Joint Dip Angle Research Articles

Experimental and numerical study of the failure process and energy mechanisms of rock-like materials containing cross un-persistent joints under uniaxial compression.

Joints and fissures in natural rocks have a significant influence on the stability of the rock mass, and it is often necessary to evaluate strength failure and crack evolution behavior. In this paper, based on experimental tests and numerical simulation (PFC2D), the macro-mechanical behavior and energy mechanism of jointed rock-like specimens with cross non-persistent joints under uniaxial loading were investigated. The focus was to study the effect of joint dip angle α and intersection angle γ on the characteristic stress, the coalescence modes and the energy release of jointed rock-like specimens. For specimens with γ = 30° and 45°, the UCS (uniaxial compression strength), CIS (crack initiation stress) and CDiS (critical dilatancy stress) increase as α increases from 0° to 75°. When γ = 60° and 75°, the UCS, CIS and CDiS increase as α increases from 0° to 60° and decrease when α is over 60°. Both the inclination angle α and intersection angle γ have great influence on the failure pattern of pre-cracked specimens. With different α and γ, specimens exhibit 4 kinds of failure patterns. Both the experimental and numerical results show that the energy of a specimen has similar trends with characteristic stress as α increases.

Failure behavior around a circular opening in a rock mass with non-persistent joints: A parallel-bond stress corrosion approach

The stability of underground excavations is influenced by discontinuities interspaced in surrounding rock masses as well as the stress condition. In this work, a numerical study was undertaken on the failure behavior around a circular opening in a rock mass having non-persistent open joints using PFC software package. A parallel-bond stress corrosion (PSC) approach was incorporated to drive the failure of rock mass around the circular opening, such that the whole progressive failure process after excavation was reproduced. Based on the determined micro parameters for intact material and joint segments, the failure process around the circular opening agrees very well with that obtained through laboratory experiment. A subsequent parametric study was then carried out to look into the influence of lateral pressure coefficient, joint dip angle and joint persistency on the failure pattern and crack evolution of the rock mass around the circular opening. Three failure patterns identified are step path failure, planar failure and rotation failure depending on the lateral pressure coefficient. Moreover, the increment of joint dip angle and joint persistency aggravates the rock mass failure around the opening. This study offers guideline on stability estimation of underground excavations.

Dynamic mechanical properties of artificial jointed rock samples subjected to cyclic triaxial loading

The mechanical behavior of artificial jointed rock samples with different joint dip angles was analysed on the basis of the results of cyclic triaxial tests with 1.0Hz frequency under different stress amplitudes and different confining pressures. It was found that the dynamic strength decreases with increasing joint dip angles and increases with increasing confining pressures. The stress ratio (the ratio of the maximum stress of the cyclic loading to the static triaxial compressive strength) can comprehensively reflect the influences of joint dip angles, confining pressures and stress amplitudes. For all samples with different joint dip angles, confining pressures and stress amplitudes, the number of cycles at failure decreases with increasing stress ratios. And the evolutionary characteristics of the residual axial strain and residual dilatancy with loading cycles, as well as the evolution law of damages, are determined by the stress ratios. As the stress ratio changes, the evolution laws of the residual strain and the damages form three kinds of situations. A damage variable is defined using the sum of absolute values of the volumetric contraction and dilatancy, which can consider the initial fatigue damage of jointed rocks under cyclic loading and reflect laws of the dynamic damage evolution. During each loading cycle, the samples are slightly compacted and then dilate largely in the reloading stage, while dilate slightly and are largely compacted when unloading. The volumetric strain exhibits a series of significant changing rule within a cycle and with the increasing number of cycles, which contributes to reveal the mesoscopic damage mechanism in the cyclic loading condition. Finally, the confining pressure and the joint dip angle strongly influence the dynamic deformation and failure mechanism of jointed rock samples by affecting the generating of the weak zone along the joint plane and the initiation and propagation of micro-cracks.

Experimental and numerical study on crack propagation and deformation around underground opening in jointed rock masses

Understanding the process of crack propagation and deformation around underground openings is a key issue in several engineering fields, such as tunneling, mining, and radioactive waste disposal facilities. Many failures of underground openings are commonly induced by the geological discontinuities of host rock masses, which contain the existing discontinuities and the new cracks generated during the construction of underground structures. In this study, base friction tests were conducted to investigate the influence of dip angle of layered joints on the stability of circular openings in jointed rock masses and a series of numerical simulations utilizing an originally developed code based on distinct element method (DEM) were performed on the experimentbased and extended numerical models, respectively. The results show that the main propagation direction of newly generated cracks is approximately perpendicular to the joint dip angle. For the brittle host rock masses, the deformation around the underground openings is governed by the tensile failure of host rock masses. A decrease in joint dip angle gives rise to an increase of plastic failure zone in the host rock masses. The models with lower joint dip angles could generate a larger number of cracks. The maximum displacement observed at the left shoulder of openings is approximately 1.8–2 times of the minimum displacement at the right side wall of circular opening. The influences of the opening shape on the main propagation direction of newly generated cracks could be negligible. Due to the stress concentration at the sharp corner of square openings, a larger area of plastic zones is developed, which leads to obvious increment of displacements around the underground openings.

