Numerical simulations of failure behavior around a circular opening in a non-persistently jointed rock mass under biaxial compression

Pre-existing discontinuities change the mechanical properties of rock masses,and further influence failure behavior around an underground opening.In present study,the failure behavior in both Inner and Outer zones around a circular opening in a non-persistently jointed rock mass under biaxial compression was investigated through numerical simulations.First,the micro parameters of the PFC^(3D) model were carefully calibrated using the macro mechanical properties determined in physical experiments implemented on jointed rock models.Then,a parametrical study was undertaken of the effect of stress condition,joint dip angle and joint persistency.Under low initial stress,the confining stress improves the mechanical behavior of the surrounding rock masses;while under high initial stress,the surrounding rock mass failed immediately following excavation.At small dip angles the cracks around the circular opening developed generally outwards in a step-path failure pattern;whereas,at high dip angles the surrounding rock mass failed in an instantaneous intact rock failure pattern.Moreover,the stability of the rock mass around the circular opening deteriorated significantly with increasing joint persistency.

A 3D microseismic data-driven damage model for jointed rock mass under hydro-mechanical coupling conditions and its application

Rock mass is a fractured porous medium usually subjected to complex geostress and fluid pressure simultaneously.Moreover,the properties of rock mass change in time and space due to mining-induced fractures.Therefore,it is always challenging to accurately measure rock mass properties.In this study,a three-dimensional(3D)microseismic(MS)data-driven damage model for jointed rock mass under hydro-mechanical coupling conditions is proposed.It is a 3D finite element model that takes seepage,damage and stress field effects into account jointly.Multiple factors(i.e.joints,water and microseismicity)are used to optimize the rock mass mechanical parameters at different scales.The model is applied in Shirengou iron mine to study the damage evolution of rock mass and assess the crown pillar stability during the transition from open-pit to underground mining.It is found that the damage pattern is mostly controlled by the structure,water and rock mass parameters.The damage pattern is evidently different from the two-dimensional result and is more consistent with the field observations.This difference is caused by the MS-derived damage acting on the rock mass.MS data are responsible for gradually correcting the damage zone,changing the direction in which it expands,and promoting it to evolve close to reality.For the crown pillar,the proposed model yields a more trustworthy safety factor.In order to guarantee the stability of the pillar,it is suggested to take waterproof and reinforcement measures in areas with a high degree of damage.

Strength and deformation characteristics of irregular columnar jointed rock mass: A combined experimental and theoretical study

The irregularity of jointed network poses a challenge to the determination of field mechanical param-eters of columnar jointed rock mass(CJRM),and a reasonable prediction of deformation and strength characteristics of CJRM is important for engineering construction.The Voronoi diagram and three-dimensional printing technology were used to make an irregular columnar jointed mold,and the irregular CJRM(ICJRM)specimens with different dip directions and dip angles were prepared.Uniaxial compression tests were performed,and the anisotropic strength and deformation characteristics of ICJRM were described.The failure modes and mechanisms were revealed in accordance with the final appearances of the ICJRM specimens.Based on the model test results,the empirical correlations for determining the field deformation and strength parameters of CJRM were derived using the dip angle and modified joint factor.The proposed empirical equations were used in the Baihetan Project,and the calculated mechanical parameters were compared with the field test results and those obtained from the tunneling quality index method.Results showed that the deformation parameters determined by the two proposed methods are all consistent with the field test results,and these two methods can also estimate the strength parameters effectively.

Fracture behaviors of columnar jointed rock mass using interface mechanics theorem

For a special geological structure of columnar jointed rock mass(CJRM),its mechanical properties are strongly affected by the columnar joints.To describe the fracture behaviors of CJRM using the basic theories of interface mechanics for composite materials,the interface stresses of the vertical and horizontal joints,which are the two primary joints in the CJRM under triaxial compression,are studied,and their mathematical expressions are derived based on the superposition principle.Based on the obtained interface stresses of the vertical and horizontal joints in the CJRM,the crack initiation of the joint interface in the CJRM is studied using the maximum circumferential stress theory in fracture mechanics.Moreover,based on this investigation,the fracture behaviors of CJRM are analyzed.According to the results of similar material physical model tests for the CJRM,the theoretical study is verified.Finally,the influence of the mechanical parameters of the CJRM on the joint interface stress is discussed comprehensively.

