Hybrid Finite-discrete Element Research Articles

Y-Mat: an improved hybrid finite-discrete element code for addressing geotechnical and geological engineering problems

PurposeContinua and discontinua coexist in natural rock materials. This paper aims to present an improved approach for addressing the mechanical response of rock masses based on the combined finite-discrete element method (FDEM) proposed by Munjiza.Design/methodology/approachSeveral algorithms have been programmed in the new approach. The algorithms include (1) a simpler and more efficient algorithm to calculate the contact force; (2) An algorithm for tangential contact force closer to the actual physical process; (3) a plastic yielding criterion (e.g. Mohr-Coulomb) to modify the elastic stress for fitting the mechanical behavior of elastoplastic materials; and (4) a complete code for the mechanical calculation to be implemented in Matrix Laboratory (MATLAB).FindingsThree case studies, including two standard laboratory experiments (uniaxial compression and Brazilian split test) and one engineering-scale anti-dip slop model, are presented to illustrate the feasibility of the Y-Mat code and its ability to deal with multi-scale rock mechanics problems. The results, including the progressive failure process, failure mode and trajectory of each case, are acceptable compared to other corresponding studies. It is shown that, the code is capable of modeling geotechnical and geological engineering problems.Originality/valueThis article gives an improved FDEM-based numerical calculation code. And, feasibility of the code is verified through three cases. It can effectively solve the geotechnical and geological engineering problems.

Hybrid Finite-Discrete Element Modelling of Various Rock Fracture Modes during Three Conventional Bending Tests

The numerical techniques for modelling the rock fracture have been briefly reviewed. A hybrid finite-discrete element method (HFDEM) is proposed to simulate various fracture types of rock. A fracture model is implemented in the HFDEM for simulation of the three main fracture types. In addition, the influence of the strain rate is considered during the HFDEM modelling rock behavior. Then, two typical rock mechanism tests are employed to calibrate the HFDEM. The proposed method has well modelled the rock fracture processes and can obtain reasonable stress distribution and force–displacement curves. After that, the HFDEM is used to model three convention bending tests. The obtained rock fracture processes indicates that the HFDEM can simulate various fracture types. The obtained rock strengths and fracture toughness indicate that the HFDEM can reflect the influence of the strain rate. It is concluded that the HFDEM can model the entire and complete rock fracture process during the three convention bending tests, and it also can capture the rock’s behavior on the strain rate.

Hydro-mechanical modelling of gas transport in clayey host rocks for nuclear waste repositories

Host rocks, as the final impediment to waste migration, play a significant role in the nuclear waste repositories. Modelling of gas migration in saturated host rocks as well as its coupled hydro-mechanical (HM) behaviors is of importance to the assessment of geological disposal facilities. A comprehensive literature review is carried out on the models for simulating gas migration in saturated rock materials. Several aspects have been provided and discussed, including the material properties and experimental interpretations, governing equations, constitutive models for hydraulic and mechanical processes, fracture propagation models. Specifically, these models are discussed in detail with respect to their performance in describing the recorded experimental behaviors. It is found that the visco-capillary two phase flow model with enriched intrinsic permeability is commonly used to describe the advective gas transport in saturated host rocks. The embedded fracture model (EFM) or enriched EFM seems to be the most favored model as it accounts for the fracturing mechanism which is more representative to implicitly simulate the preferential gas pathways. To describe the mechanical behavior of rocks during the gas migration process, linear/nonlinear elastic, elastoplastic and damage models have been included in the mechanical processes. The HM models incorporating plasticity or damage may not be applicable in most experimental studies, which are not competent to simulate the key experimental behavior associated with the development of gas preferential pathways. Current models that can explicitly describe the gas induced micro-fracturing are seldomly reported, the existed ones are not able to represent all the key experimental behaviors related to preferential gas flow. Advanced approaches, i.e., phase field (PF) method, extended finite element method (XFEM), discrete element method (DEM), hybrid finite discrete element method (FDEM) can be integrated with cohesive zone model (CZM) to provide some promising aspects in representing the development of preferential gas pathways. Lastly, the conclusions and recommendations for future modelling are given.

