critical for development of natural gas resources in tight sands and gas shales [1-8]. Hydraulic fracturing can be significantly influenced by the geometry and properties of pre-existing natural fractures. While the geometry of hydraulic fractures is driven primarily by the in situ stress field, rock mass anisotropy, and natural fractures in particular, can determine the details of hydrofrac location, size, and orientation. Hydraulic fracture size can be limited by leak-off to natural fractures, but can also be increased where the hydrofrac can extend by propagation of new and reactivation of natural fractures, rather than expending energy on intact rock breakage. This paper describes the development and verification of a discrete fracture network (DFN) approach for modeling the interaction between natural fractures and hydraulic fractures during the hydro fracturing process, see (Figure 1). 2. ASSUMED HYDRAULIC FRACTURE MECHANICS The propagation of hydraulic fractures is assumed to be controlled by: ? The reservoir in situ effective stress, defined by the total stress tensor and reservoir pressure. ? The rock matrix strength, deformability, heterogeneity and anisotropy. ? The geometry, mechanical, and flow properties of the natural fracture system. ? The configuration and operation of the hydraulic injection process itself. 2.1 In Situ Effective Stress Since hydraulic fracture propagation generally occurs in tension, the minimum principal stress determines both the direction and extent of the hydraulic fracture. In many tectonic settings, the vertical stress is the major principal stress, with the maximum and minimum horizontal stresses on the order of 60% or more of the lithostatic stress. Hydraulic fractures propagate where the effective normal stress on the plane of hydrofracturing is less than the tensile strength of the rock in that direction, reffered to as the “rock toughness” (Zobak, 2007). For sedimentary rocks such as shale, this toughness can be highly anisotropic, such that the direction of hydraulic fracture propagation can deviate from the normal to the normal to the direction of minimum stress (i.e, the direction of maximum horizontal stress), see (Warpinski, 1982). This same fracture fluid pressure propagates into the connected natural fracture system. Where the resulting effective minimum stress is less than the fracture toughness, natural fractures can open and extend in tension. Where the resulting effective stress state exceeds the shear strength, the natural fractures are reactivated, and can move and potentially propagate in shear.
- North America > United States > Texas (0.28)
- North America > United States > Kansas > Butler County (0.24)
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Geology > Rock Type > Sedimentary Rock > Clastic Rock > Mudrock > Shale (0.56)
- North America > United States > Texas > Haynesville Shale Formation (0.99)
- North America > United States > Louisiana > Haynesville Shale Formation (0.99)
- North America > United States > Arkansas > Haynesville Shale Formation (0.99)
ABSTRACT: The propagation of fracture in rock is associated with a process zone in the form of a localized region of damage. Digital image correlation was employed to observe the process zone based on the measurements of the surface displacement field. A Berea sandstone beam with a center notch was tested in three-point bending and a charge-coupled device camera was used to acquire digital images. The images were concentrated on the area surrounding the notch and were processed using a crosscorrelation algorithm based on the Fast Fourier Transform approach. The process zone was identified from the detailed measurements of the displacements related to the region surrounding the tip of the notch. In particular, the initiation of fracture was characterized by a region of localized damage. Furthermore, a traction-free part of the fracture was found from the measured displacement profile after sufficient propagation. 1. INTRODUCTION It is observed that a nonlinear process zone is developed near the fracture front for so-called quasi-brittle materials such as rock and concrete, which are materials that exhibit significant microcracking. The development of the nonlinear zone has a fundamental influence on the mechanical response of a structure at failure, such that the global behavior is related to characteristics of the zone within the structure [1, 2]. Thus, the investigation of the fracture process zone (FPZ) is key to understanding the failure response [3]. The FPZ in quasi-brittle materials is characterized by progressive damage with material softening, which is caused by microcracking, crack deflection, void formation, interface breakage, and other phenomena. The FPZ is sometimes separated into two parts: (1) a bridging zone and (2) a microcracking zone, but this separation is somewhat artificial. Crack bridging (ligament connection) is known to form at the crack tip, such that bridging is a result of the weak interface between unbroken crystals, aggregates, etc. The region of microcracks that are not connected following the bridging zone is considered the microcracking zone. The initiation and propagation of fracture in quasi-brittle materials are associated with the development of the FPZ, and the size is generally large enough such that it has to be considered in the study of structural response. Fig. 1 illustrates a conceptual representation of a fracture with a traction-free portion and the FPZ represented by a cohesive zone where the restraining traction acts on the separating surfaces [4]. The traction is due to the influence of crack tortuosity or unbroken ligaments, which may be viewed as a function of separation distance. 2. DIGITAL IMAGE CORRELATION (DIC)2.1. Introduction Conceptually, DIC is a particle tracking method that can be used to determine displacements of speckles in a digital image. One of the early papers on the use of a numerical based analysis of digital images to estimate the displacement field was published in 1982 [5]. The digital images of speckle patterns were related to reference and current configurations. The concept of small regions, referred to as subsets, was introduced in the comparison of the images.
