Hydraulic Conductivity Of Rock Mass Research Articles

Coupling stress and transmissivity to define equivalent directional hydraulic conductivity of fractured rocks

A DFN (Discrete Fracture Network) modelling approach is developed to couple stresses with fracture transmissivities and to evaluate large scale rock mass hydraulic conductivity. The transmissivity-stress coupling relies on a negative exponential correlation between normal stress acting on a fracture and fracture transmissivity, bounded by residual and maximal apertures. The remote stresses and the local stress fluctuations induced by the fractures themselves are combined in a semi-analytical approach to compute the normal stress acting on each fracture of a DFN. Directional equivalent hydraulic conductivities are numerically computed in all directions from a spherical permeameter setup. The resulting properties are first a cloud of points, where each point defines a direction and an equivalent hydraulic conductivity. The distribution of equivalent hydraulic conductivities is analyzed to define mean values, preferential directions and anisotropy ratio. The entire workflow is developed in the numerical platform DFN.lab. The capacity of the method to investigate the impact of the in-situ stresses on the rock mass hydraulic conductivity is illustrated for fracturing and in-situ stress conditions similar to the conditions at the Forsmark site in Sweden. We find that the stress fluctuations induced by the fractures have a significant impact on the resulting hydraulic conductivity field. They limit the anisotropy ratio to values close to a factor of 3 while the transmissivity distribution is correlated to orientations and spans several orders of magnitude. Sensitivity analyses, performed by changing the parameters of the transmissivity-stress law, show quantitatively how the directional hydraulic conductivities are rather controlled by the orientations of the in-situ stresses or by the underlying connectivity structure of the DFN.

An analytical–numerical method for the hydraulic–mechanical coupling analysis of time-dependent behavior of pressurized tunnels: Impact of an excavation damaged zone

The issue of pressurized tunnels has been sufficiently investigated in the literature; nevertheless, the long-term behavior of the problem, which is the case in very squeezing conditions, remains unexplored. From a practical standpoint, the development of a damaged zone around the tunnel, due to the poor quality of blasting for excavation, imposes an additional challenge in predicting tunnel responses. This paper introduces an analytical approach addressing both water seepage and the excavation damaged zone (EDZ) concerning the long-term behavior of lined circular pressurized tunnels. To achieve this goal, the rheological behaviors of the initial rock mass and damaged zone are modeled using the Burgers model, along with considering the time-dependent variation in the rock mass hydraulic conductivity. After the validation of the model, the conducted parametric studies reveal that rock mass and lining hydraulic conductivities, EDZ radius, and the Burger shear modulus are the most influential parameters.

Rock mass permeability evolution during triaxial shearing based on three-dimensional distinct element model (3DEC) simulation

The evolution and change in jointed rock mass hydraulic conductivity (krm) during triaxial compression was investigated using the discrete element method code 3DEC bonded block model approach. The code’s fluid flow logic was used to simulate flow tests at various stages, from the initial confined state through to the post-peak (i.e., failed) state. A target percent of joint contacts were randomly “broken” to produce different initial fracture intensities. The rock mass fluid flow behaviour was compared with laboratory flow tests of thermally brecciated granular marble. A new joint constitutive model was developed to account for nonlinear, hysteretic load–displacement behaviour to accurately reproduce changes in joint aperture due to complex inter-block stress changes. The simulations provided quantifiable measurements of krm evolution with axial strain throughout the test for several rock mass quality conditions. The evolution of krm with loading was found to be nonlinear, with significant increases associated with yielding-induced dilation, followed by relatively minor changes in the post-peak loading range. These trends are similar to the laboratory tests conducted on intact rock and on rock joints. The findings will be useful for improving groundwater flow models for stability assessment of rock slopes, particularly where significant shearing is expected.

Assessment of engineering geology and grouting applications in Yalnızardıç Dam Site (Antalya, Türkiye)

The Yalnızardıç roller compacted concrete (RCC) dam, constructed in 2015, is located in the Mediterranean region of Turkey, intended for electric power production. It is approximately 303 m long and 92 m in height. In this paper, it is aimed to determine the performance of the grouting method, conducted to control the water seepage and improve the foundation of the Yalnızardıç dam. The engineering properties of the site were introduced by geological mapping, drilling, and laboratory tests. The seepage of the foundation was determined by the in-situ Lugeon test method, also referred to as a water pressure test, which is estimate the mean hydraulic conductivity of rock mass. As a result of the tests, high-permeable and permeable zones were determined in the dam axis area. To make a barrier layer for these permeable zones, grout curtains were built in the left abutment, thalweg and right abutment with two lines of grouting holes which is proposed according to the Lugeon test and rock quality designation (RQD) results. Consolidation grouts were built to increase the density of the soil which makes the strength of the dam's foundation. After the groutings, check boreholes were drilled and water pressure tests were performed to control the seepage along the foundation and dam site. The results indicated that average Lugeon (LU) values reduced, and the values were within the recommended limit. The process shows considerable accuracy in evaluating grouting efficiency.

