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Europe
Abstract: Recent regulations introduced a limit state approach to geotechnical design by using representative values of the actions and of the strength parameters, partial safety factors that affect them, and by including safety margins in the calculation models. Moreover, Eurocode 7 stresses the importance of making use of test results for establishing ground parameters, and so rock joint shear tests are set to play a relevant role in the assessment of the shear strength required in the design of important projects, such as concrete dams, large bridge foundations, slopes, or underground excavations. In this keynote, joint shear tests are described, along with a presentation of their equipments and different procedures. Test results and calculations for the assessment of relevant shear strength parameters are illustrated, and several topics regarding sampling and variability are discussed. Opportunity is taken to present a practical apparatus allowing to perform simple shear tests (push tests) under very low normal stresses with advantages over tilt or pull tests. 1 INTRODUCTION Certain projects, such as concrete dams, rock slopes, or relatively shallow under-ground works, require the design of geotechnical works in rock masses where stresses are low when compared with the intact rock strength. In these cases, stability is structurally controlled by the shear strength of kinematically unfavourable rock discontinuities (joints, bedding planes, shear zones, faults, and cleavage or foliation planes). The analysis of this type of limit state requires the estimation of the shear strength of the rock joints, which is usually done by means of shear tests (Goodman 1976; Hoek & Brown 1977, Muralha 2007). Though Eurocode 7 is intended to be applied mainly to common civil engineering works, it establishes a comprehensive framework for the design of any kind of structure, such as underground caverns, tunnels, slopes, and dam or large bridge foundations.
- Europe (1.00)
- South America > Brazil (0.28)
Abstract: Rock support is associated with the rock mass quality as well as the in the state of situ stresses. A safety factor, defined by the strength of the support system and the load exerted on it, is usually used for rock support design. This design method is based on the principle of structure mechanics. It is appropriate to do so in a loadcontrolled situation, such as rock falls under gravity, but it is not valid to use the traditional strength-load safety factor for support design in high stress rock conditions under which instability usually involves stress-induced rock failure. In this case, the principle of rock support should be to absorb the energy released from the rock instead to equilibrate the weight of falling blocks. In this paper, the failure modes of rock under different stress conditions are first viewed. The philosophy and principles of rock support design are then presented for different rock conditions. Finally, yield support elements used so far in civil and mining engineering are briefly reviewed. Keywords: rock support, support design, pressure arch, yield rock support, steel set, rock bolt, energy-absorbing rock bolt, rock excavation. 1 INTRODUCTION Instability is always a concern in underground excavations. The stability of an underground opening, such as a tunnel, cavern or mine stope, is mainly governed by three factors: the quality of the rock mass, the in situ stress state and the size and geometry of the opening. Of these three factors, the in situ stress state is the factor that plays the crucial role in the stability of a rock mass. The type of excavation disturbance is mainly associated with the magnitudes of the in situ stresses with respect to the quality of the rock mass.
- Europe (1.00)
- North America > Canada > Quebec (0.28)
- Materials > Metals & Mining (0.67)
- Energy > Oil & Gas (0.46)
- Europe > United Kingdom > England > London Basin (0.91)
- North America > Canada (0.89)
Abstract: In the Carrara Marble industrial extraction district the number of the under-ground quarries is increasing due to the convergence of several concerns: safety, environmental impact, mining optimization. These aspects led to the opening of large underground quarries. In 1994 rockburst phanomena occurred in two panels of the Carlone underground quarry,, which is located about 450 m below the topographic surface. In recent years a series of studies have been developed in order to understand and overcome the rockbursts. In this paper a detailed review of the geostructural and geomechanical setting of the Carlone quarry is presented along with a. 2D FEM analysis, both aimed at verifying the stress distributions around the underground openings during the excavation process, in order to better to understand the origin of the occurred rockburst. 1 INTRODUCTION The exploitation of Carrara Marble dates back to the Roman era, if not even further back to the Etruscans (Dolci 1980, Capuzzi 1984, Bruschi et al. 2004). In the last fifty years the rate of Carrara Marble exploitation has experienced a sudden increase, at present, about 4.5 Gtons of Carrara Marble is excavated every year, main-ly from a few large open pit and wide underground quarries. In the year 1973 a new underground quarry (Carlone quarry) was opened right in the middle of a tunnel of the old dismissed Marble Railway, which was excavated by drill&blasting in the last decades of the XIX century. In the early 1990's, two of the Carlone excavation panels suffered from localized rockbursts (Coli 1995, Ferrero et al. 2009, Coli et al. 2010, Gullรฌ et al. 2010). A series of studies aimed at understanding the triggering mechanism of this phe-nomena have been carried out by the Authors in the last few years.