Development and application of a three-dimensional GPGPU-parallelized FDEM for modelling rock fragmentation by blast

Numerical modelling of rock fragmentation by blast is an important step to the optimum fragmentation. Combined finite-discrete element method (FDEM) has recently been proven to be one of the most promising methods for modelling the fracture and fragmentation of rocks under various loading conditions including rock blasting. However, almost all FDEM modellings of rock blasts are conducted in two-dimension. Correspondingly, this study implements a three-dimensional (3D) hybrid finite-discrete element method (HFDEM) parallelized on the basis of the general-purpose graphic-processing-unit (GPGPU) to model the fracture and fragmentation process of rock by blast. The main components of HFDEM3D include robust fracturing algorithm, efficient contact activation approach and various implementations of gas and rock interactions, which are firstly briefly reviewed here and introduced in detail in authors’ various former publications. The GPGPU-parallelized HFDEM3D is then applied to model the rock fracture and fragmentation process by the detonation of a single borehole and simultaneous and consecutive detonations of multiple boreholes.

Experimental and numerical studies on failure behaviours of sandstones subject to freeze-thaw cycles

The freeze-thaw induced damage of rock affects the durability and serviceability of geo-structures, especially those constructed in the regions frequently impacted by climatic changes. A series of laboratory tests including P-wave velocity tests, freeze-thaw tests, uniaxial compression strength (UCS) tests and Brazilian tensile strength (BTS) tests are conducted to investigate the physical-mechanical properties and failure behaviours of Tasmanian sandstones subjected to various freeze-thaw cycles. It is observed that the P-wave velocity, BTS and UCS of the sandstone decrease as the number of freeze-thaw cycles increases, in which the decreasing rate from 0 to 20 freeze-thaw cycles was more pronounced than that from 20 to 40 and 40 to 60 freeze-thaw cycles. Moreover, it is found that the main failure mode of the sandstone changes from axial splitting to shearing along a single plane in the UCS tests and from central smooth fractures to a central zigzag fracture in the BTS tests with the number of freeze-thaw cycles increasing. Three-dimensional (3D) numerical modellings are then conducted using a self-developed 3D hybrid finite-discrete element method (HFDEM) parallelized on the basis of general-purpose graphic processing units (GPGPU) to further investigate the failure mechanisms of Tasmanian sandstones subjected to various freeze-thaw cycles in the UCS and BTS tests. The 3D numerical modellings agree very well with the experimental observations that the physical-mechanical parameters of the sandstone degrade with the increasing number of the freeze-thaw cycles. Moreover, the 3D numerical modellings reveal the deterioration and failure mechanisms of sandstones subjected to various freeze-thaw cycles. For the sandstone specimens without subjecting to freeze-thaw cycles, axial splitting is the main failure pattern while tensile and mixed-mode damages are the dominant failure mechanism in the UCS tests. For the sandstone subjecting to various freeze-thaw cycles, the increasing number of freeze-thaw cycles causes the macroscopic cracks to propagate, interact and coalesce in the shear behaviour resulting in the final shear fracture pattern in the UCS test. The 3D numerical modellings of the BTS test show that, although, for both the models with and without subjecting to freezing and thawing cycles, a central fracture is the eventual failure pattern, the failure surface becomes more zigzag as the number of freeze-thaw cycles increases.

Hybrid finite–discrete element modelling of rock fracture process in intact and notched Brazilian disc tests

A hybrid finite–discrete element method is proposed to model the rock fracture behaviour under various loading rates. Three fracture models are proposed to predict the fracture initiation and propagation for modelling the transition from continuum to discontinuum. The modelling transition from continuum to discontinuum makes the hybrid method superior to the traditional continuum-based finite element method and discontinuum-based discrete element method. Moreover, the hybrid method considers the effect of the loading rate by implementing an empirical relationship between the static strengths and the dynamic strengths derived from the dynamic rock fracture experiments. Then, the Brazilian tensile strength tests are modelled to calibrate the proposed method under various loading rate, and demonstrate its ability in modelling the dynamic rock behaviours. The Notched Brazilian Disc tests are modelled to illustrate the capabilities of the proposed method in modelling different fracture modes. The hybrid finite element method has well modelled the stress propagation, fracture initiation and propagation, even the pure mode I, pure mode II and mixed-mode I–II fractures. It is concluded that the hybrid finite element method is superior to the continuum-based finite element method and discontinuum-based discrete element method in modelling the fracture behaviours of rock under various loading rates.

Modeling concrete fracturing using a hybrid finite-discrete element method

The hybrid Finite-Discrete Element (FDEM) approach combines aspects of both finite elements and discrete elements with fracture mechanics principles, and therefore it is well suited for realistic simulation of quasi-brittle materials. Notwithstanding, in the literature its application for the analysis of concrete is rather limited. In this paper, the proprietary FDEM code ELFEN is used to model concrete specimens under uniaxial compression and indirect tension (Brazilian tests) of different sizes. The results show that phenomena such as size effect and influence of strain-rate are captured using this modeling technique. In addition, a preliminary model of a slab subjected to dynamic shear punching due to progressive collapse is presented. The resulting fracturing pattern of the impacted slab is similar to observations from actual collapse.

