Synthetic Rock Mass Research Articles

A 3D synthetic rock mass numerical method for characterizations of rock mass and excavation damage zone near tunnels

In practice, a damage zone is generally formed after tunnel excavation in jointed rock mass. This damage zone is closely related to rock mass properties and requires careful examination in order for cost effective supporting designs. In this research, a synthetic rock mass (SRM) numerical method is applied for characterizations of the jointed rock mass and excavation damage zone (EDZ) near underground tunnels in 3D. The SRM model consists of bonded particles and simulates deformation and crack propagation of the rock mass through interactions between these particles. The effects of joint stiffness and distribution on the rock mass properties are systematically examined by comparing the numerical data with an empirical geological strength index (GSI) system and an associated Hoek-Brown strength criterion. The numerical results suggest that rock mass properties are comparable to the empirical GSI/Hoek-Brown system only when inclined joints are simulated in the rock mass subjected to axial loading. The rock mass is strengthened and the empirical GSI/Hoek-Brown characterization becomes inappropriate when the joints are less favorable to shear sliding. The SRM method is then applied for characterizations of tunnel EDZ. It appears that the depth and location of the EDZ are a function of the tunnel orientation, joints, and in situ stresses. The EDZ depth is expected to be higher when inclined joints are simulated. The EDZ area is reduced when the joints in the rock mass are horizontally and vertically distributed.

Numerical Study of the Influence of Joint Angle on the Failure Behavior of Randomly and Nonpersistently Jointed Rock Mass

The generation of nonpersistent joints in numerical specimens is not easy to achieve in some geotechnical numerical software. In this paper, the synthetic rock mass modeling method was used to construct a group of numerical specimens based on triangular grain-based model and random joint set (RJS) model in universal discrete element code. The randomly distributed nonpersistent joints have seven dominant inclination angles. Then the unconfined compressive strength (UCS) tests of the seven groups of specimens were carried out numerically to obtain some insight into the influence of dominant inclination angle on strength behaviors, failure modes and fracture development. The inclination angles of the nonpersistent RJS have fluctuant effect on the UCS of the specimens. The maximum UCS appeared when the RJS angle is 90°, and small UCS values appeared when the RJS angles are 45°, 60° and 75°. With decreasing RJS angle, the brittleness gradually changes into ductility, and strain hardening occurs. During the whole uniaxial compression process, shear fractures take the dominant role during the preliminary stage and then reduce gradually, while the tensile fractures have an inverse variation trend during the initial stage and significantly increase when the axial strain exceeds 0.6% and then keeps growing until the compression ending. The nonpersistent RJSs with small and large angles have little contribution to the shear fracture development, and only the RJSs with median inclination angles (45°, 60°) significantly promote the shear fracture propagation and development, while the RJS makes an increasing contribution to the tensile fracture propagation and development with the increase in the RJS inclination angle.

Investigation of mechanical properties of jointed granite under compression using lattice-spring-based synthetic rock mass modeling approach

Joint waviness is one of the important parameters of joint that need to be considered in civil and mining engineering design of structures built in or on jointed rock masses. However, this parameter is often overlooked by designers. In this study the mechanical properties of laboratory scale jointed granite with both planar and non-planar joints of different joint configurations are investigated using the lattice-spring-based synthetic rock mass (LS-SRM) modeling approach. Jointed granite models are calibrated using the laboratory test results and the calibrated lattice model parameters are used to perform both conventional triaxial and true triaxial compression tests. In addition, the influences of joint orientation, intensity, and persistence on the strength and deformability of jointed granite with both planar and non-planar joints are investigated. The simulation results show that the strength and deformation modulus of the jointed rock decrease with the increase of joint intensity and persistence. The strength and deformation modulus of the jointed rock are higher for the ones with non-planar joints. Under the true triaxial loading condition, the strength and deformation modulus of the jointed granite depend strongly on the magnitude and orientation of the intermediate principal stress.