- North America > United States > West Virginia (0.25)
- North America > United States > Pennsylvania (0.25)
- North America > United States > Ohio (0.25)
- North America > United States > Kentucky (0.25)
ABSTRACT: This paper explains the engineering Waste Injection (WI) assurance process and shows examples where its application successfully supports the development of major drilling projects where WI was a critical part of the operation. The methodology followed by the process gives the tools to manage responsible and safely a permanent disposal of the drilling waste while maintaining drilling pace and schedules. The process begins with a full feasibility study FEED where accurate geomechanical modeling is required to define the optimal operational conditions to achieve success during the injection, disposal formation, containment zones, capacity, and operational parameters are defined in the study. The next stage is the calibration of the model with a full injectivity test into the target formation. Once the injection operation begins, monitoring injection and decline pressures allows total control and verification of the injection parameters to maintain control of the waste disposal domain. Injection pressure analysis, during and post injection, determines how the fracture system is behaving as WI progresses. Recalculation of the main geomechanical formation parameters based on the formation pressure response needs to be conducted in order to obtain accurate forecast of the disposal domain and formation capacity. As conclusion, more than 28 MM bbls of waste have been successfully injected worldwide, reducing considerably the impact to the environment compared to conventional disposal options. 1. INTRODUCTION Waste Injection, WI, operations were initiated at the end of the 80's decade, when reduced volumes of oily waste combined with drilled cuttings where injected into the subsurface through tubular or annulus spaces. Some of these operations could be considered just like pumping jobs without the understanding of the real injection process neither the behavior nor response of the disposal formation. These procedures lead, in some instances, to non productive time events such as well plugging, disposal fracture screen outs, elevated operational pressures above equipment capabilities and in few cases even to more dramatic environmental impacts due to contamination caused by injected waste breaking to protected zones, i.e. aquifers or reaching to the sea bed. These events caused liabilities that generated negative economical effects, forcing organizations to create an engineering program that analyzes the overall process and helps understanding the subsurface waste injection and all its implications. Initially this program focused on monitoring the injection operation and defining the operational procedures required to guarantee the injectivity into the subsurface, nowadays the program has evolved to a fully systematic engineering assessment tool that uses the hydraulic fracturing theory to define the subsurface behavior of the fracture system created while pumping the waste into the selected disposal formation, the process is recognized in the E&P industry as the WI assurance process. 2. WASTE INJECTION PROCESS The search for new hydrocarbon resources has been pushing the exploration and drilling to new frontiers with extreme and sensitive conditions. Essentially WI was the response to the necessity of innovative technologies that allow E&P companies to drill in remote geographical areas under challenging conditions where traditional waste management techniques cannot be used.