Estimating the Hydraulic Conductivity of jointed rock mass using Genetic Programming

Ground engineering through injection of cementitious grouts or polymer resins is an integral part of fractured rock mass stabilisation by improving its maximum load carrying capacity, stiffness, cohesiveness and reduction in permeability. To optimise product delivery and achieve a high penetrability of product into the rock mass, it is imperative to understand the permeability characteristics or hydraulic conductivity of the rock mass. The hydraulic conductivity of the rock mass also dictates the type of polyurethane (PU) or cement-based products to be injected. For this purpose, a Packer test has to be conducted to measure the hydraulic conductivity of a rock formation which is an expensive and time-consuming process. Alternatively, a number of empirical methods that use rock mass classifications and rock joint properties Rock Quality Designation (RQD), Q-system (Q), Geological Strength Index (GSI), Joint Spacing (JS), Joint Aperture (a) proposed by a number of researchers that are capable of predicting hydraulic conductivity (HC). In this research, a predictive model between rock mass properties and HC is proposed using new approach - genetic programming (GP). For this purpose, a database of rock mass parameters including RQD, Q, GSI, JS, Joint Aperture (a), Second Permeability Index (SPI) and Packer test results available in the literature is established. The database is split into randomly selected training and testing sets. To assess the fitting quality, the sum of the absolute difference is used, while maximum depth on trees is set to control the bloat of the model. The performance is assessed with four statistical criteria and three GP models using different input combinations are proposed. These models have been converted into simple mathematical equations to calculate HC based on collected input data. In summary, two out of three models have successfully predicted HC with high correlation to the actual HC (R2 of testing sets ≈ 0.92). Therefore, this study has shown the feasibility of applying GP models into future prediction of HC for the initial phase of rock grouting design.

Fluid flow of various configurations in a rock-proppant-system

A high rock mass permeability is essential for developing an enhanced geothermal system and commonly achieved through the fracture conductivity. Hydraulic stimulation of fractures in combination with the use of proppants can significantly increase and maintain rock mass permeability. To investigate and quantify the effect of proppants, which are so far not commonly used to enhance deep geothermal reservoirs, on the rock mass hydraulic conductivity we performed various sets of laboratory experiments. We performed long-term creep deformation tests on a stack of rock samples with simultaneous a) fluid flow and b) acidification and fluid flow, in both cases with and without a proppant-filled fracture between the stacked rock samples. A central borehole served as fluid inlet into the lower sample of the stack. Two more boreholes in the lower sample allowed fluid flow. Sample stacks were placed in a Hoek cell at elevated confining pressures to prevent fluid flow along the sidewalls of the cylindrical sample setup. We compared the axial deformation and injection pressures between the experimental setups. The laboratory tests are part of the ZoKrateS Project, which aims at showing the feasibility of enhancing fractured carbonate rock mass by proppant placement for geothermal applications.

Inversion method of hydraulic conductivity for steady-state problem based on reduced-order model constructed by improved greedy sampling method

To solve the time-consuming parameter inversion problem based on swarm evolution algorithm, this study proposes an improved greedy sampling method (GSM) (IGSM) based on model reduction technology to rapidly estimate the hydraulic conductivity of steady-state seepage problem. Different from the traditional GSM, IGSM neither needs to estimate the posterior error bound of the reduced-order model (ROM) nor does it require a training sample set to train the ROM. Instead, the observation data are directly applied to the training process of ROM. Each iterative process of IGSM includes a parameter inversion process to obtain the parameters that best match the observation information and use it to update the current ROM. The improved differential evolution algorithm (e.g., MMRDE) is used as the search algorithm, and the resulting algorithm is denoted as IGSM-MMRDE. After comparative testing, IGSM-MMRDE was found to have higher accuracy and lower calculation cost than GSM-MMRDE. In addition, IGSM-MMRDE is also capable of high-dimensional parameter inversion problems and parameter inversion with a wider search range. After applying IGSM-MMRDE to the inversion of the rock mass hydraulic conductivity of the natural seepage field in the dam site area of a hydropower station, IGSM-MMRDE converges quickly and takes much less time than ADINA-MMRDE based on the full-order model (approximately 0.31% of ADINA-MMRDE). The performance results are even better than the ADINA-MMRDE using the fixed evolutionary generation. Therefore, the advantages of using IGSM in the rapid inversion of the initial seepage field of large-scale projects are extremely evident.