Scale effect of mechanical properties of jointed rock mass: A numerical study based on particle flow code

The synthetic rock mass (SRM) were used to investigate the influence of specimen size on the mechanical properties of jointed rock mass. The SRM were established based on parallel bond model (PBM) and smooth joint model (SJM) and the scaled rock specimens were sampled in two SRMs considering three sampling locations. The research results show that the smaller the initial fracture density is, the greater the uniaxial compressive strength (UCS), elastic modulus (E) is when compared with the same sampling location. The mechanical properties of rock specimens obtained by different sampling methods in different SRMs have different scale effects. The strength of rock specimens with more new cracks is not necessarily less than that of rock specimens with fewer new cracks and the failure of rock is caused by the formation of macro-fracture surface.

Investigating the Failure Mechanism of Jointed Rock Slopes Based on Discrete Element Method

This paper presents a comprehensive engineering method to investigate the failure mechanism of the jointed rock slopes. The field geology survey is first carried out to obtain the slope joint data. A joint network model considering the structural complexity of rock mass is generated in the PFC software. The synthetic rock mass (SRM) approach for simulating the mechanical behavior of jointed rock mass is employed, in which the flat‐jointed bonded‐particle model (FJM) and smooth joint contact model (SJM) represent intact rock and joints, respectively. Subsequently, the effect of microparameters on macromechanical properties of rock is investigated for parameter calibration. Moreover, the scale effect is analyzed by multiscale numerical tests, and the representative elementary volume (REV) size in the selected research area is found as 16 m × 16 m × 16 m. The microparameters of the SRM model are calibrated to match the mechanical properties of the engineering rock mass. Finally, an engineering case from Shuichang open‐pit mine is analyzed and the failure process of the slope during the excavation process from micro‐ to macroscale is obtained. It has been found that failure occurs at the bottom of the slope and gradually develops upwards. The overall failure of the slope is dominated by the shallow local tension fracture and wedge failure.

Numerical Analysis of the Anisotropy and Scale Effects on the Strength Characteristics of Defected Rockmass

Discontinuous defect in the rockmass is a key influential factor in controlling the strength behavior, and how to estimate the anisotropic strength and scale effect on the defected rockmass is the remaining challenging focus in engineering application. In the present study, intact tuff samples cored from the Xiabeishan tunnel engineering in situ are conducted by experiment tests (i.e., uniaxial compression test, triaxial compression test, and Brazilian tensile test) to obtain the corresponding mechanical parameters. Results from the numerical simulations using the particle flow code (PFC) by the flat‐jointed model (FJM) are performed to match the macroparameters from experimental results. It is observed that numerical results have good agreement with the macroscopic mechanical parameters of intact samples including UCS, BTS, triaxial compression strength, and corresponding deformation parameters. Finally, a series of uniaxial and confining compression tests are conducted by using a synthetic rockmass (SRM) method which is coupled with the discrete element method (DEM) and discrete fracture network (DFN). Then, the anisotropy and scale effects on the strength characteristics of the defected rockmass are investigated. The results show that defects have a vital effect on the failure mode and strength behavior of the rockmass in the research region. The strength parameters are changed with the specimen size. The REV size of the considered defected rockmass is regarded as 5 × 10 m, and this size is also influenced by the confinement level. The anisotropy of macroscopic strength parameters is found in the considered defected rockmass, whose stress‐strain curves and failure modes are also discussed.

Analysis of Rock β‐Dynamic Parameters and the Stability of Earthquake Dangerous Rocks Based on PFC

