Showing papers on "Discontinuity (geotechnical engineering) published in 2008"

Using close range terrestrial digital photogrammetry for 3-D rock slope modeling and discontinuity mapping in the United States

Abstract: Close range terrestrial digital photogrammetry has evolved into a useful remote characterization tool for rock slope practitioners, particularly in situations where manual discontinuity measurement may be impractical or dangerous. The primary products are high-resolution 3-D photographic surface models that have several advantages over terrestrial laser scanner point clouds, including cost, equipment portability and the ability to include discontinuities that appear as linear traces rather than flat planar surfaces. Models can be made with currently available off-the-shelf digital cameras while commercially available software provides virtual mapping capabilities that allow practitioners to identify, map, and quickly calculate the orientation of modeled discontinuities using the same experience-based logical process used by traditional fieldworkers. Comparison of manually measured and model-derived discontinuity orientations shows good agreement both between individual discontinuities and entire sets of discontinuities. Application of the technique is illustrated using five recent mining and civil engineering projects in the United States.

Determination and analysis of long-wavelength transition zone structure using SS precursors

Abstract: SUMMARY Global mapping of 410 and 660 km discontinuity topography and transition zone thickness has proven to be a powerful tool for constraining mantle chemistry, dynamics and mineralogy. Numerous seismic and mineral physics studies suggest that the 410 km discontinuity results from the phase change of olivine to wadsleyite and the 660 km discontinuity results from the phase change of ringwoodite to perovskite and magnesiowustite. Underside reflections of the 410 and 660 km discontinuities arrive as precursors to SS. With the recent development of a semi-automated method of determining SS arrivals, we have more than tripled the Flanagan and Shearer (1998a) data set of handpicked SS waveforms. We are able to increase resolution by stacking waveforms in 5 ◦ rather than 10 ◦ radius bins as well as increasing data coverage significantly in the southern hemisphere. The resulting SS-S410S and SS-S660S times are heavily influenced by upper-mantle velocity structure. We perform a joint inversion for discontinuity topography and velocity heterogeneity as well as performing a simple velocity correction to the precursor differential times and find little difference between the two methods. The 660 km discontinuity topography and transition zone thickness are correlated with velocities in the transition zone whereas the 410 km discontinuity topography is not. In addition, the 410 km discontinuity topography is not correlated with the 660 km discontinuity topography, rather anticorrelated, as expected due to the opposite signs of the Clapeyron slopes of their respective phase changes. These results suggest that, whereas the topography of 660 km discontinuity could be dominated by thermal effects, the topography of the 410 km discontinuity is likely dominated by compositional effects. In addition, unlike previous studies which find less topography on the 410 km discontinuity than on the 660 km discontinuity, our 410 and 660 km topography have similar amplitudes.

Terminology for structural discontinuities

Abstract: Strain localization structures such as fractures, stylolites, and deformation bands have important effects on reservoir performance but lack a consistent terminology. Advances in the recognition and interpretation of such structures now motivate a comprehensive framework that stresses their similarities instead of their differences. We review and assess the classical terms for localized geologic structures, followed by a comprehensive nomenclature that accounts for joints, faults, fractures, anticracks, shear zones, and deformation bands in compact and high-porosity rocks. Geologic structural discontinuities are defined by their lengths and by the sense and rate of displacement change across them. For example, structural discontinuities having negligible thickness, and consequently, a discontinuous displacement across them, are called sharp discontinuities. Depending on the sense of displacement (opening, shearing, or closing), these structures are called cracks, faults, or anticracks. However, structural discontinuities having a measurable thickness in outcrop or hand specimen and a continuous change in displacement across them are called tabular discontinuities. Correspondingly, these types of deformation bands are called dilation bands, shear bands, or compaction bands. The class of structural discontinuity, i.e., sharp or tabular, depends on the properties of the deforming rock. Consistent characteristics and patterns of these structural discontinuities, and their displacement-length scaling relations, demonstrate the rich yet consistent varieties of strain localization that are manifested in crustal rocks in general, and reservoir rocks in particular.