Initial Assessment on Hydraulic Conductivity of Rock Mass and its Size Effect Using Drilling Data

Predicting hydraulic conductivity and characterizing its size effect in jointed rock masses is fundamental to various geoscience and engineering disciplines. A new empirical method, using drilling data, is established to predict the hydraulic conductivity of a rock mass in the scanline direction. The effect of the scanline unit length on the distribution characteristics of hydraulic conductivity for boreholes in jointed rock masses is studied. In addition, the scanline length of boreholes was used to study the size effect of hydraulic conductivity of jointed rock masses, and a new model was proposed to determine the size effect of hydraulic conductivity. A mathematical method for representative volume element (RVE) determination was proposed based on the proposed size effect model of hydraulic conductivity. Some packer tests were conducted to validate the developed method for predicting hydraulic conductivity. The results show that (a) the predicted hydraulic conductivities are close to those determined from the packer tests; (b) the scanline unit lengths influence the distribution characteristics of hydraulic conductivity; (c) a large but fluctuating increase in hydraulic conductivity with increasing scanline length, and the fluctuation decreases with fewer fractures and joints; (d) the mathematical method for predicting RVE and KRVE is effective to make an initial evaluation on jointed rock masses with high fracture frequency.

Hydrogeological Assessment and Modified Conceptual Model of a Dinaric Karst Island Aquifer

Vis Island is situated in southern Croatia. The island is mostly composed of karstified carbonate rocks and belongs to the Dinaric karst region, which is a locus typicus of karst landforms. Located far from the mainland, Vis island has maintained a successful water supply from its own karst aquifer for decades. Hydrogeological research has been undertaken to protect this excellent karst aquifer by establishing sanitary protection zones and to explore the possibility of increasing the pumping yield. New groundwater velocity data obtained via a tracer test were in accordance with the rock mass hydraulic conductivity calculated from previous pumping tests. The hydrochemical interpretation indicated several different phenomena, from carbonate and sulfate rock dissolution to seawater mixing with groundwater. A conceptual model of the island’s aquifer was improved, and two main catchments were delineated according to tracer test results, and connected with the geological setting, hydrochemical data, and new climatological insights. Such an approach is applicable for similar karst aquifers, in which topographic and hydrogeological divides usually do not coincide.

A Method for Determining the Hydraulic Conductivity of Rock Mass with Fractures of Limited Length

The stringent environmental requirements make bedrock a resource for hydraulic construction. A method for calculating the hydraulic conductivity of a symmetric rock mass with fractures of limited length is described. Curves of the hydraulic conductivity versus the width and length of fractures are presented. A general method for combining the calculated vales of hydraulic conductivity and the data of standard hydraulic testing is considered.

Hydrogeology Tests Performed in Shazaoyuan Candidate Site for China’s HLW Geodisposal

Rock mass hydraulic conductivity is one of the most important indexes for candidate site quantitative assessment of high-level radioactive waste disposal. 178 hydrogeology tests, mainly constant head injection test and pulse test, were performed in the 4 boreholes drilled in Shazaoyuan candidate site. Reliable hydraulic conductivity was obtained with transient curve matching methods. The result shows that the hydraulic conductivity was high heterogeneous; the maximum hydraulic conductivity was 9.30×10-8m/s and 93% of the sections were lower than 10-8m/s. Shazaoyuan candidate site can be inferred as one of the most potential sites for the geological disposal of China’s HLW in the view of low permeability.

