Jointed Rock Block Research Articles

Effect of joints on microwave fracturing of the Bukit Timah granite using an open‐ended antenna

AbstractMicrowave fracturing of rocks before mechanical breakage could improve the performance of mechanical excavators and reduce environmental impacts. Previous research focused on the microwave fracturing of intact rock blocks. By using an open‐ended antenna, this paper investigates the effect of pre‐existing joints on the microwave fracturing of the Singapore Bukit Timah granite blocks. The results show that the specimens are weakened in the manners of cracking, spalling, melting, or a combination of them. The crack number and the total crack length produced by microwave treatment of jointed rock blocks are slightly smaller than those in the intact rock blocks. The interaction between joints and microwave‐induced cracks can be summarized into the following four patterns: (1) microwave‐induced cracks become arrested so that the crack propagation is terminated; (2) microwave‐induced cracks penetrate the joints and continue to propagate; (3) microwave‐induced cracks become deflected along the joints; and (4) microwave‐induced cracks propagate forward following the joints. The smaller the approach angle between the microwave‐induced crack and the pre‐existing joint is, the more microwave‐induced cracks tend to be arrested at the joint. Increasing the approach angle between the microwave‐induced crack and the joint can increase the chance of microwave‐induced crack penetrating the joint. The results also show that the smaller the distance is between the microwave radiation point and the joint, the easier it is for microwave‐induced cracks to penetrate the joints; otherwise, the microwave‐induced crack is more likely to be arrested at the pre‐existing joint.

Analysis and prediction of brittle failure in rock blocks having a circular tunnel under uniaxial compression using acoustic Emission technique: laboratory testing and numerical simulation

In this research, the failure mechanism and anomalous behavior of intact and jointed rock block having a circular tunnel under compression are studied. This was done by monitoring the progressive failure of a rock tunnel subjected to uniaxial loading. The tests were conducted in sandstone blocks and “Acoustic Emission” (AE) technique was used to identify the crack damage and other failure attributes. Three cases have been considered in the research, i.e. tunnel in the intact rock, with horizontal joints, and with vertical joint sets. Images of progressive failure, acoustic signals, and applied loads were simultaneously recorded during the test. The intact block demonstrates continuous crack generation while the block with horizontal joint set shows a stepwise cracking pattern. In the third case where the vertical joints were employed, the deformation was largely roof failure and joint perpendicular extension was dominant. The AE events show that a sudden drop and then a quiet period of seismic “Ib value” could be considered as the precursors to forecast the rockburst hazard. The paper also compares the results of the physical model test with a 2D finite element model. The compared results show good agreement between the physical and numerical models.

Modelling of thermo-mechanical effects in a rock quarry wall induced by near-surface temperature fluctuations

Mechanical response of rock masses to the periodical fluctuation of surface temperatures is a relevant topic to be focused on for the comprehension of short- to long-term stability of rock slopes continuously stressed by slight yet periodical actions. To better understand how preparatory factors can lead rock masses toward failure, thermal and strain monitoring activities are ongoing in a selected quarry wall, where an experimental test-site was implemented on a jointed rock block. The role of pervasive joints in perturbing the heating-cooling process of the rock mass and their related effects was already output by direct and remote monitoring. For these purposes, FDM numerical modelling was focused on the comprehension and simulation of induced mechanical effects due to cyclical thermal forcings over a simplified 2D half-space. Therefore, the role of rock mass joints on both heat propagation and resulting thermo-mechanical effects was analysed, basing on monitoring data, field experiments and numerical modelling. Numerical results output the role of periodic daily, annual and compound thermal input in inducing irreversible deformation along joints under different rock mass jointing configuration, as a function of the intrinsic period of thermal input and joint attitude. Temperature distribution across joints was also analysed as a possible constraining effect for time and space distribution of yielding. A sensitivity analysis of heat propagation conditions was performed in order to infer the role of joint sets on induced thermo-mechanical strains at the rock block scale. A non-negligible influence on surficial as well as deeper portions of the rock mass was observed, highlighting how near-surface temperature fluctuations can drive the long-term evolution of rock block conditions towards failure.

Thermal Response of Jointed Rock Masses Inferred from Infrared Thermographic Surveying (Acuto Test-Site, Italy).

