Showing papers on "Discontinuity (geotechnical engineering) published in 2000"

Topography of the transition zone seismic discontinuities

Abstract: Phase transformations in mantle mineralogies probably cause the transition zone seismic discontinuities, nominally at 410, 520, and 660 km depth. Thermodynamic principles govern phase transformations, making them sensitive to changes in the mantle's ambient conditions through the thermodynamic response: Changes in temperature or composition shift the transformation to a different pressure, creating topography on a level discontinuity. With this use as an exploratory tool for the mantle in mind, we review the basic seismological and phase equilibrium concepts underlying the detection and interpretation of the seismic wave arrivals associated with the transition zone discontinuities. Reviewing the evidence for and against the phase transition model, we conclude that it is viable and describe how discontinuities have been and can be used to probe the physical and chemical state of the transition zone.

Discontinuity Surfaces, Clinoforms, and Facies Architecture in a Wave-Dominated, Shoreface-Shelf Parasequence

Abstract: Detailed outcrop analysis of a wave-dominated, shoreface-shelf parasequence exposed in the Book Cliffs, Utah reveals minor stratigraphic discontinuities, which define clinoforms. Two types of discontinuity are recognized, each with a distinctive lithologic character and geometry. Nondepositional discontinuities are marked by an abrupt decrease in the thickness and amalgamation of storm-generated event beds, and are interpreted to record hiatuses in sedimentation. These discontinuities define clinoforms with a concave-upward geometry that dip gently (0.02-0.58°) over distances of 800-6000 m down depositional dip. Erosional discontinuities are marked by an abrupt increase in event-bed amalgamation, grain size, and sand content, and are interpreted as enhanced storm-wave scours. These discontinuities define more steeply dipping (0.22-0.95°), concave-upward clinoforms that extend over 100-1600 m down depositional dip and 500-1500 m along depositional strike. The distribution and amalgamation of minor stratigraphic discontinuities defines linear zones of distinctive facies architecture that are oriented parallel to the paleoshoreline trend. Using the simple assumption that the shoreface-shelf equilibrium profile remained approximately constant for each type of discontinuity throughout shoreface migration (the "Bruun rule"), intra-parasequence facies architecture can be speculatively interpreted in terms of shoreline trajectory, which reflects the balance between sediment supply and accommodation. The resulting interpretations support the notion that shoreline trajectory exerts a strong control on intra-parasequence facies architecture and preservation of the shoreface-shelf profile.

Modeling of Crack Initiation, Propagation and Coalescence in Uniaxial Compression

Abstract: ¶Cracks that initiate from pre-existing discontinuities can link with other cracks or with other discontinuities and produce failure in a rock mass. The Displacement Discontinuity Method (DDM), FROCK, is used in this investigation to model experimental observations on pre-cracked specimens of gypsum. In these experiments two fractures, which were either both open or closed, were placed through the thickness of the specimens, and detailed observations of the cracking process were performed as the specimens were loaded in uniaxial compression. The following aspects are studied for both open and closed fractures: 1) crack initiation stress; 2) direction and propagation of the new cracks; 3) type of coalescence and stress at which it occurs. Modeling is done considering the actual size of the specimens. Relations between the direction of initiation for each type of crack, the orientation of the initial fractures, and the type and coalescence are established. In addition, comparisons between results from experiments and predictions from the model are presented. The numerical results are in agreement with the experiments.

Recognition and Interpretation of Polygenic Discontinuity Surfaces in the Middle Cretaceous Shu’aiba, Nahr Umr, and Natih Formations of Northern Oman

Abstract: Discontinuity surfaces that recorded superposition of marine hardground and subaerial exposure stages are common in the Middle Cretaceous of northern Oman. These surfaces formed during periods of rapid sea-level drop. The marine hardground stages are dominant in the field, whereas the subaerial exposure stage is documented in circumstantial petrographic, geochemical, and biological evidence. The record of a shoaling phase prior to exposure is commonly subtle and incomplete; supratidal deposits are conspicuously absent. Porosity in the limestones underlying the discontinuities is rearranged during subaerial exposure and subsequent burial and hence the permeability of large volumes of limestone is affected at a variety of scales. During marine hardground stages, carbonate cements, iron oxides, and manganese occludes some of the existing pore space. During burial, these intervals may thus have acted as either seals or efficient conduits of fluid flow. The surfaces under study in the Shu’aiba, Nahr Umr, and Natih formations are spaced ten to few tens of meters apart and many of them were traced laterally over distances of 100 kilometers and more between sections at Jebel Akhdar and in the Foothills. This implies that they play an important, but poorly understood role in compartmentalization of carbonate reservoir rocks.

