Permeability Of Fractured Rock Masses Research Articles

A comparative study on the influences of fracture intersections on the prediction of rock mass permeability

Geometric predictive methods are efficient for determining the permeability of fractured rock masses, which evaluate the contribution of each fracture to the flow. However, the influences of fracture intersections are ignored during the calculation. In this study, influences of fracture intersections on the permeability prediction are studied. The result shows that the predicted flow rate can only have a good accuracy when there is an infinite-length fracture with a larger aperture. Based on this conclusion, three inferences for the applicability of geometric predictive methods are proposed: (a) for fracture networks where all fractures have infinite length, the predicted permeability can have a good accuracy; (b) for fracture networks containing both infinite-length and finite-length fractures, the predicted permeability is only accurate when the infinite-length fracture have a larger aperture; and (c) for fracture networks where all the fractures have finite length, the error rates of predicted permeability increase as the aperture ratios of fracture sets increase. Sixty-three sets of discrete fracture network models are generated for verification, and these numerical results validated our proposed inferences.

Estimating the permeability of fractured rock masses for design of construction dewatering systems

A comprehensive data on dewatering and pumping-well systems installed in fractured rock is compiled and analyzed to estimate the permeability of fractured rock masses. The data covers construction activities of different nature and dewatering depths, and considered the richest of its kind. Due to the relatively small volume and the nature of rock masses involved in the analyzed systems, the effect of having predominant oriented fracture sets, and consequently different directional permeabilities (i.e., scale-dependent permeabilities) are minimal. As a result, the current study is a clear advancement on state-of-the art given that the vast majority of the related studies available in the literature are presented to estimate the directional permeability of rock masses. The analyses showed a significant discrepancy between the coefficients of permeability estimated from field packer and falling-head tests and those back-calculated from the actual discharge pumped out of rock masses using dewatering or pumping-well systems. Back-calculated permeabilities can be up to five orders of magnitude larger than the measured values. Such discrepancy increases with increasing permeability of rock mass. The sensitivity of the estimated permeability to the difference in rock formation, utilized fracturing index, test type, and depth of test is studied. A data-driven approach is used to develop a novel correlation between the measured and back-calculated coefficients of permeability. Accordingly, a procedure is proposed to estimate the permeability of fractured rock masses involved in construction dewatering as a function of the measured permeability and rock degree of fracturing.

Analysis of Coupled Seepage and Temperature Fields of Fractured Porous Rock Mass under Brine–Liquid Nitrogen Freezing

The existence of fracture flow has an undesirable effect on the creation of the frozen wall. Brine and liquid nitrogen combined freezing technology can ensure the safety of freezing engineering, reduce the construction period and save cost. Considering the permeability of the rock matrix, fluid exchange and Darcy–Stokes coupling effect between the rock matrix and fracture, a thermo-hydraulic model of the fractured porous rock mass under water seepage is herein established. The interfacial seepage field characteristics of fractured rock mass under different fluid flow models and interface conditions are compared. The numerical simulations of the initial brine freezing and liquid nitrogen reinforcement freezing are carried out. The results show that the overall permeability of fractured rock mass computed by free flow considering the Darcy–Stokes effect is greater than that computed by the Cubic law. The limit seepage velocity of the intact rock mass in brine freezing is 2.5 m/d, and that of fractured rock mass decreases to 1 m/d. The fracture aperture and groundwater seepage velocity are directly proportional to the closure time of the frozen wall. Liquid nitrogen freezing can seal water quickly and shorten the closure time of the frozen wall when the seepage velocity of the fractured rock mass is greater than the limit seepage velocity, and the rapid cooling of the upstream region plays an important role in the formation of the frozen wall in fractured rock mass.

Interval Prediction of the Permeability of Granite Bodies in a High-Level Radioactive Waste Disposal Site Using LSTM-RNNs and Probability Distribution

During long-term geological tectonic processes, multiple fractures are often developed in the rock mass of high-level radioactive waste disposal sites, which provide channels for release of radioactive material or radionuclides. Studies on the permeability of fractured rock masses are essential for the selection and evaluation of geological disposal sites. With traditional methods, observation and operation of fractured rock mass penetration is time-consuming and costly. However, it is possible to improve the process using new methods. Based on the penetration characteristics of fractured rock mass, and using machine learning techniques, this study has created a prediction model of the fractured rock mass permeability based on select physical and mechanical parameters. Using the correlation coefficients developed by Pearson, Spearman, and Kendall, the proposed framework was first used to analyze the correlation between the physical and mechanical parameters and permeability and determine the model input parameters. Then, a comparison model was created for permeability prediction using four different machine-learning algorithms. The algorithm hyper-parameters are determined by a ten-fold cross-validation. Finally, the permeability interval prediction values are obtained by comparing and selecting the prediction results and probability distribution density function. Overall, the computational results indicate the framework proposed in this paper outperforms the other benchmarking machine learning algorithms through case studies in Beishan District, Gansu, China.