The Mediterranean region is affected by considerable daily and seasonal temperature variations due to intense solar radiation. In mid-seasons, thermal excursions can exceed tens of degrees thus influencing the long-term behaviour of jointed rock masses acting as a preparatory factor for rock slope instabilities. In order to evaluate the thermal response of a densely jointed rock-block, monitoring has been in operation since 2016 by direct and remote sensing techniques in an abandoned quarry in Acuto (central Italy). Monthly InfraRed Thermographic (IRT) surveys were carried out on its exposed faces and along sections of interest across monitored main joints. The results highlight the daily and seasonal cyclical behaviour, constraining amplitudes and rates of heating and cooling phases. The temperature time-series revealed the effect of sun radiation and exposure on thermal response of the rock-block, which mainly depends on the seasonal conditions. The influence of opened joints in the heat propagation is revealed by the differential heating experienced across it, which was verified under 1D and 2D analysis. IRT has proved to be a valid monitoring technique in supporting traditional approaches, for the definition of the surficial temperature distribution on rock masses or stone building materials.

Effect of Joint Micro Mechanical Parameters on a Jointed Rock Block Behavior Adjacent to an Underground Excavation: A Particle Flow Approach

The particle flow code PFC3D was utilized to investigate mechanical behavior of jointed rock blocks, having non-persistent filled joints, located adjacent to an underground excavation. The focus of this investigation was to study the effect of filled joint micro-mechanical properties on the failure mode and strength of jointed rock blocks by varying the joint orientation. The smooth joint model was incorporated to create non-persistent filled joints having the following micro-mechanical parameters: the joint normal stiffness, joint shear stiffness, bonded joint friction angle, bonded joint cohesion, joint friction coefficient and joint tensile strength. The joint normal and shear stiffness were found to affect the mechanical behavior of jointed rock blocks having joint dip angles less than or equal to 45°. Increase of the bonded joint cohesion increased the strength significantly of jointed blocks having dip angles 30° through 90°. Increase of the bonded joint friction angle increased the strength significantly of jointed blocks having dip angles 30° through 60°. Increase of the joint friction coefficient slightly increased the strength of jointed blocks having dip angles 30° through 60°. Effect of the joint tensile strength on the mechanical behavior of jointed blocks was found to be negligible apart from the jointed block which had 90° dip angle, which showed slight affect. The applied stress path in the conducted study resulted in more shear fractures compared to the tensile fractures in the joint segments. The results indicated the importance of using proper micro-mechanical parameter values to obtain realistic behavior of jointed rock masses in investigating stability of underground excavations using PFC3D.

Dynamic Fragmentation of Jointed Rock Blocks During Rockslide‐Avalanches: Insights From Discrete Element Analyses

AbstractThe dynamic fragmentation of jointed rock blocks during rockslide avalanches has been investigated by discrete element method simulations for a multiple arrangement of a rock block sliding over a simple slope geometry. The rock blocks are released along an inclined sliding plane and subsequently collide onto a flat horizontal plane at a sharp kink point. The contact force chains generated by the impact appear initially at the bottom frontal corner of the rock block and then propagate radially upward to the top rear part of the block. The jointed rock blocks exhibit evident contact force concentration and discontinuity of force wave propagation near the joint, associating with high energy dissipation of granular dynamics. The corresponding force wave propagation velocity can be less than 200 m/s, which is much smaller than that of an intact rock (1,316 m/s). The concentration of contact forces at the bottom leads to high rock fragmentation intensity and momentum boosts, facilitating the spreading of many fine fragments to the distal ends. However, the upper rock block exhibits very low rock fragmentation intensity but high energy dissipation due to intensive friction and damping, resulting in the deposition of large fragments near the slope toe. The size and shape of large fragments are closely related to the orientation and distribution of the block joints. The cumulative fragment size distribution can be well fitted by the Weibull's distribution function, with very gentle and steep curvatures at the fine and coarse size ranges, respectively. The numerical results of fragment size distribution can match well some experimental and field observations.

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...

Numerical simulation of a jointed rock block mechanical behavior adjacent to an underground excavation and comparison with physical model test results

Mechanical behavior of a jointed rock mass with non-persistent joints located adjacent to a free surface on the wall of an excavation was simulated under without and with support stress on the free surface using approximately 0.5m cubical synthetic jointed rock blocks having 9 non-persistent joints of length 0.5m, width 0.1m and a certain orientation arranged in an en echelon and a symmetrical pattern using PFC3D software package. The joint orientation was changed from one block to another to study the effect of joint orientation on strength, deformability and failure modes of the jointed blocks. First the micro-mechanical parameters of the PFC3D model were calibrated using the macro mechanical properties of the synthetic intact standard cylindrical specimens and macro mechanical properties of a limited number of physical experiments performed on synthetic jointed rock blocks of approximately 0.5m cubes. Under no support stress, the synthetic jointed rock blocks exhibited the same three failure modes: (a) intact rock failure, (b) step-path failure and (c) planar failure under both physical experiments and numerical simulations for different orientations. The jointed blocks which failed under intact rock failure mode and planar or step-path failure mode produced high and low jointed block strengths, respectively. Three phases of convergence of free surface were discovered. The joint orientation and support stress played important roles on convergence magnitude. The average increment of jointed block strength turned out to be about 10, 7.9 and 6.6 times the support stress when support stresses of 0.06MPa, 0.20MPa and 0.40MPa were applied, respectively. The modeling results offer some guideline in support design for underground excavations.