ABSTRACT: The danger of rock slope failure has been recognised for a long time and is a constant concern in earthquake prone regions and where valleys are over-steepened by rapid tectonic uplift. In this paper, the so called combined finite-discrete element, FEMDEM, method is employed to model a simple planar failure. FEMDEM [1] modelling technology combines the multi-body particle interaction and motion modelling (i.e. Discrete Element Model, DEM) with the ability to model internal deformation of arbitrary shape (Finite Element Model, FEM). In this paper, the failure of a simple planar block in a 50 m high slope is simulated using FEMDEM. The 285 m2 basal plane is at 45 degrees and the Coulomb friction coefficient is set at 0.95 to initiate instability. Internal deformation of the block during accelerated sliding causes crack initiation and propagation. Two simulations of the sliding of a perched rock block are presented, the only difference in conditions being two different intact tensile strengths, 1MPa and 5MPa. They show a wide size distribution of boulders in the run-out material for the stronger 5MPa rock which is not observed in our simulations using rock with tensile strengths of 1 MPa. The size distribution of fragments and kinetic energy, two important parameters, for example in mining and quarrying, are analysed in this paper. 1. INTRODUCTION The danger of rock slope failure has been recognised for a long time and their effects have been well documented throughout history. The rock slope failure is a constant concern in earthquake-prone regions and where valleys are over-steepened by rapid tectonic uplift, e.g. recent earthquake triggered landslides in Wenchuan, China blocked roads; damaged and destroyed homes; locally disrupted rivers, etc. With increases in computing power, rock slope failure has been investigated numerically since the 1970s. Conventional numerical modelling methods can be divided into two categories: continuum and discontinuum methods. Continuum methods, e.g. finite element method and finite difference method, were mainly used in early numerical analyses of rock slopes [2-4]. Discontinuum methods have been rapidly developing since the first discontinuum method - discrete element method (DEM) was invented in 1979 [5]. In 1989, Shi and Goodman [6] invented discontinuous deformation analysis (DDA). These two methods were introduced to simulate the rock slope failure in recent years[7]. For example, Cleary [8, 9] used their classical spherical DEM and clustered non-spherical DEM codes to study the mechanism of rock avalanching. DDA was used to analyse the stability of a reservoir rock slope during a period in which reservoir water levels were varying [10]. Stead [11] pointed out that most of models which are purely continuum or purely discontinuum may not be adequate to simulate this phenomenon. A comprehensive Combined Finite-Discrete Element Method (FEMDEM) invented by Munjiza in the early 1990's has been used to deepen the understanding of rock slope failure mechanisms by Stead [7, 11]. Using a FEMDEM proprietary code, ELFEN (Rockfield), Stead [11] demonstrated that FEMDEM is able to simulate not only the failure initiation stage but also its progressive development.
- Asia > China (0.34)
- North America > United States (0.28)
ABSTRACT: This paper aims to quantify, through numerical simulations, the geometry of an axisymmetric sub-surface crack subject to a uniform pressure simulating fluid or gas pressure. Two algorithms based on the displacement discontinuity method are employed: one that relies on symmetry, as the fracture is discretized into ring elements, and another one in which the fracture surface is represented by flat, triangular elements. The fracture propagation is modeled by adding new elements to the existing fracture, with the inclination of a new element determined from the maximum tensile stress criterion. The numerical results are compared with available results of simulations of bowl-shaped fractures and laboratory experiments. 1. INTRODUCTION Near-surface gas blasting [1], pull-out tests of short anchor bolts [2], preconditioning of rock mass by hydraulic fracturing to enhance caving operations [5], remediation of contaminated soils [6], and formation of magma-driven sills [7] are examples of geo-engineering or natural processes that involve the propagation of a crack near a free surface. A particular feature of these processes is that the propagating fracture curves towards the free surface to eventually daylight. Modeling of the fracture path is important for predicting the fracture daylighting signature [3] and understanding the failure mechanisms [2]. This paper is concerned with the computation of the path of a sub-surface crack subjected to an axisymmetric loading with respect to the site of fracture initiation, see Fig. 1. Under these particular conditions, the crack may be visualized as “bowl”-shaped [3,8,1], whose curvature is essentially controlled, in the absence of surface loads, by the ratio ? of the far-field normal stress s parallel to the free surface to the characteristic stress KIcH-¹/ ², where KIc is the material toughness and H is the initial fracture depth [9] (Fig. 1). As confirmed in laboratory experiments [3,4], curving of the fracture towards the free surface is more pronounced with decreasing value of ? . This paper aims to quantify, through numerical simulations, the geometry of a sub-surface crack subject to a uniform pressure simulating fluid or gas pressure (Fig.1). The simulations are performed for the case of a zero far-field stress (s = 0), for which ? = 0. According to the experimental findings [3,4], the assumption of uniform pressure does not place a significant constraint on the propagation path and thus the results of the present simulations can be viewed as a good approximation for more general loadings. Two algorithms based on the displacement discontinuity method (DDM) [10,11] are used for the simulations; one invokes symmetry, the other is a full three-dimensional implementation of DDM. In the first case, the fracture is discretized into ring elements and in the second case, the fracture surface is represented by flat, triangular elements. The fracture propagation and curving is modeled by adding new elements to the existing fracture, with the inclination of a new element determined from the maximum tensile stress criterion. The numerical results are compared with available results of simulations of bowl-shaped fractures [1] and laboratory experiments [3,4].