Quantifying strength and permeability of fractured rock mass using 3D bonded block numerical model

In this research, a 3D bonded block model (BBM) is applied to numerically quantify the strength and permeability of fractured rock mass. The reliability of the BBM is initially verified through comparisons between conceptual models and theoretical and laboratory results. A series of rock mass models incorporating complex fracture networks is then constructed, and the strengths are quantified accordingly. The rock mass is weakened when low fracture stiffness and friction angle are assigned, and steep fractures are simulated. In both cases the low strength is attributed to the decreasing frictional resistance of the initial fractures. The rock mass is further weakened when more persistent fractures are simulated, in which the weakening effects of the fractures are amplified. The numerical models are compared with the widely used Hoek-Brown strength criterion. The strengths resulting from the two methods converge only when steep and less persistent fractures are simulated. Numerical simulations also suggest that in general, the permeability is positively related to the strength for rock masses that are impermeable in the pre-peak loading stage. In such cases additional flow paths are formed and attributed to failed rock bridges. The research works provide new options to quantify the strength and permeability of fractured rock mass, and offer opportunities to examine relations between these parameters.

Impact of fracture length distribution on seepage characteristics of fractured rock masses

This study presents a numerical investigation on the effect of fracture length distribution on the equivalent permeability of fractured rock masses. Three-dimension discrete fracture network (DFN) models with different fracture length distributions were generated using Monte Carlo method. The equivalent permeability was calculated by simplifying the 3D DFN models to equivalent pipe network (EPN) models. The results show that the equivalent permeability becomes less scattered with increasing side length of sub-models (L n), showing a clear tendency of reaching a representative element volume (REV) size. The root mean square (RMS) = 0.2 is utilized as the threshold to determine the REV size. The REV size of the DFN model is approximately 4.17×104 m3 in which the power exponent (a) of fracture length distribution is 4.5. The fracture density (P 32) decreases exponentially with the increment of a. This is because the number of short fractures in the DFN model with a larger a is larger. The P 32 of the DFN model is positively correlated to the equivalent permeability.

Flow Characteristics of Deep Fractured Rock Mass under Water-Rock Mass Coupling

The deeply fractured rock mass exists in the environment of in-situ stress and high seepage pressure, and they are prone to seepage-slipping failure. Therefore, it is of excellent engineering value and theoretical significance to study the nonlinear seepage evolution of fractured rock mass under water-rock mass coupling. This paper taking the persistent fracture granite as a tested sample, a series of seepage tests of persistent fracture granite under different seepage pressures were carried out. The nonlinear seepage evolution of fractured granite with different JRC values under water-rock mass coupling was studied, and the 3D reconstruction of the fracture surface was carried out after the experiment. The influence of the roughness degree of fracture surface on the permeability of fractured rock mass is investigated. Based on the experimental results, the non-darcy flow behavior of the fractured granite is analyzed by using the quadratic function-type Forchheimer formula. The results show that the permeability of the fractured granite increases nonlinearly with the increase of seepage gradient when the volume flow rate over 20mm3/s; The research results are of great significance for the study of nonlinear seepage evolution of rock mass with different JRC values under high in-situ stress and high seepage pressure.

Estimating fractured rock effective permeability using discrete fracture networks constrained by electrical resistivity data

Although the permeability of fractured rock mass is a fundamentally important property for the safe construction of civil and mining engineering structures such as tunnels, in situ characterization of permeability without resorting to hydraulic tests is difficult. For rapid, wide-area estimation, a method that can be conducted at a field-scale using geological and geophysical investigation data is proposed. The method is not based on conventional hydraulic test results. Instead, it combines the stochastic generation of fracture networks with the crack tensor theory. The most important parameter for this method is the fracture length distribution. Although the distribution parameters in the DFN model are assigned through sampling, a bias is generally experienced because of the limited sampling area. To improve the estimation of such parameters, in-situ electrical resistivity data and a symmetric self-consistent method are used to constrain the fracture length distribution. The proposed method is applied to the fractured crystalline rock mass of the Mizunami Underground Research Laboratory (URL) in the Tono area of central Japan. Its effectiveness and correctness are demonstrated through good correspondence of the derived effective permeability with the in-situ measured permeability.