Seismic migration processing of P‐SV converted phases for mantle discontinuity structure beneath the Snake River Plain, western United States

Abstract: We experiment with backprojection migration processing of teleseismic receiver functions from the Snake River Plain (SRP) broadband seismic experiment. Previous analyses of data from this experiment have used a common midpoint (CMP) stacking approach, a method widely applied for analysis of P-SV converted phases (receiver functions) to obtain high-resolution imaging of upper mantle discontinuities. The CMP technique assumes that all P-SV conversions are produced by flat-lying structures and may not properly image dipping, curved, or laterally discontinuous interfaces. In this paper we adopt a backprojection migration scheme to solve for an array of point scatterers that best produces the large suite of observed receiver functions. We first perform synthetic experiments that illustrate the potential improvement of migration processing over CMP stacks. Application of the migration processing to the SRP data set shows most of the major features as in the original CMP work, but with a weaker 410-km discontinuity and a more intermittent discontinuity at 250 km apparent depth. Random resampling tests are also performed to assess the robustness of subtle features in our discontinuity images. These tests show that a 20-km elevation of the 660-km discontinuity directly beneath the Snake River Plain is robust, but that the variations in 410-km discontinuity topography that we observe are not stable upon resampling. “Bright spots” near 250 km apparent depth are robust upon resampling, but interpretation of these features is complicated by possible sidelobe artifacts from topside Moho reverberations.

Shear Capacity of Reinforced Concrete Deep Beams

Abstract: A mechanism analysis of shear failure of simply supported reinforced concrete deep beams is presented. Concrete and steel reinforcement are modeled as rigid perfectly plastic materials. The failure modes are idealized as an assemblage of rigid blocks separated by failure zones of displacement discontinuity. The shear strength of deep beams is derived as a function of the location of the instantaneous center of relative rotation of moving blocks. Minimization of the developed function gives the shear capacity of deep beams. Comparisons of the predicted shear capacity of numerous deep beams show good agreement with results obtained from experiments. A parametric study of main variables affecting shear strength of deep beams is conducted. The present model shows that the shear-span-to-depth ratio has more influence on the shear capacity than the span-to-depth ratio and as the former increases, the shear strength decreases. Increasing main longitudinal bottom reinforcement increases the shear capacity up to a...

Discontinuity model for internal transport barrier formation in reversed magnetic shear plasmas

Abstract: It is becoming clear that tokamak anomalous transport is dominated by radially extended non-local modes which originate from strong toroidal coupling of rational surfaces in non-uniform plasmas. To aid in understanding the internal transport barrier (ITB) formed in reversed magnetic shear experiments, in addition to the well known shear flow effect, the article points out an important non-local effect and/or finite size effect which comes from the complex behaviour of the mode over a finite radial region around the minimum q (safety factor) surface. The non-local mode, which is characterized by its radial extent and the degree of tilting in the poloidal direction (Δr, θ0), changes its structure depending on the sign of the magnetic shear, and as a result such modes are weakly excited across the qmin surface. This leads to a discontinuity or gap which disconnects the phase relation in the global wave structure across the qmin surface. Once such a discontinuity (or gap) is formed, transport suppression occurs and therefore a transport barrier can be expected near the qmin surface. The existence of this discontinuity is confirmed through use of a toroidal particle simulation. It is also shown that whether such a discontinuity is efficiently established depends on the presence of the radial electric field and the related plasma shear flow.

FE Modeling of Strain Localization in Soft Rock

Abstract: A finite-element (FE) model of localized deformation in soft rock taking a strong discontinuity approach is presented. The model is formulated within the context of rate-independent, nonassociated Drucker-Prager plasticity with nonlinear cohesion hardening/softening. Strain localization is modeled as a jump in the displacement field and simulated within the framework of the FE method using the standard Galerkin approximation. The model is used to simulate the load-displacement behavior of Gosford sandstone deforming in plane strain, focusing on the prediction of the stress levels necessary to initiate strain localization, based on the strong and weak discontinuity criteria (jumps in displacement and strain, respectively), and on the demonstration of mesh-independence of the FE solutions in the bifurcated state. For the sandstone, the onset of weak discontinuity is detected first, before the onset of strong discontinuity, suggesting a possible coupling of the two types of discontinuities in the strain-softening regime.

