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Collaborating Authors
Geologic modeling
ABSTRACT Presently the survey of absolute displacements of targets fixed at the tunnel wall is the state of the art in performance monitoring of tunnels. Monitoring data are used to assess the stabilization process of the tunnel, more recently also for the short term prediction of the ground conditions ahead of the face. The displacements do not substantially vary in rock mass conditions with nearly constant properties and influencing factors. On the other hand, changes in the rock mass structure or properties result in changes in the displacement characteristics. For nearly constant conditions, the displacement trends show minor fluctuations within a certain normal range, due to minor variations in the rock mass properties. Deviations from this range are clear indicators for changing conditions ahead of the face or outside of the tunnel. To identify such trend deviations the normal range of the trend lines becomes crucial. Geostatistical methods allow an automatic identification of trends along the tunnel. Using data from completed tunnel projects a "reference trend table" can be established. By comparing actual observed trend characteristics to this reference table changing ground conditions ahead of the face can be identified and hence, the change in the displacement characteristics and magnitudes can be predicted. 1 INTRODUCTION The uncertainties in the geological conditions and ground parameters require an observational approach for safe and economical tunnel construction. Several conditions must be fulfilled for a successful application of the observational method (Peck 1969, Schubert 2008). One of the requirements is the assessment of possible behaviors and the establishment of their acceptable limits during design. This includes the identification of potential failure modes, as well as the determination of deformation characteristics and magnitudes. As the ground in general is all but homogeneous, continuous, and isotropic, simple homogeneous models usually do not provide enough insight to establish a realistic "normal behavior" for structured and heterogeneous ground conditions. It is also unrealistic to think that sophisticated numerical models can be used for an entire project during the design. A reasonable way to produce expected realistic ground behaviors is to first use simplified models to determine the range of expected displacements, and then modify the results with the help of expert knowledge. During construction the measurement results contain all influences of the ground structure, stresses, and interaction between ground and support. The previously established characteristic behaviors for certain conditions are compared to the monitoring results. In case of agreement it can be established that the observed behavior is "normal". Deviations from the expected behavior can have various reasons. One may be that the behavior during design was not assessed correctly. In this case, a refinement of the model is required. Another reason for behavior deviating from the expected can be a change in the ground conditions ahead of the face. It is meanwhile well known that trends of displacement vector orientations can be used to predict changing ground conditions ahead of the face (Schubert & Budil 1995, Steindorfer 1997, Jeon et al. 2005).
- Data Science & Engineering Analytics > Information Management and Systems (1.00)
- Reservoir Description and Dynamics > Reservoir Characterization > Geologic modeling (0.49)
ABSTRACT This study investigates the influence of fracture properties on Dense Non-Aqueous Phase Liquids (DNAPL)-water flow in rock fractures. The aperture distributions of fractures in sandstone and shale specimens of Alberta Paskapoo Formation were obtained from the X-ray computed tomography (CT) technique. Then, the geostatistical parameters of each aperture distribution were estimated, and isotropy or anisotropy nature of the aperture distributions was identified. Equiprobable aperture distributions were generated from stochastic simulation to observe the affect of anisotropy of aperture distribution on DNAPL migration parameters. One set of fractures was generated using geostatistical properties similar to that of measured aperture distribution and the other set was generated by interchanging the highest and the lowest spatial correlation directions of the measured aperture distribution. DNAPL-water flow process was simulated in each generated fracture by utilizing the invasion percolation approach. The affects of anisotropy of aperture distribution on the main flow parameter, the capillary pressure–saturation relationship, was determined from the result of the simulated flow process. 1 INTRODUCTION Rock fracture is a crucial geological feature in subsurface contaminant migration process. The affects of fracture characteristics on migration process of Dense Non-Aqueous Phase Liquids (DNAPL), a critical ground water contaminant in many industrialized cities, are the main focus of this study. The investigation is restricted for single rock fractures, which act as the fundamental elements in understanding the flow behavior in fracture networks. 1.1 Fracture aperture distribution Determination of aperture distribution is essential to estimate the fracture characteristics and flow behavior of a single fracture. Some early researches proposed to estimate the aperture distribution by obtaining a cast of the void space. In several other studies fracture replicas were first constructed from a transparent material and the aperture distribution was determined by means of light transmission. The use of a profilometer or a nuclear magnetic resonance imaging (NMRI) technique is another method of measuring the fracture apertures. X-ray computer tomography (CT) is an attractive technique for determination of morphology of rock fractures (Johns et al., 1993; Keller, 1997; Bertels et al., 2001; Muralidharan et al., 2004; Walters, 1995) due to its convenience of use and non-destructive nature. Goestatistics have been used in studies of single fractures for characterization of fracture surfaces (Marache et al., 2002) or for generation of aperture distributions (Moreno et al., 1988; Tsang and Tsang, 1989; Pruess and Tsang, 1990). The spatial continuity is an important property of a fracture aperture distribution and it can be quantitatively estimated by geostatistical parameters. 1.2 DNAPL-water flow in rock fracture The DNAPLs migrate deep into the subsurface due to their high densities and accumulate on the bedrock. If the bedrock is fractured, DNAPLs can enter into the fractures. However, for entry, the DNAPL pressure have to be higher than the capillary forces between water and DNAPL at the fracture entrance (Kueper and McWhorter, 1991). Once entered, the migration of DNAPL in variable aperture fractures is controlled by the capillary forces (Reitsma and Kueper, 1994).