Upper mantle seismic shear discontinuities of the Pacific

Abstract: [1] Shear wave impedance discontinuities are inventoried for eight paths connecting circum-Pacific earthquakes with seismic stations on Hawaii and Oahu. In addition to the transition zone discontinuities, we observe a consistent impedance decrease at a depth of ∼80 km that marks the transition from the fast seismic lid to the low-velocity zone. The interval over which this transition occurs is less than 30 km. The requisite impedance decrease, ascribed almost entirely to diminished velocities in the low-velocity zone, exceeds predictions for peridotite chemistries of appropriate lithospheric age. In all cases, the transition is better matched by lithosphere of roughly half of the true age. Four paths show clear evidence of the X discontinuity, an impedance increase of 3–8% near 300 km depth. The only viable explanations would require extensive eclogite or basalt-depleted mantle in the upper mantle of the western Pacific. The impedance contrast of the 410-km discontinuity, which depends on the modal fraction of mantle olivine, does not signal the presence of either, although a mixture of the two remains a possibility. We find no evidence of a low-velocity layer atop of the 410-km discontinuity. If present, it is either consistently thin (≤15 km), highly variable in thickness (topography in excess of 60 km) or has little impedance contrast (≤2–3%) with overlying mantle. The apparent absence of melt atop the 410-km discontinuity, an intermittent and weak 520-km discontinuity, and a thinned transition zone are consistent with relatively dry conditions in the deep upper mantle of the central Pacific.

Detailed nature of the 660 km region of the mantle from global receiver function data

Abstract: [1] We use a global data set of receiver functions to study the range of observations for the region at the base of the mantle transition zone, around the 660 km discontinuity. A variety of signals has already been observed in regional studies; here we characterize the breadth of such observations on a global scale. While the 410 km discontinuity is usually found to be simple in shape, the 660 km discontinuity often shows broad or asymmetric peaks and sometimes multiple signals. Unlike previous studies on the subject, we show that complexity of structure is not limited to subduction zones. Reflectivity synthetics are used to show that the observed signals can be generated by structures with combinations of sharp discontinuities and velocity gradients around 660 km depth. Mineral physics suggests that at 410 km depth only the olivine phase transition is important. In contrast, at 660 km depth both the olivine and nonolivine transitions generate multiple discontinuities and gradients, which could be visible to Pds signals. Seismic observations of a simple 410 km signal and a complex 660 km signal might therefore be expected from the predictions of mineral physics. However, a mantle of uniform composition fails to explain the variety of signals seen in our data set, requiring laterally varying chemical composition at the base of the mantle transition zone.

Splitting of the 520-Kilometer Seismic Discontinuity and Chemical Heterogeneity in the Mantle

Abstract: Seismic studies indicate that beneath some regions the 520-kilometer seismic discontinuity in Earth's mantle splits into two separate discontinuities (at approximately 500 kilometers and approximately 560 kilometers). The discontinuity near 500 kilometers is most likely caused by the (Mg,Fe)2SiO4 beta-to-gamma phase transformation. We show that the formation of CaSiO3 perovskite from garnet can cause the deeper discontinuity, and by determining the temperature dependence for this reaction we demonstrate that regional variations in splitting of the discontinuity arise from variability in the calcium concentration of the mantle rather than from temperature changes. This discontinuity therefore is sensitive to large-scale chemical heterogeneity. Its occurrence and variability yield regional information on the fertility of the mantle or the proportion of recycled oceanic crust.

Discontinuity in Jupiter's main auroral oval

Abstract: [1] On the basis of a series of FUV Hubble Space Telescope images obtained between 1997 and 2007 it is shown that there is a segment of the main auroral oval where the emission drops significantly from a few hundreds to a few tens of kiloRayleigh, forming a discontinuity in the oval. It is shown that the discontinuity is present in both hemispheres and confined in magnetic local time. Its equatorial source is located in the prenoon and early noon sector. The main auroral oval is associated with the ionosphere-magnetosphere coupling current system which is related to the breakdown of corotation in the middle magnetosphere. Necessary for the electron precipitation in the ionosphere and the formation of the main auroral oval is the presence of upward field-aligned currents, carried by downward moving electrons. Field-aligned currents inferred by Pioneer, Voyager and Galileo in situ observations in the near equatorial plane showed evidence of reduced or/and downward field-aligned currents in the prenoon and early afternoon sector, the location of the equatorial source of the discontinuity. Additionally, we estimate the precipitation energy flux in the ionosphere, for a typical reduced upward field-aligned current value at that region, which is found to be within the range of the observed brightness of the discontinuity. Field aligned current distributions in the ionosphere based on magnetohydrodynamic simulations of the interaction between the solar wind and the magnetosphere have predicted a region of downward currents implying a discontinuity at the main auroral oval emission, in very good agreement with the HST observations presented in this work.

