Groundwater Flow In Fractured Rock Research Articles

Groundwater flow through fractured rocks and seepage control in geotechnical engineering: Theories and practices

Groundwater flow through fractured rocks has been recognized as an important issue in many geotechnical engineering practices. Several key aspects of fundamental mechanisms, numerical modeling and engineering applications of flow in fractured rocks are discussed. First, the microscopic mechanisms of fluid flow in fractured rocks, especially under the complex conditions of non-Darcian flow, multiphase flow, rock dissolution, and particle transport, have been revealed through a combined effort of visualized experiments and theoretical analysis. Then, laboratory and field methods of characterizing hydraulic properties (e.g. intrinsic permeability, inertial permeability, and unsaturated flow parameters) of fractured rocks in different flow regimes have been proposed. Subsequently, high-performance numerical simulation approaches for large-scale modeling of groundwater flow in fractured rocks and aquifers have been developed. Numerical procedures for optimization design of seepage control systems in various settings have also been proposed. Mechanisms of coupled hydro-mechanical processes and control of flow-induced deformation have been discussed. Finally, three case studies are presented to illustrate the applications of the improved theoretical understanding, characterization methods, modeling approaches, and seepage and deformation control strategies to geotechnical engineering projects.

Geophysical for granitic joint patern and subsurface hydrology related to slope instability

The groundwater flow in fractured rock is an important process that allows to understand the hydrological dynamics at hillslope scales as well the increase of pore pressure at specific points in the soil-rock contact. Exfiltration from bedrock fractures on the failure surfaces and stability problems related to structural characteristics of rocks are common in the highland region of Rio de Janeiro, which were identified in a landslide scar resulting from a translational shallow landslide at the soil-bedrock interface triggered by the extreme rainfall events in January 2011. Considering the temporary springs from bedrock fractures in fresh landslide scars this research aims to analyze, through Ground Penetration Radar (GPR), the structure of the rock and the pattern of fractures in subsurface to understandand the hole the existence of a network of fractures in the slope hydrogeological dynamics and possible consequences for its stability. GPR results proved the existence of fissural suspended aquifers near the surface and sub-vertical tectonic fractures acting as geological barriers that create saturation zones inducing the formation of suspended discontinuous aquifers. It is possible that the fracture systems are determinant for the deflagration of mass movement in the region, mainly of translational shallow types, on slopes covered by saprolites overlying fractured rocks.

Modelling Methods Attached to the Research of Groundwater Flow in Fractured Rocks – Theories, Laboratory and Numerical Modelling

There are serious efforts worldwide to better understand and model groundwater flow in fractured rocks and karst aquifers. This study summarizes the major theories and idealizations used for describe flow in fractured rocks and presents a laboratory model and numerical models which were made for this purpose. The laboratory model originally made by Ollős and Nemeth in 1960 was rebuilt in MODFLOW-CFP. The usability of both modelling method was analysed. Based on the experience of this modelling an existing cave system was modelled with CFP. The Molnar Janos Cave – a karst cave almost filled with water – was analysed with the tool of numerical modelling to better understand the flow in cave conduits.

Modelling of Groundwater Flow in Fractured Rocks

Traditional groundwater flow modelling is based on the Darcy's law which is valid when the flow is laminar. However, the water flow in fractured aquifers can be non-laminar in discontinuities and in this case the Darcy's law is not valid. That is why a numerical solver is needed which is able to model both laminar and non-laminar flows. The MODFLOW-CFP by USGS is one of the programs that can be used for this issue.Additionally to numerical experiments physical modelling can be also a useful and effective tool to explore the flow in fractured media in details. Besides some international published case studies exists a less-known Hungarian conduit model by Öllős & Németh (1960). In this study the Öllős-Németh's conduit model was rebuilt in the modified MODFLOW-CFP (CFPv2). The goal of this modelling study was to reach good agreement between the results of the numerical model and the measurements. After having studied many numerical cases and by means of sensibility analysis, the CFP model was verified. Based on the verification the model is applicable to simulate non-laminar flows in fractured rocks not only in laboratory-scale but even in real cases.Following the international practice the MODFLOW-CFP is a usable tool to learn more about a well-developed karst aquifer in Hungary. Using the numerical model the flow in this karst area can be described in detail and more precisely.

