Properties Of Fractured Rock Masses Research Articles

Experimental study on the size effect and anisotropy in mechanical properties of fractured rock mass based on 3D printing

The size effect and anisotropy of fractured rock mass mechanical parameters are difficult problems to solve in the field of engineering. This study taken full advantage of sand 3D printing technology to batch-reproduce complex structures and prepare rock-like specimens with different sizes and rotation directions. The size effect and anisotropy of the fractured rock mass were investigated. Digital image correlation was used to conduct noncontact full-field observations of the specimen. The results showed that the sand 3D printing specimens have the advantages of high precision and brittleness and can reproduce the deformations and failure processes of natural rock. The compressive strength and failure mode of the fractured rock have significant size effect characteristics. Assessing the size effect on the compressive strength of the specimen, 100 × 100 mm is the mechanical representative elementary volume (mREV) scale. The compressive strength and failure mode of the fractured rock under the mREV scale show obvious anisotropy. Based on the size effect of fracture density, 80 × 80 mm was identified as the geometric representative elementary volume (gREV) scale. The gREV scale was smaller than and close to the mREV scale was found. In addition, the differentiation rate of standard deviation (DRSD) is defined to provide information on the dispersion of the strain field and the precursor information of crack initiation during the deformation and fracture of the rock mass. This study provides a new method for laboratory experimental studies on the size effect and anisotropy of complex fractured rock masses from the perspective of 3D printing.

A method to interpret fracture aperture of rock slope using adaptive shape and unmanned aerial vehicle multi-angle nap-of-the-object photogrammetry

The aperture of natural rock fractures significantly affects the deformation and strength properties of rock masses, as well as the hydrodynamic properties of fractured rock masses. The conventional measurement methods are inadequate for collecting data on high-steep rock slopes in complex mountainous regions. This study establishes a high-resolution three-dimensional model of a rock slope using unmanned aerial vehicle (UAV) multi-angle nap-of-the-object photogrammetry to obtain edge feature points of fractures. Fracture opening morphology is characterized using coordinate projection and transformation. Fracture central axis is determined using vertical measuring lines, allowing for the interpretation of aperture of adaptive fracture shape. The feasibility and reliability of the new method are verified at a construction site of a railway in southeast Tibet, China. The study shows that the fracture aperture has a significant interval effect and size effect. The optimal sampling length for fractures is approximately 0.5–1 m, and the optimal aperture interpretation results can be achieved when the measuring line spacing is 1% of the sampling length. Tensile fractures in the study area generally have larger apertures than shear fractures, and their tendency to increase with slope height is also greater than that of shear fractures. The aperture of tensile fractures is generally positively correlated with their trace length, while the correlation between the aperture of shear fractures and their trace length appears to be weak. Fractures of different orientations exhibit certain differences in their distribution of aperture, but generally follow the forms of normal, log-normal, and gamma distributions. This study provides essential data support for rock and slope stability evaluation, which is of significant practical importance.

More is different: On the emergence of collective phenomena in fractured rocks

Fractures widely exist in crustal rocks and form complex networks dominating the bulk behaviour of geological media. Thus, understanding how fracture networks affect subsurface processes/phenomena is highly relevant to many rock engineering applications. However, the large-scale behaviour of a fractured rock mass consisting of numerous fractures and rocks cannot be predicted by simple applications of the knowledge of individual fractures and/or rocks, due to upscaling complexities involving the hierarchy of scales, heterogeneities, and physical mechanisms as well as the possible emergence of qualitatively different macroscopic properties. In other words, macroscopic phenomena in fractured rocks arise from the many-body effects (i.e. collective behaviour) of numerous interacting fractures and rocks, such that the emergent properties at the fracture system scale are much richer than those of individual components. Hence, more is different! This paper gives a discussion on the mechanism of emergence in fractured media from a combined statistical physics and rock mechanics perspective, and further presents a multiscale conceptual framework to link microscopic responses of single fractures/rocks to macroscopic behaviour of rock masses consisting of many fractures and rocks. This framework can serve as a useful tool to bridge experimentally-established constitutive relationships of fracture/rock samples at the laboratory scale to phenomenologically-observed macroscopic properties of fractured rock masses at the site scale.