Continuity and Discontinuity

Abstract: PROF. J. K. ROBERTSON, professor of physics, Queen's University, Kingston, Ontario, in his presidential address to the Royal Society of Canada de-livered on May 21, 1945, argues that the notions of continuity and discontinuity as used in physics and other sciences are complementary and not just contradictory. When, in the eighteenth century, physical theory was occupied with the continuous aspect of things it was natural to suppose that the structure of the world must be either continuous or discontinuous, not both. Therefore, since continuity was apparent, there could be no discontinuity anywhere, as Leibniz argued. The chemical atomic theory and the kinetic theory of gases of the nineteenth century brought out the aspect of discontinuity for the first time, and recent sub-atomic physics has gone a stage further. At every stage, however, as Prof. Robertson emphasizes, theory really requires both continuity and discontinuity. This was first realized in the case of light, which in some aspects is continuous, and in others discontinuous ; and, later, electrons were found to play a similar dual part. Granted that the single electron is discrete, indefinite and indeterminate, it should not be inferred that the continuity, definiteness and strict causality found in the behaviour of large numbers of electrons are not equally genuine aspects of the physical world. Prof. Robertson refers briefly to the way in which the biological theory of evolution requires both a continuous long-term aspect and a discontinuous short-term one ; and he ends by asking the question, whether new discoveries in science which look like discontinuities in the historical development of thought are not also in other respects examples of the continuity of thought.

Fuzzy spectral clustering for identification of rock discontinuity sets

Abstract: The characterization of rock masses for engineering applications generally includes analyses for the identification of discontinuity sets and the characterization of their orientation which are commonly performed using hemispherical projections of discontinuity unit normal vectors (see e.g., Priest, 1985, 1993b; Harrison, 1992). Visual inspection of contour plots of orientation density computed by counting the number of unit normal vectors that fall inside a reference circle have been traditionally employed for such task. However, they have been found to present problems due to sampling bias (Terzaghi, 1965; Priest, 1993a); to clustering results heavily depending on the size of the reference circle (Harrison, 1992); and to subjectivity in the interpretation of the results (Mahtab and Yegulalp, 1982; Priest, 1993b; Hammah and Curran, 1998). Such problems have lead to a recent interest in the development of alternative techniques for automatic identification of rock discontinuity sets based on their orientation. For instance, spectral clustering methods that use eigenvectors of matrices constructed using measures of similarity between the data points have been recently employed for clustering of rock discontinuities (Jimenez-Rodriguez and Sitar, 2006).

Influence of Stochastic Discontinuity Network Parameters on the Formation of Removable Blocks in Rock Slopes

Abstract: We study the effects of discontinuity network parameters on the formation of removable wedges in rock slopes. Discontinuities are simulated using the Poisson disk model, and removable wedges are identified using block theory. The formation of removable wedges of different sizes is assumed to follow a Poisson process. Poisson regression and Monte Carlo simulation are then used to identify statistically relevant parameters of the model, and to study the effects that variations in their values have on formation of removable blocks. The sensitivity of the results as a function of the mean orientations of the discontinuity sets forming the blocks is also studied by means of a parametric study. The volumetric intensity of discontinuities in the rock mass is found to have a significant impact on the computed estimates of removable block formation. As predicted by theory, our results indicate that, everything else being equal, the expected rate of formation of removable wedges is proportional to the square of the intensity measure. Estimates are also sensitive to changes in discontinuity size, especially in cases in which discontinuities are smaller than one to two times the height of the slope. The interaction between the mean size of discontinuities and the coefficient of variation of discontinuity sizes is found to be significant as well. Finally, results of our sensitivity analysis suggest that the orientation of discontinuity sets significantly affects the rate of formation of removable blocks in rock slopes.

