Showing papers on "Discontinuity (geotechnical engineering) published in 1993"

Sharpness of upper-mantle discontinuities determined from high-frequency reflections

Abstract: AN understanding of the nature of seismic discontinuities in the Earth's upper mantle is important for understanding mantle processes: in particular, the amplitude and sharpness of these discontinuities are critical for assessing models of upper-mantle phase changes and chemical layering. So far, seismic studies aimed at determining the thickness and lateral variability of upper-mantle discontinuities have yielded equivocal results, particularly for the discontinuity at 410km depth1,2. Here we present short-period (0.8–2.0 s) recordings of upper-mantle precursors to the seismic phase P′P′ (PKPPKP) from two South American earthquakes recorded by the ∼700-station short-period array in California. Our results show that the 410- and 660-km discontinuities beneath the Indian Ocean are locally simple and sharp, corresponding to transi-tion zones of 4 km or less. These observations pose problems for mineral physics models3–5, which predict a transitional thickness greater than 6 km for the peridotite to β-spinel phase transition. In contrast to the results of long-period studies6,7, we observe no short-period arrivals from near 520 km depth.

Theoretical investigation of phenomena caused by heterogeneous surface tension in solids

Abstract: From the analysis of the mechanical equilibrium of an interface between two different media (the generalized Laplace equation) it follows that, in addition to the discontinuity of the normal stress, there exists a discontinuity of the tangential stress across this interface due to the surface tension gradient. Using the solution of the corresponding problem for a system “elastic solid-elastic surface” the generalization of the Cassie equation for the contact angle on the heterogeneous surface is obtained and the level of stress arising from the interaction of surface-active melt with metal is estimated.

Global mapping of upper mantle reflectors from long-period SS precursors

Abstract: SUMMARY Long-period precursors to SS resulting from underside reflections off upper mantle discontinuities (SdS where d is the discontinuity depth) can be used to map the global distribution and depth of these reflectors. We analyse 5,884 long-period seismograms from the Global Digital Seismograph Network (1976-1987, shallow sources, transverse component) in order to identify SdS arrivals. Corrections for velocity dispersion, topography and crustal thickness at the SS bounce point, and lateral variation in mantle velocity are critical for obtaining accurate estimates of discontinuity depths. The 410 and 660 km discontinuities are observed at average depths of 413 and 653 km, and exhibit large-scale coherent patterns of topography with depth variations up to 40 km. These patterns are roughly correlated with recent tomographic models, with fast anomalies in the transition zone associated with highs in the 410 km discontinuity and lows in the 660 km discontinuity, a result consistent with laboratory measurements of Clapeyron slopes for the appropriate phase changes. The best resolved feature in these maps is a trough in the 660km discontinuity in the northwest Pacific, which appears to be associated with the subduction zones in this region. Amplitude variations in SdS arrivals are not correlated with discontinuity depths and probably result from focusing and defocusing effects along the ray paths. The SdS arrivals suggest the presence of regional reflectors in the upper mantle above 400 km. However, only the strongest of these features are above probable noise levels due to sampling inadequacies.

Fluid Flow in Discontinuous Rocks

Abstract: Rock and rock mass permeability. Rock discontinuity characterization. Fluid flow in a single discontinuity. Connectivity, scale effects and percolation theory. Permeability and depth in rock masses. Numerical methods of modelling flow. Index.

A detailed map of the 660-kilometer discontinuity beneath the izu-bonin subduction zone.

Abstract: Dynamical processes in the Earth's mantle, such as cold downwelling at subduction zones, cause deformations of the solid-state phase change that produces a seismic discontinuity near a depth of 660 kilometers. Observations of short-period, shear-to-compressional wave conversions produced at the discontinuity yield a detailed map of deformation beneath the Izu-Bonin subduction zone. The discontinuity is depressed by about 60 kilometers beneath the coldest part of the subducted slab, with a deformation profile consistent with the expected thermal signature of the slab, the experimentally determined Clapeyron slope of the phase transition, and the regional tectonic history.

