Showing papers on "Discontinuity (geotechnical engineering) published in 2004"

Post-Perovskite Phase Transition in MgSiO3

Abstract: In situ x-ray diffraction measurements of MgSiO3 were performed at high pressure and temperature similar to the conditions at Earth9s core-mantle boundary. Results demonstrate that MgSiO3 perovskite transforms to a new high-pressure form with stacked SiO6-octahedral sheet structure above 125 gigapascals and 2500 kelvin (2700-kilometer depth near the base of the mantle) with an increase in density of 1.0 to 1.2%. The origin of the D″ seismic discontinuity may be attributed to this post-perovskite phase transition. The new phase may have large elastic anisotropy and develop preferred orientation with platy crystal shape in the shear flow that can cause strong seismic anisotropy below the D″ discontinuity.

Low-velocity zone atop the 410-km seismic discontinuity in the northwestern United States

Abstract: The seismic discontinuity at 410 km depth in the Earth's mantle is generally attributed to the phase transition of (Mg,Fe)_2SiO_4 from the olivine to wadsleyite structure. Variation in the depth of this discontinuity is often taken as a proxy for mantle temperature owing to its response to thermal perturbations. For example, a cold anomaly would elevate the 410-km discontinuity, because of its positive Clapeyron slope, whereas a warm anomaly would depress the discontinuity. But trade-offs between seismic wave-speed heterogeneity and discontinuity topography often inhibit detailed analysis of these discontinuities, and structure often appears very complicated. Here we simultaneously model seismic refracted waves and scattered waves from the 410-km discontinuity in the western United States to constrain structure in the region. We find a low-velocity zone, with a shear-wave velocity drop of 5%, on top of the 410-km discontinuity beneath the northwestern United States, extending from southwestern Oregon to the northern Basin and Range province. This low-velocity zone has a thickness that varies from 20 to 90 km with rapid lateral variations. Its spatial extent coincides with both an anomalous composition of overlying volcanism and seismic 'receiver-function' observations observed above the region. We interpret the low-velocity zone as a compositional anomaly, possibly due to a dense partial-melt layer, which may be linked to prior subduction of the Farallon plate and back-arc extension. The existence of such a layer could be indicative of high water content in the Earth's transition zone.

The elasticity of the MgSiO3 post-perovskite phase in the Earth's lowermost mantle.

Abstract: MgSiO3 perovskite has been assumed to be the dominant component of the Earth's lower mantle, although this phase alone cannot explain the discontinuity in seismic velocities observed 200–300 km above the core–mantle boundary (the D″ discontinuity) or the polarization anisotropy observed in the lowermost mantle1. Experimental and theoretical studies that have attempted to attribute these phenomena to a phase transition in the perovskite phase have tended to simply confirm the stability of the perovskite phase2,3,4,5,6. However, recent in situ X-ray diffraction measurements have revealed7 a transition to a ‘post-perovskite’ phase above 125 GPa and 2,500 K—conditions close to those at the D″ discontinuity. Here we show the results of first-principles calculations of the structure, stability and elasticity of both phases at zero temperature. We find that the post-perovskite phase becomes the stable phase above 98 GPa, and may be responsible for the observed seismic discontinuity and anisotropy in the lowermost mantle. Although our ground-state calculations of the unit cell do not include the effects of temperature and minor elements, they do provide a consistent explanation for a number of properties of the D″ layer.

Post-Perovskite Phase Transition

Abstract: In situ x-ray diffraction measurements of MgSiO 3 were performed at high pressure and temperature similar to the conditions at Earth’s core-mantle boundary. Results demonstrate that MgSiO 3 perovskite transforms to a new high-pressure form with stacked SiO6-octahedral sheet structure above 125 gigapascals and 2500 kelvin (2700-kilometer depth near the base of the mantle) with an increase in density of 1.0 to 1.2%. The origin of the D seismic discontinuity may be attributed to this post-perovskite phase transition. The new phase may have large elastic anisotropy and develop preferred orientation with platy crystal shape in the shear flow that can cause strong seismic anisotropy below the D discontinuity.