Mesoparameters of rock materials are the main factors affecting the macromechanical properties of dangerous rock slopes. Based on the principle of particle flow and synthetic rock mass technology (SRM), the influence of mesoparameters on macromechanical properties is investigated by calibrating mesoparameters of rock materials at depth for a rock sequence in Beichuan Qiang Autonomous County, Sichuan Province, China. By combining these parameters with conventional and dynamic cycle triaxial tests, sensitivity analysis of rock β‐parameters was completed. As a result, the reliability of mesoparameters in the simulation of dangerous rocks is strengthened, providing a basis to examine the failure mechanism of earthquake dangerous rocks in this region. Results indicate that, in the triaxial test, sandstone failed in tension, and brittleness gradually weakened as confining pressure increased. Mudstone recorded shear failure, and the characteristic value of brittle attenuation showed a V‐shaped change with increasing confining pressure. Under cyclic loading, cracks had a degrading effect on the damping ration (β) and the damping coefficient (C) of sandstone. Mudstone recorded relatively low β and low brittleness whilst sandstone had high β and high brittleness. In rock materials, βn is more sensitive than βs in mechanical properties. When the value of the βn‐parameter was between 0.2 and 0.3 and the value of the βs‐parameter was between 0.2 and 0.6, rock brittleness was more stable, and the reflected macroscopic mechanical properties were the most authentic. By using a deepened mesoparameter trial adjustment method, the failure mode of the Particle Flow Code (PFC) dangerous rock model near provincial highway 205, simulated under conditions for the Wenchuan earthquake, indicated a tensile fracture‐horizontal slip failure. The simulated failure mode was consistent with that of real dangerous rocks, with the failure trend being concentrated between the first and the third layer of the rock mass.

Estimation of REV for fractured rock masses based on Geological Strength Index

The representative elementary volume (REV) of fractured rock masses is a significant index to investigate the rock mass behaviors in the continuum mechanics. In this research, a new indicator to estimate the REV size of fractured rock masses based on the Geological Strength Index (GSI) is proposed. For this purpose, a new method that combines the PFC-based synthetic rock mass (SRM) model with the Hoek-Brown (HB) failure criterion is proposed to investigate the strength and deformation properties of fractured rock masses under biaxial stress conditions. Extensive numerical analyses are carried out to estimate variation of the uniaxial compression strength (UCS), deformation modulus (E) and GSI of the Brunswick mine rock mass with increasing the size of the SRM models up to a REV size. Results show that the GSI-based indicator gives relatively larger REV size compared with the traditional UCS or deformation modulus (E) based indicators. Compared with the traditional indicators, the proposed GSI-based indicator has merits of not only reflecting the geometrical characteristics of rock structures but also containing both geometrical and mechanical properties of discontinuities.

A DFN–DEM Multi-scale Modeling Approach for Simulating Tunnel Excavation Response in Jointed Rock Masses

Based on the concept of the representative elementary volume (REV) and the synthetic rock mass (SRM) modeling technique, a DFN–DEM multi-scale modeling approach is proposed for modeling excavation responses in jointed rock masses. Discrete fracture networks (DFNs) are generated using MoFrac. For a given volume of jointed rock masses, multi-scale DFN models are constructed according to the hierarchical order of fracture size. Based on the DFN models of various scales, equivalent rock mass properties are obtained using 3DEC SRM models. A tunnel excavation simulation using data from the Aspo TAS08 tunnel is conducted to demonstrate the applicability of the proposed multi-scale modeling approach. The modeling results show that it is efficient to model the mechanical response of the rock mass using the proposed approach. The proposed approach has the advantages of both equivalent continuum and discontinuum methods, with a higher degree of accuracy compared with the pure continuum approach and a less computational effort compared with the pure discontinuum approach.

Numerical characterization for rock mass integrating GSI/Hoek-Brown system and synthetic rock mass method

Rock mass characterization is a key issue to derive mechanical properties of jointed rock mass. In this research, the strength, modulus and post-peak behaviour of rock masses with varied joint orientations and confining stresses are characterized by integrating an empirical geological strength index (GSI)/Hoek-Brown system and a synthetic rock mass (SRM) numerical method. The SRM model is a 3D distinct element model supporting explicit simulation of discontinuous joints and crack propagation in the rock mass. The rock mass structure as denoted by a discrete fracture network is correlated to the GSI system through a quantitative factor of rock block size. The strength and modulus are then derived from the GSI/Hoek-Brown system and used as references for a series of synthetic rock mass models. A comparative study shows that when the predominant joints are steeply inclined in the rock mass subjected to axial loading, the empirical GSI/Hoek-Brown system provides reliable mechanical properties. When the joints are less favorable to shear sliding, the GSI/Hoek-Brown system underestimates the rock mass strength. Cracking characteristics are also investigated in selected SRM models, where few cracks are simulated when inclined joints are inserted in the rock mass and low confining stresses are applied. This research finally shows the method to construct continuum rock mass models using the numerical parameters of the SRM models. The numerical results suggest that the global failure zone is thin when the post-peak is elasto-brittle, and the failure zone is much wider when the post-peak is elasto-plastic. The involvement of the additional failed zones is responsible for the post-peak ductility. The paper is meant to explore new methods to derive high quality geo-data for jointed rock mass.