Wave diffraction, amplification and differential motion near strong lateral discontinuities

Abstract: Macroseismic observations during various historical and recent earthquakes consistently show an increase of damage on narrow stripes located along strong lateral discontinuities, i.e., along contacts between two materials with very different rigidities. This paper presents a series of numerical investigations into this issue. The model consists of a semi-infinite planar, soft layer embedded in a stiffer bedrock, and impinged upon by vertically incident plane SH waves. Computations are based on a finite-difference scheme including anelastic attenuation. Two basic phenomena are observed on such simple lateral discontinuities: the well-known 1D (vertical) resonance, and an efficient wave diffraction from the discontinuity towards the softer side. They induce frequency-dependent amplification and a significant differential motion. A parameter study considering various impedance contrast and damping values shows that the amplification is primarily controlled by the impedance contrast, and to a lesser degree by sediment damping. The amplitude exhibits a slight maximum near the discontinuity, but always remains comparable (within 30%) to the 1D value. Differential motion is also controlled by the impedance contrast. It always exhibits a sharp peak in the immediate vicinity of the discontinuity, the level of which does not depend on sediment damping, and reaches significant values even for moderate incident motion. It is concluded that reported observations of increased damage near such geological structures are very likely connected with effects of differential motion on structures.

Observational test of hot flow anomaly formation by the interaction of a magnetic discontinuity with the bow shock

Abstract: The formation of a hot flow anomaly (HFA) observed near the Earth's bow shock appears to be due to the interaction between the bow shock and an impinging discontinuity in the upstream plasma Recent single-particle and 2D hybrid numerical studies have suggested that such an interaction will produce an HFA only if the motional electric field in the ambient plasma points toward the discontinuity, thereby focusing shock-reflected ions into it We perform a test of this electric field orientation for a set of nine HFA events observed by the ISEE spacecraft and described previously in the literature Under the assumption that the discontinuities were tangential, the predicted electric field orientation is found on at least one side of all nine observed HFAs (on the trailing edge of seven and the leading edge of five, and on both sides of three events) Further, there is evidence that asymmetries in the observed magnetic field signatures are related to the orientation of the motional electric field The events in which the electric field points toward the discontinuity on both sides tend to be those with fairly symmetric flanking magnetic field enhancements

Description of Complex and Sharp Interface during Shock Wave Interaction with Liquid Drop

Abstract: A method of treating a sharp discontinuity with a density function is proposed. The surface of the density function is captured within a grid cell throughout the calculation even when the discontinuity surface is largely distorted. This description is made possible by the CIP (cubic-interpolated propagation) method combined with variable transformation. This scheme can be used for detecting a sharp boundary in compressible fluid as well as incompressible fluid. Lower order schemes with the same procedure cannot reproduce the result. This scheme is applied to the interaction of a shock wave with a liquid drop. The liquid mass is conserved within an error of 0.15%.

The maximum distortional strain energy density criterion for shear fracture propagation With applications to the growth paths of En Échelon faults

Abstract: A maximum distortional strain energy density criterion is proposed for shear fracture propagation under mixed mode loading conditions It is shown that an isolated mode-II shear fracture would propagate in its own plane However, the growth paths of two en echelon shear fractures deviate from straight lines due to crack interaction Using a displacement discontinuity boundary element method, we show that the shear fracture propagation paths of two en echelon mode-II faults always converge toward each other regardless of the sense of shear and fault step In contrast, opening (mode-I) fracture growth paths of two en echelon faults depend on the sense of stepping and shearing The predicted propagation paths of en echelon shear fractures agree well with those observed in the field

Seismic constraints on mantle flow and topography of the 660-km discontinuity: evidence for whole-mantle convection

Abstract: Recent seismic models of three-dimensional mantle structure and topography on the transition-zone seismic discontinuities permit the direct evaluation of the buoyancy forces that drive large-scale mantle flow. This buoyancy distribution, coupled with radial viscosity models that are consistent with the observed geoid, predicts flow patterns with significant vertical flow through the 660-km phase boundary. The observed topography on the 660-km discontinuity acts to inhibit convection across the boundary, but is two to four times too small to prevent whole-mantle convection.