Cluster observations of hot flow anomalies

Abstract: [1] On 2 April 2002, Cluster entered the solar wind at high northern latitudes and observed a series of disturbances upstream of the bow shock We suggest that these were signatures of a sequence of hot flow anomalies (HFAs), observed at different stages of development We estimate the orientation and motion of the HFAs and structures within them using multispacecraft data and examine the variation of plasma flow in the context of the orientation of the underlying discontinuities We demonstrate that the motional electric field at each disturbance was in a direction such that it would act to focus ions reflected from the bow shock toward the discontinuity, consistent with current understanding of HFA formation The first disturbance consisted of hot plasma flowing along the estimated discontinuity plane during the core of the event, together with a weakened solar wind beam Toward the edge of the event, compressed solar wind plasma flowed approximately perpendicular to the discontinuity plane, consistent with expansion of the hot plasma We suggest that the presence of multiple ion distributions signifies that the HFA was at an early stage of its evolution A second HFA, which had a more complex signature, contained a single ion distribution within the event core, which is the more typical signature of an HFA It also contained a large velocity deflection, consistent with expansion of the plasma perpendicular to the discontinuity plane The final HFA event we present is associated with a shock encounter, demonstrating the level of complexity that can be associated with the bow shock response to changing upstream conditions

Configuration of subducting Philippine Sea plate beneath southwest Japan revealed from receiver function analysis based on the multivariate autoregressive model

Abstract: [1] We construct a detailed image of velocity discontinuity beneath southwest Japan by using receiver functions. We first propose an improved receiver function estimation method based on the statistical multivariate autoregressive model. Then we apply the new method to the teleseismic waveform data recorded by the high-density seismograph network in southwest Japan. The results show a clear velocity discontinuity at 30 km depth beneath the southern Shikoku region. This discontinuity, corresponding to the boundary between the oceanic crust and the high-velocity layer (the oceanic Moho) of the Philippine Sea plate (PHS), continues down to the north indicating that the aseismic PHS extends to the central Chugoku region. The continental Moho is also clearly imaged beneath the Chugoku region. The depth contour of the PHS shows a rather complicated feature with ridges and valleys. The most significant ridge is located around longitude 133°E, west of which the contours are basically directed north-south, changing to west-east to the east. Beneath the western part of this ridge, hypocenters of microearthquakes are located above this velocity discontinuity, while earthquakes mainly occur below the discontinuity, within the oceanic mantle, beneath the eastern part of the ridge.

Experimental observations on the crush characteristics of AA6061 T4 and T6 structural square tubes with and without circular discontinuities

Abstract: Quasi-static compressive testing of extruded aluminum alloy 6061-T4 and 6061-T6 square cross-sectional tubular specimens, with and without the presence of dual centrally located circular hole discontinuities, was completed to investigate the load management and energy absorption characteristics of these structural members. The tubular geometries selected (tube lengths of 200 mm and 300 mm, wall thickness of 3.15 mm, and nominal side width of 38.1 mm) had parameters which result in predicted global bending collapse under compressive loading. Centrally located circular holes, machined into the two opposing walls of the tubes, were used as crush initiators to commence the plastic buckling process. Holes with diameters of 7.1 mm and 14.2 mm were considered. In addition to progressive buckling, collapse modes involving cracking and splitting were observed in many tests and are characterized using photographs of the experimental process. The collapse modes and energy absorption of the crush test specimens were found to be dependent largely on material properties and to a lesser extent on the diameter of the discontinuity. Significant increases in the crush force efficiency, up to a maximum of approximately 22%, were observed for the shorter length tube geometry. However, the presence of the circular discontinuity within the AA6061 T4 300 mm tubes slightly degraded the crush force efficiency compared to the same length tubular member without any discontinuity. Energy absorption capabilities were substantially improved for all AA6061- T6 specimens containing a circular discontinuity relative to the specimens without any crush initiators. With the addition of the discontinuity, energy absorption was improved for the 200 mm length AA6061-T4 specimens but slightly decreased for the 300 mm length members.

An Unusual Discontinuity in the X-Ray Surface Brightness Profile of NGC 507: Evidence of an Abundance Gradient?