Multiscale hierarchical analysis of rock mass and prediction of its mechanical and hydraulic properties

Engineering geological and hydro-geological characteristics of foundation rock and surrounding rock mass are the main factors that affect the stability of underground engineering. This paper presents the concept of multiscale hierarchical digital rock mass models to describe the rock mass, including its structures in different scales and corresponding scale dependence. Four scales including regional scale, engineering scale, laboratory scale and microscale are determined, and the corresponding scale-dependent geological structures and their characterization methods are provided. Image analysis and processing method, geostatistics and Monte Carlo simulation technique are used to establish the multiscale hierarchical digital rock mass models, in which the main micro- and macro-structures of rock mass in different geological units and scales are reflected and connected. A computer code is developed for numerically analyzing the strength, fracture behavior and hydraulic conductivity of rock mass using the multiscale hierarchical digital models. Using the models and methods provided in this paper, the geological information of rock mass in different geological units and scales can be considered sufficiently, and the influence of downscale characteristics (such as meso-scale) on the upscale characteristics (such as engineering scale) can be calculated by considering the discrete geological structures in the downscale model as equivalent continuous media in the upscale model. Thus the mechanical and hydraulic properties of rock mass may be evaluated rationally and precisely. The multiscale hierarchical digital rock mass models and the corresponding methods proposed in this paper provide a unified and simple solution for determining the mechanical and hydraulic properties of rock mass in different scales.

ESTIMATION OF GROUNDWATER INFLOW SITUATION USING FUZZY LOGIC: A CASE STUDY (BEHESHTABAD WATER CONVEYING TUNNEL, IRAN)

Water inflow is one of the most important challenges in the underground excavations. In addition to inducing working conditions and environmental problems, it decreases the stability and quality of the surrounding rocks. The direct method of measuring rock mass hydraulic conductivity consists of drilling the boreholes and observing the rate of fluid lost in the boreholes. Applying this method is still problematic due to the depth of underground spaces, and also the groundwater level covering them. Therefore, many researchers have tried to predict the water inflow indirectly. This paper attempts to predict the groundwater conditions in the Beheshtabad tunnel (in Iran) using the fuzzy inference system based on the datasets acquired from the preliminary exploration studies. 250 datasets for the Beheshtabad tunnel were used out of which, 200 datasets were used to develop the model and 50 were used to validate the results obtained. 90% accuracy was obtained through comparing the fuzzy estimation and actual groundwater conditions. The proposed model can be used with much less degree of complexity for prediction of the groundwater conditions as well as decreasing the overall costs of the exploration measurements, and due to these characteristics, it is applicable for most users.

A new equation for the equivalent hydraulic conductivity of rock mass around a tunnel

A new equation for the equivalent hydraulic conductivity of rock mass around a tunnel

Representativeness of jointed rock mass hydraulic conductivity obtained from packer tests for tunnel inflow rate estimate

In current engineering practice, groundwater inflow into a tunnel in a jointed rock mass is estimated using the equivalent hydraulic conductivity, which is often estimated from packer tests. In a jointed rock mass, however, the flow rate out of a packer interval is controlled mainly by the hydraulic conductivity of joints intersecting the packer interval, which in turn depends on joint characteristics and effective joint normal stress. If the state of stress, or the geological conditions, of joints intersecting a packer interval are significantly different from those intersecting the tunnel, the equivalent hydraulic conductivity estimated from the test might not be representative in estimating groundwater inflow rate into the tunnel. In this study, the potential impact of different stress distribution and geological characteristics of joints around a packer interval from that around a tunnel is discussed on the basis of the concept of hydro-mechanically coupled behavior of joints. The effect of anisotropic characteristics of the jointed rock mass around a packer interval is also discussed through numerical parametric study using the distinct element method. Based on the numerical analysis results and literature review, subsequent guidelines and recommendations in performing packer tests are provided.

An empirical model for estimating hydraulic conductivity of highly disturbed clastic sedimentary rocks in Taiwan

This paper presents the establishment of an empirical HC model for estimating rock mass hydraulic conductivity of highly disturbed clastic sedimentary rocks in Taiwan using high-resolution borehole acoustic televiewer and double packer hydraulic tests. Four geological parameters including rock quality designation (RQD), depth index (DI), gouge content designation (GCD), and lithology permeability index (LPI) were adopted for establishing the empirical HC model. To verify rationality of the proposed HC model, 22 in-situ hydraulic tests were carried out to measure the hydraulic conductivity of the highly disturbed clastic sedimentary rocks in three boreholes at two different locations in Taiwan. Besides, the model verification using another borehole data with four additional in-situ hydraulic tests from similar clastic sedimentary rocks was also conducted to further verify the feasibility of the proposed empirical HC model. The field results indicated that the rock mass in the study area has a conductivity between the order of 10 − 10 m/s and 10 − 6 m/s at the depth between 34 m and 275 m below ground surface. Results demonstrate that the empirical HC model may provide a useful tool to predict hydraulic conductivity of the highly disturbed clastic sedimentary rocks in Taiwan based on measured HC-values.