Experimental Investigation of the Influence of Joint Geometric Configurations on the Mechanical Properties of Intermittent Jointed Rock Models Under Cyclic Uniaxial Compression

Intermittent joints in rock mass are quite sensitive to cyclic loading conditions. Understanding the fatigue mechanical properties of jointed rocks is beneficial for rational design and stability analysis of rock engineering projects. This study experimentally investigated the influences of joint geometry (i.e., dip angle, persistency, density and spacing) on the fatigue mechanism of synthetic jointed rock models. Our results revealed that the stress–strain curve of jointed rock under cyclic loadings is dominated by its curve under monotonic uniaxial loadings; the terminal strain in fatigue curve is equal to the post-peak strain corresponding to the maximum cyclic stress in the monotonic stress–strain curve. The four joint geometrical parameters studied significantly affect the fatigue properties of jointed rocks, including the irreversible strains, the fatigue deformation modulus, the energy evolution, the damage variable and the crack coalescence patterns. The higher the values of the geometrical parameters, the lower the elastic energy stores in this jointed rock, the higher the fatigue damage accumulates in the first few cycles, and the lower the fatigue life. The elastic energy has certain storage limitation, at which the fatigue failure occurs. Two basic micro-cracks, i.e., tensile wing crack and shear crack, are observed in cyclic loading and unloading tests, which are controlled principally by joint dip angle and persistency. In general, shear cracks only occur in the jointed rock with higher dip angle or higher persistency, and the jointed rock is characterized by lower fatigue strength, larger damage variable and lower fatigue life.

Effect of parallel joint interaction on mechanical behavior of jointed rock mass models

To better understand the interaction of parallel joints and its effect on the mechanical behavior of jointed rock mass models, the results of a physical experiment program undertaken in the laboratory were present in this manuscript. The joint spacing and joint overlap are varied to alter the relative positions of parallel joints in geometry. Accordingly, four basic failure modes identified from the testing results are tensile failure across the joint plane, shear failure along the joint plane, tensile failure along the joint plane, and intact material failure. The wing cracks from tensile failure across the joint plane mode represent the interaction between parallel joints which depends on the joint dip angle, joint spacing and joint overlap. With the increment of parallel joint interaction, the corresponding normalized strength and maximum displacement at peak stress of jointed rock mass models reduced generally except at α=0°.

Analysis of stress distribution around tunnels by hybridized FSM and DDM considering the influences of joints parameters

The jointed rock mass behavior often plays a major role in the design of underground excavation, and their failures during excavation and in operation, are usually closely related to joints. This research attempts to evaluate the effects of two basic geometric factors influencing tunnel behavior in a jointed rock mass; joints spacing and joints orientation. A hybridized indirect boundary element code known as TFSDDM (Two-dimensional Fictitious Stress Displacement Discontinuity Method) is used to study the stress distribution around the tunnels excavated in jointed rock masses. This numerical analysis revealed that both the dip angle and spacing of joints have important influences on stress distribution on tunnel walls. For example the tensile and compressive tangential stresses at the boundary of the circular tunnel increase by reduction in the joint spacing, and by increase the dip joint angle the tensile stress in the tunnel roof decreases.

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 PFC3D 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.

Particle Flow Modeling of Rock Blocks with Nonpersistent Open Joints under Uniaxial Compression

AbstractIn this study, numerical simulation of rock blocks with nonpersistent open joints under uniaxial compression was undertaken using the particle flow modeling method. First, the micromechanical parameter values of intact material were calibrated through a trial-and-error process using macromechanical laboratory test results. Then, a back-analysis procedure was used to calibrate the joint gap and joint micromechanical parameter values using laboratory test results conducted on jointed rock blocks. Afterward, the effects of joint dip angle, joint persistency, and joint gap on the mechanical behavior of block models having nonpersistent open joints was investigated using the calibrated micromechanical parameter values. The joint dip angle and joint persistency were found to play significant roles in the failure mode, strength, and stress–strain relationship of jointed blocks. The joint gap played a significant to negligible role in the mechanical behavior of jointed block models gradually when the join...