Detailed measurement of the magnitude and orientation of thermal gradients in lined boreholes for characterizing groundwater flow in fractured rock

• TVP measures the detail variations in the thermal gradient within a lined borehole. • Components of the thermal gradient vector characterize fracture flow through rock. • Complex thermal patterns exist in a lined borehole adjacent to fractures. • Data are reproducible and consistent with three nearby multilevel installations. • Variations in “thermal stratigraphy” are similar to hydro-stratigraphic layers. Recent developments have led to revitalization of the use of temperature logging for characterizing flow through fractured rock. The sealing of boreholes using water-filled, flexible impermeable liners prevents vertical cross connection between fractures intersecting the hole and establishes a static water column with a temperature stratification that mimics that in the surrounding formation. Measurement of the temperature profile of the lined-hole, water column (using a high sensitivity single-point probe achieving resolution on the order of 0.001 °C) has identified fractures with active flow under ambient groundwater conditions (without cross connecting flow along the borehole). Detection of flow in fractures was further improved with the use of a heater to create thermal disequilibrium in the active line source (ALS) technique and eliminate normal depth limitations in the process. This paper presents another advancement; detailed measurement of the magnitude and direction of the thermal gradient to characterize flow through fractured rock. The temperature within the water column is measured along the length of the lined hole using a temperature vector probe (TVP): four high sensitivity sensors arranged in a tetrahedral pattern oriented using three directional magnetometers. Based on these data, the horizontal and vertical components of the thermal field, as well as the direction of temperature gradient are determined, typically at depth intervals of less than 0.01 m. This probe was assessed and refined by trials in over 30 lined boreholes; the results from two holes through a fractured dolostone aquifer in Guelph, Ontario are used as exampled. Since no other device exists for measuring flow magnitude and direction under the ambient flow condition created by lined holes, the performance of the TVP is assessed by examining the reproducibility of the temperature measurements through an ALS test, and by the consistency of the results relative to other types of larger-scale information from the study area. Temperature profiles were measured in lined holes under both ambient thermal conditions and subject to ALS heating of the entire length of the holes to demonstrate resolution and reproducibility. The hydraulic gradient in three-dimensional space, based on pressure measurements from three depth discrete, multilevel monitoring systems in nearby holes, was used to independently estimate variations in groundwater flow directions. The characteristics of the hydraulic and thermal regimes are compared to assess response to changes in flow in a fractured rock system. When used in the lined holes, the level of detail provided by this multi-sensor probe is much greater than that provided by a single-sensor probe and this detail strongly supports inferences concerning the relative magnitude and direction of the flow. The results of this study indicate that the details of the thermal gradient can be measured and provides superior results compared to a conventional one dimensional temperature profile, thereby substantially enhancing the characterization of groundwater flow in fractured rock.

Simulation of groundwater flow in fractured rocks using a coupled model based on the method of domain decomposition

Groundwater flow in fractured rocks is modeled using a coupled model based on the domain decomposition method. In the model, the fractured porous medium is divided into two non-overlapping sub-domains. One is the rock matrix, in which the medium is described using a continuum model. The other consists of deep fractures and fissure zones, where the medium is described using a discrete fracture network (DFN) model. The two models are coupled through the continuity of the hydraulic heads and fluxes on the common boundaries. The coupled model is used to simulate groundwater flow in a hydropower station. The results show that the model simulates groundwater levels that are in agreement with the measured groundwater levels. Furthermore, the model’s parameters relating to deep fractures and fissure zones are verified by comparing three different models (the continuum model, coupled model, and DFN model). The results show that the coupled model can capture and duplicate the hydrogeological conditions in the study domain, whereas the continuum model overestimates and the DFN model underestimates the measured hydraulic heads. A sensitivity analysis shows that fracture aperture has a considerable effect on the groundwater level. So, when the fracture aperture is large, the coupled model or DFN model is more appropriate than the continuum model in the fracture domain.