Local sharpness and shear wave speed jump across the 660‐km discontinuity

Abstract: We examine vertical component short-period teleseismic seismograms from earthquakes in the Izu-Bonin subduction zone recorded by networks in the western United States for phases associated with conversions from mantle discontinuities. The dominant phase in the stacked P coda is the result of near source S-to-P conversions from a subhorizontal discontinuity at a depth ranging from 650 km to 745 km. We previously used 88 timings of this phase, called S660P to determine the topography of the 660-km discontinuity. Employing the 17 best recorded S660P phases, we modeled the S660P amplitude accounting for attenuation and correcting for three-dimensional discontinuity topography. Just to the east of the Izu-Bonin subduction zone, the 660-km discontinuity is sharp (<10 km) and the 5-wave velocity contrast across the discontinuity is 0.60±0.11 km s−1. This value is 60% larger than the preliminary reference Earth model (PREM) value (70% larger than in the iasp91 and ak135 models) and, unlike estimates from reflected SH waves, is independent of the estimated density contrast at 660 km. The large 5-wave velocity contrast inferred here is consistent with recent mineral physics experiments and extrapolations if the mantle at 660 km depth and a few hundred kilometers east of the slab is at near normal mantle temperatures and contains between 3% and 5% cation aluminum, as expected in a pyrolitic mantle.

A 2-D meshless model for jointed rock structures

Abstract: According to the characteristic structural features of jointed rock structures, a meshless model is proposed for the mechanics analysis of jointed rock structures based on the moving least-squares interpolants. In this model, a jointed rock structure is regarded as a system of relatively intact rock blocks connected by joints or planes of discontinuity; these rock blocks are modelled by general shaped anisotropic blocks while these joints and planes of discontinuity are modelled by interfaces. The displacement field of each block is constructed by the moving least-squares interpolants with an array of points distributed in the block. To deal with the discontinuities of rock structures, the displacement fields are constructed to be discontinuous between blocks. The displacement fields and their gradients are continuous in each block, hence no post processing is required for the output of strains and stresses. The finite element mesh is totally unnecessary, so the time-consuming mesh generation is avoided. The rate of convergence can exceed that of finite elements significantly, and a high resolution of localized steep gradients can be achieved. Furthermore, the discontinuities of rock structures are also fully taken into consideration. The present method is developed for two-dimensional linear elastic analysis of jointed rock structures, and can be extended to three-dimensional and non-linear analysis. Copyright © 2000 John Wiley & Sons, Ltd.

Nondestructive evaluation of the severity of discontinuities in flat conductive materials by an eddy-current transducer with orthogonal coils

Abstract: This work presents two methods for nondestructive evaluation of the severity of material discontinuities in nonmagnetic conductive plates by eddy currents generated by a transducer with orthogonal coils. The first method deals with long rectilinear discontinuities located beneath the surface under test and having a negligible width. It is based on the geometrical theory of diffraction of electromagnetic waves generated by the transducer. The conditions under which a real transducer satisfies the diffraction model hypotheses are specified. In this case, the experimental results are in good agreement with the theoretical ones. The second method concerns small volumetric discontinuities whose shape and size are evaluated by an eddy-current holographic procedure. When the discontinuity size is comparable with that of the transducer, this permits reconstruction of the shape and determination of the depth of the discontinuity beneath the surface under test.

Overlay registration error measurement made simultaneously for more than two semiconductor wafer layers

Abstract: An improved target and a technique for using it to measure registration relative to each other of more than two layers of a semiconductor wafer. At least first, second and third layers are formed to overlay each other. A first pattern is provided in a designated location of the first layer. A second pattern is provided in a designated location of the second layer, such second pattern having a given shape and a given size, and having at least one discontinuity formed therein at a predetermined location. A third pattern is provided in a designated location of the third layer, such third pattern having the given shape and the given size of the second pattern, and having at least one discontinuity formed therein at a predetermined location, wherein a portion of each one of the second and third patterns fits within the at least one discontinuity in the other when the second and third layers are in registration with each other.

Effects induced by an earthquake on its fault plane:a boundary element study

Abstract: SUMMARY Mechanical eiects left by a model earthquake on its fault plane, in the post-seismic phase, are investigated employing the 'displacement discontinuity method'. Simple crack models, characterized by the release of a constant, unidirectional shear traction are investigated ¢rst. Both slip componentsparallel and normal to the traction directionare found to be non-vanishing and to depend on fault depth, dip, aspect ratio and fault plane geometry. The rake of the slip vector is similarly found to depend on depth and dip.The fault plane is found to suier some small rotation and bending, which may be responsible for the indentation of a transform tectonic margin, particularly if cumulative eiects are considered. Very signi¢cant normal stress components are left over the shallow portion of the fault surface after an earthquake: these are tensile for thrust faults, compressive for normal faults and are typically comparable in size to the stress drop. These normal stresses can easily be computed for more realistic seismic source models, in which a variable slip is assigned; normal stresses are induced in these cases too, and positive shear stresses may even be induced on the fault plane in regions of high slip gradient. Several observations can be explained from the present model: low-dip thrust faults and high-dip normal faults are found to be facilitated, according to the Coulomb failure criterion, in repetitive earthquake cycles; the shape of dip-slip faults near the surface is predicted to be upward-concave; and the shallower aftershock activity generally found in the hanging block of a thrust event can be explained by 'unclamping' mechanisms.