Experimental Study on Grouting Seepage Characteristics of Single-Fractured Rock Masses with Different Inclination Angles under Three-Dimensional Stress

In underground engineering, the fracture dip angle and three-dimensional stress have great influence on the permeability and mechanical properties of fractured rock mass. Firstly, single-fracture samples with similar three-dimensional fractal dimensions and different dip angles were selected by using CT scanning technology. Then, the grouting material seepage tests under different dip angles and different three-dimensional stresses were carried out by self-developed single-fracture stress-seepage coupling true triaxial test system. The experimental data of stress-strain curve, grouting pressure, and grouting material flow rate of single-fracture specimen under different fracture dip angles and three-dimensional stress were obtained. The results show that the three-dimensional fractal dimension can be used as an index to measure the influence of fracture factors on grouting seepage test. The third principal stress σ 3 had a greater influence on the fracture width than the first principal stress σ 1 . The larger the fracture inclination, the lower the strength of the single-fracture specimen. As the third principal stress σ 3 increases, the grouting pressure increased while the grouting flow decreased. With the increase of the fracture inclination angle, the influence of σ 1 on the hydraulic conductivity became larger, while the influence of σ 3 on the hydraulic conductivity became smaller. Moreover, the expression of hydraulic conductivity of single-fracture specimen with different dip angles under three-dimensional stress was obtained by nonlinear fitting of hydraulic conductivity data.

Numerical study of related factors affecting mechanical properties of fractured rock mass and its sensitivity analysis

Numerical study of related factors affecting mechanical properties of fractured rock mass and its sensitivity analysis

Development and verification of three-dimensional equivalent discrete fracture network modelling based on the finite element method

The discrete fracture network (DFN) method is essential for estimating the mechanical properties of naturally fractured rock masses. A three-dimensional (3D) equivalent DFN model was proposed and thoroughly validated for fractured rock masses. In the equivalent DFN modelling, the Monte Carlo method and 3D rock failure process analysis (RFPA3D) software were used. Additionally, the fracture and its spatial distribution were described explicitly, and the strength and deformability of complex fractured rock masses were investigated. A case study was conducted at the dam site area of Lianghekou Hydropower Station's the left bank slope. The ShapeMetriX3D system was used to acquire the geometric parameters and probability distribution of fractures via digital photogrammetry. Additionally, a series of numerical models were created by combining the proposed equivalent DFN method with the fracture geometry information from the case study. Based on the concept of scale effect and anisotropy, the representative element volume (REV) and mechanical parameters of fractured rock masses were determined. The mechanical parameters of slope rock masses obtained by the proposed DFN modelling and generalized Hoek Brown strength criterion were compared. Furthermore, the applicability and reliability of the proposed model were validated using site data and numerical verifications. The proposed 3D equivalent DFN modelling can analyze the scale effect and anisotropy of mechanical properties of fractured rock masses effectively. Finally, the influence of water head on the hydraulic characteristics of fractured rock masses was analyzed based on the proposed model. Results showed that fractures were the main pathway of water.

Comparative study on interconnectivity between three-dimensional and two-dimensional discrete fracture networks: A perspective based on percolation theory

Two-dimensional (2-D) discrete fracture networks (DFNs) are widely used to study the hydraulic properties of fractured rock masses. There are few studies on the reliability of 2-D DFNs reflecting the hydraulic properties of three-dimensional (3-D) rock masses. This study provides a new perspective based on percolation theory to study the interconnectivity between 3-D and 2-D DFNs quantitatively. For isotropic fractured rock masses, the corresponding value of the number of fractures in per sampled volume (P30) when 3-D DFNs achieve the percolation threshold is theoretically derived. Additionally, the corresponding value of P30 of the original 3-D model when its 2-D DFNs on cutting planes achieve the percolation threshold is also derived for comparison. These theoretical formulas show that: (a) The corresponding values of P30 at percolation threshold for both 3-D and 2-D DFNs are only related to the fracture diameter and inversely proportional to its third power; and (b) The corresponding value of P30 when 2-D DFNs achieve the percolation threshold is 6.5 times of the corresponding value of P30 when its original 3-D DFNs achieve the percolation threshold. Numerical codes and experiments based on Particle Flow Code and Matlab are developed to verify the derived theoretical equations, and the results show that the theoretical equations are validated.