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.

Experimental Investigation on Permeability Evolution of Granite Samples Containing a Grout Infilled Fracture under Triaxial Compression

Grouting has been widely used in rock engineering to improve the stability and strength of fractured rock mass, as well as to decrease the permeability of fractured rock mass. Fractures infilled with grout may endure different in-situ stress conditions during the construction and operation life of tunnels and other underground spaces in fractured rocks. To study the stress induced changes in permeability of fractures infilled with grout, we carried out triaxial compression tests on the fractured rock samples infilled with grout, during which the permeability of fractured rock samples was measured. Both planar and rough fractures with different apertures were tested. The results showed that the permeability of infilled fracture samples increases with increasing grout width, but the strength of infilled fracture samples decreases with increasing grout width. The evolution of permeability is highly related with the volumetric strain. The permeability decreases slightly with increasing deviatoric stress from 0 to crack damage stress, followed by drastic enhancement in permeability with further increasing deviatoric stress. The sample’s permeability is enhanced by about 5~10 times at the peak deviatoric stress compared with the initial value at 5MPa deviatoric stress.

INVESTIGATIONS ON REPRESENTATIVE ELEMENTARY VOLUME AND DIRECTIONAL PERMEABILITY OF FRACTAL-BASED FRACTURE NETWORKS USING POLYGON SUB-MODELS

Since the directional permeability of fractured rock masses is significantly dependent on the geometric properties of fractures, in this work, a numerical study was performed to analyze the relationships between them, in which fracture length follows a fractal distribution. A method to estimate the representative elementary volume (REV) size and directional permeability ([Formula: see text] by extracting regular polygon sub-models with different orientation angles ([Formula: see text] and side lengths ([Formula: see text] from an original discrete fracture network (DFN) model was developed. The results show that the fracture number has a power law relationship with the fracture length and the evolution of the exponent agrees well with that reported in previous studies, which confirms the reliability of the proposed fractal length distribution and stochastically generated DFN models. The [Formula: see text] varies significantly due to the influence of random numbers utilized to generate fracture location, orientation and length when [Formula: see text] is small. When [Formula: see text] exceeds some certain values, [Formula: see text] holds a constant value despite of [Formula: see text], in which the model scale is regarded as the REV size and the corresponding area of DFN model is represented by [Formula: see text] (in 2D). The directional permeability contours for DFN models plotted in the polar coordinate system approximate to circles when the model size is greater than the REV size. The [Formula: see text] decreases with the increment of fractal dimension of fracture length distribution ([Formula: see text]. However, the decreasing rate of [Formula: see text] (79.5%) when [Formula: see text] increases from 1.4 to 1.5 changes more significantly than that (34.8%) when [Formula: see text] increases from 1.5 to 1.6 for regular hexagon sub-models. This indicates that the small non-persistent fractures dominate the preferential flow paths; thereafter, the flow rate distribution becomes more homogeneous when [Formula: see text] exceeds a certain value (i.e. 1.5). A larger [Formula: see text] results in a denser fracture network and a stronger conductivity.

A numerical study of shear-induced evolutions of geometric and hydraulic properties of self-affine rough-walled rock fractures

Understanding the nonlinear flow regime of fluids through rock fractures is very important for accurately predicting the permeability of fractured rock masses. This study numerically generated rough fracture surfaces using the modified successive random addition algorithm and simulated fluid flow through void spaces induced by shear by solving the Navior-Stokes equations. The evolutions of geometric and hydraulic properties of 3D rough fractures during the whole shearing process are analyzed. The results show that both the asperity height and the aperture induced by shear follow Gaussian distributions. With the increments of shear displacement and/or surface roughness of fractures, both the mean aperture and standard deviation of aperture increase following exponential functions. When the shear displacement is small (i.e., 3 mm), the streamlines are very tortuous bypassing the contacts, showing obvious channeling flow phenomenon due to the small mean aperture and a large number of contacts. The critical Reynolds number increases versus shear displacement with a gradually decreasing rate. The rougher surface of fractures represented by a smaller Hurst exponent gives rise to a larger critical Reynolds number. The streamline distributions indicate that the flow anisotropy at a smaller shear displacement is more significant than that at a larger shear displacement. The permeability in the direction perpendicular to shear direction is always larger than that parallel to shear direction. This is reasonable because during shearing, the upper surface contacts the lower surface on the up slope of the asperities, which blocks the fluid flow along the direction parallel to shear direction; however, the void spaces induced by shear on the down slope provides available flow paths, enhancing the permeability in the direction perpendicular to shear direction.

