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ABSTRACT This paper presents the results of a laboratory investigation of the thermomechanical behaviour of anisotropic rock. The tests were performed on natural (Tournemire shale) using special triaxial cell able to control and to go to high temperature. The range of temperatures that were investigated is from 20 Cยฐ to 250 Cยฐ. (20, 100, 150, 200 and 250 Cยฐ), and the range of confining pressures is from 0 MPa to 20MPa. (0, 5, 10, and 20 MPa). The influence of temperature on their mechanical behaviour was investigated for drained tests. Anisotropic elastic response and plastic deformation have been investigated. It seems that, the thermomechanical behaviour of the Tournemire shale is anisotropic and strongly depends on confining pressure and loading orientation at the applied temperature. Hydrostatic compressibility tests (in the perpendicular orientation ฮธ = 90ยฐ) allowed to present the thermal effect on the mechanical behaviour of this rock. In this range of temperatures, the deformability and strength of this rock were found also to be strongly dependent of temperature. 1 INTRODUCTION Many underground works, such as chemical and nuclear waste storage, oil boreholes, injection and production activity, are located in anisotropic rocks and based on the study of thermomechanical behaviour. The design and stability analysis of such structures require knowledge of the deformation and failure of these rock materials. A large number of experimental investigations involving the laboratory tests have been performed. However, most of experimental data reported in the literature is obtained from the tests carried out between the ambient temperature (20 Cยฐ) and (100 Cยฐ), especially for the clay (Behrooz 2003; Campanella 1968; Cekerevac 2004; De Bruyn 1996; Delage 2000; Del Olmo 1996; Hueckel 2002; Imbert 2005). For a higher temperature, few experimental results are available in the literature; this may be due to the technical difficulties in the experimental works. The group of sedimentary rocks, termed shales, represents a particular interest in oil industry. Experimental investigations are still necessary to have a better understanding of the thermomechanical behaviour of these materials. In the oil industry, the exploitation of heavy oil by the technical injection of vapour at high temperature, the rocks of the reservoir are subjected to coupled thermal and hydromechanical efforts. So it is necessary to study the thermo-mechanical behaviour of these materials subjected to variations of temperature in order to study the mechanical stability of the petroleum reservoirs. The object of this study consists in carrying out new experimental study of the thermomechanical behaviour of the saturated stiff shales subjected to high temperatures (until 250 Cยฐ) and to compressive stresses. The main aim was to carry out extensive laboratory experiments on the thermomechanical behaviour of Tournemire shale. The emphasis is given to investigating thermal effect on the elastic response, plastic flow and failure behaviour of the shale. Experimental results presented here provide a data base for the development of thermoelastoplastic modelling and failure criteria. 2 EXPERIMENTAL STUDIES For the sake of realize this experimental laboratory investigation performed on Tournemire shale.
- Research Report > New Finding (0.54)
- Research Report > Experimental Study (0.54)
- Geology > Rock Type > Sedimentary Rock > Clastic Rock > Mudrock > Shale (1.00)
- Geology > Geological Subdiscipline > Geomechanics (1.00)
ABSTRACT Although a number of numerical simulation models were developed and improved in the theory and programs for jointed rock mass, there are many drawbacks to the simulation techniques, such as discontinuity modeling and free expansion. Instead of the spring model of discontinuity, a new numerical model is developed based on the essences of deformability and mechanical characteristics of discontinuity. Discontinuity is considered as a nonrenewable and volumechangeable plastic medium. In addition, Numerical errors between the linear displacement functions and the exact solutions can accumulate when the block rotates continuously. This cause a change in block size called the free expansion phenomenon. Here, the finite rotation theory is adopted to address the drawback of free expansion. Finally, jointed rock mass is considered as a two-media model. The coupling effects between blocks and discontinuities are established. These developments of simulation techniques can reveal the essences of rock mass and improve analysis accuracy for simulation. 1 INTRODUCTION The rock mass is essentially a discontinuous medium due to the presence of various discontinuities at different scales and orientations, such as faults, bedding and joints. Its stability is significantly affected by the geometrical distribution and geological properties of the discontinuities. There are almost no closed-form solutions to solve the complicated problem for jointed rock mass. Therefore, in recent decades, a number of numerical simulation models based on a prior assumption of discontinuity were developed, such as DEM (Cundall 1971; Itasca 1999), DDA (Shi 1989) and BSM (Wang 1993). And there are much improvement in the theory and programs. However, there are still many drawbacks to the numerical simulations for jointed rock masses. In this paper, the authors address two major drawbacks regarding discontinuity model and free expansion. 2 DISCONTINUITY MODEL 2.1 Previous discontinuity model Rock blocks are of static equilibrium state in nature stress field. But some rock block may slide, rotate, fall or simultaneously slide and rotate when it is disturbed. Rock block is subject to not only gravity and outside force but also frictional constraint. It is difficult to determine failure mode for block by reason of three dimension distribution of outside force and friction. Many researchers simplify the calculation based on the theory of concurrent forces. However, one cannot calculate the normal force on each discontinuity when contact occurs on more than three non-parallel discontinuities, or on two or more parallel discontinuities. In order to overcome the reaction force indeterminacy, one needs to introduce the deformability of the discontinuities. For example, DEM, DDA and BSM use normal and tangential springs at the intersection point between a discontinuity face and a block's vertex. And they consider the thickness of discontinuity as 0. The interactive forces between discontinuity and block are based on the โinvasionโ. Although this discontinuity model can address the problem of discontinuity simulation, it does not reveal the essences of discontinuity, such as discontinuity deformability, mechanical characteristics. 2.2 Discontinuity description In practical rock engineering, the discontinuities have various geometrical shape and different thickness as shown in Figure 1.