- Asia (1.00)
- Europe > Italy (0.30)
- North America > United States (0.28)
Abstract: The Swedish Nuclear Fuel and Waste management Co. is planning for a future nuclear waste repository, and there is a need of describing the rock conditions accurately to fulfill the high demands on site descriptions for design and safety assessment purposes. One of the important factors is the in situ rock stress conditions, in terms of orientation and magnitude. This paper gives a description of the effort made to develop a robust method specifically for determination of stress orientation. The methodology, which has been tested with promising results, is to study thermally induced borehole breakouts. The orientation of breakouts found in ordinary cored boreholes at the investigated repository site Forsmark, is fairly constant but the breakout occurrence is limited and varying. A study concerning correlation between breakouts and the drilling conditions and rock types has therefore been performed. A quite strong degree of correlation is found between breakout occurrence and rock type and rock alteration. Some degree of correlation is also found between breakouts and drill water flow and breakouts and drill core bit changes. These findings lead to recommendations concerning the continued investigations and method development. 1 BACKGROUND The Swedish Nuclear Fuel and Waste management Co. (SKB) is planning for a future underground nuclear waste repository located in Forsmark (SKB 2009). The design and construction of a repository for spent nuclear fuel must consider the site condi-tions that may impact the long-term safety of a repository. Many of the constraints that are needed to ensure the safe performance of a final repository facility with respect to radionuclide containment are unique for the repository, and the designers have to find a design and layout that meets both the operational requirements for such facility and the long-term safety requirements related to nuclear-waste containment.
- Geology > Rock Type (1.00)
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Water & Waste Management > Solid Waste Management (1.00)
- Energy > Power Industry > Utilities > Nuclear (1.00)
- Energy > Oil & Gas > Upstream (1.00)
Abstract: Stress determination methods based on rock memory the Deformation Rate Analysis (DRA) is analysed. It is traditionally assumed that stress memory is only induced when the rock is subjected to stresses sufficiently high to produce crack initiation or propagation. We present experiments and a model showing that the DRA can also detect lower stresses, insufficient to induce any damage accumulation in the rock. The mechanism of the low stress rock memory is based on the frictional sliding over the pre-existing cracks or interfaces. We model the rock with pre-existing cracks or interfaces using a simplified rheological model that includes friction and viscous elements. We demonstrated that the frictional sliding mechanism does produce inflections in the DRA curve. The detectible stress levels fall between the minimum and the maximum cohesions. An increase in the number of cracks, which is essentially an increase in the sample size, can widen the interval of detectible stresses; it also produces smoother DRA curves and subsequently reduces the accuracy of the stress reconstruction. The accuracy of the stress determination also depends upon a combination of the rheological parameters of the rock and cracks and the loading rate, which is an important (and essentially the only) controlling parameter. 1 INTRODUCTION Understanding the current in situ stress state is critical for the design and performance of underground excavations and open pits in rock masses (e.g., Fairhurst 2003, Dight, 2011). Amongst the many stress determination methods available, the rock memory methods have a natural advantage in that they can used on cores that are accumulated in great numbers at the exploration stage and do not require stress recalculation from the strain measurements and hence avoid the tedious determination of generally anisotropic elastic moduli of the rock (Dight & Dyskin 2007).