Abstract: We present results from a 45 ks Chandra ACIS-I observation of the nearby (z = 0.01646) NGC 507 group. The X-ray surface brightness profile of the outer region of the group is well described by an isothermal β-profile consistent with earlier ROSAT observations. We find a sharp edge or discontinuity in the radial surface brightness profile 55 kpc east and southeast of NGC 507 covering an ~125° arc. At the sharpest part of the discontinuity, the surface brightness declines by a factor of ~2 over a distance of 6.9 kpc (~15''). The inner and outer gas temperatures across the discontinuity differ by only about 0.2 keV (interior and exterior gas temperatures are 1.2 and 1.4 keV, respectively). Spectral analysis indicates that there is a gradient in the elemental abundance across the discontinuity, and comparison with the low-resolution NRAO VLA Sky Survey radio maps suggests that the discontinuity is aligned with a low surface brightness radio lobe. We conclude that the appearance of this discontinuity may be the result of two phenomena: the transonic ( ~ 0.75-1) expansion/inflation of the radio lobe close to our line of sight and the transport of high-abundance material from the center of the galaxy due to this expansion.

Lateral variation of the D″ discontinuity beneath the Cocos Plate

Abstract: [1] Broadband shear wave signals from 15 deep South American earthquakes reveal small-scale variations of the D″ shear velocity discontinuity beneath the Cocos Plate. Deconvolution by average event source wavelets allows multiple event and California station combinations to be double-array stacked in 4 geographically separate reflection point bins with lateral dimensions of about 200 km. Reflections from the top of D″ are apparent in all 4 bins, with significant variations in timing and strength relative to ScS. The data are compatible with a discontinuous shear velocity increase 264 km above the core-mantle boundary that varies laterally in strength from 0.9% to 2.6%, with corresponding lateral velocity variations within D″.

Assessing fracture occurrence using "weighted fracturing density": a step towards estimating rock instability hazard

Abstract: . Based on the assumption that major class of rock instabilities are created by discontinuities, a method is proposed to estimate the fracture density by means of a digital elevation model (DEM). By using the mean orientation, the mean spacing and the mean trace length of discontinuity sets potentially involved in slope instabilities and a DEM, it is possible to calculate the mean number of discontinuities of a given set per cell of the DEM. This would allow for an estimation of the probability of the presence of at least one discontinuity in a given area or simply in a topographic cell of the DEM. This analysis highlights sites potentially affected by rockslides within a region. Depending on the available data, the mean number can be calculated either by area, or along a line parallel to the mean apparent spacing. The effective use of the probability of occurrence is dependent on the size of the discontinuities because short and closely spaced discontinuities will have a 100% probability of occurrence in each favorable location. The a posteriori prediction of a recent rockslide is discussed as an example.

Microstructure and fracture in asphalt mixtures using a boundary element approach

Abstract: This paper describes the use of the displacement discontinuity boundary element method to model the microstructure of asphalt mixtures. The problem of indirect tension test (IDT) was used to demonstrate the application of the method in the modeling of the cracking behavior of asphalt mixtures. An IDT sample was modeled by three sets of displacement discontinuity boundary elements based on location and function: (1) on the periphery of the specimen, (2) inside the specimen to represent the aggregate structure, and (3) internal fracture path elements inside the aggregate structure. The mastic was modeled by a nonlinear failure law. The simulation of the cracking behavior of the IDT test with this method showed a good agreement with laboratory observations. The predicted stress-strain curves matched the experimental results for both vertical compressive stress and horizontal tensile stress. Crack patterns from these simulations were also similar to the patterns observed visually in the laboratory. In summary, explicit fracture modeling has shown promise as a tool for studying the crack growth and localization of asphalt mixtures.

Geometry of interactions in complex bodies

Abstract: We analyze geometrical structures necessary to represent bulk and surface interactions of standard and substructural nature in complex bodies. Our attention is mainly focused on the influence of diffuse interfaces on sharp discontinuity surfaces. In analyzing this phenomenon, we prove the covariance of surface balances of standard and substructural interactions.

New design of the magnetic fluid linear pump to reduce the discontinuities of the pumping forces

Abstract: In this research, a new design to reduce the discontinuities of the pumping forces of the magnetic fluid linear pump (MFLP) is studied. Continuous pumping of the newly designed MFLP by using ac current with a phase shift increases pumping efficiency and reduces the pumping shock. Forming shapes of the magnetic fluid at the intermediate state are computed and compared to measurement. Because the back flow of the fluid is reduced remarkably, 4-yoke ac driving is more efficient than 7-yoke dc driving. The size, weight, and pumping discontinuity is also reduced.

