Strength Of Jointed Rock Mass Research Articles

Weakening mechanism of shear strength of jointed rock mass considering the filling characteristics

Weakening mechanism of shear strength of jointed rock mass considering the filling characteristics

Numerical simulation study on shear resistance of anchorage joints considering tensile–shear fracture criterion of 2G-NPR bolt

2G-NPR bolt (the 2nd generation Negative Poisson’s Ratio bolt) is a new type of bolt with high strength, high toughness and no yield platform. It has significant effects on improving the shear strength of jointed rock mass and controlling the stability of surrounding rock. To achieve an accurate simulation of bolted joint shear tests, we have studied a numerical simulation method that takes into account the 2G-NPR bolt's tensile–shear fracture criterion. Firstly, the indoor experimental study on the tensile–shear mechanical properties of 2G-NPR bolt is carried out to explore its mechanical properties under different tensile–shear angles, and the fracture criterion of 2G-NPR bolt considering the tensile–shear angle is established. Then, a three-dimensional numerical simulation method considering the tensile–shear mechanical constitutive and fracture criterion of 2G-NPR bolt, the elastoplastic mechanical behavior of surrounding rock and the damage and deterioration of grouting body is proposed. The feasibility and accuracy of the method are verified by comparing with the indoor shear test results of 2G-NPR bolt anchorage joints. Finally, based on the numerical simulation results, the deformation and stress of the bolt, the distribution of the plastic zone of the rock mass, the stress distribution and the damage of the grouting body are analyzed in detail. The research results can provide a good reference value for the practical engineering application and shear mechanical performance analysis of 2G-NPR bolt.

An Improved Method for Estimating the Strength of Jointed Rock Mass Using Drilling Technology

During underground excavations for civil and mining engineering purposes, the variations in mechanical properties of jointed rock mass are important, especially for the design of underground structures. The unreasonable evaluation of strength properties for rock masses can lead to the structure instability of underground construct, which subsequently causes collapse accidents in underground engineering and poses a serious threat to the structures safety. Rock mass classification system is an effective method for determining the strength parameters for rock masses. Firstly, the existing approaches for determining the strength parameters of rock masses are reviewed using classification systems such as the joint factor, jointing index methods, rock mass rating (RMR), Q-system (Q), geological strength index (GSI), and rock quality designation (RQD). Since in some cases, rock quality designation (RQD) is the only information available, a comparative study with other methods then is conducted to verify the limitation and deficiencies of RQD method. On this basis, a newly developed method from RQD classification system based on drilling special energy is established to estimate the rock mass strength. Finally, the developed method is applied in the tunnel engineering in the Daheba hydropower station in Hanjiang to the Weihe River project of China, and the results are compared with those from the previous works. The in situ test results show that the developed method can give appropriate values compared to various suggested relationships from RMR, Q, and GSI. The results can be treated as an important design reference for the prevention and treatment of surrounding rock instability and underground excavations in the Daheba hydropower station.

Fractal Statistical Study on the Strength of Jointed Rock Mass

The failure probability and strength of jointed and fractured rock mass under water pressure action place important constraints on the safety and stability of rock mass engineering. In this study, two strength calculation models for rock mass joint rupture failure and shear slip failure are established according to different relative sizes and working conditions, and the effects of fractal dimension, water pressure, and number of cracks on the strength and failure probability of rock mass are analyzed. The most unfavorable dip angle under shear-slip conditions is calculated. The proposed models accurately reflect the failure probability and strength of jointed rock mass, and different strength calculation methods should be selected according to the specific working conditions. The effect of water pressure on the strength of jointed rock mass is sensitive to the failure mode. Reasonable drainage should be carried out according to the different failure modes when constructing tunnels with abundant groundwater, and certain drainage or water plugging construction schemes are not necessarily applicable to all cases. The dip angle at which joint fissures are most prone to shear slip is given as π/4 + φ/2, where φ is the jointed rock mass strength parameter. An increase in the fractal dimensions, external stress, and number of fractures will enhance the rock mass failure probability regardless of failure mode. Controlled blasting and advanced grouting reinforcement measures are recommended when a tunnel traverses through a joint fissure zone to reduce the number of cracks generated in the surrounding rock and thus improve its strength and stability.