- Asia > Japan (0.46)
- Oceania > Australia > Western Australia (0.29)
- Asia > Japan > Chubu > Sea of Japan > Japan Basin > Yamato Basin (0.99)
- Europe > United Kingdom > England > London Basin (0.91)
Abstract: The mechanical and hydraulical behaviour of rock mass is governed by the presence of discontinuities. The simulation of fracture growth and interaction may help understanding the general mechanical and hydraulic rock mass behavior to changes in boundary conditions in rock engineering applications. In this investigation the numerical simulation of crack propagation and interaction of fractures in uniaxial and biaxial stress fields using a fracture mechanics based software is presented. For the analyses, the geomecon fracture mechanics research code roxol was used. roxol is a 2D hydro-fracture-mechanical finite element code designed for simulating the initiation and propagation of fractures in geological media. The elastic deformation is modelled using an orthotropic Hookeยดs law. Fracture propagation criteria comprise minimum principle stress and minimum tangential stress. In this investigation, the experimental results by Bobet and Einstein (1998) are analysed in a series of simulations using roxol. Bobet and Einstein studied distinct differences in the fracture interaction and coalescence behaviour of gypsum specimens containing two flaws. Using the different fracture propagation criteria these physical experiments were mirrored using roxol. The study at hand compares the fracture evolution and interaction from numerical experiments to the physical experiments by Bobet and Einstein, and discusses the implications for the validity of the fracture propagation criteria in the simulation of rock breakdown processes. 1 INTRODUCTION In classical rock mechanics, rock is viewed as a continuous flawless material. Rock strength is described by empirical criteria based on observations of failure, mostly in the laboratory. The most frequently used criteria are the Mohr-Coulomb, Hoek-Brown, Mogi, and Lade Duncan criteria and modifications or extensions of them (Benz and Schwab, 2008). However, the hypothesis of continuity does not hold for rock. Rock material is a discontinuous combination of solid matter, pores, cracks and fractures.
Abstract: Large scale stress redistribution around longwall panels in coal mines put rock masses in vicinity of the underground excavations close to failure. While immediate failure is reflected for example by instantaneous seismic events there is also a delayed response of the rock mass as noted from decaying seismicity during non-operating times. Sandstone samples from the hanging wall of coal seam in the Ruhr coal mining district in Germany have been subjected to conventional strength, creep and relaxation tests, respectively. From creep and relaxation tests estimates of time dependent strength properties are derived. It was found from creep tests that below a certain stress or strain level the samples showed negligible creep as evaluated by inspection of the axial, lateral and volumetric strain over time. This stress level in the order of 75 % of the failure stress was used to delineate the potential for delayed failure around the under excavation. Strain rates increase significantly above this stress level and will eventually lead to the short term failure of the rock. Numerical modeling was employed to delineate zones of states of stress around underground longwall coal panels at depth of approximately 1200 m which are prone to time dependent failure. INTRODUCTION The knowledge about time dependent deformation in a mining environment is necessary to ensure safe and economic working conditions for miners. The temporal pattern of seismic events reflects the time dependent behavior of rock under excavation- induced stress changes. The need for understanding time dependent rock behavior as well as the need for a simple, mine-proof tool for estimating the time of stable rock conditions is evident. There exists a vast body of knowledge on time dependent behavior of salt rock as well as many theoretical approaches towards that problem (Cristescu, 1993) but little is known for hard rocks.