Simulating tunnel support integrity using FEM and FDEM based on laboratory test data

The West Vaughan Sewer System (WVSS) tunnel, at the time of writing, is planned to be constructed in Toronto, Ontario, Canada. A geotechnical program was conducted to characterize the mechanical properties of the anisotropic bedrock, Georgian Bay shale, and was determined to exhibit weak rock strength properties, moderate to fair anisotropy, and extremely low abrasivity. Following characterization, the shale was numerically calibrated using the hybrid finite-discrete element method (FDEM) and used to qualitatively simulate the WVSS tunnel. The major advantage of FDEM is its ability to explicitly simulate fracturing in geomaterials allowing for the excavation damaged zone (EDZ) to be estimated. In addition, anisotropic elastic and fracture models were used to appropriately simulate shale. The objective of the simulations is to explicitly compare the difference between the finite element method (FEM) and FDEM in estimating the deformation and stress in the rock mass and support liner. In FDEM simulations where the virgin stress conditions did not cause the rock to fracture, the FEM and FDEM results are very similar. However, in simulations where the rock is fractured, predominantly consisting of bedding plane slippage due to the orientation of the maximum in-situ stress relative to the strata, several key differences are evident. The liner simulated in FDEM has an asymmetric distribution of stress and strain, liner deformation is approximately 60% greater, stresses can be up to four times larger, and the stresses in the liner are purely compressive.

An Investigation of the Factors Controlling the Mechanical Behaviour of Slender Naturally Fractured Pillars

Previous studies on the strength and deformability of pillars in hard-rock mines have demonstrated that the influence of natural fractures diminishes with increasing pillar width. Jointing conditions and fracture intensity/intersections play a critical role in the behaviour of slender pillars (width-to-height ratio less than 0.5). An important challenge is the characterization of rock mass damage as a function of the natural fracture network and loading conditions. In this paper, we present advanced pre-processing methodologies to measure fracture intensity and fracture density prior to loading and relate those measures to the evolution of damage (intensity and density) with increasing pillar load. A hybrid finite-discrete element method (FDEM) with fracture mechanics capabilities is used to simulate the mechanical behaviour of synthetic pillar models with different embedded discrete fracture traces of similar total length. Newly developed post-processing methodologies are used to capture the location of damage events, track rock mass damage intensity and density as the simulation progresses and relate those changes to the modelled pillar strength. We believe the results of this study provide useful insights on rock mass damage evolution.

Numerical study on abrasive machining of rock using FDEM method

Rock machining has been widely used in many industries. In order to better understand the fracture and removal mechanism for rock materials during abrasive machining, a two-dimensional scratch model was developed using the hybrid finite-discrete element method (FDEM). The crack propagation during the abrasive machining process was simulated by inserting zero-thickness cohesive elements. Based on the model, the rock fragment, cutting forces as well as damage layer under different cutting parameters and abrasive angle were investigated by a simulation study. It was found that the fracture of cohesive elements was dominated by tensile loading (mode Ⅰ) at low cutting speed and cutting depth. With the increase of cutting depth and cutting speed, shear loading (mode Ⅱ) was responsible for the fracture mode of the cohesive elements in the primary chipping zone during abrasive machining of rock materials. Both the cutting forces and the thickness of damage layer increased with the increase of cutting speed and cutting depth, which is consistent with the simulation results obtained by finite element method (FEM) and discrete element method (DEM). The inertia effect in dynamic loadings was believed to be the main reason for the increase of cutting forces at higher cutting speeds. The results demonstrated the feasibility and reliability of FDEM in rock machining simulation.