Lab Test of Brittle Behavior of Marble Sampled At Great Depth of Jinping II Hydropower Project
Chu, W.J. (HydroChina Huadong Engineering Corporation) | Zhang, C.S. (HydroChina Huadong Engineering Corporation) | Li, L.Q. (HydroChina Huadong Engineering Corporation) | Zhu, H.C. (Itasca Consulting China Ltd.)
ABSTRACT: The Jinping II hydropower project consists of four headrace tunnels, the length of each tunnel approximately is 16.7 km with running parallel to each other and crossing the Jinping Mountain. The four headrace tunnels are designated 1 to 4, respectively. Tunnel 1 and 3 are being excavated by Drill & Blast methods (Section Diameter 13m) and Tunnels 2 and 4 are being mainly constructed by TBM (Section Diameter 12.4m). The average overburden above each tunnel is 2000m, the maximum overburden is 2500m. The main rock along the tunnels is Triassic marble. Laboratory results from uniaxial compression tests performed on Jinping marble samples indicate that a number of stages in the progressive failure process of rock can be identified through the combined analysis of strain measurement and acoustic emission (AE) data. Testing focused on identifying the crack initiation stress threshold sci and crack damage stress thresholds scd. Results from the testing showed that the properties of the acoustic events are markedly different before and after crack initiation. Testing results also showed that all of the marble samples are damaged during coring. 1. INTRODUCTION It has been well-documented that brittle failure of rock could be a result of the initialization and growth of micro-scale cracks. Understanding of crack initiation and propagation is fundamental for engineering practice in brittle rock under high stress condition. Crack initialization and propagation can be identified with critical indexes on a stress-strain curve obtained from lab test on specimen. Those indexed or stress thresholds are: ?sci: Crack initialization limit as the stress level corresponding to the onset of cracking; ?scd Damage strength or a stress level under which non-linear response starts as a result of accelerating growth of cracks; ? sf Peak strength which has been widely understood. A separated paper prepared by H.C. Zhu for this symposium describes briefly the Jinping II Hydropower project. This project consisting of seven tunnels in total is currently being driven in brittle marble mostly. Responses of brittle marble to excavation have been a great concern for driving the tunnels at such great depth. A comprehensive geomechanics study campaign has been launched, including on-site Mine-by test to investigate ground response while TBM with 12.4m in diameter is driving by and a series of lab tests of marble specimen to study the complete stress-strain response curve, brittle behavior, subcritical crack growth, and etc.. This paper presents the results of lab tests completed at the early stage of this campaign. 2. SELECTION OF METHOD DETERMINING DAMAGE THRESHOLDS A few approaches have been proposed to determine the values of rock parameters describing cracking behaviors. Generally damage thresholds of sci and scd can be estimated from a complete stress-strain curve, or identified by acoustic emission events recorded while compressing a specimen. However difficulty may be encountered in practice because of complex stress-strain behavior and cracking responses to loading. For instance, the existence of specimen damage prior to loading can be one of the reasons accounting for such complexity.
