Showing papers on "Discontinuity (geotechnical engineering) published in 1988"

The Existence of Two Stages in the Transition to Three-Dimensionality of a Cylinder Wake

Abstract: The transition to three‐dimensionality in the near wake of a circular cylinder involves two successive transitions, each of which corresponds with a discontinuity in the Strouhal–Reynolds number relationship. The first discontinuity [between Reynolds numbers (Re) of 170 to 180] is associated with the inception of vortex loops, and it is hysteretic. The second discontinuity (between Re=230 to 260) corresponds with a change to a finer‐scale streamwise vortex structure. At this discontinuity there is no hysteresis, and it is suggested that two modes of vortex shedding alternate in time.

The role of tectonic grain size reduction in the rheological stratification of the lithosphere

Abstract: With the aid of published constitutive flow laws for common crustal and upper mantle rock types, it is possible to construct curves showing variation of rock strength with depth in the continental lithosphere, assuming a particular crustal stratification of rock types and a particular geothermal gradient. Such exercises lead to the conclusion that marked changes in strength can occur at major petrological interfaces, such as the Moho discontinuity, which are therefore likely to be associated with a tendency for major, sub-horizontal detachments to form. Constitutive flow laws used for such purposes are usually assumed to represent steady-state flow, at constant rock microstructure. We argue that perhaps the most significant tectonic displacements of rock masses occur across major, localized shear zones, characterized by intense grain-size reduction, and illustrate this by reference to deformation in the metabasic and ultrabasic rocks of the lower part of the Ivrea Zone of northern Italy, which is believed to provide a section through what was once part of the deep continental crust. Calculations of the variation of the flow strength with depth of rocks should therefore use flow laws which include provision for strain dependent variations of strength, arising from strain dependent microstructural changes. We use the results of recent studies of grain-size dependent flow of olivine to estimate strain dependent variations in the flow strength of the sub-continental upper mantle, arising from dynamic recrystallization. From this we argue that after an initial period of flow at high stress, parts of the upper mantle can become very weak, if grain size is sufficiently reduced. Under favourable circumstances, flow can be concentrated immediately below the Moho, or in a band below the Moho. Such behaviour is consistent with our observations of the behaviour of dunites in the Ivrea Zone, and with some interpretations of upper mantle seismic reflectors.

Evidence for a sharp 670‐km discontinuity as inferred from P‐to‐S converted waves

Abstract: The fine structure of the upper mantle discontinuities is investigated using observations of converted waves on short-period and broadband seismograms. Using a stacking technique to analyze the P wave coda of teleseismic records, evidence is found for coherent near-receiver P-to-S converted phases generated by the 400- and 670-km discontinuity beneath a number of the stations used in this study. Variations in travel time, slowness, and amplitude of these phases as observed among the stacks for stations of the Regional Seismic Test Network in the United States and of the Network of Autonomously Recording Seismographs in western Europe are very likely the expression of upper mantle heterogeneity. Observations of coherent converted phases from the 400-km discontinuity are fewer in number than of phases converted at the 670-km discontinuity, suggesting that the latter is more pronounced. Some of the seismograms, especially of station RSCP, show extremely high-amplitude P-to-S converted phases from the 670-km discontinuity. These seismograms allow a detailed waveform comparison of the converted phase with the direct P phase and present evidence for a sharp 670-km discontinuity.

Subduction zone earthquakes and stress in slabs

Abstract: The pattern of seismicity as a function of depth in the world, and the orientation of stress axes of deep and intermediate earthquakes, are explained using viscous fluid models of subducting slabs, with a barrier in the mantle at 670 km. 670 km is the depth of a seismic discontinuity, and also the depth below which earthquakes do not occur. The barrier in the models can be a viscosity increase of an order of magnitude or more, or a chemical discontinuity where vertical velocity is zero. LongN versus depth, whereN is the number of earthquakes, shows (1) a linear decrease to about 250–300 km depth, (2) a minimum near that depth, and (3) an increase thereafter. Stress magnitude in a subducting slab versus depth, for a wide variety of models, shows the same pattern. Since there is some experimental evidence thatN is proportional toeκσ, where κ is a constant and σ is the stress magnitude, the agreement is encouraging. In addition, the models predict down-dip compression in the slab at depths below 400 km. This has been observed in earlier studies of earthquake stress axes, and we have confirmed it via a survey of events occurring since 1977 which have been analysed by moment tensor inversion. At intermediate depths, the models predict an approximate but not precise state of down-dip tension when the slab is dipping. The observations do not show an unambiguous state of down-dip tension at intermediate depths, but in the majority of regions the state of stress is decidedly closer to down-dip tension than it is to down-dip compression. Chemical discontinuities above 670 km, or phase transitions with an elevation of the boundary in the slab, predict, when incorporated into the models, stress peaks which are not mirrored in the profile of seismicity versus depth. Models with an asthenosphere and mesosphere of appropriate viscosity can not only explain the state of stress observed in double Benioff zones, but also yield stress magnitude profiles consistent with observed seismicity. Models where a nonlinear rheology is used are qualitatively consistent with the linear models.