Integration of hydrological and ecological modelling for the assessment of a nuclear waste repository

The Swedish Nuclear Fuel and Waste Management Co. (SKB) is currently investigating two sites in Sweden as potential locations for underground disposal of spent nuclear fuel. The site investigations cover geology, chemistry, hydrology and ecology, among several other disciplines. Strong emphasis is put on the characterisation of properties and processes affecting the water-born transport and retention of radionuclides along potential flow paths from repository depth (ca. 500 m below ground) to ground surface, and the dispersal and accumulation of the radionuclides in the surface system. The modelling performed to support the site descriptions and the safety assessments utilises a wide range of hydrological modelling tools. These include tools primarily developed for modelling groundwater flow in fractured rock, and modelling tools that handle surface and subsurface flows, water uptake in vegetation and the interactions with the atmosphere. The couplings between the hydrogeology, the near surface hydrology and the surface ecosystems are especially important in this integrated modelling. This paper highlights some ecohydrological aspects of this coupled modelling and provides example results from site descriptive and safety assessment modelling.

Detecting subsurface groundwater flow in fractured rock using self-potential (SP) methods

In the wine district of Clare Valley in South Australia, the natural-voltage SP signal generated by fluid flow in fractured rock during a pumping test was carefully monitored by time-continuous measurements. From ten electrode locations surrounding the pumping well, the drawdown cone produced by pumping was determined on the basis of the SP measurements together with laboratory measurements of the grain-boundary ζ (zeta)-potential. Such measurements allowed calculation of the fractured-rock aquifer's transmissivity and average permeability. Results were confirmed by piezometer measurements to the extent that data were available. The study has revealed that SP signals generated during pumping tests are of a complex nature. However, if the pumping test is sufficiently long to allow the signal to stabilise, and careful field procedures are in place, then SP measurements have the potential to add significant hydrogeological information. SP measurements are relatively easy and cheap, and are, contrary to traditional methods, not restricted to the locations of existing piezometers, which is particularly useful in fractured-rock aquifers.

割れ目系岩盤地下水に関する調査・解析法の現状

Methods of investigation and analysis for groundwater flow in fractured rocks were reviewed from the viewpoint of the scales and objectives. Research on study styles with three categories; theories, field and laboratory experiments of groundwater in fractured rocks from 366 papers in three international journals and two Japanese journals published since 1990 to 2000 reveals that 34 % of papers were discussed theories without data of field and laboratory experiments. As for investigation methods, in-situ tracer experiments and borehole tests etc. were described. The practical techniques for laboratory experiments were illustrated with some experiments. Then, analysis methods including theories and problems of both continuous and discontinuous models were reviewed. The behavior of groundwater at the intersection of the fracture aperture has a significant influence on the dispersion mechanism in the network model. It is suggested that studying this phenomenon would be an important research theme. Future studies integrated theoretical study, field and laboratory experiments would be requested on the groundwater flow in fractured rocks. Key Words : fractured rock, tracer method, laboratory experiment, fracture intersection, discrete model

Comparison of alternative modelling approaches for groundwater flow in fractured rock

In performance assessment studies of radioactive waste disposal in crystalline rocks, one source of uncertainty is the appropriateness of conceptual models of the physical processes contributing to the potential transport of radionuclides. The Alternative Models Project (AMP) evaluates the uncertainty of models of groundwater flow, an uncertainty that arises from alternative conceptualisations of groundwater movement in fractured media. The AMP considers three modelling approaches for simulating flow and advective transport from the waste canisters to the biosphere: Stochastic Continuum, Discrete Fracture Network, and Channel Network. Each approach addresses spatial variability via Monte Carlo simulation, whose realisations are summarised by the statistics of three simplified measures of geosphere performance: travel time, transport resistance (a function of travel distance, flow-wetted surface per volume of rock, and Darcy velocity along a flowpath), and canister flux (Darcy velocity at repository depth). The AMP uses a common reference case defined by a specific model domain, boundary conditions, and layout of a hypothetical repository, with a consistent set of summary statistics to facilitate the comparison of the three approaches. The three modelling approaches predict similar median travel times and median canister fluxes, but dissimilar variability. The three modelling approaches also predict similar values for minimum travel time and maximum canister flux, and predict similar locations for particles exiting the geosphere. The results suggest that the problem specifications (i.e. boundary conditions and gross hydrogeology) constrain the flow modelling, limiting the impact of this conceptual uncertainty on performance assessment.