- Asia > China (0.69)
- North America (0.47)
ABSTRACT: Hydraulic fracturing in naturally fractured reservoirs can be significantly different from that in non-fractured reservoirs. New diagnostic tools have verified complicated network of stimulated fractures in naturally fractured reservoirs. Here, we are trying to give a physical explanation of erratic changes in the orientation of hydraulic fracture growth. When hydraulic fractures intersect sealed natural fractures, a kink is formed in the fracture path. These kinks are the locations of stress singularity which makes them a secondary choice for fracture propagation. Results demonstrate that fracture pattern complexity is strongly controlled by the magnitude of anisotropy of in situ stresses, rock toughness, fracture cement strength as well as geometry of natural fractures. 1. INTRODUCTION Large volumes of natural gas exist in tight fractured reservoirs. Hydraulic fracturing is the most effective stimulating technique to augment recovery from these fractured reservoirs[1]. Hydraulic fracturing is a common technique not just for enhancing hydrocarbon production but also geothermal energy extraction [2]. It is also widely used for other purposes like hazardous solid waste disposal [3], measurement of in-situ stresses [4], and fault reactivation in mining [5]. Hydraulic fractures which are naturally induced by pressurized fluid in the host rock are also observed in outcrops as joints [6] and veins [7], as well as magma-driven dikes [8]. During the last two decades, a gigantic amount of natural gas has been found in low-permeability fractured reservoirs around the world. Because of the low permeability of these formations and the low conductivity of the natural fracture networks, stimulation techniques such as hydraulic fracturing are necessary to make economic production possible. The low conductivity of the natural fracture system could be caused by occluding cements that precipitated during the diagenesis process [9]. The fact that natural fractures might be sealed by cements does not mean that they can be ignored while designing well completion processes. Cemented natural fractures can still act as weak paths for fracture growth. However, the presence of pre-existing natural fractures is not always advantageous [10]. New diagnostic tools developed during the last decade strongly demonstrate the existence of multiple fracture propagation or multi-stranded hydraulic fractures in naturally fractured reservoirs. Dynamic fracture mechanics indicates that only in cases where fracture propagation speed is comparable to the seismic velocity (more precisely, the Rayleigh wave speed) of the rock, crack tip branching (shown in Figure 1) will occur [11]. However, field data demonstrate that hydraulic fractures propagate at much less speeds than seismic wave speeds [12], so multi-branched fracturing should not exist in a homogeneous, isotropic, intact rock mass. For instance, hydraulic fracturing in the Minami-Nagoka gas field encountered premature screen-out [13]. Based on their report, only 20% of the designed volume of proppant was placed in the reservoir. Fracturing pressure analysis typically attributes screens-outs to either excessive fluid leakoff or insufficient fracture width. However, Sato et al. [14] did not observe excessive fluid leakoff. Additionally, the extremely high net pressure (4000 psi) would imply a relatively wide fracture if only a single-stranded hydraulic fracture is assumed.
- Research Report > New Finding (0.34)
- Research Report > Experimental Study (0.34)
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Geology > Rock Type > Sedimentary Rock > Clastic Rock > Sandstone (0.46)
- North America > United States > West Virginia > Appalachian Basin (0.99)
- North America > United States > Virginia > Appalachian Basin (0.99)
- North America > United States > Texas > Sabine Uplift > Carthage Cotton Valley Field > Cotton Valley Group Formation > Cotton Valley Sand Formation (0.99)
- (10 more...)