An assessment of hydrogeological parameters on the karstic island of Dugi Otok, Croatia

Summary Extensive explorations have been performed in the northern part of the island of Dugi Otok in the Adriatic Sea (Croatia) with the purpose of extraction of fresh and/or brackish water. The island is predominantly composed of karstified carbonate rocks of the Cretaceous age. The explorations included geological and hydrogeological mapping of the area, geophysical surveys (by means of the geoelectric tomography method, electrical resistivity sounding, and surface seismic refraction), exploratory core drilling, hydrochemical in situ and laboratory explorations, and the pumping test with the calculation of rock mass parameters. Based on the obtained results, it was concluded that it is possible to exploit the groundwater with a pumping quantity of 5 l/s without intrusion of salt water into the fresh-water lens. Choosing a pumping regime based on the parameters of the aquifer (rock mass hydraulic conductivity order of magnitude 10−4–10−6 m/s), and knowing the parameters of the well, as well as the tidal efficiency, should reduce the possibility of intrusion to the minimum. Because of the systematic quality of the performed explorations and obtained results, the island of Dugi Otok can serve as a typical example for such kinds of exploration works on other karstic islands. Hydrogeological composition behaves partly according to Ghyben–Herzberg‘s law, but because of the nature of the karstic hydrogeological system, such a statement needs to be accepted approximately because of the highly complicated relationships.

Rock mass hydraulic conductivity of the Bukit Timah granite, Singapore

Field tests were conducted to determine the rock mass hydraulic conductivity through double packer constant pressure, rising and falling head tests in boreholes. The field results indicated that the Bukit Timah granite is massive and has the hydraulic conductivity of 10−10–10−8m/s, with isolated discontinuity zones having the hydraulic conductivity of 10−7–10−6m/s. The rock mass hydraulic conductivity generally decreases with increasing depth. The field data provide a base for assessment and prediction of water inflow to rock tunnels.

Evaluation of excavation-induced changes in rock permeability

The nature of the changes in permeability around an underground opening induced by blast-damage or stress-relief is an important factor in the design of nuclear waste repositories in rocks such as granite or basalt. Changes in stress and corresponding changes in rock mass hydraulic conductivity are analyzed for a circular shaft using simplifying assumptions regarding fracture geometry and initial stress state. Stress distributions around the shaft are calculated using closed-form soltions and are related to changes in hydraulic conductivity using a form of the cubic law for flow through fractured media The analysis predicts that hydraulic conductivity may be increased, close to the wall of the opening, by two to three orders of magnitude over the far-field value solely in response to stress relief. The zone in which hydraulic conductivity is increased by at least one order of magnitude over the far-field value is limited to within one to two radii from the opening. Blasting may result in large increases in hydraulic conductivity immediately adjacent to an opening but blast damage may be limited to within 0.3 m of the opening using controlled blasting techniques with low perimeter charge weights.

Comparison of laboratory, in situ, and rock mass measurements of the hydraulic conductivity of metamorphic rock at the Savannah River Plant near Aiken, South Carolina

In situ testing of exploratory wells in metamorphic rock indicates that two types of fracturing occur in the rock mass. Rock containing small openings that permit only extremely slow movement of water is termed virtually impermeable rock. Rock containing openings of sufficient size to permit transmission of water at a significantly faster rate is termed hydraulically transmissive rock. Laboratory methods are unsuitable for measuring hydraulic conductivity in hydraulically transmissive rock; however, for the virtually impermeable rock, values comparable to those of the in situ tests are obtained. The hydraulic conductivity of the rock mass over a large region is calculated by using the hydraulic gradient, porosity, and regional velocity. This velocity is determined by dividing the inferred travel distance by the age of water, which is determined by the helium content of the water. This rock mass hydraulic conductivity value is between the values measured for the two types of fractures but is closer to the measured value for the virtually impermeable rock. This relationship is attributed to the control of the regional flow rate by the virtually impermeable rock where the discrete fractures do not form a continuous open connection through the entire rock mass. Thus laboratory methods of measuring permeability in metamorphic rock are of value if they are properly applied.