- Geology > Geological Subdiscipline > Geomechanics (0.92)
- Geology > Rock Type > Sedimentary Rock > Clastic Rock > Sandstone (0.30)
- Well Completion > Hydraulic Fracturing (1.00)
- Reservoir Description and Dynamics > Reservoir Characterization > Geologic modeling (1.00)
- Reservoir Description and Dynamics > Formation Evaluation & Management (1.00)
Geomechanical Stability And Integrity of Radioactive Waste Repositories In Salt Rock
Heusermann, S. (Federal Institute for Geosciences and Natural Resources) | Fahland, S. (Federal Institute for Geosciences and Natural Resources) | Eickemeier, R. (Federal Institute for Geosciences and Natural Resources)
ABSTRACT To prove the suitability and safety of underground structures for the disposal of radioactive wastes, an extensive safety analysis has to be carried out. Basic steps of the analysis are the geological modelling of the entire structure including the host rock, the overburden and the repository geometry, the idealization of the geology as well as the geomechanical modelling taking into account the modelling of the underground structure and the appropriate material models with respect to creep and dilatancy of rock salt. For the geomechanical analysis the ANSALT finite-element code was compared to the JIFE code which allows the consideration of large three-dimensional structures with complex inelastic material behaviour. To establish the finite-element models needed for stability and integrity calculations, the geological models are simplified with respect to homogenous rock layers with uniform creep behaviour. The modelling results are basic values for the evaluation of the stability of the repository and the long-term integrity of the geological barrier. Concerning the barrier integrity, two geomechanical criteria are considered, the dilatancy criterion and the frac criterion. As an example of application, the analysis of the central part of the Morsleben site, used for the disposal of low and medium radioactive waste, is described. 1 INTRODUCTION Over the last three decades, the Federal Institute for Geosciences and Natural Resources (BGR), Germany, has been carrying out extensive geoscientific research and practical project work on domal salt structures to prove their suitability for the disposal of high and low level radioactive wastes. The objective of a waste repository, i.e., prevention of hazardous substances from entering the biosphere, is attained through the use of a system of barriers. Public acceptance of a waste repository depends on the assurance that these barriers are sufficient to provide the necessary protection. Therefore, the safety analysis has a prime importance to the planning and authorization of a repository. The natural geological barrier is an important part of the multiple-barrier system of repositories. Thus, the load-bearing capacity and geomechanical integrity of the rock, its geological and tectonic stability, and its geochemical and hydrogeological development are important aspects of the safety analysis. It is, therefore, not only an engineering problem, but must include geological aspects. The safety analysis must be based on a safety concept that takes into consideration the possibilities for failure that could occur during excavation, operation, and post-operation phases, as well as measures to avoid such failures. The safety analysis must include several steps as geological investigations to provide the basic geological data, mine observations and mining experience, geotechnical in-situ measurements to provide the necessary parameters of the host rock and the overburden, monitoring of the long-term rock behaviour, geomechanical laboratory investigations to determine the relevant properties of the rock and to develop adequate material models, geomechanical and, if required, thermomechanical or hydromechanical model calculations to analyse the stability and integrity of the structure and the repository, as well as evaluation and assessment of the safety taking all geological, experimental and theoretical investigation results into account (Langer & Heusermann 2001).
- Water & Waste Management > Solid Waste Management (1.00)
- Energy > Power Industry > Utilities > Nuclear (1.00)
- Reservoir Description and Dynamics > Reservoir Characterization > Reservoir geomechanics (1.00)
- Reservoir Description and Dynamics > Reservoir Characterization > Geologic modeling (1.00)
- Health, Safety, Environment & Sustainability > Environment > Waste management (1.00)
- Health, Safety, Environment & Sustainability > Environment > Naturally occurring radioactive materials (1.00)