ABSTRACT: Stress and permeability variations around a wellbore and in the reservoir are of much interest in petroleum and geothermal reservoir development. Water injection causes large changes in pore pressure, temperature, and stress in hot reservoirs that in turn impact rock permeability. In this paper, two- and three-dimensional finite element methods are developed for thermoporo- mechanical coupled reservoir simulation with damage mechanics and stress dependent permeability. Convective heat transfer is considered to reflect the influence of increased fluid velocity in the damage phase of rock deformation. Damage mechanics is applied to capture the alteration of elastic modulus due to the crack initiation, micro-void growth and fracture propagation. Results show effective stress relaxation in the damage phase and its concentration at the interface between the damaged phase and the intact rock. The models presented promise to be effective tools for the analysis of stress induced micro-seismicity and fracture propagation in geothermal and petroleum reservoirs. 1. INTRODUCTION Water injection in geothermal reservoir involves coupled rock deformation and fluid flow as described in Biot's poroelastic theory [1]. Thermal and chemical effects can also be significant in this context [2]. The influence of fluid flow and temperature change around the wellbore on the stress variations in the reservoir can be described using thermo-poroelasticity. This influence is often computed based on a linear rock behavior without rock failure. The assumptions of linear elastic rock skeleton and constant permeability have limitations for use in predicting the real behavior of the reservoir rock. Generally, the strain-stress behavior of rocks in triaxial tests shows hardening and post-peak softening. This behavior depends on the rock type, pore pressure, stress conditions, and temperature. The continuum damage mechanics approach is one of the methods that can capture the hardening and softening behavior of the rock. The continuum damage mechanics was first introduced by Kachanov and since has been developed by many researchers [3-8] who have investigated inelastic behavior caused by crack initiation, micro-void growth, and fracture propagation. Also, the evolution of rock damage in the presence of poroelastic and thermoporoelastic effects has been considered. Selvadurai [8] studied influence of damage and permeability in porous rock. His results showed a significant permeability alteration caused by damage evolution in consolidation problems. Tang et al. proposed a damage and permeability model based on experimental strain-stress observations and permeability measurements [6-7]. The permeability variations induced by altered stress and rock failure has been studied by many researchers [e.g., 9-12], and relations have been suggested between permeability change and micro-crack and void evolution [12] showing that for granite permeability can increase by a factor of four. Other studies present different magnitudes for the increase in the level of permeability depending on rock type and experimental conditions. In this work, we present the development of finite element models to study the influence of thermo-poromechanical coupling on rock damage evolution and permeability variation. The damage model used corresponds to the brittle rock failure behavior with crack initiation, micro-void growth and permanent deformation prior to fracture.
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Geology > Rock Type > Sedimentary Rock > Clastic Rock (0.47)
Simulation of Rock Slope Failures With the Numerical Manifold Method
Ning, Y.J. (School of Civil and Environmental Engineering, Nanyang Technological University) | An, X.M. (School of Civil and Environmental Engineering, Nanyang Technological University) | Ma, G.W. (School of Civil and Environmental Engineering, Nanyang Technological University, School of Civil and Resource Engineering, University of Western Australia)
ABSTRACT: In this paper, the numerical manifold method has been extended to simulate the practical rock slope failure process. The Mohr-Coulomb criterion with a tensile cutoff is employed to predict the crack initiation and propagation. A cover-division strategy is adopted to realize the fracturing process. Compared with the traditionally adopted element-division strategy, the cover-division strategy can avoid the mesh dependency to some extent because of the non-local nature of the stress. Algorithms are implemented to treat the manifold elements, the physical covers and the loops during the fracturing process. The developed program has been calibrated through a Brazilian test, and then applied to simulate the progressive failure of rock slopes with non-persistent joints. Numerical results indicate that it is able to capture the fracturing in intact rock bridge and finally allow the kinematic release. The numerical manifold method enabling fracturing is promising for such problems and deserves to be further developed for more complex applications in the future. 1. INTRODUCTION Rock mass is a complex geological material which differs greatly from most other engineering materials because rock mass is often highly discontinuous. The presence of discontinuities in the rock mass, which usually appear in the form of faults, joints or bedding planes, is quite a challenge for numerical simulation. One of the main tasks of numerical modeling is to represent such discontinuities physically in the computer model. To simulate the mechanical behavior of rock mass such as the discontinuous deformation, crack propagation and fragmentation, as well as large-scale displacement is very challenging work. With the increasing needs to design and evaluate practical engineering structures, such as the rock tunnels and rock slopes, a wide spectrum of numerical approaches have been developed to account for various engineering investigations. The existing numerical methods fall into two main categorizes: continuum-based methods, such as the finite difference method (FDM), the finite element method (FEM), the boundary element method (BEM), various meshfree methods, etc, and the discontinuum-based methods, such as the distinct element method with typical codes UDEC in a two-dimensional setting and 3DEC in a three-dimensional setting, and the discontinuous deformation analysis (DDA) method. The continuum hypothesis in the continuum-based approaches is only valid as long as the characteristic length of the engineering problem is much larger than its representative volume. The characteristic length is defined by the smallest dimension of either the entire problem or a part of the problem. Then, it is necessary to introduce special elements into the conventional continuum-based approaches, namely modified continuous numerical model, to facilitate the ability of discontinuous deformation analysis. For example, various joint elements or interface element models [1-5] have been implemented into the FEM to account for the fractures. Despite these efforts, treatment of fractures and fracture growth remains limited in the continuum-based approaches. Block rotations, complete detachment and large-scale opening still cannot be properly treated. The number of discontinuities which can be dealt with is limited. The propagation of fractures requires remeshing, which complicates the simulation.