Geometry nature of hydraulic fracture propagation from oriented perforations and implications for directional hydraulic fracturing

Abstract In this study, hydromechanical hybrid finite-discrete element method (FDEM) models were employed to investigate hydraulic fracturing from oriented perforations at the microscale. Numerical calibrations were first used to obtain the microproperties that can represent the realistic behavior of sandstone. The fracture morphology and breakdown pressure obtained from the numerical hydraulic fracturing show reasonable agreements with the experimental results, indicating that the numerical results are convincing. Then, this method was applied to investigate the effects of the differential stress, perforation angle, perforation length, and injection rate on both the geometry nature of the hydraulic fractures (HFs) and the breakdown pressure. The perforation orientation, differential stress, and injection rate are found to strongly affect the breakdown pressure and HF geometry. However, the perforation length shows a weaker effect, especially when the perforation length is larger than the wellbore diameter. Furthermore, small-scale simulations were performed to investigate the formation of connected fractures developed from multiple perforated wellbores under the concept of directional hydraulic fracturing (DHF). The different fracture propagations and geometries in the DHF model indicate that conclusions from studying hydraulic fracturing from a single perforated wellbore may provide a limited reference for hydraulic fracturing from multiple perforated wellbores. The stress shadow effects during HF interactions were identified as the primary factor that contributed to these differences. Numerical results also provide new information on the roles of several factors (differential stress, injection rate, perforation orientation, and injection sequence) in DHF. DHF could take advantage of stress shadow effects by optimizing related factors. Based on the numerical results, some implications on the design of DHF, from small-scale simulations to field-scale applications, are discussed.

Interactive roles of geometrical distribution and geomechanical deformation of fracture networks in fluid flow through fractured geological media

In this study, the combined effects of geometrical distribution and geomechanical deformation of fracture networks on fluid flow through fractured geological media are investigated numerically. We consider a finite-sized model domain in which the geometry of fracture systems follows a power-law length scaling. The geomechanical response of the fractured rock is simulated using a hybrid finite-discrete element model, which can capture the deformation of intact rocks, the interaction of matrix blocks, the displacement of discrete fractures and the propagation of new cracks. Under far-field stress loading, the locally variable stress distribution in the fractured rock leads to a stress-dependent variable aperture field controlled by compression-induced closure and shear-induced dilatancy of rough fractures. The equivalent permeability of the deformed fractured rock is calculated by solving for the fracture-matrix flow considering the cubic relationship between fracture aperture and flow rate at each local fracture segment. We report that the geometrical connectivity of fracture networks plays a critical role in the hydromechanical processes in fractured rocks. A well-connected fracture system under a high stress ratio condition exhibits intense frictional sliding and large fracture dilation/opening, leading to greater rock mass permeability. However, a disconnected fracture network accommodates much less fracture shearing and opening, and has much lower bulk permeability. We further propose an analytical solution for the relationship between the equivalent permeability of fractured rocks and the connectivity metric (i.e. percolation parameter) of fracture networks, which yields an excellent match to the numerical results. We infer that fluid flow through a well-connected system is governed by traversing channels (forming an “in parallel” architecture) and thus equivalent permeability is sensitive to stress loading (due to stress-dependent fracture permeability), whilst fluid flow through a disconnected system is more ruled by matrix (linking isolated clusters “in series”) and has much less stress dependency.

Aspects of the hybrid finite discrete element simulation technology in science and engineering

Aspects of the hybrid finite discrete element simulation technology in science and engineering

GPGPU-parallelised hybrid finite-discrete element modelling of rock chipping and fragmentation process in mechanical cutting

Mechanical cutting provides one of the most flexible and environmentally friendly excavation methods. It has attracted numerous efforts to model the rock chipping and fragmentation process, especially using the explicit finite element method (FEM) and bonded particle model (BPM), in order to improve cutting efficiency. This study investigates the application of a general-purpose graphic-processing-unit parallelised hybrid finite-discrete element method (FDEM) which enjoys the advantages of both explicit FEM and BPM, in modelling the rock chipping and fragmentation process in the rock scratch test of mechanical rock cutting. The input parameters of FDEM are determined through a calibration procedure of modelling conventional Brazilian tensile and uniaxial compressive tests of limestone. A series of scratch tests with various cutting velocities, cutter rake angles and cutting depths is then modelled using FDEM with calibrated input parameters. A few cycles of cutter/rock interactions, including their engagement and detachment process, are modelled for each case, which is conducted for the first time to the best knowledge of the authors, thanks to the general purpose graphic processing units (GPGPU) parallelisation. The failure mechanism, cutting force, chipping morphology and effect of various factors on them are discussed on the basis of the modelled results. Finally, it is concluded that GPGPU-parallelised FDEM provides a powerful tool to further study rock cutting and improve cutting efficiencies since it can explicitly capture different fracture mechanisms contributing to the rock chipping as well as chip formation and the separation process in mechanical cutting. Moreover, it is concluded that chipping is mostly owed to the mix-mode I-II fracture in all cases although mode II cracks and mode I cracks are the dominant failures in rock cutting with shallow and deep cutting depths, respectively. The chip morphology is found to be a function of cutter velocity, cutting depth and cutter rake angle.