Integration of three dimensional discontinuous deformation analysis (DDA) with binocular photogrammetry for stability analysis of tunnels in blocky rockmass

This paper demonstrates and evaluates an integrated system coupling 3-dimensional binocular photogrammetry and discontinuous deformation analysis (DDA) for stability analysis of tunnels in blocky rockmass. 3D DDA is an effective numerical method for discontinuous and large displacement problems. In stability analysis of rockmass, especially for tunnels, one of the primary bottlenecks in 3D DDA is generating discontinuous models. For tunnel constructions requiring small interval time for blasting, traditional geotechnical data collection approaches such as hand-mapping and geologic compass are always lacking in efficiency and accuracy. 3D photogrammetry is an economical and efficient method for collecting geometric features and surface data. An integrated system connecting geometrical data acquisition and discontinuous numerical analysis automatically, is valid for improving safety in tunnel construction. This system includes photogrammetry module, modeling module and analysis module. In the photogrammetry module, binocular photogrammetry devices and image reconstruction technique are implemented for geometric data of rockmass. Then location, dip direction and dip angle of joints as well as other geometric information of tunnels are input into the modeling module to generate 3-dimensional discontinuous models. The analysis module using 3-dimensional DDA to evaluate the stability of the surrounding rock in tunnels. The integrated system is implemented in an engineering instance, Suocaopo Tunnel in Guizhou, China.

High internal pressure induced fracture patterns in rock masses surrounding caverns: Experimental study using physical model tests

This research has experimented with artificial rock specimens that contain a circular or oblong hole using a novel physical modeling approach to investigate the failure behavior of rock masses surrounding a cavern with high internal pressure. The knowledge of fracture initiation and propagation in rock masses is crucial to the selection of underground storage construction sites and a stability evaluation method. Specifically, this research attempts to investigate the fracture initiation and propagation direction of the rock mass that encloses an underground gas/air storage cavern with high internal pressure under varying control conditions. The experiments are carried out with 200-time scaled-down artificial rock specimens under varying stress ratio (ki), overburden stress (σvi) and the joint's dip angle (α). The internal pressure applied to the hole is gradually increased until the fracture is initiated and propagates. Photogrammetric analysis is utilized to identify the rock mass response induced by increasing internal pressure and the occurrence of failure path. The experimental results indicate that the fracture initiation point and propagation direction of failure path are strongly influenced by the in-situ stress ratio, k. The findings also reveal that the site with an initial in-situ stress ratio greater than one is suitable for use as a high-internal-pressure underground cavern. In addition, in the presence of a nearby joint, the site with a smaller dip angle joint is more desirable. For the same in-situ stress ratio, an oblong-shaped cavern is more stable than a circular cavern (tunnel) of similar size, although both cavern configurations exhibit the similar failure patterns.

Mechanical and fracture behavior of rock mass with parallel concentrated joints with different dip angle and number based on PFC simulation

Rock mass is an important engineering material. In hydropower engineering, rock mass of bank slope controlled the stability of an arch dam. However, mechanical characteristics of the rock mass are not only affected by lithology, but also joints. On the basis of field geological survey, this paper built rock mass material containing parallel concentrated joints with different dip angle, different number under different stress conditions by PFC (Particle Flow Code) numerical simulation. Next, we analyzed mechanical property and fracture features of this rock mass. The following achievements have been obtained through this research. (1) When dip angle of joints is 15° and 30°, with the increase of joints number, peak strength of rock mass has not changed much. But when dip angle increase to 45°, especially increase to 60° and 75°, peak strength of rock mass decreased obviously with the increase of joints number. (2) With the increase of confining stress, peak strengths of all rock mass have different degree of improvement, especially the rock mass with dip angle of 75°. (3) Under the condition of no confining stress, dip angle of joints is low and joint number is small, existence of joints has little influence on fracture mode of rock mass, but when joints number increase to 5, tensile deformation firstly happened at joints zone and further resulted in tension fracture of the whole rock mass. When dip angle of joints increases to 45°, fracture presented as shear along joints, and with increase of joints number, strength of rock mass is weakened caused by shear-tension fracture zone along joints. When dip angle of joints increases to 60° and 75°, deformation and fracture model presented as tension fracture zone along concentrated joints. (4) Influence of increase of confining stress on fracture modes is to weaken joints' control function and to reduce the width of fracture zone. Furthermore, increase of confining stress translated deformation mode from tension to shear.