Origin of the derivative discontinuity in density functional theory

Abstract: The local electron-interaction potential of Kohn-Sham density functional theory is representative of electron correlations due to the Pauli exclusion principle, Coulomb repulsion, and correlation-kinetic effects which are a consequence of the correlation contribution to the kinetic energy. This potential exhibits a discontinuity as the electron number passes through an integer value. We show, via quantal density functional theory, that the physical origin of the discontinuity is the correlation-kinetic effect. The magnitude of the discontinuity is derived to be the work done to move an electron in a conservative field representative of this effect. An explanation is given as to how these correlation-kinetic effects give rise to such a discontinuity. Further, how this understanding relates to that within traditional Kohn-Sham theory and other previous explanations of the discontinuity is also discussed.

Evolutionarity of MHD Discontinuities

Abstract: Of concern to us is the issue of the stability of MHD discontinuities with respect to their decomposition into more than one discontinuity. To answer this question small perturbations must be imposed on the discontinuity surface. If they do not instantaneously lead to large changes of the discontinuity, then the discontinuity is termed evolutionary.

Formation of a compound slow shock‐rotational discontinuity structure

Abstract: Whang et al. [1996, 1997] reported observations of a new double discontinuity in the interplanetary space and magnetotail. The double discontinuity is a compound structure of a slow shock (SS) followed by an adjoining rotational discontinuity (RD). In a pressure isotropic plasma the SS-RD compound structure cannot be maintained, because the normal flow speed downstream of an SS is less than the Alfven speed and the downstream RD will overtake the SS. However, the Alfven speed in a pressure anisotropic plasma may be highly reduced by an increased value of P‖/P⊥. We suggest that the observed SS-RD compound structure may be related to this anisotropic effect. In the hybrid simulations of a slow shock, the back streaming ions will result in high P‖ and relatively low P⊥ in the foreshock region. This leads to a region with highly reduced local intermediate wave speed. A rotational discontinuity propagating toward the slow shock from the downstream region would be slowed down in the high P‖ region and attached to the SS transition region to form a structure resembling a double discontinuity. We present hybrid simulations to examine possible results of the interaction between an RD and an SS, and the formation of a compound discontinuity. Our simulations show that there are two different types of interactions: (1) An RD passes through and propagates upstream of an SS, when a weak SS cannot provide enough high anisotropy (P‖/P⊥ > 1); and (2) a compound SS-RD-SS structure is formed in which an RD is trapped inside the transition region of a strong SS. In this compound structure, the SS is divided into two portions by the RD. However, this SS-RD-SS structure is somewhat different from the observed double discontinuity in which the RD is attached to the downstream part of a slow shock.

Some aspects of vibration of an elastic body with a breathing discontinuity of material

Abstract: We present the results of analytical, numerical, and experimental investigations of some vibration damage indicators in resonance, sub-, and superresonance vibrations of an elastic body with a "breathing" discontinuity of material over a wide range of values of the excitation-to-natural frequency ratio of the vibrating system (from 0.25 to 2.0). We estimate the applicability limits of the analytical solutions obtained.

Discontinuity Data Analysis From Oriented Boreholes

Abstract: This paper presents ongoing research in improvement of analytical tools for the characteriza- tion of rock mass structure from oriented borehole discontinuity data. Using automated multivariate cluster analysis and the "three dimensional stereonet" provides for fast and objective characterization of oriented core discontinuity data. Improvements to the CYL software include three new methods of cluster analysis, incor- poration of roughness as well as orientation and spacing as variables, additional visualization modes, and an automated method to split the data set into different Geotechnical Mapping Units (GMU). In addition the pro- gram facilitates the selection of optimum drilling angle for boreholes.

Numerical analysis of mixed mode fracture in concrete using interface elements

Abstract: The main objective of the present paper is to analyze the effect of cracks and their propagation on the mechanical behaviour of a structure fabricated with a quasibrittle mate- rial. The analysis is restricted to two dimensions, and therefore, the fracture mechanisms of interest are those of mode I (pure tension) and mode II (pure shear). The analysis has been performed by developing and implementing a model based on non-linear fracture mechanics, and using it for simulating results from a extensive experimental study on beams with centric and eccentric notches of high and normal strength concretes. The model consists of a cohesive crack model for non-planar tension-shear fracture, which is implemented with the discrete crack approach, where the discontinuity is modeled in the con- text of the finite element method through interface elements (or joint elements). The fracture behaviour is simulated through the constitutive law of the interface, which represents the co- hesive crack stress-separation relation. A fracture criterion, together with a flow rule and a softening law, defines the constitutive model.