AE Monitoring of Hydraulic Fracturing Laboratory Experi-ment With Supercritical And Liquid State CO2
Ishida, T. (Kyoto University) | Niwa, T. (Kyoto University) | Aoyagi, K. (Kyoto University) | Yamakawa, A. (Kyoto University) | Chen, Y. (Kyoto University) | Fukahori, D. (Kyoto University) | Murata, S. (Kyoto University) | Chen, Q. (3D Geoscience, Inc) | Nakayama, Y. (3D Geoscience, Inc)
Abstract: CO2 (Carbon dioxide) is often used to enhance and stimulate oil recovery in depleted petroleum reservoirs. Its behavior in rock is interesting also in projects of hot dry rock geothermal energy extraction and of carbon dioxide capture and storage. CO2 usually becomes SC (Supercritical) in underground for depth larger than 1,000m, while it becomes L (Liquid) in geological condition having low temperature. The viscosity of SC-CO2 is around 2 %, while that in L-CO2 is around 10 % of that of wa-ter. The difference in viscosity may cause difference in induced cracks and fluid flow in the cracks. As the first step to clarify the difference, we conducted hydraulic frac-turing experiments using 17 cm cubic granite blocks under the hydrostatic pressure of 1 MPa. The breakdown pressure and distribution of located AE (acoustic emission) sources with injection of SC-CO2 and L-CO2 were compared.In the experiments, SC-CO2 tended to generate thinner and wavelike cracks with more secondary branches than L-CO2. The tendency that the breakdown pressure with SC-CO2 is lower than that with L-CO2 was also observed. 1 INTRODUCTION CO2 (Carbon dioxide) is often used for miscible flooding of enhanced oil recovery in depleted petroleum reservoirs. It is considered to use CO2 as fracturing fluid for well stimulation for its advantage of elimination of formation damage and residual fractur-ing fluid (Sinal & Lancaster, 1987 and Liao et al. 2009). Also in hot dry rock geo-thermal energy extraction, a concept to use CO2 for fracturing and circulating fluid is proposed, because of reducing the circulating pumping power requirements and elimi-nating scaling in the surface piping due to its inability to dissolve mineral species (Brown, 2000). Recently, also in shale gas reservoir, enhanced gas recovery by injecting CO2 with advantage of CO2 sequestration has been examined as a feasibility (Kalantari-Dahaghi, 2010).
- Energy > Oil & Gas > Upstream (1.00)
- Energy > Renewable > Geothermal > Geothermal Resource > Hot Dry Rock (0.45)
- North America > United States > Texas > Fort Worth Basin > Barnett Shale Formation (0.99)
- Europe > Norway > North Sea > Central North Sea > South Viking Graben > PL 046 > Utsira Formation (0.99)
Abstract: The disposal of used nuclear fuel using the KBS-3 concept requires the excavation of thousands of 1.8-m diameter 8 m deep boreholes. The heat generating spent fuel placed in these boreholes will increase the stresses on the boundary of these boreholes. Three in-situ experiments have been carried out to assess the performance of the borehole wall as the excavation-induced and thermally-induced stresses are applied. The three experiments utilized similar configurations and concepts. The APSE and CAPS experiments were carried out in heterogeneous igneous crystalline rock mass. The heterogeneity is caused by alteration, sealed fractures, my-lonite shear zone and open flowing fractures. The APSE experiment utilized external heaters while CAPS used a central heater. The diameter of the APSE experiment was more than 3 times greater than the diameter of the CAPS experiments. Despite the different loading path and differences in scale the rock mass spalling strength deter-mined from the CAPS and APSE results was essentially the same, approximately 50% of the intact laboratory uniaxial compressive strength. Both experiments also showed that a small confining pressure applied to the wall of the borehole was sufficient to control fracture propagation associated with the spalling process. The POSE Experiment was the first attempt to establish the spalling strength for a heterogeneous mixture of strongly foliated mica gneiss with coarse grained pegmatite granite veins (Migmatite). The rock mass is essentially massive, i.e., no open frac-tures. The results from the POSE experiment are still being reviewed as the experi-ment is still in progress. The results obtained to date are not as easy to interpret as those obtained in APSE and CAPS, in part because there is still some uncertainty as to the orientation of the in-situ stress in the vicinity of the experiment.
- Europe (0.47)
- North America > Canada > Alberta (0.28)
- Geology > Rock Type > Igneous Rock (1.00)
- Geology > Geological Subdiscipline > Geomechanics (0.70)