A Preliminary Investigation on the Role of Brittle Fracture in the Kinematics of the 2014 San Leo Landslide

The stability of high rock slopes is largely controlled by the location and orientation of geological features, such as faults, folds, joints, and bedding planes, which can induce structurally controlled slope instability. Under certain conditions, slope kinematics may vary with time, as propagation of existing fractures due to brittle failure may allow development of fully persistent release surfaces. In this paper, the progressive accumulation of brittle damage that occurred prior to and during the 2014 San Leo landslide (northern Italy) is investigated using a synthetic rock mass (SRM) approach. Mapping of brittle fractures, rock bridge failures, and major structures is undertaken using terrestrial laser scanning, photogrammetry, and high-resolution photography. Numerical analyses are conducted to investigate the role of intact rock fracturing on the evolution of kinematic freedom using the two-dimensional Finite-discrete element method (FDEM) code Elfen, and the three-dimensional lattice-spring scheme code Slope Model. Numerical analyses show that the gradual erosion of clay-rich material below the base of the plateau drives the brittle propagation of fractures within the rock mass, until a fully persistent, subvertical rupture surface form, causing toppling of fault-bounded rock columns. This study clearly highlights the potential role of intact rock fracturing on the slope kinematics, and the interaction between intact rock strength, structural geology, and slope morphology.

Investigating Hydraulic Fracturing Complexity in Naturally Fractured Rock Masses Using Fully Coupled Multiscale Numerical Modeling

Naturally fractured rock mass is highly inhomogeneous and contains geological discontinuities at various length scales. Hydraulic fracture stimulation in such a medium could result in complex fracture systems instead of simple planar fractures. In this study, we carried out fully coupled multiscale numerical analysis to investigate some key coupled processes of fluid-driven fracture propagation in naturally fractured rock mass. The numerical analysis follows the concept of the synthetic rock mass (SRM) method initially developed in the discrete element method (DEM). We introduce a total of five case study examples, including fracture initiation and near wellbore tortuosity, hydraulic fracture interaction with natural fractures, multi-stage hydraulic fracturing with discrete fracture network (DFN), in-fill well fracturing and frac hits after depletion-induced stress change, and induced seismicity associated with fault reactivation. Through those case studies, we demonstrate that with an advanced numerical modeling tool, the complex fracturing associated with hydraulic fracturing in naturally fractured rock mass can be qualitatively analyzed and the extent of various uncertainties can be assessed.

A method for numerical simulation based on microseismic information and the interpretation of hard rock fracture

As an effective technique for detecting microfractures in rock mass, microseismic monitoring (MS) plays an important role in obtaining and interpreting early warning signs of rock engineering hazards. Although microseismic events provide abundant source information (e.g., source parameters and mechanisms) of rock mass fracture, the validity of interpreting rock fracture behavior is restricted due to the discreteness of the temporal-spatial distribution of microseismic events. Feeding back the microseismic information to numeric simulation and calibrating a rock fracture model can improve the simulation accuracy and can effectively interpret the progressive fracture effect in the cluster of microseismic events. Based on this, a microseismic-based method and its implementation steps for numerical simulation and the interpretation of hard rock fracture is proposed in this paper. Indexes resembling the source parameters of the fracture source are introduced in the method to ensure its capability in simulating microseismicity. In the engineering case study of a rockburst hazard, the source mechanism information is used to improve the synthetic rock mass model with a more accurate rock-mass structure. Therefore, the simulated microseismicity is matched well with the measured microseismicity, and the progressive fracturing effect of the rockburst hazard in the cluster of microseismic events is interpreted well by the evolution of indexes that characterize the continuous mechanical effect (stress components, volumetric strain and kinetic energy). This method provides a new way to interpret early warning signs of rock engineering hazards and provides an alternative approach that combines rock mechanics and microseismicity.