An Analysis of Gas Tightness Around Unlined Storage Cavern Using a Discrete Fracture Flow FEM Model

ABSTRACT Water curtain method has been utilized to prevent the escape of gas from storage caverns excavated in the rock. The water curtain method appears to be increasingly used in various forms of storage caverns. Fluid flow in crystalline fractured rock where most of water curtains are constructed is basically controlled by the geometry of fracture network and the properties of the individual fracture. The concept of discrete fracture flow is more appropriate approach for analyzing the stability of water curtain system. In order to analyze the groundwater flow in fracture network, we have developed an unsteady state discrete fracture flowing FEM simulator and have applied it to underground LPG storage facilities at Ulsan, Korea. In this site the hydraulic head of observation well Y was monitored as −7.5 m. With the continuum model concept for the fractured rock the hydraulic head of well Y was calculated as −18.62 m, whereas −4 m was estimated with discrete fracture flow model. The result showed that the discrete fracture flow model was more reasonable for the analysis of groundwater flow in fractured rock than the continuum model concept.

Application of seepage flow models to a drainage project in fractured rock

Various theoretical approaches are used to model groundwater flow in fractured rock. This paper presents the application of several approaches to the restoration of the drainage of Rofla tunnel, Grisons, Switzerland. In this tunnel it became necessary to take measures against the washing out of calcium carbonates from the tunnel lining cement, because the calcium carbonate clogged up the existing drainage tubes leading to increased rock water pressures on the inside arch of the tunnel. Drainage boreholes were drilled on a section of the tunnel and their influence on the water pressures was monitored. On the basis of the geological survey different seepage flow models were established to reproduce the measured water pressures. The models were then used to predict the future water pressures acting on the tunnel lining after restoration. Thus, the efficacy of the different drainage proposals could be predicted and therefore optimised. Finally, the accuracy of the predictions is discussed and illustrated using the measurements in the test section.

The nature of groundwater flow in fractured rock: Evidence from the isotopic and chemical evolution of recrystallized fracture calcites from the Canadian Precambrian Shield

The isotope geochemistry of fracture calcites in three Precambrian plutons on the Canadian Shield has been investigated in order to understand the paleohydrogeological conditions in fractured crystalline rock. Fracture calcites of ancient hydrothermal origins in the Chalk River and East Bull Lake plutons exhibit 18O enrichment and 13C depletion trends resulting from recent low-temperature calcite recrystallization under open-system conditions for oxygen, but semiclosed for carbon, and under extremely variable time-integrated, water/rock (calcite) ratios. This process causes recycling of elements with calcite distribution coefficients > 1 (rare earths, manganese, and possibly iron) from the precursor calcite to younger calcites as well as calcite control over the 87Sr 86Sr ratio of the groundwater within the Chalk River pluton. The large but variable water/rock (calcite) mole ratios calculated from the shifts in the stable isotopic compositions of fracture calcites are compatible with fracture flow models that invoke flow channeling within single fractures that also contain regions of immobile porosity. Quantification of isotopic shifts resulting from recrystallization requires that the initial isotopic composition of the precursor hydrothermal calcite be well constrained. Unlike the Chalk River and East Bull Lake plutons, hydrothermal fracture calcites in the White Lake pluton, which intrudes marble-rich country rocks of the Grenville Supergroup, have highly variable δ 13C and δ 18O values. This is attributed to mixing of carbon from magmatic and sedimentary reservoirs, and to oxygen isotopic exchange between hydrothermal fluids and carbonate country rocks at the time of intrusion. Convective circulation of meteoric groundwater from the surrounding 13C-rich carbonate rocks (up to 6.6%.) resulted in δ 13C values as heavy as 3.0%. for the fracture calcites in the pluton.

Hydrologic happenings

Shlomo Neuman, University of Arizona, has been selected to be the Birdsall Distinguished Lecturer in Hydrogeology, Geological Society of America, for the 1986–1987 academic year. Neuman is an expert on groundwater flow in fractured rocks and groundwater modeling and will be prepared to give one to three lectures at the institutions that he visits. The bequest of John Birdsall, hydrogeologist, U.S. Geological Survey, provides for transportation costs, and the host institution pays the living costs for the lecturer during his or her visit. If you would like Neuman to visit your institution, please contact L.F. Konikow, Lecturer Tour Coordinator, U.S. Geological Survey, 431 National Center, Reston, VA 22092; telephone: 703‐648‐5878.