Upscaling of fractured rock mass properties – An example comparing Discrete Fracture Network (DFN) modeling and empirical relations based on engineering rock mass classifications

In this study a comparison of Discrete Fracture Network (DFN) modeling and empirical relationships based on conventional engineering rock mass classifications is carried out in order to determine the elastic properties of fractured rock masses. Uncertainties are considered by a stochastic description of the fracture network geometry and a probabilistic approach, utilizing Monte Carlo simulation techniques, for the various other input parameters. The proposed workflow is applied to two sandstone outcrops. Geometric information on the fracture systems is obtained from terrestrial laser scanning providing the input parameters (orientation, size and intensity) for each fracture set including probability density functions required for DFN modeling. After adding the mechanical properties of intact rock and fractures obtained by laboratory tests, the DFN approach allows to calculate values for rock mass deformation modulus and Poisson's ratio assuming either isotropy or vertical-transverse isotropy. These estimates are compared to values derived from empirical relationships based on engineering rock mass classifications. The comparison of the two approaches and, in particular, of the different empirical relationships reveals a significant scatter in the estimated rock mass properties. Only those empirical equations which incorporate intact rock properties in addition to the classification score yield plausible results, i.e., a pronounced weakening due to the fractures, and are in agreement with the estimates of the DFN approach. The additional effort of generating a DFN model shows its benefits if an anisotropic rock mass and/or spatially varying properties have to be considered, thus, providing a comprehensive hydromechanical characterization of anisotropic fractured rock masses.

Hydromechanical rock mass characterization using discrete fracture network models – a case study based on terrestrial laser scanning and rock mechanical testing

Understanding the anisotropic hydraulic and mechanical properties of fractured rock masses is of great importance for a safe and optimal utilisation of the subsurface. Two sandstone quarries are utilized to obtain fracture network characteristics by Terrestrial Laser Scanning (TLS) producing 3d point cloud data. Semiautomatic analysis of the point clouds provides the probability density functions for each of the fracture parameters used as stochastic input for a Discrete Fracture Network (DFN) model. Rock mechanical laboratory tests are carried out to determine the mechanical properties of the intact rock and fractures. These parameters are then combined in the DFN model to calculate spatially variable tensors for permeability, Young’s modulus and Poisson’s ratio. Thereby, the spatial resolution of the tensor description is adapted to the grid size which can be used in further hydromechanical models. The approach allows to populate these models with more realistic parameters which incorporate also the effect of fractures on the rock mass behaviour. Obtained results are subsequently compared with conventional engineering rock mass classifications. The applied workflow allows for upscaling of rock properties determined in the laboratory to the anisotropic rock mass properties required for further hydromechanical modelling on larger scales, e.g., the reservoir scale.

The effects of hydro-mechanical coupling on hydraulic properties of fractured rock mass in unidirectional and radial flow configurations

The effects of hydro-mechanical coupling on hydraulic properties of fractured rock mass in unidirectional and radial flow configurations

Correlations between Geometric Properties and Permeability of 2D Fracture Networks

The equivalent permeability of fractured rock masses plays an important role in understanding the fluid flow and solute transport properties in underground engineering, yet the effective predictive models have not been proposed. This study established mathematical expressions to link permeability of 2D fracture networks to the geometric properties of fractured rock masses, including number density of fracture lines, total length of fractures per square meter, and fractal dimensions of fracture network structures and intersections. The results show that the equivalent permeability has power law relationships with the geometric properties of fracture networks. The fractal dimensions that can be easily obtained from an engineering site can be used to predict the permeability of a rock fracture network. When the fractal dimensions of fracture network structures and intersections exceed the critical values, the effect of randomness of fracture locations is negligible. The equivalent permeability of a fracture network increases with the increment of fracture density and/or fractal dimensions proportionally.