Crack Growth And Coalescence Mechanism In Granite Material Containing Two Surface Flaws Under Uniaxial Compression
Yin, P. (Civil and Structural Engineering Dept. of The Hong Kong Polytechnic University) | Wong, R.H.C. (Civil and Structural Engineering Dept. of The Hong Kong Polytechnic University) | Chau, K.T. (Civil and Structural Engineering Dept. of The Hong Kong Polytechnic University)
ABSTRACT: The main purpose of this study is to investigate the crack growth process and the coalescence mechanisms of two parallel pre-existing 3-D surface flaws under uniaxial compression in real rocks. In this study, the flaw angle, flaw length and the distance between two surface flaws (bridge length) are fixed. The bridge angle (the relative inclination between two flaws) is varied from 45º to 90º. Two observation systems (CCD camera and acoustic emission (AE) system) were used to study the propagation of cracks in the specimen. It was observed that petal cracks initiated along the interior surface of the flaw front. The propagation of the petal cracks is in three-dimensional curve shape towards to the surface of the specimen and extended to the bridge area. The coalescence mechanism depends on the bridge angle and bridge length. When the bridge angle is 90º with the bridge length equal to the flaw length, coalescence occurred. The coalescence crack is formed by the mixed mode of tensile cracks and petal cracks. But when the bridge angle is 45º with the same bridge length, no coalescence occurred. Further experimental study is required for coalescence mechanism of variables not fully investigated in the current study. 1. INTRODUCTION Natural discontinuities such as faults, joints or cracks are the geological fractures exist in the form of enechelon array in parallel discrete segments. A number of theoretical and experimental studies has been carried out by modelling the natural discontinues as penetrative (2-D) fractures (or flaws) to investigate its failure process and the coalescence mechanisms [1-5]. In nature, pre-existing fractures exist in threedimensional (3-D) type either fully embedding in rock mass (defined as 3-D internal flaws) or semiembedding in rock (defined as 3-D surface flaws). The crack growth and coalescence mechanisms of 3-D flaws are more complicated. A number of theoretical and experimental studies has been done by Dyskin and his co-authors [6-8] on 3-D internal flaws using both PMMA and cement material. They reported that unlike 2-D flaws, wing cracks (mode ?) sprouted from the two tips of the initial flaw and then wrapped around it. Wing cracks grew to about the length of the initial flaw and then stopped. The stress for wing crack initiation was affected by the angle of initial flaw where the shallow incline angle required lesser stress for wing crack initiation than that of the steeper angles. For the study of growth and interaction of multiple 3-D flaws, Dyskin et al [6] created specimens with two to several aligned inclined flaws. They reported that if the distance between two flaws was less than four times of the radii, a third large tensile fracture appeared suddenly and tended to split the sample. The results from Dyskin and his co-authors on the 3-D internal flaw are useful initial studies. They used three point bending method to produce a 3-D surface flaw. The created 3-D surface flaw extended into the specimen at about 73% of the thickness of the specimen (Fig. 1a).
- Geology > Geological Subdiscipline > Geomechanics (0.67)
- Geology > Rock Type > Igneous Rock > Granite (0.41)