Investigation of long-wavelength elastic wave propagation through wet bentonite-filled rock joints

The saturation of the compacted bentonite buffer in the deep geological repository can cause bentonite swelling,intrusion into rock fractures,and erosion.Inevitably,erosion and subsequent bentonite mass loss due to groundwater inflow can aggravate the overall integrity of the engineered barrier system.Therefore,the coupled hydro-mechanical interaction between the buffer and rock during groundwater inflow and bentonite intrusion should be evaluated to guarantee the long-term safety of deep geological disposal.This study investigated the effect of bentonite erosion and intrusion on the elastic wave propagation characteristics in jointed rocks using a quasi-static resonant column test.Jointed rock specimens with different joint conditions(i.e.joint surface saturation and bentonite filling)were prepared using granite rock discs sampled from the Korea Underground Research Tunnel(KURT)and Gyeongju bentonite.The long-wavelength longitudinal and shear wave velocities were measured under different normal stress levels.A Hertzian-type power model was used to fit the wave velocities,and the relationship between the two fitted parameters provided the trend of joint conditions.Numerical simulations using three-dimensional distinct element code(3DEC)were conducted to better understand how the long-wavelength wave propagates through wet bentonite-filled rock joints.

Shear strength criteria for rock,rock joints,rockfill and rock masses:Problems and some solutions

Although many intact rock types can be very strong,a critical confining pressure can eventually be reached in triaxial testing,such that the Mohr shear strength envelope becomes horizontal.This critical state has recently been better defined,and correct curvature or correct deviation from linear Mohr-Coulomb(MC) has finally been found.Standard shear testing procedures for rock joints,using multiple testing of the same sample,in case of insufficient samples,can be shown to exaggerate apparent cohesion.Even rough joints do not have any cohesion,but instead have very high friction angles at low stress,due to strong dilation.Rock masses,implying problems of large-scale interaction with engineering structures,may have both cohesive and frictional strength components.However,it is not correct to add these,following linear M-C or nonlinear Hoek-Brown(H-B) standard routines.Cohesion is broken at small strain,while friction is mobilized at larger strain and remains to the end of the shear deformation.The criterion 'c then σn tan φ' should replace 'c plus σn tan φ' for improved fit to reality.Transformation of principal stresses to a shear plane seems to ignore mobilized dilation,and caused great experimental difficulties until understood.There seems to be plenty of room for continued research,so that errors of judgement of the last 50 years can be corrected.

Numerical investigation for anisotropy of compressive strength of rock mass with multiple natural joints

Based on Rock Failure Process Analysis model RFPA , the evolutionary processesof failure process of rock mass with multiple natural joints were simulated.Numericalsimulations show that anisotropy of compressive strength of jointed rock mass varieswith the number of natural joints and inclination of natural joints.As the number of naturaljoints in rock mass increases, the anisotropy becomes less and less.It is justifiable to treatapproximately rock mass containing six or more natural joints instead of four or more jointsthat was described in literature of Hoek and Brown as isotropy.

Analysis of the Elastic-Plastic Dynamic Finite Element on a Jointed Rock Mass

目的研究节理岩体的弹塑性动态本构关系，并进行数值模拟．方法考虑动荷作用过程中，应力随时间变化这一应力历史，取单元为分离体，根据静力平衡原则，求出与之相应的节点集中力，再计算节点位移，得到节理岩体弹塑性动态有限元方程．结果给出了一种节理岩体在动荷作用下的有限元分析方法．结论通过动态有限元分析，有效地解决了节理岩体在爆炸载荷作用下的大塑性变形问题，并对爆破范围进行了数值模拟。

Responses of jointed rock masses subjected to impact loading

Impact-induced damage to jointed rock masses has important consequences in various mining and civil engineering applications. This paper reports a numerical investigation to address the responses of jointed rock masses subjected to impact loading. It also focuses on the static and dynamic properties of an intact rock derived from a series of laboratory tests on meta-sandstone samples from a quarry in Nova Scotia, Canada. A distinct element code(PFC2D) was used to generate a bonded particle model(BPM) to simulate both the static and dynamic properties of the intact rock. The calibrated BPM was then used to construct large-scale jointed rock mass samples by incorporating discrete joint networks of multiple joint intensities into the intact rock matrix represented by the BPM. Finally, the impact-induced damage inflicted by a rigid projectile particle on the jointed rock mass samples was determined through the use of the numerical model. The simulation results show that joints play an important role in the impactinduced rock mass damage where higher joint intensity results in more damage to the rock mass. This is mainly attributed to variations of stress wave propagation in jointed rock masses as compared to intact rock devoid of joints.