Sharpness of the mantle discontinuities

Abstract: Seismic estimates of sharpness of the mantle discontinuities are important for constraining models of composition and temperature in the deep Earth but these data are difficult to obtain. We explore a possibility to determine sharpness of the major mantle discontinuities (those at depths around 400 and 650 km) from the broad-band records of phases converted from P to S underneath the receiver. Our estimates are obtained from a comparative analysis of waveforms of the converted phases and those of the P waves in the teleseismic records of BRV (Kazakhstan), GRF (Germany), NRE0 (Norway) and YKW (Canada). The data indicate that the shape of the pulse converted at the 400-km discontinuity is close to that of the P wave whereas the pulse of the 650-km conversion is substantially broader. We infer from this comparison that the 400-km discontinuity is sharp (width of the transition zone is less than 5–7 km). The 650-km transition is modelled by a linear gradient zone 20–30 km thick.

Seismic discontinuity at the top of d : a world-wide feature ?

Abstract: Considering the growing number of observations of a seismic discontinuity at the top of D″, some 300 km above the core-mantle boundary, we investigate the scenario of whether this feature could be global. We show that the mean first arrivals of P-waves, as used to build global models, can be fit by the two branches of a discontinuous model. We argue that the intermittence of the detection of the reflector is due to lateral variations of a global discontinuity. These observations put strong constraints on the type of chemical layering that could explain the discontinuity. We speculate that the properties of the D″ discontinuity are better explained in terms of a new phase transition.

Lithospheric stresses associated with nontransform offsets of the Mid-Atlantic Ridge: Implications from a finite element analysis

Abstract: Nontransform offsets are a fundamental aspect of the offset geometry exhibited along the mid-oceanic ridge system, independent of spreading rate. Along the slow/intermediate opening (<40 mm/y full rate) Mid-Atlantic Ridge these offsets of the ridge axis range in length from less than 10 km to approximately 30 km and vary in age offset from 0.5 to 2.0 m.y. The variable morphotectonic geometries associated with these discontinuities indicate that horizontal shear strains are accommodated by both extensional and strike-slip tectonism and that the geometries are unstable in time. In many cases, there appears to be an evolutionary relationship between transform fault boundaries and nontransform offsets as the result of prolonged differential asymmetric spreading between adjoining ridge segments. The finite element method is used to study the complex stress field associated with these small-offset discontinuities of ridges with slow (30 mm/y) and fast (100 mm/y) total opening rates. A plane stress plate model examines the variation in the horizontal tectonic stress field produced by offsets with different lengths and changes in the ratio of a ridge-normal tensile stress resisting plate separation to a shear stress resisting relative plate motion along the discontinuity. The predicted fault patterns based on the calculated stress field are compared with seafloor observations in terms of the morphotectonic patterns and evolution of nontransform offsets. For a slow spreading rate, the analysis shows that all structural geometries observed can be modeled by a range of offset lengths (5, 10, 20, 30, and 40 km) and by a ridge-normal stress 3 to 5 times greater than the discontinuity shear stress. These findings suggest that nontransform offsets are zones of mechanical weakness relative to the surrounding lithosphere. An offset length between 10 and 20 km is predicted to be the threshold length for maintaining a transform fault geometry. As inferred from ridge axis morphology, there seems to be a strong link between the magnitude of the stress ratio and the time varying magmatic activity along and between ridge segments. While our models are consistent with a weak discontinuity shear stress relative to the ridge-normal stress to explain the geometries of nontransform offsets of slow-spreading centers, a weaker ridge-normal stress to discontinuity shear stress most closely models the development of an overlapping spreading center geometry, the distinctive geometry of nontransform offsets of spreading centers opening at fast rates. This difference is attributed to magma supply along-axis, relatively continuous for fast-spreading centers and intermittent for slow-spreading centers, and a preexisting zone of mechanical weakness linked to the evolution of nontransform offsets from transform faults on slow-spreading centers.

Magnetic and tectonic studies of the dueling propagating spreading centers at 20°40′S on the East Pacific Rise: Evidence for crustal rotations