Remote Characterisation of Surface Roughness of Rock Discontinuities

Abstract: The roughness of a discontinuity may be an important factor influencing shear strength of the discontinuity. Determining the roughness of a discontinuity surface and providing a numerical measure of the roughness generally requires considerable effort. The development of new, easily accessible tools for 3D imaging has provided effective fast and accurate means of creating 3D surface models from which roughness measurements can be readily obtained. These new techniques are versatile enough to enable a user to measure roughness along any desired profile and at any resolution up to the limit of the spatial subtense of an image pixel on the discontinuity surface. This paper reviews the use of 3D imaging for the measurement of roughness and provides some examples of the measurements.

Evidence for nonhydrostatic gas motions in the hot interstellar medium of centaurus a

Abstract: We present preliminary results from a deep (600 ks) Chandra observation of the hot interstellar medium of the nearby early-type galaxy Centaurus A. We find a surface brightness discontinuity in the gas ~3.5 kpc from the nucleus spanning a 120? arc. The temperature of the gas is -->0.60 ? 0.05 keV ( -->0.68 ? 0.10 keV) interior (exterior) to the discontinuity. The elemental abundance is poorly constrained by the spectral fits, but if the abundance is constant across the discontinuity, there is a factor of -->2.3 ? 0.4 pressure jump across the discontinuity. This would imply that the gas is moving at -->470 ? 100 km s?1, or Mach -->1.0 ? 0.2 ( -->1.2 ? 0.2) relative to the sound speed of the gas external (internal) to the discontinuity. Alternatively, pressure balance could be maintained if there is a large (factor of ~7) discontinuity in the elemental abundance. We suggest that the observed discontinuity is the result of nonhydrostatic motion of the gas core (i.e., sloshing) due to the recent merger. In this situation, both gas motions and abundance gradients are important in the visibility of the discontinuity. Cen A is in the late stages of merging with a small late-type galaxy, and a large discontinuity in density and abundance across a short distance demonstrates that the gas of the two galaxies remains poorly mixed, even several hundred million years after the merger. The pressure discontinuity may have had a profound influence on the temporal evolution of the kiloparsec-scale jet. The jet could have decollimated, crossing the discontinuity and thereby forming the northeast radio lobe.

Evidence for Non-Hydrostatic Gas Motions in the Hot ISM of Centaurus A

Abstract: We present preliminary results from a deep (600 ks) {\em Chandra} observation of the hot interstellar medium of the nearby early-type galaxy Centaurus A (Cen A). We find a surface brightness discontinuity in the gas $\sim$3.5 kpc from the nucleus spanning a 120$^\circ$ arc. The temperature of the gas is 0.60$\pm$0.05 and 0.68$\pm$0.10 keV, interior and exterior to the discontinuity, respectively. The elemental abundance is poorly constrained by the spectral fits, but if the abundance is constant across the discontinuity, there is a factor of 2.3$\pm$0.4 pressure jump across the discontinuity. This would imply that the gas is moving at 470$\pm$100 km s$^{-1}$, or Mach 1.0$\pm$0.2 (1.2$\pm$0.2) relative to the sound speed of the gas external (internal) to the discontinuity. Alternatively, pressure balance could be maintained if there is a large (factor of $\sim$7) discontinuity in the elemental abundance. We suggest that the observed discontinuity is the result of non-hydrostatic motion of the gas core (i.e. sloshing) due to the recent merger. In this situation, both gas motions and abundance gradients are important in the visibility of the discontinuity. Cen A is in the late stages of merging with a small late-type galaxy, and a large discontinuity in density and abundance across a short distance demonstrates that the gas of the two galaxies remains poorly mixed even several hundred million years after the merger. The pressure discontinuity may have had a profound influence on the temporal evolution of the kpc-scale jet. The jet could have decollimated crossing the discontinuity and thereby forming the northeast radio lobe.

Mountain Waves in Two-Layer Sheared Flows: Critical-Level Effects, Wave Reflection, and Drag Enhancement

Abstract: Internal gravity waves generated in two-layer stratified shear flows over mountains are investigated here using linear theory and numerical simulations. The impact on the gravity wave drag of wind profiles with constant unidirectional or directional shear up to a certain height and zero shear above, with and without critical levels, is evaluated. This kind of wind profile, which is more realistic than the constant shear extending indefinitely assumed in many analytical studies, leads to important modifications in the drag behavior due to wave reflection at the shear discontinuity and wave filtering by critical levels. In inviscid, nonrotating, and hydrostatic conditions, linear theory predicts that the drag behaves asymmetrically for backward and forward shear flows. These differences primarily depend on the fraction of wavenumbers that pass through their critical level before they are reflected by the shear discontinuity. If this fraction is large, the drag variation is not too different from th...