Numerical studies ~ of loss of ellipticity near singularities in an elastic material

Abstract: Numerical solutions of anti-plane shear crack problems and screw dislocation problems are presented for materials in which the equilibrium equation varies in type locally from elliptic to hyperbolic as a result of deformation. These results show the emergence of surfaces of discontinuity in the displacement field in some materials. In other materials they show a chaotic mixture of elliptic phases at intermediate distances from the singularity. A statistical analysis applied to the numerical solutions demonstrates the role of elastic stability in the mechanics of these deformations.

The acquisition of spatial knowledge under conditions of temporospatial discontinuity.

Abstract: Three experiments were conducted to examine the effect of temporospatial discontinuity on route learning. Discontinuity was created by showing subjects in a route-learning task a set of environmental scenes comprising a route in scrambled, rather than logically sequenced, order. Results from Experiments 1 and 2 indicated that discontinuity during learning affected the accuracy and the quantity of the route knowledge acquired, but suggested that the processes underlying acquisition were relatively robust. Results from Experiment 3 suggested that learning routes under conditions of discontinuity leads to a greater exaggeration of distance between route segments than is the case without discontinuity.

Sp phases from the Australian upper mantle

Abstract: SUMMARY Routinely published source mechanisms of large earthquakes greatly facilitate the selection of data that may be suitable for the study of S to P conversions from the upper mantle. This is demonstrated for three stations of the Global Digital Seismograph Network located at Narrogin (NWAO), Charters Towers (CTAO) and Tasmania University (TAU) in Australia. Of ten events that were selected, eight showed precursors to S, SKS and ScS on long-period vertical-component seismograms at epicentral distances between 70" and 91". The precursors are interpreted as Sp conversions from the upper mantle beneath Australia and surrounding areas. Synthetic seismograms that were calculated for the PREM model of Dziewonski and Anderson (1981) show good agreement with many details of the data. The main precursor arrivals are compatible with S to P conversions from a seismic discontinuity at 400 km depth. Relative to conversions from the '400-km' seismic discontinuity, Sp phases from the '670-km' discontinuity have much smaller amplitudes both in the synthetics and data in the epicentral distance range from 75" to 85". It is suggested that Sp phases from the '670-km' discontinuity can best be observed at epicentral distances beyond 89". This is confirmed by one observation at A = 90.6" made at CTAO. Within the limits of resolution of long-period data, there is no indication for strong lateral variations in the upper mantle discontinuity at 400 km between various geological provinces of Australia and surrounding areas. There are indications for the presence of S to P converted phases from a depth near 220 km. Their appearance is variable so that they are difficult to explain by S to P conversions at a seismic discontinuity at this depth.

Theoretical and Experimental Investigation of Open Microstrip Gap Discontinuities

Abstract: A theoretical analysis of open discontinuities in microwave planar circuits using integral equations technique solved by the moment method is described. Characterization of the dicontinuity by an equival ent scattering matrix is then accurately evaluated from the determnation of the current distribution along the line containing the discontinuity under different load conditions. An experimental method, which is capable to determine directly the discontinuity equivalent circuit parameters, is proposed. A good agreement between theoretical and experimental results is reported.

Spherical elastic-plastic wave solutions

Abstract: Two closed-form radially symmetric elastic-plastic wave solutions, corresponding to spherical cavity loadings, are compared to numerical solutions constructed by a finite difference code. An infinitesimal strain theory, with plastic incompressibility, a Mises yield condition and either perfectly plastic flow or parabolic work hardening, allows wave function representations in both elastically and plastically deforming regions. The solutions examined are for (a) the loading problem in which the cavity wall pressure is raised instantaneously to a value p0, sufficient to induce plastic yielding, then maintained atP0, and (b) the unloading problem when the wall pressure is released continuously after the instantaneous step to P0. In the former case there is a region of continuous plastic loading extending from the cavity wall to the diverging plastic discontinuity front which persists for a finite time until the plastic discontinuity is annulled. In the latter case, provided that the initial pressure decrease rate exceeds a critical value, the region behind the diverging plastic discontinuity front is purely elastic. In both cases the numerical solution smears the plastic discontinuity, and while the maintained plastic loading in case (a) allows the maximum pressure to be attained over a finite distance, this is prevented by the elastic unloading in case (b) and the following release wave is significantly distorted. It is shown that suitable ramping of the applied load produces an excellent analytic-numeric agreement unless the continuous unloading is too rapid.