Strength criterion of rock mass considering the damage and effect of joint dip angle

It is highly significant to theoretically assess the effect, under load, of initial stress and structure on the mass damage of rock mass. In this reported study, first a multi-factor coupling damage constitutive model under the action of joint-load was established by fully considering the non-uniformity, anisotropy and initial structure of a rock mass based on the Weibull distribution and D-P criterion. The relationship between the damage evolution and joint angle in the rock mass was elaborated. Then, a jointed rock mass strength criterion was built in line with the D–P criterion and the limit state of rock mass failure by the method of multivariate function total differential as based on the constitutive model. The results showed that the established constitutive model was in good agreement with the test results, which accurately reflected the damage characteristics of jointed rock mass during the entire loading process. The initial damage value of the rock mass increased with increasing joint dip angles, and the damage evolution of the jointed rock mass could be divided into the initial, stable, accelerated and failure damage stages. Comparing the results of this approach with other methods it was found that the strength criterion better reflected the effects of minimum principal stress σ3, volume stress σm, shear stress J21/2 and joint dip angle β on rock mass strength than other existing strength criteria, which showed that the proposed method offered important guiding principles for the engineering practice.

Hybrid machine learning-based triaxial jointed rock mass strength

Hybrid machine learning-based triaxial jointed rock mass strength

Roughness Evaluation for Distinguishing Fresh and Sheared Rock Joint Surfaces with Different Sampling Intervals

Abstract The subtle alteration of surface geometry from a fresh surface to a sheared surface usually results in a considerable variation in the shear strength of jointed rock mass. Through profilin...

3-D Rock Mass Strength Criteria—A Review of the Current Status

The presence of complex discontinuity patterns, the inherent statistical nature of their geometrical parameters, the uncertainties involved in the estimation of the discontinuity geometrical and geo-mechanical properties and complex three dimensional (3-D) in-situ stress make the accurate prediction of rock mass strength a difficult task. It has been a great challenge for the rock mechanics and rock engineering professions to develop a rock mass strength criterion in three dimensions that incorporates the effect of the minor and intermediate principal stresses and captures the scale dependent and anisotropic properties resulting from the discontinuity geometry parameters, such as the number of discontinuity sets, 3-D discontinuity intensity, and the distributions of the discontinuity orientation and size. Rock mechanics and rock engineering researchers have dealt with this topic for more than 55 years. The paper provides a critical review of the current state of the art regarding 3-D jointed rock mass strength criteria. The shortcomings of several rock mass strength criteria are discussed. The historic development of rock mass strength criteria that incorporate the effect of the minor and intermediate principal stresses and capture the scale dependent and anisotropic properties is presented. The most advanced 3-D rock mass strength criteria currently available in the literature are presented, including suggested future improvements.

A Simple Three-Dimensional Failure Criterion for Jointed Rock Masses under True Triaxial Compression

The joint configuration and the intermediate principal stress have a significant influence on the strength of rock masses in underground engineering. A simple three-dimensional failure criterion is developed in this study to predict the true triaxial strength of jointed rock masses. The proposed failure criterion in the deviatoric and meridian planes adopts the elliptic and hyperbolic forms to approximate the Willam–Warnke and Mohr–Coulomb failure criterion, respectively. The four parameters in the proposed failure criterion have close relationships with the cohesion and the internal friction angle and can be linked with the joint inclination angle using a cosine function. Two suits of true triaxial strength data are collected to validate the correctness of the proposed failure criterion. Compared with other failure criteria, the proposed failure criterion is more reasonable and acceptable to describe the strength of jointed rock masses.

Strength anisotropy of jointed rock slope considering mining damage: a numerical analysis method

A combination of Hoek–Brown strength criterion and geological strength index (GSI) rock mass classification is a common method to determine rock mass strength. However, the conventional method involving the determination of disturbance coefficient (D) cannot reflect the anisotropic characteristics of strength of jointed rock mass when considering mining damage. A numerical analysis method is proposed to determine the value of D using jointed rock damage tensor and mining damage evolution simulation. The anisotropy of rock mass strength at different mining depths for a high jointed rock slope in Dagushan open-pit mine was analyzed by mining damage simulation. Additionally, the Rosenblueth moment point estimation principle was combined with Monte-Carlo method to obtain the random strength of a jointed rock mass considering the effects of mining probability damage. The results show that the influence of the unloading due to excavation to stress state of rock mass is relatively obvious within 100 m away from the slope surface, however, the area of drastic change of damage is limited to a range of ∼50 m. Moreover, the reliability of local slope, analyzed using limit equilibrium method (Janbu), shows that consideration of spatiotemporal anisotropy of rock mass with mining damage is closer to reality.