Investigation of mechanical properties and crack propagation in pre-cracked marbles using lattice-spring-based synthetic rock mass (LS-SRM) modeling approach

In this study, progressive failure and mechanical properties of pre-cracked marbles with planar or non-planar cracks are studied using the lattice-spring-based synthetic rock mass (LS-SRM) modeling approach. The modeling results show that a combination of wing, anti-wing, shear, and secondary cracks are observed in models with pre-existing planar cracks. Complex cracking mechanism is observed in models with pre-existing non-planar cracks. Both the peak strength and the deformation modulus decrease with the increase of the crack angle, crack length, ligament angle, ligament length, and number of cracks. The influence of crack waviness on the mechanical properties of cracked synthetic model is high only when the crack persistency is large.

Discrete Element Modeling of Strength and Failure Behavior of Transversely Isotropic Rock under Uniaxial Compression

Abstract This paper presents a discrete element modeling (DEM) with particle flow code 2D (PFC2D) for simulating the mechanical behavior of artificial transversely isotropic rock under uniaxial compression. The DEM model is established with the approach of synthetic rock mass (SRM) modeling, i.e. embedding joints of weak plane to the bonded particle model (BPM) of intact rock for simulating the joint rock. Sensitivity of joint micro-parameters is analyzed to provide references for calibrating micro-parameters of artificial transversely isotropic rock in PFC2D. The uniaxial compression test of DEM model displays U-shape variation of uniaxial compressive strength (UCS) along with bedding angle which is same to that of artificial transversely isotropic rock, a non-ignorable diversity in UCS value compared with the artificial transversely isotropic rock is also presented, while the normalized values of them are of well-matching. The obtained information of type, counts and distribution of micro-crack in PFC2D after DEM model failure indicate a remarkable impact on failure behavior of transversely isotropic rock, which cannot be surveyed in laboratory test. Farther more, the bond force field and particle displacement field reveal that separation zones caused by tension arise at bedding angle below 45°, while the shear zone occurs at bedding angle above 45°.

Efficient 3D probabilistic stability analysis of rock tunnels using a Lattice Model and cloud computing

In this paper, the Classic Lattice Spring Model is developed in order to be applied to probabilistic stability analyses of rock tunnels in low in situ stress environments, where the falling of blocks is the main failure mechanism. For this objective, the method is combined with the Synthetic Rock Mass technique and Barton-Bandis joint constitutive model, giving it a series of advantages: simulations with deformable blocks; possible formation of tension cracks; and an appropriate constitutive model for the mechanical behavior of joints. The use of cloud computing technology is proposed for efficient probabilistic analyses using the Monte Carlo simulation. A case study is presented based on an unsupported section of a shallow rock tunnel excavated in the state of Espírito Santo, Brazil. A 3D probabilistic stability analysis of the tunnel is performed with the proposed methodology. From the results, a positional probability map is elaborated, which indicates the likelihood of block failure around the excavation cross-section. The unstable zones indicated by the map are then compared to the failed region of the real tunnel so as to demonstrate the competence of the methodology.

Numerical investigation of the mechanical properties of coal masses with T-junctions cleat networks under uniaxial compression

Evaluating the mechanical properties of coal masses is essential to the further understanding of coal bumps, pillar design, roadway support design and movement of overlying strata, etc. It is generally understood that the cleat network of a coal mass plays a major role in determining its mechanical behavior. Based on an extensive review of the characteristics of coal cleats, a novel method was proposed to generate a realistic model of T-junctions cleat network. Using the synthetic rock mass (SRM) approach, five sets of cubic cleat networks, with uniform sizes of 1 × 1 × 1 m3, were initially generated. Afterwards, smaller cubic cleat networks, with edge lengths of 5 cm, 10 cm, and 15 cm, were randomly cut from the large cubic cleat networks and then embedded into bonded-particle models (BPMs) to form SRM coal specimens of varying sizes. The SRM coal specimens were tested under unaxial compression conditions and the results of the tests were compared with the experimentally determined mechanical properties. The scale effect of uniaxial compression strength (UCS), relationships between failure modes and stress-strain behaviors, and the principal causes of fluctuations in the stress-strain curves were intensively investigated. The results demonstrated that the existence and intensity of scale effect mainly depends on sampling methods and cleat network geometry. SRM coal specimens whose failure modes were intact matrix failure and planar failure, and block failure and step-path failure exhibit typical strain softening behavior and fluctuating ductile behavior, respectively, in their stress-strain curves. Moreover, the fluctuations in the stress-strain curves are attributed to the changes in cleat aperture throughout the uniaxial compression tests.