Hybrid finite-discrete element modelling of rock fracture during conventional compressive and tensile strength tests under quasi-static and dynamic loading conditions

A hybrid finite-discrete element method is proposed to model the rock fracture under various loading conditions. The key component of the hybrid method, i.e. transition from continuum to discontinuum through fracture and fragmentation, is introduced in detail. An empirical relationship between the static strengths and the dynamic strengths derived from the dynamic rock fracture experiments is implemented in the hybrid method to model the effect of loading rate. The hybrid method is calibrated by modelling the Brazilian tensile strength (BTS) test, uniaxial compressive strength (UCS) and notched Brazilian dis tests. Then the hybrid method is employed to model the dynamic rock fracture process in UCS and BTS tests. The proposed method has well modelled the dynamic rock fracture and fragmentation processes and captured the effect of loading rate on rock strengths. It is concluded that the hybrid finite-discrete element is a valuable tool to study the dynamic rock fracture as it takes the advantages of the continuum and discontinuum based method, and considers the effect of loading rate.

The Role of In Situ Stress in Organizing Flow Pathways in Natural Fracture Networks at the Percolation Threshold

We investigated the effect of in situ stresses on fluid flow in a natural fracture network. The fracture network model is based on an actual critically connected (i.e., close to the percolation threshold) fracture pattern mapped from a field outcrop. We derive stress-dependent fracture aperture fields using a hybrid finite-discrete element method. We analyze the changes of aperture distribution and fluid flow field with variations of in situ stress orientation and magnitude. Our simulations show that an isotropic stress loading tends to reduce fracture apertures and suppress fluid flow, resulting in a decrease of equivalent permeability of the fractured rock. Anisotropic stresses may cause a significant amount of sliding of fracture walls accompanied with shear-induced dilation along some preferentially oriented fractures, resulting in enhanced flow heterogeneity and channelization. When the differential stress is further elevated, fracture propagation becomes prevailing and creates some new flow paths via linking preexisting natural fractures, which attempts to increase the bulk permeability but attenuates the flow channelization. Comparing to the shear-induced dilation effect, it appears that the propagation of new cracks leads to a more prominent permeability enhancement for the natural fracture system. The results have particularly important implications for predicting the hydraulic responses of fractured rocks to in situ stress fields and may provide useful guidance for the strategy design of geofluid production from naturally fractured reservoirs.

The Performance of Axially Loaded, Fully Grouted Rock Bolts Based on Pull-Out Experiments Utilizing Fiber Optics Technology and Associated Numerical Modelling of Such Support Elements

Rock bolt is a support system extensively utilized in Civil and Mining Engineering applications, especially for underground excavations projects. The main function of a rock bolt is to stabilize the rock mass around the opening of an excavation by fastening to further stable formations. Previous technical work developed pull-out testing in order to improve understanding regarding the mechanical response within this system. However, due to limitations with conventional laboratory methods of capturing strain, there still exists a lack of understanding in this mechanical response at the micro-scale; more specifically, in the detailed strain profile distribution along fully grouted rock bolts. Aiming to address this knowledge gap, two research programmes were conducted and will be addressed in this paper: numerical modelling of pull-out tests through finite element method (FEM) and hybrid finite-discrete element method (FDEM), and laboratory pull-out tests on concrete samples utilizing a newly developed distributed optical strain sensing technology that provides a spatial resolution of 0.65 mm in order to capture the strain along the rock bolt. Experimental and modelling results were compared to evaluate the numerical approaches feasibility and their correspondence with the experiments. FDEM modelling demonstrated ability in capturing general mechanical behavior, while the FEM software seems to be a good option to be utilized as a design tool, yet improvements to the interaction model are necessary to achieve a better calibration with experimental results.

Development of a 3D Hybrid Finite-Discrete Element Simulator Based on GPGPU-Parallelized Computation for Modelling Rock Fracturing Under Quasi-Static and Dynamic Loading Conditions