Quantifying the hydraulic properties of fractured rock masses along a borehole using composite geological indices: A case study in the mid and upper Choshui River Basin in Central Taiwan

Comprehensive information on vertical variations in hydraulic conductivity along a borehole is indispensable for characterizing the complexity of the hydraulic properties of fractured bedrock aquifers or for use as input data for groundwater modeling. This study presents a practical method for addressing various engineering concerns (e.g., project budget, completion time, manpower, and direct measurement of hydraulic properties from rock core specimens) when obtaining detailed and continuous hydraulic conductivity data along a borehole. Based on in-situ hydrogeological data collected from 26 boreholes located in the Choshui River Basin of Central Taiwan, eight individual and six composite geological indices were investigated to determine their correlations with the hydraulic conductivity of fractured rocks through bivariate analysis. The correlation analysis results indicate that all composite geological indices have higher correlations with the hydraulic conductivity than individual indices do. Moreover, the greater the number of individual geological indices integrated into one composite index, the higher the correlation. Based on the correlation results, quantification models for predicting the hydraulic conductivity using the composite geological indices were developed using regression analysis techniques. The performances of various statistical models were evaluated and compared. The regression analysis results for all six predictive models show that a power law relationship exists between each composite geological index and the hydraulic conductivity, and that the coefficient of determination ranges from 0.77 to 0.88. Therefore, the newly developed models can serve as an alternative for obtaining vertical hydraulic conductivity data when the budget for onsite hydraulic test is limited. • Models for predicting the hydraulic conductivity in boreholes are developed. • Eight individual and six composite geological indices are used. • All composite indices have high correlations with hydraulic conductivity. • The developed models are proven to be reliable.

Shear behaviors of granite fractures immersed in chemical solutions

Highly mineralized and acid/alkaline groundwater is critical to the short- or long-term stability of fractured rock masses in geological engineering. Understanding the impact of water-rock interactions on mechanical properties of fractured rock masses is important but challenging. This study investigated the physicochemical interactions between chemical solutions and granite fractures which can directly control the mechanical behaviors of fractured rock masses. Direct shear tests were performed on artificial granite fracture samples immersed in chemical solutions with different pHs for 30 and 150 days. The results show that the solution of pH = 2 has the most significant influence on fracture shear properties, followed by the solutions of pH = 12 and pH = 7. The deionized water has the minimal influence on fracture shear properties. The shear properties of fractures immersed in acid and alkaline solutions are sensitive to immersion time, whereas the shear properties of fractures immersed in neutral solutions are not significantly affected by immersion time. The weakening mechanisms of the water-rock chemical interaction are different in different chemical solutions. Dissolution mainly works in acid solutions, whereas the competing minerals dissolution and precipitation/crystallization occur in alkaline solutions. This is why alkaline solutions have less significant influence on fracture shear properties than acid solutions.

EVALUATING THE EFFECT OF APERTURE VARIATION ON THE HYDRAULIC PROPERTIES OF THE THREE-DIMENSIONAL FRACTAL-LIKE TREE NETWORK MODEL