Determination of geological strength index of jointed rock mass based on image processing

The geological strength index(GSI) system,widely used for the design and practice of mining process,is a unique rock mass classification system related to the rock mass strength and deformation parameters based on the generalized Hoek-Brown and Mohr-Coulomb failure criteria.The GSI can be estimated using standard chart and field observations of rock mass blockiness and discontinuity surface conditions.The GSI value gives a numerical representation of the overall geotechnical quality of the rock mass.In this study,we propose a method to determine the GSI quantitatively using photographic images of in situ jointed rock mass with image processing technology,fractal theory and artificial neural network(ANN).We employ the GSI system to characterize the jointed rock mass around the working in a coal mine.The relative error between the proposed value and the given value in the GSI chart is less than 3.6%.

Strength of massive to moderately jointed hard rock masses

The Hoek-Brown (HB) failure criterion and the geological strength index (GSI) were developed for the estimation of rock mass strength in jointed and blocky ground where rock mass failure is dominated by sliding along open joints and rotation of rock blocks. In massive, veined and moderately jointed rock in which rock blocks cannot form without failure of intact rock, the approach to obtain HB parameters must be modified. Typical situations when these modifications are required include the design of pillars, excavation and cavern stability, strainburst potential assessment, and tunnel support in deep underground conditions (around σ1/σci > 0.15, where σ1 is the major principal compressive stress and σci is the unconfined compressive strength of the homogeneous rock) in hard brittle rocks with GSI ≥ 65. In this article, the strength of massive to moderately jointed hard rock masses is investigated, and an approach is presented to estimate the rock mass strength envelope using laboratory data from uniaxial and triaxial compressive strength tests without reliance on the HB-GSI equations. The data from tests on specimens obtained from massive to moderately jointed heterogeneous (veined) rock masses are used to obtain the rock and rock mass strengths at confining stress ranges that are relevant for deep tunnelling and mining;and a methodology is presented for this purpose from laboratory data alone. By directly obtaining the equivalent HB rock mass strength envelope for massive to moderately jointed rock from laboratory tests, the HB-GSI rock mass strength estimation approach is complemented for conditions where the GSIequations are not applicable. Guidance is also provided on how to apply the proposed approach when laboratory test data are not or not yet available.

Macro and meso analysis of jointed rock mass triaxial compression test by using equivalent rock mass(ERM) technique

Methods that can efficiently model the effects of rock joints on rock mass behavior can be beneficial in rock engineering. The suitability of equivalent rock mass(ERM) technique based upon particle methods is investigated. The ERM methodology is first validated by comparing calculated and experimental data of lab triaxial compression test on a set of cylindrical rock mass samples, each containing a single joint oriented in various dip angles. The simulated results are then used to study the stress-strain nonlinearity and failure mechanism as a function of the joint dip angle and confining stress. The anisotropy and size effects are also investigated by using multi-scale cubic ERM models subjected to triaxial compression test. The deformation and failure behavior are found to be influenced by joint degradation, the micro-crack formation in the intact rock, the interaction between two joints, and the interactions of micro-cracks and joints.

Effect of layered joints on rockburst in deep tunnels

The existence of joints in the surrounding rock mass has a considerable efect on tunnel rockbursts.Herein,we studied the efect of layered joints with diferent inclination angles and spacings on rockburst in deep tunnels and investigated the failure area,deformation process of the surrounding rock mass,stress change inside the surrounding rock mass,velocity of the failed rock,and the kinetic energy of the failure.The failure type of the surrounding rock mass can thus be determined.The results showed that the intensity of rockburst increases as rock quality designation(RQD)decreases,while the deformation rate of the surrounding rock mass frst increases and then decreases.The deformation rate exhibits a turning point between RQD=50 and 70,below which the deformation rate of the surrounding rock mass gradually decreases,ultimately ceasing to be a rockburst.Rockburst always occurs perpendicular to the direction of the joint.Whenσ_(x)=σ_(y),as the joint inclination angle changes from 45°to 90°,the intensity of a rockburst frst decreases(from 45°to 60°),and then increases(from 60°to 90°).When combined with the evolution law of stress and strain energy,the rockburst process can be divided into four stages.

Dynamic-static coupling analysis on rockburst mechanism in jointed rock mass

A numerical code called RFPA-Dynamics was used to study the rockburst mechanism under dynamic load based on coupled static-dynamic analysis.The results show that dynamic disturbance has a very distinct triggering effect on rockburst.Under the dynamic load,rockburst is motivated by tensile stress formed by the overlapping of dynamic waves in the form of instantaneous open and cutting through of cracks in weak planes and pre-damaged areas.Meanwhile,the orientation of joint sets has an obvious leading effect on rockburst locations.Finally,a higher initial static stress state before dynamic loading can cause more pre-damaged area,thus leading to a larger rockburst scope.