Dry rock avalanche propagation: unconstrained flow experiments with granular materials and blocks at small scale

Abstract: Rock avalanches are catastrophic phenomena which are not yet exhaustively understood. They consist of rock mass movements of more than one million cubic metres, involving a great amount of energy and travelling farther than expected with a normal sliding friction law. The present study has as main purpose to investigate the propagation mechanisms involved in rock avalanche processes and to identify parameters influencing velocity and deposit characteristics by means of laboratory experiments, i.e. dry unconstrained flows of granular materials and blocks at small scale (bricks) down an inclined board which ends with a horizontal accumulation zone. Two main experimental campaigns have been carried out. The first represents a preliminary study which has been useful to test the experimental set-up, to improve the measuring devices and to assess significant factors governing propagation of granular avalanches. Fall height, volume, materials used (sand or gravel), releasing geometry and the number of consecutive releases have been varied and their influence on front mass velocity and on deposit characteristics has been studied. In the second experimental campaign the varied parameters are fall height, volume, material (aquarium gravel and small bricks), slope inclination, base friction coefficient and the regularity of the pathway (sharp or curved discontinuity at the toe). Bricks are randomly poured into the releasing container before failure (loose mass) or piled orderly (structured mass). Furthermore, it has been possible to compute the morphology of the final deposit and the position of its centre of mass thanks to a new optical technique, the fringe projection method, recently developed and adapted to the laboratory conditions. The analyses of this extensive set of parameters put in evidence the importance of the nature of the released material, the structure of the mass before failure and the topography, i.e. the slope and the regularity of the pathway. Factors causing longer runouts are: larger volume, greater fall height, lower coefficient of friction, higher slope angle, the use of bricks ordered in piles and a smoother discontinuity at the toe of the slope. Morphology is dependent on the type of material used: sand or gravel; gravel deposit seems closer to real cases. There is also a considerable difference in deposit morphology when the event is the consequence of one large volume released at once or of a progressive failure. In the latter case final deposit characteristics depend on the individual smaller volumes. By analysing the velocity of the mass front as it enters the accumulation zone, it is possible to see that a transfer of momentum occurs between the rear approaching part and the front one slowing down ahead, inducing an excessive travel distance. In the case of piled bricks the regime is mainly frictional (energy dissipated essentially by friction at the base) on the inclined panel, where the mass remains relatively structured, and then frictional-collisional (energy dissipated also by friction and collisions within the mass) in the accumulation zone. The abrupt change of flow direction seems to be the cause of the shattering of the mass activating this passage to a different regime. On the other hand, both regimes can be found from the beginning of the slope in the case of loose materials, i.e. gravel and random bricks. The length is independent from the fall height. A marked difference is detected for tests with a curved connection at the toe which show high nondimensional values of the length and of the runout against the cubic root of the volume, closer to the ones of real cases. Statistical analyses confirm the considerations made.

The Cádiz-Alicante Fault: an important discontinuity in the Betic Cordillera

Abstract: During the Burdigalian and the middle Miocene the Cadiz-Alicante Fault facilitated the continuity of the westward displacement of the Betic Internal Zone, simultaneously dragging part of the Subbetic. Its position locally coincides with the contact between the Internal Zone and the External, but especially in the west part affects only the Subbetic. From the late Miocene the crustal discontinuity formed by this fault aided the subsidence of the depocentres controlled by NNW-SSE faults, acting as a backstop for the extension to the north. This fault constitutes an important discontinuity in the Betic Cordillera and, although there is no proof of earthquakes deeper than 7 km linked to the fault, a volcanoclastic emission occurred during the Burdigalian, apparently related to the fault. This implies that the fault cut deeply, a proposition easily assumed if it is considered that the subsidence of some of the depocentres limited by the fault is on the order of 3 km and even more, requiring a compensation depth greater than 10 km.