Interaction of arbitrary multiple cracks in an infinite plate

Abstract: In this article, the interaction of arbitrary multiple cracks in an infinite plate is examined by means of a boundary element method consisting of the non-singular displacement discontinuity element presented by Crouch and Starfied and the crack tip displacement discontinuity elements proposed by the present author. In the boundary element implementation, the left or the right crack tip element is placed locally at the corresponding left or right crack tip on top of the constant displacement discontinuity elements that cover the entire crack surface and the other boundaries. The present numerical results of stress intensity factors (SIFs) for some typical multiple crack problems show that the numerical approach is simple yet very accurate for analysing the interaction of arbitrary multiple cracks in an infinite plate.

Fault filter for seismic discontinuity data

Abstract: Various methods are disclosed for identifying faults in a seismic data volume. In some method embodiments, the fault identification method comprises determining a planarity value for each of multiple positions of an analysis window in the data volume. The planarity value may be indicative of the planarity of discontinuities in the analysis window, and may be further filtered by limits on the verticality and centrality of the discontinuities. Thus a filter may be determined for suppressing relatively non-planar, relatively non-vertical, and relatively un-centered discontinuities from a discontinuity display, thereby enhancing a display of faults present in the seismic data volume.

The thickness and internal structure of Fireweed rock glacier, Alaska, U.S.A., as determined by geophysical methods

Abstract: Geophysical investigations on rock glaciers are often difficult because rock glaciers are covered by an unconsolidated debris mantle a few meters thick, are typically <50 m thick and are composed of an ice-rock mixture of unknown composition Transient electromagnetics (TEM) is a method that allows some of these difficulties to be minimized, and data collection is relatively efficient TEM, with calibration from terminus exposure, was used to determine the thickness (∼60 m) of Fireweed rock glacier, Alaska, USA, under complex valley geometry A conductive layer beneath the rock glacier was identified, and its distribution is consistent with a till-like layer Seismic refraction, used to resolve the debris-mantle thickness (2-4 m), suggests the presence of a discontinuity at 18-28 m depth within the rock glacier The discontinuity is also indicated in the radio-echo sounding and the TEM data, but to a lesser extent This discontinuity is important because the motion of the rock glacier may occur across this as a shear plane

Automated identification and characterisation of discontinuity sets in outcropping rock masses using 3D terrestrial laser scan survey techniques

Abstract: It is generally accepted that the identification and characterisation of discontinuities in discontinuous rock masses is one of the most important aspects in rock mass modelling. Traditional manual field survey methods for gathering discontinuity properties are biased, hazardous, difficult and time-consuming. This paper describes a computer approach, based on terrestrial laser scan data, which seems promising as an alternative survey technique. A terrestrial laser scanner can create rapidly, a highly accurate 3D point cloud models of any outcropping rock mass. The point cloud model can be converted into a virtual 3D surface using digital surface reconstruction. This paper describes a method using clustering algorithms, that allow for automated identification and calculation of different discontinuity sets using these virtual rock surfaces. Not only is laser scanning a safer and faster surveying technique, it also provides a more accurate, precise and reliable basis for discontinuity identification and characterisation.

The Effect of Scale and Criticality in Rock Slope Stability

Abstract: Summary The apparent shear strength of rock discontinuities is considerably smaller than that of small scale samples. At the same time, the sliding behavior is characterized, in situ, by marked instabilities, with the typical features of Critical Phenomena. Contact mechanics permits to calculate normal and tangential forces at any point, and to follow the stick-slip transition for arbitrary loading histories. On the other hand, the above aspects are not captured by the classical theories, including those based on roughness indices. We argue that the multiscale topology of contact domains plays a fundamental role in determining the behavior of rock joints. In particular, experiments and numerical simulations show that these domains are lacunar sets with fractal dimension smaller than 2.0. This provides peculiar scaling of normal and tangential pressures at the interface, and the consequent size-dependence of the apparent friction coefficient. Moreover, we implement Renormalization Group to determine the critical point (e.g. the critical shear force) when rock sliding occurs. We show that the critical force is less than the one predicted by the classical Coulomb’s theory, and that it depends on the specimen size and on the topology of the interface. The same reasoning can be extended to other phenomena, e.g., to the rupture of brittle materials.