Joint Elasticity Effect on the Failure Behaviours of Rock Masses using a Discrete Element Model

It is widely accepted that the mechanical properties and failure behaviours of a rock mass are largely dependent upon the geometrical and mechanical properties of discontinuities. The effect of joint elasticity on the failure behaviours of rock masses is investigated using a discrete element model, namely, the synthetic rock mass model. Here, uniaxial compression tests of the numerical model are carried out for the rock mass model with a persistent joint to analyse the role of joint elasticity in the failure process with various joint orientations, β. A strong correlation between the joint elasticity and failure strength is found from the simulation results: a positive relationship when the joint orientation β < φ j ; a negative relationship when the joint orientation φ j < β < 90 ° ; and a very limited effect when the joint orientation β = 90 ° . Additionally, it is shown that the joint elasticity is the governing factor in the transition of failure modes, especially from the sliding failure mode along the joint to the mixed sliding-tensile failure mode.

Improving DFN-geomechanical model integration using a novel automated approach

In the past decade, numerical modelling has been increasingly used for simulating the mechanical behavior of naturally fractured rock masses. The synthetic rock mass approach often adopted has three main components: data collection and characterisation; discrete fracture network (DFN) modelling; and finally, geomechanical modelling to simulate rock mass behaviour. The focus of this paper is the integration of the DFN with geomechanical models, with emphasis on the numerical aspects of embedding rather complex 2D fracture networks within a finite element mesh. A new method is introduced that simplifies fracture networks, while maintaining important fracture properties such as orientation, connectivity, and intensity. The resulting finite element mesh is of sufficient quality to both improve computational time and eliminate spurious results that may be associated with the presence of highly distorted elements. The proposed method uses a web-based code with two integral components (DFNAnalyzer and DFNCleaner) that analyse the properties of the original fracture network, and then perform simplifications to ensure an acceptable (user defined) mesh quality is achieved.

Time and technique of rehabilitation for large deformation of tunnels in jointed rock masses based on FDM and DEM numerical modeling

When large deformation occurs in a tunnel, resulting in the primary lining intruding into the tunnel clearance, an appropriate treatment method is critically important to ensure that the tunnel can be completed on time and safely. This paper investigates the optimal treatment timing and technique for replacing the deformed primary lining. For this purpose, a two-stage numerical approach is employed for a severe tunnel collapse accident occurring on the Jinhong Highway. In the first stage, a global tunnel model is constructed using the finite difference method (FDM). The parameters are calibrated by the synthetic rock mass (SRM) approach, laboratory testing and field data. The continuously yielding (CY) joint model is used in the SRM, and its parameters are derived by the joint direct shear test. In the second stage, a local model encompassing the discrete fracture network (DFN) is constructed using the discrete element method (DEM) in the collapsed zone. Based on the DEM, the influence of the primary lining rehabilitation on the failure of the rock masses is analyzed. The local model is used to study the optimal rehabilitation timing, the optimum rehabilitation duration and the reasonable length of each replacement cycle. The following three conclusions are drawn. (1) The optimal rehabilitation timing is when the deformation of the rock masses reaches 90% of the total deformation. (2) There is a time threshold for the duration of the rehabilitation; if the duration is less than this value, the impact on the surrounding rock is small and almost the same. However, if the duration exceeds this value, the damage to the rock mass will increase as the duration increases. (3) The reasonable length of every replacement cycle should be less than the steel support spacing.