Permeability characteristics of fractured rock mass: a case study of the Dongjiahe coal mine

The intricate distribution of fractures in rock mass leads to the anisotropy of permeability characteristics of the rock mass, which affects the overall stability of rock mass. Aiming at the water inrush from floor in the deep mining of the Dongjiahe coal mine, the distribution and development law of fractures in coal mine rock mass are determined by combining the acoustic emission information from the RFPA software and the in-site microseismic monitoring data. The representative element volume (REV) of permeability of fractured rock mass is determined by numerical simulation. The results show that: (1) As the increase of surrounding rock pressure, the permeability coefficient is decreasing, as well as the principal values of permeability. (2) Under the same surrounding rock pressure, the surrounding rock pressure has little influence on the principal directions of permeability; when the surrounding rock pressure is different, the principal directions of permeability changes greatly with the change of surrounding rock pressure. (3) With the increase of the trace length, the principal values of permeability dramatically increase and the size of REV gradually decreases. When the trace length reaches a certain size, the influence on the REV can be ignored.

Estimation of the REV Size and Equivalent Permeability Coefficient of Fractured Rock Masses with an Emphasis on Comparing the Radial and Unidirectional Flow Configurations

A method to estimate the representative elementary volume (REV) size for the permeability and equivalent permeability coefficient of rock mass with a radial flow configuration was developed. The estimations of the REV size and equivalent permeability for the rock mass around an underground oil storage facility using a radial flow configuration were compared with those using a unidirectional flow configuration. The REV sizes estimated using the unidirectional flow configuration are much higher than those estimated using the radial flow configuration. The equivalent permeability coefficient estimated using the radial flow configuration is unique, while those estimated using the unidirectional flow configuration depend on the boundary conditions and flow directions. The influences of the fracture trace length, spacing and gap on the REV size and equivalent permeability coefficient were investigated. The REV size for the permeability of fractured rock mass increases with increasing the mean trace length and fracture spacing. The influence of the fracture gap length on the REV size is insignificant. The equivalent permeability coefficient decreases with the fracture spacing, while the influences of the fracture trace length and gap length are not determinate. The applicability of the proposed method to the prediction of groundwater inflow into rock caverns was verified using the measured groundwater inflow into the facility. The permeability coefficient estimated using the radial flow configuration is more similar to the representative equivalent permeability coefficient than those estimated with different boundary conditions using the unidirectional flow configuration.

Permeability variations within mining-induced fractured rock mass and its influence on groundwater inrush

This paper is concerned with the evaluation of permeability of fractured rock mass due to the cover stress re-establishment, which is a major factor in controlling water and gas flow rate induced by mining operations in fractured rock. The case study considered in this paper is based on the results of observations of groundwater inrush and a decrease in water inflow from the fractured roof strata due to mining advancing in the Taiping Coalmine, Shandong Province, China. A conceptual model of an effective porous media was used to assess the permeability distribution in the fractured zone induced by coal mining. The cover stress re-establishment in gob fractured rock mass was evaluated using an empirical formula based on the surface subsidence. A simplified conceptual model of the fractured zone was used to evaluate the deformation of fractured zone along with the evaluation of changes in the rock permeability above the gob due to the cover stress re-establishment. These data were then used to calculate the water inflow rate into the panel. Predicted water inflow rates have been found to be in good agreement with those from monitoring data. This study improved the understanding of the mechanisms of the post-mining cover stress re-establishment on permeability change of the overburden fracture rock strata. These results can then be applied for numerical simulations of the process of overburden failure and consequent groundwater inrush due to coal mining.

Investigating the permeability of fractured rock masses and the origin of water in a mine tunnel in Shandong Province, China.

The coastal Sanshandao mine is threatened by the overlying Quaternary water and seawater. Following an introduction to the geology and hydrogeological conditions in the mine area, a detailed hydrogeological survey and sampling were conducted and hydrochemical and stable isotopic (δ2H and δ18O) tests on various waters were carried out to characterize the origin of water in the mine tunnels. Investigation and statistical analysis indicated that the northwest-trending fractures with large dip angles and long trace lengths are well developed in the northeast compared with those in the southwest of the mine. The permeability coefficients of the rock masses are in the range 4.19×10(-8)-2.25×10(-5) m/s, indicating that the fractured rock masses have generally low permeability. The seepage water had higher values of EC, total dissolved solids, and concentrations of most elements than the seawater and saline groundwater. Besides, the isotope composition of the waters indicated that the seepage water was more isotopically enriched than seawater but less than brine. The proportions of the three different sources were calculated based on hydrochemical and isotopic analyses. Overall, the mine water was composed of 72% seawater, 14.8% brine, and 13.2% atmospheric precipitation, respectively. Therefore, some preventive measures are essential to avoid the probability of seawater inrush.