Numerical investigation into strength and deformability of veined rock mass

Veined rock masses can be found in porphyry copper deposits like at the El Teniente mine in Chile. Rock masses with cemented joints or veins cannot be accurately represented using the methods commonly applied to predict the deformability and strength of jointed rock masses. Laboratory tests on veined rock andesite from the El Teniente mine have shown that the veins control the fracturing process and strength. In this work, the behavior of veined rock was studied using a numerical model, built in the finite element software ABAQUS. In this analysis, the rock mass was modeled as an assemblage of blocks bonded by layers, representing the intact rock and veins respectively. Uniaxial compression tests on veined core-size specimens were simulated and their results compared to experimental data. The results show good agreement with the experiments in terms of strength, stiffness and mode of failure. After this, a veined rock mass volume was modeled using the orientation of mapped and post-processed discontinuities as input. Simulations of uniaxial and triaxial tests were performed on the modeled veined rock mass. The observed failure mode confirms experimental results, showing that at low confinement the failure is concentrated in the veins and the rupture of intact rock becomes significant at higher confinement. The obtained strength envelope of the veined rock mass is non-linear and its shape is independent of the strength of the intact rock; however, the Hoek-Brown criterion did not fit the results. The residual strength of the veined rock approximated the residual strength of the intact rock, agreeing with previously reported observations in jointed rock.

Influence of degree of interlock on confined strength of jointed hard rock masses

The strength of jointed rock mass is strongly controlled by the degree of interlock between its constituent rock blocks. The degree of interlock constrains the kinematic freedom of individual rock blocks to rotate and slide along the block forming joints. The Hoek–Brown (HB) failure criterion and the geological strength index (GSI) were developed based on experiences from mine slopes and tunneling projects in moderately to poorly interlocked jointed rock masses. It has since then been demonstrated that the approach to estimate the HB strength parameters based on the GSI strength scaling equations (called the ‘GSI strength equations’) tends to underestimate the confined peak strength of highly interlocked jointed rock masses (i.e. GSI > 65), where the rock mass is often non-persistently jointed, and the intact rock blocks are strong and brittle. The estimation of the confined strength of such rock masses is relevant when designing mine pillars and abutments at great depths, where the confining pressure is high enough to prevent block rotation and free sliding on block boundaries. In this article, a grain-based distinct element modeling approach is used to simulate jointed rock masses of various degrees of interlock and to investigate the influences of block shape, joint persistence and joint surface condition on the confined peak strengths. The focus is on non-persistently jointed and blocky (persistently jointed) rock masses, consisting of hard and homogeneous rock blocks devoid of any strength degrading defects such as veins. The results from this investigation confirm that the GSI strength equations underestimate the confined strength of highly interlocked and non-persistently jointed rock masses. Moreover, the GSI strength equations are found to be valid to estimate the confined strength of persistently jointed rock masses with smooth and non-dilatant joint surfaces.

Slope stability assessment of an open pit using lattice-spring-based synthetic rock mass (LS-SRM) modeling approach

Abstract Discontinuity waviness is one of the most important properties that influence shear strength of jointed rock masses, and it should be incorporated into numerical models for slope stability assessment. However, in most existing numerical modeling tools, discontinuities are often simplified into planar surfaces. Discrete fracture network modeling tools such as MoFrac allow the simulation of non-planar discontinuities which can be incorporated into lattice-spring-based geomechanical software such as Slope Model for slope stability assessment. In this study, the slope failure of the south wall at Cadia Hill open pit mine is simulated using the lattice-spring-based synthetic rock mass (LS-SRM) modeling approach. First, the slope model is calibrated using field displacement monitoring data, and then the influence of different discontinuity configurations on the stability of the slope is investigated. The modeling results show that the slope with non-planar discontinuities is comparatively more stable than the ones with planar discontinuities. In addition, the slope becomes increasingly unstable with the increases of discontinuity intensity and size. At greater pit depth with higher in situ stress, both the slope models with planar and non-planar discontinuities experience localized failures due to very high stress concentrations, and the slope model with planar discontinuities is more deformable and less stable than that with non-planar discontinuities.

Effects of High Temperatures and Loading Rates on the Splitting Tensile Strength of Jointed Rock Mass

The splitting tensile tests on the artificial jointed rock mass specimens were carried out using cylindrical samples after being exposed to high temperatures. The effects of the dip angle of joint plane, the radial elastic modulus, the radial peak strain, the damage variable D, the loading rate and the high temperature on the tensile strength of the jointed rock mass were investigated. Test results show that: (1) when the past-exposed high temperature is the same and the dip angle β (the angle between the joint plane and the horizontal plane) of joint plane increases from 0° to 90°, both the tensile strength and the radial elastic modulus first decrease and then increase. However, the damage variable D increases with the dip angle β rising from 0° to 60°, and decreases with the dip angle β elevating from 60° to 90°. (2) When both the dip angle β and the loading rate are the same, as the exposed high temperature increases from room temperature 25 to 400 °C, the tensile strength, the radial elastic modulus decreases and the radial peak strain all gradually decrease, whereas the damage variable D increases gradually. (3) When both the dip angle β and the past-exposed temperature are the same, as the loading rate increases from 0.05 to 5.0 mm/min, the tensile strength and the radial elastic modulus both increase gradually, whereas the damage variable D decreases gradually.