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.

Experimental and Numerical Simulations of Jointed Rock Block Strength under Uniaxial Loading

To simulate brittle rocks, a mixture of sand, plaster of paris, and water was used as a model material. Thin galvanized sheets were used to create joints in blocks made out of the model material. To investigate the failure modes and strength, 30 × 12.5 × 8.6 cm jointed model material blocks having different joint geometry configurations were subjected to uniaxial compressive loading. Results indicated three failure modes: (1) tensile failure through intact material; (2) combined shear and tensile failure or only shear failure on joints; and (3) mixed failure of the above two modes depending on the joint geometry. The fracture tensor component in a certain direction quantifies the directional effect of the joint geometry, including number of fracture sets, fracture density, and probability distributions for size and orientation of these fracture sets. Results obtained from the experiments were used to develop a strongly nonlinear relation between the fracture tensor component and the jointed block strength...

Physical and particle flow modeling of jointed rock block behavior under uniaxial loading

Abstract Laboratory experiments and numerical simulations, using Particle Flow Code (PFC3D ), were performed to study the behavior of jointed blocks of model material under uniaxial loading. The effect of joint geometry parameters on the uniaxial compressive strength of jointed blocks was investigated and this paper presents the results of the experiments and numerical simulations. The fracture tensor component in a given direction is used to quantify the combined directional effect of joint geometry parameters including joint density, orientation and size distributions, and the number of joint sets. The variation of the uniaxial compressive strength of the jointed blocks of the model material with the fracture tensor component was investigated. Both the laboratory experiments and the numerical simulations showed that the uniaxial block strength decreases in a nonlinear manner with increasing values of the fracture tensor component. It was observed that joint geometry configuration controls the mode of failure of the jointed blocks and three modes of failure were identified, namely (a) tensile splitting through the intact material, (b) failure by sliding along the joint plane and/or by displacement normal to the joint plane and, (c) mixed mode failure involving both the failure mechanisms in (a) and (b). It has also been shown that with careful parameter calibration procedures, PFC3D could be used to model the strength behavior of jointed blocks of rock.

Effect of Block Size and Joint Geometry on Jointed Rock Hydraulics and REV

The effect of joint geometry parameters and the size of jointed rock block on the hydraulic properties of jointed rock, including the equivalent continuum behavior, was investigated through numerical experimentation. The chance to reach equivalent continuum behavior for a rock mass having a certain joint configuration increases with the increase of block size. The REV (representative elementary volume) size to show hydraulic equivalent continuum behavior for a jointed rock system was found to depend on the orientation of the joint sets; the REV size seems to decrease with increasing joint density and joint size. The REV does not seem to exist for some rock masses having joint systems with low relative orientation angles (systems with two joint sets) and low densities. The average block permeability (K0) value at the REV size for a joint system increases with increase in joint size and joint density. The equivalent continuum behavior of a joint system can be expressed with respect to a cut-off value for the first invariant of fracture tensor (F0). The block size corresponding to the aforementioned cut-off F0 (between 10 and 30 for the joint systems investigated in this study) can be considered to provide the REV size for a given joint configuration. A threshold value for F0 (a joint network having two perpendicular joint sets produced 2.75) can be used to find the chance for a given joint network to have nonzero block permeability. A strong power functional relation seems to exist between the directional permeability and the fracture tensor component for the connected joint configuration when rock blocks contain minor discontinuities.

Effect of Joint Geometry and Transmissivity on Jointed Rock Hydraulics

The effect of joint geometry parameters (joint orientation, joint density, and joint size) and joint transmissivity on the hydraulic properties of jointed rock blocks was examined in two dimensions using joint networks with two joint sets through numerical experiments. The chance for equivalent continuum hydraulic behavior and the average block permeability of jointed rock were found to be very sensitive to the relative orientation of the joint sets in a joint network system. Although both the equivalent continuum behavior and the average block permeability seem to be little affected by the change of the standard deviation of the joint orientation distribution, the degree of permeability anisotropy seems to depend on the standard deviation of the joint orientation especially for anisotropic joint systems. Chance for the equivalent continuum behavior and the average block permeability were found to increase with the increase of joint density and joint size. The chance for equivalent continuum behavior and the permeability anisotropy were found to depend on the distribution of joint hydraulic conductivity. The influence of the joint hydraulic conductivity is more prominent for joint systems with high joint density and large joint sizes. The greater the variation of the hydraulic conductivity value among the joints in a joint set, the lesser the chance for the medium to behave like an equivalent continuum porous medium.