IMITATING STUDY ON BLASTING EFFECT OF JOINTED ROCK MASSES

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Particle flow study on strength and meso-mechanism of Brazilian splitting test for jointed rock mass

A discrete element method(DEM) called particle flow code(PFC2D) was used to construct a model for Brazilian disc splitting test in the present study. Based on the experimental results of intact Brazilian disc of rock-like material, a set of micro-parameters in PFC2 Dthat reflected the macro-mechanical behavior of rock-like materials were obtained. And then PFC2 Dwas used to simulate Brazilian splitting test for jointed rock mass specimens and specimen containing a central straight notch. The effect of joint angle and notch angle on the tensile strength and failure mode of jointed rock specimens was detailed analyzed. In order to reveal the meso-mechanical mechanism of crack coalescence, displacement trend lines were applied to analyze the displacement evolution during the crack initiation and propagation. The investigated conclusions can be described as follows.(1) The tensile strength of jointed rock mass disc specimen is dependent to the joint angle. As the joint angle increases, the tensile strength of jointed rock specimen takes on a nonlinear variance.(2) The tensile strength of jointed rock mass disc specimen containing a central straight notch distributes as a function of both joint angle and notch angle.(3) Three major failure modes, i.e., pure tensile failure,shear failure and mixed tension and shear failure mode are observed in jointed rock mass disc specimens under Brazilian test.(4) The notch angle and joint angle play important roles on crack initiation and propagation characteristics of jointed rock mass disc specimen containing a central straight notch under Brazilian test.

Numerical investigation on the sensitivity of jointed rock mass strength to various factors

The mechanical properties of jointed rock masses, such as strength, deformation and the failure mechanism, can be understood only by studying the sensitivity of jointed rock mass strength (both the peak and residual strengths) to the factors that affect it. An orthogonal design of uniaxial compression tests was simulated on eighteen groups of jointed rock specimens having different geometric and mechanical properties using RFPA2D (Rock Failure Process Analysis) code. The results show that the peak strength is controlled by the geometric parameters of the joints, but that the residual strength is controlled by the mechanical prop- erties of the joint interfaces. The failure mode of jointed rock specimens is mainly shear failure. Joint quantity, or density, is the most important index that affects jointed rock mass strength and engineering quality.

Determination of mechanical parameters for jointed rock masses

Combining with empirical method, laboratory test and numerical simulation, a comprehensive system was presented to determine the mechanical parameters of jointed rock masses. The system has the following four functions: (1) Based on the field investigation of joints, the system can consider rock mass structures, by using network simulation technology. (2) Rock samples are conducted by numerical simulation with the input engineering mechanical parameters of rocks and joints obtained from laboratory tests. (3) The whole stress-strain curve of jointed rock masses under certain normal stress can be plotted from numerical simulation, and then the shear strength parameters of jointed rock masses can be obtained from the whole stress-strain curves under different normal stresses. (4) The statistical values of mechanical parameters of jointed rock masses can be determined according to numerical simulation. Based on the statistical values, combining with engineering experiences and geological investigations, the comprehensive mechanical parameters of jointed rock masses can be achieved finally. Several cases are presented to prove the engineering feasibility and suitability of this system.

Parameter analysis of anchor bolt support for large-span and jointed rock mass

In order to obtain the optimal parameters of anchor bolt supporting system for large-span and jointed rock mass in Kaiyang Phosphor Mine, it is expensive and unavailable with the method of in-situ experiments. This paper describes a numerical modeling with discrete element method for the supporting effects of different type of anchor bolts. The anchor bolts with variant length of 0.5m, 0.8m, 1.0m, diameter of 10mm, 15mm, 20mm, setting spacing of 3.0m, 2.5m, 2.0m, and setting angle of 10°, 20°, 30°, are simulated respectively. The results show that there exist optimal parameters of anchor bolt support for large-span and jointed rock mass. For the bolt support of the concerning, the optimal length is 2.53.5m, the diameter is 2535mm, the spacing is 0.50.6m, and the setting angle is 105°.

Distinct element modelling of fracture plan control in continuum and jointed rock mass in presplitting method of surface mining

Controlled blasting techniques are used to control overbreak and to aid in the stability of the remaining rock formation. Presplitting is one of the most common methods which is used in many open pit mining and surface blast design. The purpose of presplitting is to form a fracture plane across which the radial cracks from the production blast cannot travel. The purpose of this study is to investigate of effect of presplitting on the generation of a smooth wall in continuum and jointed rock mass. The 2D distinct element code was used to simulate the presplitting in a rock slope. The blast load history as a function of time was applied to the inner wall of each blasthole. Important parameters that were considered in the analysis were stress tensor and fracturing pattern. The blast loading magnitude and blasthole spacing and jointing pattern were found to be very significant in the final results.