An analytical model for strength of jointed rock masses

An analytical model of rock mass with non-persistent joints is utilized to develop linear and nonlinear failure criteria based on intact rock strength, joint shear strength and joint persistence. The analytical model matches the solutions for the strength of fracture-free and fully jointed rocks and provides a reasonable transition of strength for rocks with non-persistent joints. Consistent with empirical evidence, the model predicts that increasing joint intensity and decreasing joint shear strength cause a reduction in the cohesive and frictional components of rock mass strength. Joint persistence in the analytical model has not only a physical definition as the normalized jointed area, but also a mechanical interpretation as the normalized reduction in strength due to the presence of joints. The relationship between the analytical model and the empirical Hoek-Brown failure criterion with Geological Strength Index (GSI) is explored. The analytical model of rock mass is utilized to develop and calibrate a quantitative GSI chart. The analytical model is validated using the results of comprehensive tests on undisturbed samples of naturally jointed Panguna andesite and thermally granulated Carrara marble. Practical application of the analytical model is illustrated by numerical back analysis of displacements around the Nathpa Jhakri powerhouse cavern.

Shear Strength of Jointed Rock Mass with Different Joint Surface Characteristic in Subsea Tunnel

Jin, Q.; Hu, J.; Liu, H.-L.; Song, S.-G.; Liu, J.-P., and Cui, L.-Y., 2019. Peak shear strength of jointed rock mass in subsea tunnel. In: Gong, D.; Zhu, H., and Liu, R. (eds.), Selected Topics in Coastal Research: Engineering, Industry, Economy, and Sustainable Development. Journal of Coastal Research, Special Issue No. 94, pp. 301–305. Coconut Creek (Florida), ISSN 0749-0208.Jointed surrounding rock mass environment is often formed under seafloor sedimentary conditions. Since the mechanical properties of joints are important to control the stability of rock masses, the peak shear strength for the joints has always been the focus of relevant research. In this paper, artificially prepared jointed rock masses with different joint surface characteristic were used to perform multiple shear tests to obtain corresponding shear strength data. The research results showed that there were obvious relations between the shear strength of the joint surface morphology. With the increase of shear times, the maximum undulating angle and the roughness of the joint contour decreased and the shear strength decreased correspondingly.

Experimental and Numerical Study of the Uniaxial Compressive Stress-Strain Relationship of a Rock Mass with Two Parallel Joints

AbstractA “rock bridge”, defined as the closest distance between two joints in a rock mass, is an important feature affecting the jointed rock mass strength. Artificial jointed rock specimens with two parallel joint fractures were tested under uniaxial compression and numerical simulations were carried out to study the effects of the inclination of the rock bridge, the dip angle of the joint, rock bridge length, and the length of joints on the strength of the jointed rock mass. Research results show: (1) When the length of the joint fracture, the length of the rock bridge, and the inclination of the rock bridge stay unchanged, the uniaxial compressive strength of the specimen gradually increases as the inclination of the joint fracture increases from 0°to 90°. (2) When the length of the joint fracture, the length of the rock bridge, and the inclination of the joint fracture stay unchanged, the uniaxial compressive strength of the specimen shows variations in trends with the inclination of the rock bridge increasing from 30° to 150° (3). In the case when the joint is angled from the vertical loading direction, when the dip angle of the joint fracture, the inclination of the rock bridge, and the length of the rock bridge stay unchanged, the uniaxial compressive strength of the specimen gradually decreases with an increasing length of joint fracture. When the dip angle of the joint fracture, the inclination of the rock bridge, and the length of the joint fracture stay unchanged, the uniaxial compressive strength of the specimen does not show a clear trend with an increase of the length of the rock bridge.