Hydraulic properties of rock fractures are important issues for many geoengineering practices. Previous studies have revealed that natural rock fractures have variable apertures that could significantly influence the permeability of single rock fractures, yet the effect of aperture variation on the hydraulic properties of fractured rock masses has received little attention. The present study implemented a series of flow calculations on two kinds of 3D fractal-like tree network models constituted by fractures with heterogeneous aperture distributions and parallel plates with uniform apertures to gain insight into the effect of aperture variation on the hydraulic properties of fractured rock masses. The fluid flows through the two kinds of models are simulated using a developed numerical code, whose validity is verified by comparisons with the analytical solutions and previous experimental results. The effects of heterogeneous aperture distributions on the permeability of the models are illustrated by changing the geometrical parameters including the aperture ratio and mean aperture of the initial fracture branch. The results show that a significant channeling flow is observed in the 3D fractal-like tree network models constituted by fractures with heterogeneous aperture distributions. These preferential flow paths are mainly located in the large aperture channels, bypassing the contacts and/or low aperture barriers. The equivalent permeability of the 3D fractal-like tree network models increases with increasing fracture aperture ratio and/or increasing mean aperture of the initial fracture branch. The permeability ratio of the model of fractures with heterogeneous aperture distributions to the model with uniform mean apertures is always smaller than 1.0, and the ratio increases following a logarithmic relationship as the mean aperture of the initial fracture branch increases. A regression function is proposed to estimate the equivalent permeability of the model of fractures with heterogeneous aperture distributions. The deviations between the predicted permeability and the simulated/experimental results are less than half of the order of magnitude, which verifies that the proposed equation is capable of predicting the equivalent permeability of 3D fractal-like tree networks as a first-order estimation.

Investigation of thermal-induced damage in fractured rock mass by coupled FEM-DEM method

In this study, a 3D coupled FEM-DEM method was adopted to study the influence of thermal-induced damage on the mechanical properties of fractured rock mass. In this method, the rock matrix was discretized into bulk elements bridged by cohesive elements, and the preexisting fractures were represented by cohesive elements. The thermal-induced damage process was modeled through two parts, i.e., thermal conduction and thermal-mechanical coupling. For thermal conduction process, a proposed 3D thermal-cohesive coupled model with a fracture aperture-dependent interfacial thermal conductivity was adopted to simulate the thermal conduction across the fractures, which make it possible to more precisely predict the temperature distribution. Then, the thermal stress induced by temperature variation was coupled to perform the mechanical-failure calculation. Validation simulations indicated that the proposed model is capable of dealing with the thermal-induced damage problems. Then, the influences of fracture aperture on thermal-induced damage in highly fractured rock mass were numerically investigated by the Brazilian disc tests with multiple randomly distributed fractures. The results indicated that as the fracture aperture increases, the area of thermal-induced damage zone increases and the Brazilian tensile strength (BTS) decreases nonlinearly.

A new determination method for the permeability tensor of fractured rock masses

Hydraulic characteristics are important fundamental properties of fractured rock masses and involve various geoscience and engineering disciplines. The determination of the permeability tensor is the basic input of the equivalent continuum (EC) model for hydraulic behavior analysis. An analytical method to determine the permeability tensor of fractured rock masses is developed based on the orientation and linear frequency of fractures along with a value of the permeability in any direction. The orientation and linear frequency of fractures can be easily sampled in practice, and the value of the permeability in some direction can be tested by a relatively cheap single-well pumping test. The key step of this method is to derive the theoretical relationship between the permeabilities in two different directions. In addition, an anisotropy index of permeability AIp is defined to quantitatively describe the permeability anisotropy. Some numerical experiments were conducted with the universal distinct element code (UDEC) to validate the developed method. The results show that: (a) the predicted values are very close to the real ones; (b) for all cases of rock masses with equal-aperture fracture sets, the mean error rate of the permeabilities in 23 different directions is approximately 10%; (c) for rock masses with different-aperture fracture sets, the mean error rate increases from 8.6% to 28.9% as the aperture ratio increases from 1 to 4; and (d) therefore, the estimation method for directional permeability proposed in this study is valid, especially for those rock masses in which the aperture ratios of the fracture sets are small (e.g., less than 3).