Numerical study to estimate the cutting power on a disc cutter in jointed rock mass

The Linear Cutting Machine (LCM) test is known to be a reliable method for designing disc cutters by estimating the penetration performance of a Tunnel Boring Machine (TBM). The LCM test is a laboratory test that uses large-scale rock blocks to estimate the real-scale load on disc cutter; the LCM test offers many advantages, but the costs of preparing the rock samples and performing the laboratory test are too high. In addition, the test results from jointed rock blocks are rarely trustworthy because of the difficulty of performing the LCM test with this type of rock sample. To compensate for the failings of the laboratory test, many studies have been conducted to develop a suitable numerical model. However, few studies have investigated the mechanism of rock fragmentation as a function of the joint spacing and the joint direction. This paper presents the results of a Discrete Element Analysis (DEA) of the cutting power exerted on a TBM disc cutter at various joint dip angles, dip directions and spacings. The results show that the cutting power must be increased when the disc cutter advances in a direction opposite to the joint dip. The numerical analysis also shows that the cutting power of a disc cutter increases as the joint dip angle decreases and the joint spacing decreases. Comparing the relationship between the cutting power of the disc cutter and the distribution pattern of the rock joints with the LCM test results can be used to increase the reliability of TBM performance predictions.

Rock Cavern Stability Analysis Under Different Hydro-Geological Conditions Using the Coupled Hydro-Mechanical Model

Rock cavern stability has a close relationship with the uncertain geological parameters, such as the in situ stress, the joint configurations, and the joint mechanical properties. Therefore, the stability of the rock cavern should be studied with variable geological conditions. In this paper, the coupled hydro-mechanical model, which is under the framework of the discontinuous deformation analysis, is developed to study the underground cavern stability when considering the hydraulic pressure after excavation. Variable geological conditions are taken into account to study their impacts on the seepage rate and the cavern stability, including the in situ stress ratio, joint spacing, and joint dip angle. In addition, the two cases with static hydraulic pressure and without hydraulic pressure are also considered for the comparison. The numerical simulations demonstrate that the coupled approach can capture the cavern behavior better than the other two approaches without the coupling effects.

Mechanical behavior of rock-like jointed blocks with multi-non-persistent joints under uniaxial loading: A particle mechanics approach

By selecting appropriate micro-mechanical parameter values through a trial and error procedure, the computer code PFC3D was used to study the macro-mechanical behavior of jointed blocks having multi-non-persistent joints with high joint density under uniaxial loading. The focus was to study the effect of joint orientation, size and joint mechanical properties on jointed block strength, deformability, stress–strain relation and failure modes at the jointed block level. Both the uniaxial compressive strength of the block, UCSB, and block deformability modulus, DMB, were found to depend heavily on the joint dip angle, β, and joint continuity factor, k. The joint particle stiffness was found to play a minor to a significant role on UCSB depending on β and k values. The joint particle stiffness was found to play a negligible to a moderate role on DMB depending on β and k values. The jointed blocks produced three types of stress–strain curves labeled as Type I through Type III. A relation seems to exist as explained in Section 4 of the paper between the types of curves and β and k values. The dominance of tensile failures over the shear failures was observed for all three types of curves based on the micro-mechanical parameter values used in the paper. The UCSB, rate of bond failures and the number of bond breakages were found to decrease as the curve type moves from Type I to Type III through Type II. The jointed blocks resulted in 4 failure modes as follows: (1) splitting failure; (2) plane failure; (3) stepped path and (4) intact material failure. The main features of each failure mode and possible relations between the failure modes, UCSB and β and k values are given in Section 5 of the paper.

Development of a probabilistic block theory analysis procedure and its application to a rock slope at a hydropower station in China

The paper deals with a development of a new procedure to perform probabilistic block theory analysis using the vector method. The procedure includes the following three main features: (a) the joint orientation is considered as a bivariate distribution including the correlation that exists between the joint dip angle and dip direction angle; (b) the variability of discontinuity shear strength is incorporated in the analysis; (c) it performs deterministic and probabilistic block theory analysis including the mode of failure (MOF), maximum safe slope angle (MSSA), factor of safety (FS) and the instability probabilities for different cut slope dip angles (CSDA) and different MOFs. A rock slope at a hydropower station in China is used as a case study to illustrate the use of the developed procedure. The slope has six sets of discontinuities. Therefore, both deterministic and probabilistic block theory analyses were conducted for 4-plane, 5-plane, 6-plane and 7-plane blocks to study the stability of the rock slope under different MOFs and CSDA. For most MOFs, the MSSA obtained through deterministic analysis were found to be significantly higher than the CSDA obtained corresponding to zero instability cumulative probability. The differences obtained between the deterministic and probabilistic analyses resulted mainly due to the variability of discontinuity orientation. The case study illustrated the importance and superiority of the probabilistic analysis over the deterministic analysis. For the investigated rock slope by selecting the CSDA of 40° and 50°, the total instability cumulative probability can be limited to less than 5% and 10%, respectively.