Dynamic constitutive model of penetrating jointed rock mass based on ZTW model

According to the characteristics of stress-strain curves of soft rock with through joints under dynamic loading and the related research results of existing dynamic constitutive models of soft rock, a dynamic uniaxial compression constitutive model for soft rock with through joints is established. The model is based on Zhu-Wang-Tang model, in which damage body, joint plane closure element and shear deformation element are connected in series to simulate the static compressive mechanical properties of jointed rock mass, and Maxwell composite element is connected in series to describe the strain rate effect of jointed rock mass. First, the micro-damage model is described assuming that the integrity of the intact rock mass follows the tensile strain criterion and the statistical damage of the micro-intensity and damage satisfying the Weibull distribution; Secondly, the Mohr-Coulomb strength criterion is introduced into the strength damage variable to modify the macroscopic damage variable according to the shear failure characteristics of the through-jointed rock mass along the Plane-straight joint plane under uniaxial compressive load. Thirdly, based on the Lemaitre equivalent strain hypothesis, a coupled damage variable (tensor) considering the macroscopic and microscopic effects is established, and then the damage viscoelastic dynamic constitutive model of the jointed soft rock is established. Finally, the model is used to discuss the effects of joint cohesion, joint internal friction angle and joint dip angle on the dynamic mechanical properties of rock mass. The results show that the uniaxial compression dynamic peak strength of jointed rock mass increases with the joint cohesion and the internal friction angle of the joint, and U-shaped with the increase of the joint inclination angle, and the minimum value is obtained at 60°.

Incorporation of uncertainty in estimating the rock mass uniaxial strength using a fuzzy inference system

Due to its complex nature and inherent uncertainty, it is very hard to directly measure the uniaxial strength of jointed rock masses. In rock mechanics, it is a necessity to employ empirical equations and numerical methods for the estimation of rock mass uniaxial strength. In addition to the existing empirical equations and numerical methods, it is essential to pursuit approaches to incorporate uncertainty in estimating rock mass uniaxial strength. One of the promising techniques pertinent to such situations is the fuzzy inference system (FIS). In the application of FIS, it is expected that the problem or system under investigation should be uncertain and fuzzy, and calculations are drawn mostly from judgment and experience, rather than mathematical expressions. This study proposes a new method of estimating the rock mass uniaxial strength using fuzzy sets theory. For this purpose, three dominant elements essential for the determination of rock mass uniaxial strength; i.e., intact rock uniaxial strength, block size, and joint surface condition, were evaluated as fuzzy variables in the analysis involving the assignment of proper membership functions. A logical and systematic combination of three input variables was employed under a Mamdani rule-based FIS environment, which explicitly takes into account the expert knowledge using built lookup charts. To check the accuracy of the FIS model estimates, an extensive data set was compiled from the published sources. This data set used in the analysis covered a wide range of rock mass types in typical rock engineering projects. It was seen that the uniaxial strength values estimated by the proposed FIS model were mostly in fair agreement (R2 = 0.77) with those estimated by well-known empirical equations.

Development of New Three-Dimensional Rock Mass Strength Criteria

Two new three-dimensional rock mass strength criteria are developed in this paper by extending an existing rock mass strength criterion. These criteria incorporate the effects of the intermediate principal stress, minimum principal stress and the anisotropy resulting from these stresses acting on the fracture system. In addition, these criteria have the capability of capturing the anisotropic and scale dependent behavior of the jointed rock mass strength by incorporating the effect of fracture geometry through the fracture tensor components. The new criteria are proposed after analyzing 284 numerical modeling results of the polyaxial, triaxial and biaxial compression tests conducted on the jointed rock blocks having one or two joint sets by the PFC3D software. Some of these simulation results were compared with experimental results to validate the developed PFC3D model that was used for numerical modeling of jointed blocks. In this research to have several samples with the same properties a synthetic rock material that is made out of a mixture of gypsum, sand and water was used. Altogether, 12 joint systems were chosen; some of them had one joint set and the rest had two joint sets. Joint sets have different dip angles varying from 15° to 45° at an interval of 15° with dip directions of 30° and 75° for the two joint sets. Each joint set also has three persistent joints with the joint spacing of 42 mm in a cubic sample of size 160 mm. The minimum and intermediate principal stress combination values were chosen based on the uniaxial compressive strength (UCS) value of the modeled intact synthetic rock. The minimum principal stress values were chosen as 0, 0.2, 0.4 and 0.6 of the UCS. For each minimum principal stress value, the intermediate principal stress value varies starting at the minimum principal stress value and increasing at an interval of 0.2 of the UCS until it is slightly lower than the strength of the sample under the biaxial loading condition with the same minimum principal stress value. To express the new rock mass strength criteria, it was also necessary to determine the intact rock strengths under the same confining stress combinations mentioned earlier. Therefore, the intact rock was also modeled for all three compression tests and the intact rock strengths were found for 33 different minimum and intermediate principal stress combinations.