Extraction and statistics of discontinuity orientation and trace length from typical fractured rock mass: A case study of the Xinchang underground research laboratory site, China

The geological parameters of rock discontinuities are of great significance for the evaluation of high-level radioactive waste repository and underground research laboratory (URL) site characteristics. To provide sufficient basis for the construction of the Xinchang URL site, this paper presents a comprehensive set of methods for acquisition of discontinuity parameters. In this set of methods, a high precision three-dimensional point cloud model of an outcrop with real geographic information is established based on close-range photogrammetry technology, from which not only planar discontinuities but also linear discontinuities can be identified and extracted by a semi-automatic method. A new fast-fuzzy clustering method is proposed to cluster discontinuity sets. The statistics method based on topographic window projection is improved to increase the rationality of estimating the geometric characteristics of fracture traces. As a preliminary validation, the results of this case study reveal that these methods have good applicability in acquiring discontinuity parameters, which is also conducive to the establishment of a deterministic-stochastic discrete fracture network (DSDFN) model to further analyze the properties of fractured rock mass. In addition, it is preliminarily found that some outcrops different locations have structures with similar geological features to a certain extent; moreover, the fractures exposed on the outcrop surface have distribution characteristics similar to those in the granite body obtained by drilling. Above all, the presented multidisciplinary methods could provide an alternative and practical approach for low-cost and accurate acquisition of discontinuity parameters, and thus, to study the properties of fractured rock mass effectively.

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

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

Strength characteristics and failure behavior of ubiquitous-joint rock-like specimens under compressive-shear stress: experimental study and digital speckle correlation method

In nature, rock mass is subjected to compression-shear effect, so it is very important to study the failure mode and mechanical properties of fractured rock mass under compression-shear effect. In order to study the influence of joint inclination on strength characteristics and failure modes of rock mass under different compression-shear angles, a series of compression-shear tests were carried out. The specimens are made of a certain proportion of fine sand, cement and water. The joints are prefabricated by inserting mica sheets and the inclination angle of joints is 0°, 15°, 30°, 45°, 60°, 75°, 90°. Digital speckle correlation method is used to analyze the strain field of the specimen during the whole loading process. The specimens are speckled on the opposite side before the experiment. In the experiment, the front and back sides of the specimen are monitored by the camera, and the images are analyzed after the experiment. The result shows that: 1) The peak shear strength of specimens mainly increases with the increase of joint inclination angle; 2) The weakening degree of shear strength caused by joint inclination angle of specimens tends to increase as the compression-stress ratio increases; and 3) The failure modes can be classified into four types: coplanar shear failure mode, inclined shear failure mode, quasi-complete shear failure mode and step shear failure mode. The digital speckle strain images better prove the failure modes obtained from the experiment.

Shear-flow coupling characteristics of a three-dimensional discrete fracture network-fault model considering stress-induced aperture variations

This study presents a three-dimensional discrete fracture network-fault (3D DFN-fault) model that contains a 3D DFN and a deformable 3D fault. The fault slip-induced shear displacement and normal displacement are taken into account. Under a constant normal stiffness (CNS) boundary condition, the normal displacement gives rise to an increase in the normal stress and a decrease in the aperture of the fractures connected to the fault, which are then incorporated into the model. Finally, a numerical code is developed to simulate fluid flow through the 3D DFN-fault model during shearing and the effects of the aperture heterogeneity, shear displacement and fracture structure on the hydraulic properties of fractured rock masses are estimated. The results show that an obvious channeling flow in the fracture networks is observed as remarkable localizations of the flow paths within limited areas of fracture planes. The aperture heterogeneity of fractures tends to hinder the overall flow through the DFN-fault model during shearing. The equivalent permeability decreases at the beginning of shear because of the stress-induced closure of fractures. With increasing the shear displacement, the equivalent permeability of the model in direction perpendicular to the fault slip direction either increases or decreases, depending on the competition of two effects: (i) the permeability is enhanced by the shear-induced dilation of the fault and the newly generated flow paths by connecting the dead-ends of fractures that are intersected to the fault; (ii) the permeability decreases due to the closure of fractures, which is influenced by the increase in the dilation-dependent normal stress. However, as the shear displacement continuously increases, the equivalent permeability in the direction parallel to the fault slip direction steadily increases.