Interpretation of the fuliji-fengxian dss profile in xiayangzi region

Abstract: The total length of the Deep Seismic Sound (DSS) profile of Fuliji-Fengxian is about 570 km. The static correction of the sedimentary layer had been made. It makes the results more reasonable and accurate. We separate the Pg wave from the head waves of basement and crystalline basement interface, and then, find their boundaries. These coincide with the geological data. The middle and lower crust are calculated in detail by the SEIS-83 program, and get the following results:1. The crust of Xiayangzi platform can be clearly divided into three layers (upper, middle, lower). The differences in each layers between this region and North China platform have been discussed.2. An interface in the middle crust in Xiayangzi region has been found. It may be produced from a high density layer or high velocity thin layer.3. No clear throw at the Moho discontinuity in the area of Tan-Lu fault has been found. It lies in the section of the upheaval batter of it. Perhaps there are some tension fractures in it.4. The complex folds in the upper palaeozoic, the volcano and the seismic zones may be related to the undulation of Moho discontinuity.

Diffraction of sound above a curved surface having an impedance discontinuity

Abstract: This article describes theory and experiments in the case of propagation of sound in the presence of an impedance discontinuity and a curved surface (or sound‐speed gradients). The theory is based on a residue series, or creeping wave, solution developed for the propagation of sound in a stratified medium under conditions favorable to the formation of a shadow zone. The creeping wave solutions on each side of the impedance discontinuity are coupled heuristically by assuming a virtual source above discontinuity. The theory is compared to controlled measurements made indoors above a curved surface having an impedance jump (hard to soft). The measurements were made using a point source driven by frequencies in the range between 0.3 and 10 kHz. The theory is also compared to measurements made outdoors above a flat ground having a mixed path (asphalt to grass) in the presence of realistic gradients. Pure tones in the frequency range between 0.5 and 5 kHz were generated from a point source and the resulting sound‐pressure levels measured on a sound level meter after suitable filtering. In all cases, there is good agreement between measurements and theory.

Comment on ``Theory of semiconductor heterojunctions: The role of quantum dipoles''

Abstract: Tersoff’s heterojunction model is critically analyzed using extensive experimental data obtained from photoemission measurements of the valence-band discontinuity.

Conduction-Band Discontinuity of GaInAs/InP Heterojunctions with Graded Donor Concentration

Abstract: We have measured the magnitude of the conduction-band discontinuity and the interface charge density for the heterojunction between VPE grown Ga0.47In0.53As and InP with graded donor concentration. The conduction-band discontinuity was obtained by C-V profiling technique, taking into account the correction for the interface charge density and the graded donor concentration. The simulation is employed to confirm the reliability of the band discontinuity obtained. We have found that the conduction-band discontinuity is 0.16±0.02 eV at R.T., which corresponds to 27% of the band gap difference.

Reply to "Comment on 'Heterojunction valence-band-discontinuity dependence on face orientation' "

Abstract: We respond to the criticisms of Van de Walle and Martin in the preceding Comment and present new calculations of the valence-band offsets.

Modelling of subsidence caused by longwall mining using finite element and displacement discontinuity methods. proceedings of the sixth international conference on numerical methods in geomechanics, 11-15 april 1988, innsbruck, austria. volumes 1 - 3

Abstract: Numerical techniques based on the finite element method and the displacement discontinuity method have been developed for predicting subsidence over longwall mine panels. A new approach based on history matching (back analysis) concept, called the "displacement approach" is presented. Material bulking due to roof collapse and subsequent failure of overlying strata is also introduced. A modified version of the displacement discontinuity method is used to simulate discrete cracks caused by mining in the overburden. The developed procedure has been verified by comparing predictions with published field observations at several mine panels. Here, the results for a mine with shallow overburden depth and for a deep mine located in northern West Virginia are presented. The study indicates that the shape and the magnitude of subsidence can be predicted reasonably well by simulating major mechanisms of roof collapse and crack propagation. (TRRL)

A constitutive model for jointed rock masses

Abstract: The presence of discontinuity in the geological media has long been recognized as an important factor influencing the mechanical behavior of the media. This paper proposed a versatile constitutive law that can model an assemblage of intact rock blocks separated by cleavage, bedding planes, joints, or other types of structural weak planes. The constitutive law is capable of modelling any number of joint sets with arbitrary orientations. Applicability of the model are evaluated by comparing the predicted results with existing model predictions. The effects of joint numbers, and joint orientations are also discussed.

Back analysis of assessing for slope stability based on displacement measurements

Abstract: SYNOPSIS This paper deals with the result of consideration and its reasonability about the mechanical model to examine the slope stability, applying the back analysis to the actual results of displacements, and the brief consideration concerning to some utilization of discontinuity characteristics of rocks by measuring techniques.