ABSTRACT: Excavations in jointed rock masses are commonly affected by the presence of keyblocks of rock formed by three or more joints on the periphery of an excavation. These blocks may be unstable if they exceed the strength of the joints unless an adequate support system is installed. Estimating the keyblock size that must be accommodated by the excavation-support system requires a model of the rock joints that considers all joint-set properties. The unit-cell method, which has previously been described in a three-dimensional form considering joint set spacing, is extended here to consider the effect of variations in joint-set orientation. Two models of tunneling cases are considered. The first case suggests that variations in joint-set orientation about a representative value makes only a small absolute difference in the probability of large keyblock formation. The relative error of the estimate is large, however, when large but unlikely keyblocks are considered. A second case is considered that produces a fairly large maximum keyblock with one joint set roughly parallel to the tunnel axis. In this case, the importance of considering variations in joint-set orientation is more clearly demonstrated.
ABSTRACT: In the analysis of discontinuity data, it is often not known initially which variables are most useful in distinguishing between the different fracture groupings or sets. Cluster analysis tools are particularly vulnerable to the presence of variables that are essentially noise variables, because such variables can mask the true cluster structure of the data leading to poor cluster recovery. It is therefore essential to reduce the influence of unimportant variables by assigning importance weights to all variables involved in a cluster analysis. One other associated problem in cluster analysis is that the different measurement scales of different variables can cause some variables to dominate a cluster analysis solely because of the magnitudes of their readings and not because of importance. Variable standardization is the technique commonly used to solve this problem. This paper looks at the issues surrounding variable weighting and standardization, and proposes algorithms for addressing the difficulties associated with it in fuzzy K-means cluster analysis.
ABSTRACT: Some implications of the static theory of linear poroelasticity for reservoir compaction are discussed. First, the relationship between the bulk compressibility and the uniaxial compaction coefficient is reviewed. Then, an expression is derived for the pore compressibility under uniaxial strain conditions. Finally, the influence of pore pressure on lateral stresses, under uniaxial strain conditions, is discussed.
ABSTRACT: Techniques are presented to quantify the rock strength of a volcano edifice (summit), enabling some understanding of failure processes to be obtained from modeling studies. Rock strength and structural information has been obtained from field studies on volcano edifices in the Cascade Range Volcanoes, Washington state and from the eastern portion of the Mexican Volcanic Belt. Field information consists of geologic mapping, point load and in-situ vane testing, and discontinuity mapping. Laboratory rock testing and RMR determination for various rock types have augmented the field data. The rock mass strength for fresh and hydrothermally altered rock was calculated for lava flow, dome and pyroclastic litholigies. Volcano modeling utilizing the distinct element method (UDEC) and the limit equilibrium technique, was used to assess stability and deformation of the edifice. Modeling results give indications of probable failure volumes, velocities and direction. The models also highlight the crucial role played by weak lithology in reducing the strength of the volcano edifice
ABSTRACT: This paper describes the geomechanical development plan established to provide data for safety assessment and conceptual models of an argillite formation studied by ANDRA in the framework of feasibility study of underground waste disposal in France. Geomechanical conceptual model of a disposal in that formation is divided into three models: the conceptual model of host formation describing the overall generic mechanical behavior; the geotechnical conceptual model for construction and design; the conceptual model of the repository describing the layer from the safety and retrievability standpoint. The content of each model is presented and analysed. In order to verify the pertinence of these models, in situ geomechanical experiments are envisaged in the Underground Research Laboratory to be constructed in the argilites formation. The strategy of numerical modeling program associated to the in situ geomechanical experiments is described.
ABSTRACT: A domal failure 12 m high has occurred in flat lying, well-bedded limestone in a large room in Salamander Cave, Indiana. The roof is analyzed as a series of laminated beams, with free slip between the individual beams of each laminated beam. The relationship between stiffness and deflection for individual beams was calculated using a cracked beam column finite element code. The analysis provides a model for evolution of the failure. Crucial to stability is the development of large axial compressive stresses in the thicker beams as they crack. Outward frictional sliding toward the free face at the nearby hillside may restrict the magnitude of the axial compressive stress. This restriction is required to explain the observed failure and it implies that the remaining roof may be near failure.
ABSTRACT: The brittle-ductile transition (BDT) in Berea sandstone is characterized by: (1) stress drops smaller than brittle tests, (2) a change from dilational to compactive behavior, (3) multiple, narrow cross-cutting cataclastic shear zones at about 35° to σ1. Shear localization via narrow cataclastic zones in BDT exhibit: (4) pervasive matrix microfracturing in BDT as opposed to microfracture 'halo' around brittle shear fractures, and (5) strong microfracture orientation parallel to σ1 in BDT away from narrow cataclastic zones instead of more random orientation associated with localized brittle failure. A decrease in the strain rate from 7.5 x 10-5sec.-1 by an order of magnitude changes the brittle-transitional boundary from about 80 MPa to about 50 MPa. The lower strain rate also shifts the effective confining pressure range for BDT towards lower confining pressures. These results suggest that the genesis of small faults or deformation bands in porous rocks may be related to BDT deformation.
ABSTRACT: The behavior of rock masses near underground openings can be influenced by the presence of major geological discontinuities. To model such discontinuities, the ground control engineer must rely on constitutive equations to describe their complex mechanical behavior. This paper presents stability analyses of an underground stope near a fault, using a recently developed non-linear model for the discontinuity. This model, called CSDS, can represent the behavior of rock discontinuities in the pre-peak as well as in the post-peak regions. A parametric investigation of the case presented shows that induced stresses around the excavation are greatly influenced by the post-peak characteristics of the fault.
ABSTRACT: A prototype computer program was developed that visualizes various kinds of geotechnical information using 3D graphics technique. The program integrates geotechnical data such as surface geology map, borehole data and geophysical data as well as man made subsurface structures. It also reads the digital map in the format of DXF and generates digital elevation model from iso-elevation line layer. All the data are put into a 3D model as 3D objects. The created 3D model can be viewed and analyses in an interactive 3D way.
ABSTRACT: This paper describes the use of analytical solutions, accurate to first order in the maximum slow of the topography, for an elastic half space with surface topography to investigate the influence of valley shape and the regional state of stress on styles of coal mine roof failures described from the Appalachian plateau of the eastern United States. The basic solutions yield results for simple sinusoidal topography, and complicated valley shapes are simulated using Fourier series superposition of solutions. Different regional states of stress are simulated by varying a coefficient of lateral earth pressure. For a given state of regional stress, however, changes in valley form yield only small changes in the magnitude and distribution of the mean normal stress and maximum shear stress. Broad-floored valleys produce significantly different displacement and strain fields than do V-notch valleys, independently of the regional state of stress, because the perturbing effects of topography vary as a function of the reciprocal valley width.