Test Study on Dynamic Mechanical Property of Jointed Rock Mass

Failure modes of jointed rock mass with different joint dip angle, joint center continuity degree, joint sets, load strain ratio and joint filling width under SHPB test are studied with model tests. The results show that failure modes and dynamic strength of jointed rock mass are much related to joint geometry. To rock mass with a single joint, its strength and failure mode are greatly controlled by the joint dip angle. The dynamic strength of the samples with joint dip angle 0° and 90°, whose failure modes are both tensile failure, is 90% and 71% of that of intact one, respectively. The dynamic strength of the samples with joint dip angle 60° is nearly zero. The dynamic strength of the samples with joint dip angle 30° and 45°, whose failure modes are mainly shear failure with partly tensile failure, is 50% and 18% of that of intact ones, respectively. The dynamic strength of the samples with 1/4, 1/2 and 4/5 joint center continuity degree is 95%, 74% and 28% of that of intact one, respectively. The dynamic strength of the samples with 1, 2 and 3 sets of joints is 54%, 23% and 10% of that of intact one, respectively. The dynamic strength of the intact and jointed samples both increases with load strain ratio, and the sensitivity to load strain ratio of the former is much higher than that of the latter, whose failure mode becomes more complicated accordingly. With increase of joint fillings width, the samples dynamic strength decreases gradually, but its failure mode does not change.

Experimental Research on Failure Modes of Specimen Containing Non-Coplanar Joints

To study the failure mechanism and failure mode of jointed rock under compressive-shear, many rock-like material specimens containing non-coplanar joints were made and a series of experiments were carried out. In the experiments, mica sheets were used as joint fillings, cement mortar was selected as rock-like material. Joints were made by inserting the mica sheet in cement mortar before initial setting. Mica sheets were left down as joint fillings. The results of experiments show that the dip angles of major joint have important influence on the failure mode of specimens. And the emerging position of wing cracks which exist in the prophase of specimens failure process changes with the dip angle. The shear strength of specimens has an important relationship with the dip angle of major joints. The smallest shear strength happens in the specimen with a joint angle of 15°, while the biggest value happens in 60°.

Test Study on Effect of Joint on Rock Mass Mechanical Property

Strength, elastic modulus and failure modes of samples with different joint dip angle, joint continuity degree, joint set and joint width under uniaxial compressive load are studied by similar material tests. To the sample with single joint, its strength, elastic modulus and failure mode are controlled by joint dip angle to much extent, and it will shear and slip along or split across joint face to fail, in which sample’s strength and elastic modulus is minimum corresponding to the first failure mode. To the jointed sample with different joint continuity degree, its strength and elastic modulus decrease with joint continuity degree, and effect on sample’s strength is much more serious. With increase of joint set, sample’s strength and elastic modulus gradually decrease. With increase of joint width, strength and elastic modulus decreases, and effect on sample’s strength is much more serious.

Unsymmetrical load effect of geologically inclined bedding strata on tunnels of passenger dedicated lines

In order to study the unsymmetrical load effect in geological bedding strata for the Muzhailing tunnel on the Lanzhou-Chongqing passenger dedicated line in China, we investigated the deformation, mechanical response and pressure of the surrounding rock and the mechanical characteristics of bolts of the tunnel. The results suggest that open zones appear at arch and invert where joints open up, when layered stratum is horizontal, or when the dip angle of inclined bedding is small. Open zones occur perpendicular to a joint. The failure mode is bending disjunction at the arch. What’s more, with the dip angle increasing, tunnel excavating would induce shear zones where joints experience a certain shear displacement, and lead to obvious geological bedding unsymmetrical load. The failure mode is shear damage. For the joint dip angle in the range of 75–90°, the failure mode is flexural crushing at the wall and vertical shear rupture above the arch. The restraining effect of two sides weakens for vertical dip. On the whole, shear failure instability trend would occur and the tunnel collapses evenly. When the angle between the bolt and structure plane is greater than 23°, bolts can enhance the shearing stiffness of joint plane. Unfortunately, in the general purpose graph of tunnel for 250 km/h of passenger dedicated lines, the bolts have equal length and spacing. The rationale behind this is worthy of further study. For inclined bedding, the surrounding rock pressure at the left wall is more than that at the right wall. In addition, lining is likely to be damaged at left shoulder and side wall. With the dip angle increasing, the unsymmetrical load gradually achieves symmetry. Asymmetry design for support is recommended to reduce the unsymmetrical load on lining disturbed by excavation.