Permeability tensor and representative elementary volume of fractured rock masses

Based on a simulation of three-dimensional fracture networks and a superposition principle of liquid dissipation energy for fractured rock masses, a model of the fracture permeability tensor is proposed. An elastic constitutive model of rock fractures, considering fracture closure and dilation during shearing, is also proposed, based on the dilation angle of the fracture. Algorithms of flow-path searching and calculation of the effective flow coefficients for fracture networks are presented, together with a discussion on the influence of geometric parameters of the fractures (trace length, spacing, aperture, orientation and the number of fracture sets) on magnitude, anisotropy of hydraulic permeability and the size of a representative elementary volume (REV). The anisotropy of hydraulic permeability of fractured rock masses is mainly affected by orientation and the number of fracture sets, and the REV size is mainly influenced by trace length, spacing and the number of fracture sets. The results of studies on REV size and the influence of in-situ stress on hydraulic conductivity of the rock mass on the slope of Jinping-I hydropower station, China, are presented using the developed models and methods. The simulation results agreed well with the results obtained from field water-pressure measurements, with an error of less than 10 %.

Effects of anisotropic permeability of fractured rock masses on underground oil storage caverns

An isotropic assumption is often applied to analyze permeability tests of in situ fractured rock mass, and the homogeneous hydraulic conductivities are then used directly in the seepage analysis. However, the hydraulic conductivities are normally anisotropic in fractured rock masses and the effects of the anisotropic permeability should be taken into account in rock engineering analysis, especially for seepage analysis of underground oil storage caverns. In this study, an underground oil storage cavern project is analyzed and the Oda’s method is used to determine the anisotropy in permeability, where the anisotropy in permeability is determined using the fracture orientation and the in situ stress information from the field survey. The horizontal hydraulic conductivities are obtained based on the geometric average hydraulic conductivities from the injection tests of six boreholes. A typical cavern unit is numerical modeled using code FLAC. The effects of anisotropic permeability on water pressure, water quantities and critical gas pressure are studied carefully. The results indicate that most calculated results based on in situ hydraulic tests with isotropic permeability assumption can be used safely in the underground oil storage cavern project.

Flow–stress coupled permeability tensor for fractured rock masses

AbstractIn this paper a new analytical model is proposed to determine the permeability tensor for fractured rock masses based on the superposition principle of liquid dissipation energy. This model relies on the geometrical characteristics of rock fractures and the corresponding fracture network, and demonstrates the coupling effect between fluid flow and stress/deformation. This model empirically considers the effect of pre‐peak shear dilation and shear contraction on the hydraulic behavior of rock fractures and can be used to determine the applicability of the continuum approach to hydro‐mechanical coupling analysis. Results of numerical analysis presented in this paper show that the new model can effectively describe the permeability of fractured rock masses, and can be applied to the coupling analysis of seepage and stress fields. Copyright © 2008 John Wiley & Sons, Ltd.

Stress-dependent permeability of fractured rock masses: a numerical study

We investigate the stress-dependent permeability issue in fractured rock masses considering the effects of nonlinear normal deformation and shear dilation of fractures using a two-dimensional distinct element method program, UDEC, based on a realistic discrete fracture network realization. A series of numerical experiments were conducted to calculate changes in the permeability of simulated fractured rock masses under various loading conditions. Numerical experiments were conducted in two ways: (1) increasing the overall stresses with a fixed ratio of horizontal to vertical stresses components; and (2) increasing the differential stresses (i.e., the difference between the horizontal and vertical stresses) while keeping the magnitude of vertical stress constant. These numerical experiments show that the permeability of fractured rocks decreases with increased stress magnitudes when the stress ratio is not large enough to cause shear dilation of fractures, whereas permeability increases with increased stress when the stress ratio is large enough. Permeability changes at low stress levels are more sensitive than at high stress levels due to the nonlinear fracture normal stress-displacement relation. Significant stress-induced channeling is observed as the shear dilation causes the concentration of fluid flow along connected shear fractures. Anisotropy of permeability emerges with the increase of differential stresses, and this anisotropy can become more prominent with the influence of shear dilation and localized flow paths. A set of empirical equations in closed-form, accounting for both normal closure and shear dilation of the fractures, is proposed to model the stress-dependent permeability. These equations prove to be in good agreement with the results obtained from our numerical experiments.