ABSTRACT An excavation slope in left abutment trough of Xiluodu arch dam has 380 meters or so. In order to ensure safety of the excavation slope, the designed excavation and reinforcement process of the slope is simulated systematically with self-developed 3D elasto-viscoplastic finite element method (FEM) analysis program based on the model of reinforced jointed rock masses. Distribution patterns of displacements, stresses and point safety factors of the rock slope and reinforcement effects under tectonic initial geostress field are analyzed and the slope stability is evaluated in each excavation step. The simulation results show that displacements of the excavated slope between 470 meter and 400 meter in elevation are relatively bigger and its yield zone extends deeper into the mountain body in the designed excavation and reinforcement scheme. Supplementary reinforcements with some pre-stress cables are suggested for strengthening the excavation slope from 470 meter to 400 meter in elevation. The numerical simulation results show that the new reinforcements help improve the stability of the excavation slope in left abutment trough and ensure the safety of the slope. 1 INTRODUCTION The Xiluodu Hydropower Project is an extreme project in China and its installed capacity is 12.6 MW, which locates on the upper reaches of the Yangtze River. The dam type is double-curvature arch dam and its maximum dam height is 278 meters. Its plan and section X-X in left bank are presented in Figure 1. Figure 1(b) shows lithological characteristics of rock masses and distributions of dominant texture planes in left bank. The rock masses mainly have four rock types according to weathering degrees: II, III1, III2, IV1. The dominant texture planes mainly include three strain-slipzoneinlayers and two joint sets. The strain-slipzoneinlayers are C7,C8,C9. Occurrences of such texture planes are presented in Table 1. After completion of excavations, the final excavation slope in left abutment trough of Xiluodu arch dam has more than 380 meters height, and its spatial shape is very complicated. Because the physical and mechanical parameters of rock masses are usually weakened for exploding construction, failures of the excavation slope are likely to occur. In order to ensure the stability and safety of the excavation slope, many supports should be utilized to reinforce the slope in a construction period, which are pre-stress cables, pre-stress bolts, systematic bolts, etc.. Which position should be reinforced, and how many bolts and cables should be adopted are focuses of engineering design and construction. Unsuitable reinforcement measures and reinforced locations will not prevent slope from instability effectively and only increase engineering investments. Consequently it has a great significance to study excavation slope stability and reinforced effects of corresponding reinforcement measures with computer aided simulation technologies before the slope excavation (FENG Xue-min, WANG Wei-ming, et al., 2004) Finite element method has gained popularity in analyzing geotechnical problems for fewer assumptions and more powerful functions (Chen, S.H., Egger, P.,1999; Chen Sheng-hong, Qing Wei-xin, Shahrour Isam, 2007). The excavation and reinforcement process of rock slope can be simulated conveniently by finite element method with suitable constitutive models of geomaterials.
- North America > Canada (0.28)
- Asia > China (0.24)
When dealing with tunnels in weak rock mass and with high overburden, the high displacements imposed on the lining dictate the application of ductile yielding elements with controllable stiffness and yield load. These properties are chosen with two goals in mind: the time-dependent strength of the shotcrete shell must not be exceeded; however the support pressure must be kept reasonably high and controllable. The attainable load-displacement lines of the ductile support elements are almost arbitrary. There are almost countless possible combinations of their stiffness and yield load, thus enabling the development of custom-tailored support systems and leaving considerable room for adapting to the encountered ground conditions. Tunneling in weak ground should be accompanied by increased efforts on monitoring the system behavior, best by a dense pattern of absolute displacement measurements. A simple technique for calculating the shotcrete utilization ratio has been developed. It applies a Newton- Raphson root-finding algorithm to determine the interpolation parameters while obeying the requirements of force equilibrium and fitting the measured displacements. The influence of non- symmetrical displacement behavior caused by heterogeneity and anisotropy of the rock mass, on the lining loading can be quantified and used for support system optimization. 1. INTRODUCTION High primary stresses associated with tectonic faulting frequently create problems during construction of Alpine base tunnels. Keeping the displacements in a range which could be sustained by the support would lead to economically unfeasible lining thickness. Ductile lining systems using in mining cannot be be transferred to traffic tunnels with their requirement of long term stability. First concepts of yielding supports for tunnels date back to the nineteen fifties (Rabcewicz 1950). The technical requirements posed on a ductile support system are quite clear:The load-displacement characteristics should be "steerable" within a broad range, allowing the avoidance of overstressing the shotcrete shell, while enabling easy modifications in order to cope with the ground heterogeneity and usually long-lasting displacement increments. The support resistance has to be reasonably high, allowing a certain amount of control over the displacement magnitude. Various types of yielding elements integrated in the lining have been developed over time, mostly based on steel and sometimes on porous cement-based materials (Schubert 2008). One of them has been developed at the Graz University of Technology (Moritz 1999), featuring an enclosed steel tube subject to controlled buckling (Figure 1), called Lining Stress Controller (LSC). Its advantage lies in an excellent general load-displacement characteristic, being highly ductile and thus dissipating the major amount of the external work, and a broadly variable yield force level and initial loading stiffness. (Figure in full paper) 2. MOTIVATION When tunneling in conditions leading to application of integrated yielding elements, great attention should be paid to comprehensive monitoring of the system behaviour. Due to the unreliability of methods measuring the kinetic quantities (e.g. forces or stresses) the measuring of the absolute displacements currently yields the most direct insight on system behaviour. Usually 5 measurement points are being used.
- Management (0.75)
- Reservoir Description and Dynamics (0.48)