As a state-of-the-art computational method for simulating rock fracturing and fragmentation, the combined finite-discrete element method (FDEM) has become widely accepted since Munjiza (2004) published his comprehensive book of FDEM. This study developed a general-purpose graphic-processing-unit (GPGPU)-parallelized FDEM using the compute unified device architecture C/C ++ based on the authors’ former sequential two-dimensional (2D) and three-dimensional (3D) Y-HFDEM IDE (integrated development environment) code. The theory and algorithm of the GPGPU-parallelized 3D Y-HFDEM IDE code are first introduced by focusing on the implementation of the contact detection algorithm, which is different from that in the sequential code, contact damping and contact friction. 3D modelling of the failure process of limestone under quasi-static loading conditions in uniaxial compressive strength (UCS) tests and Brazilian tensile strength (BTS) tests are then conducted using the GPGPU-parallelized 3D Y-HFDEM IDE code. The 3D FDEM modelling results show that mixed-mode I–II failures are the dominant failure mechanisms along the shear and splitting failure planes in the UCS and BTS models, respectively, with unstructured meshes. Pure mode I splitting failure planes and pure mode II shear failure planes are only possible in the UCS and BTS models, respectively, with structured meshes. Subsequently, 3D modelling of the dynamic fracturing of marble in dynamic Brazilian tests with a split Hopkinson pressure bar (SHPB) apparatus is conducted using the GPGPU-parallelized 3D HFDEM IDE code considering the entire SHPB testing system. The modelled failure process, final fracture pattern and time histories of the dynamic compressive wave, reflective tensile wave and transmitted compressive wave are compared quantitatively and qualitatively with those from experiments, and good agreements are achieved between them. The computing performance analysis shows the GPGPU-parallelized 3D HFDEM IDE code is 284 times faster than its sequential version and can achieve the computational complexity of O(N). The results demonstrate that the GPGPU-parallelized 3D Y-HFDEM IDE code is a valuable and powerful numerical tool for investigating rock fracturing under quasi-static and dynamic loading conditions in rock engineering applications although very fine elements with maximum element size no bigger than the length of the fracture process zone must be used in the area where fracturing process is modelled.

Mechanical analysis and interpretation of excavation damage zone formation around deep tunnels within massive rock masses using hybrid finite–discrete element approach: case of Atomic Energy of Canada Limited (AECL) Underground Research Laboratory (URL) test tunnel

The construction of an underground opening leads to changes in the in situ stress regime surrounding the excavation. The opening influences the rock mass owing to the redistribution of the stresses and results in the disturbance of the surrounding ground. At great depths, massive to slightly or moderately fractured rock masses are usually encountered, and under high stresses, they are more likely to behave in a brittle manner during an excavation. While constitutive models have been developed and proposed for the numerical simulation of such excavations using continuum mechanics, this brittle response cannot be simulated accurately enough, since the material behaviour is governed by fracture initiation and propagation. On the contrary, discontinuum approaches are more suitable in such cases. For the purposes of this paper, the brittle behaviour of hard, massive rock masses and the associated spalling failure mechanisms were simulated by employing a finite–discrete element method (FDEM) approach using Irazu software. The generated numerical model was utilized to replicate field conditions based on the observations at the Atomic Energy of Canada Limited (AECL) Underground Research Laboratory (URL) test tunnel located in Pinawa, Manitoba, Canada. The model results are compared with field observation data to explicitly demonstrate the suitability of the method.

A numerical study of stress variability in heterogeneous fractured rocks

We conduct numerical simulations to investigate the variability of local stresses in heterogeneous fractured rocks subjected to different far-field stress conditions. A realistic fracture network is constructed based on a real outcrop mapped at the Hornelen Basin in Norway. The heterogeneity of the rock material is modelled using a Weibull distribution of Young's modulus characterised by a homogeneity index m. As m decreases, the rock material becomes less homogeneous. The distribution of local stresses in the fractured rock under far-field stress loading is derived from a hybrid finite-discrete element model, and the stress variability is further analysed using a tensor-based formalism that faithfully honours the tensorial nature of stress data. The local stress perturbation is quantified using the Euclidean distance of each local stress tensor to the mean stress tensor, and the overall stress dispersion is measured using the effective variance of the entire stress tensor field. We show that the local stress field is significantly perturbed when the far-field stresses are associated with a high stress ratio and imposed at a critical direction in favour of intense sliding along preferentially-oriented fractures. The strong correlation between fracture sliding and local stress variability are further revealed from a scanline sampling analysis through the domain. Furthermore, larger perturbation of local stresses can be induced as the inhomogeneity of the rock materials increases (i.e. m decreases). Whether the stress field is dominated by fractures or matrix depends on the far-field stress state, material inhomogeneity, and fracture properties. If the rock material is highly heterogeneous, stress variability is controlled by the matrix when the far-field stress ratio is low; however, the stress distribution becomes more affected by fractures as the stress ratio increases. If the rock material is more homogeneous, the system tends to be more dominated by fractures even under a relatively low stress ratio.