Abstract: We present the results of a magnetic study of a 225 km by 240 km area centered on the dueling propagating spreading centers located at 20°40′S on the East Pacific Rise. A majority of the data used were collected during a cruise aboard the R/V Moana Wave during which continuous SeaMARC II coverage was obtained. These data were combined with additional data to produce an anomaly map which extends to anomaly-2-aged crust. A three-dimensional inversion in the presence of bathymetry was carried out for the area. The resulting magnetization distribution was interpreted and compared to side scan sonar and bathymetry data sets in order to determine the recent history of the discontinuity. The results indicate consistent asymmetric spreading faster to the east, discontinuous high magnetizations in the discordant zone associated with the discontinuity, and southward migration of the feature at a rate of 90–100 mm/yr between Jaramillo and Brunhes time (0.95 to 0.73 Ma) with slowing during the Brunhes to less than 10 mm/yr. There is also an overlapping Jaramillo isochron on the west flank and a gap in that isochron on the east flank indicating a transfer of crust during this time period from the Nazca to the Pacific plate. In addition, areas of oblique lineations possibly representing rotated crust were modelled using an inverse method which enables the specification of a nonuniform magnetization unit vector. Results from this second model support the presence of highly rotated pre-Brunhes Nazca crust within Brunhes Pacific crust which has been deformed by bookshelf faulting. This indicates at least two episodes of crustal transfer from the Nazca plate to the Pacific plate. The discontinuity appears to mark the boundary between rigid plate tectonics to the north and deformation within the Nazca plate between the discontinuity and the Easter microplate to the south. The detailed history of the discontinuity involves dueling propagation with a great deal of variation in the amount of overlap of the two ridges as well as inward and outward cutting and abandonment of the tips of both ridges.

Precursors to SS and structure of transition zone of the north‐western Pacific

Abstract: In the records of the Grafenberg observatory and of a few other stations we have detected long-period SH reflections at the underside of mantle discontinuities. The bounce points of the reflections are located in the northwestern margins of the Pacific. The data are used to evaluate S660S - S410S differential times which are directly related to the thickness of the layer between the two discontinuities. Most of the bounce points are in the area of the Japan and Kuril - Kamchatka subduction zones where a large-scale depression of the 660-km discontinuity was previously found and attributed to a cooling effect of subduction [Shearer & Masters, 1992]. According to our data the differential time in this region is 3 s less than in the IASP91 model. This anomaly suggests a thinning of the layer between the two discontinuities instead of thickening which is expected from depression of the lower discontinuity. Our analysis also indicates that there is a strong reflector in the study region at a depth of 900 km.

The Collection and Analysis of Discontinuity Orientation Data for Engineering Design, with Examples

Abstract: Publisher Summary Rock masses usually contain such features as fractures, faults, joints, bedding planes, cleavage and other mechanical defects which are here collectively referred to as discontinuities. These discontinuities can have a major influence on the deformability, strength, stability and permeability of the rock mass and thereby play a major part in dictating the design of foundations, slopes, tunnels and other structures associated with discontinuous rock masses. Discontinuity orientation data can be presented and utilized in design by two different methods. If the rock face at the design site is readily accessible, the locations and orientations of actual discontinuities can be measured and used explicitly in the design calculations. If, however, the rock face is not accessible it is necessary to measure discontinuity orientation at other rock faces, or from boreholes, and build up a statistical model that represents the discontinuity orientation characteristics of the rock mass. In most cases the statistical model is based on the fact that geological processes usually generate one or more clusters (or sets) of nearly parallel discontinuities in a given rock mass.

Modern Developments in Discontinuity Analysis – The Persistence–Connectivity Problem

Abstract: Publisher Summary Stability of rock slopes and of the rock mass around tunnels strongly depends on the persistence of discontinuities. Instability is much more likely to occur if discontinuities are fully persistent, given the usually much higher resistance of intact rock compared to that of discontinuities. Similarly, conductivity of a rock mass will be greater if a greater proportion of the discontinuities are interconnected compared to one where they are mainly isolated. Finally, if discontinuities have lower shear and normal stiffnesses compared to intact rock, the rock mass deformability will also depend on persistence. This chapter highlights a few aspects of the persistence/connectivity problem and their possible solutions.

Self-Oscillatory Modes of Fatigue Fracture and the Formation of Self-Similar Structures at the Fracture Surface

Abstract: The jump-like extension of the fatigue crack as described by P. J. E. Forsyth is considered. It is shown that such a peculiar extension regime of the fatigue crack is connected with two features: the presence of fatigue kinetic diagram discontinuity and the appearance of the self-similar intermediate asymptotic crack propagation regime with scaling properties. A model of self-similar jump-like extension of fatigue crack and the critical parameters which control such crack extension are presented.

Visco‐elastic response of joints to transmission waves

Abstract: Experiments of wave transmission across a joint were carried out to investigate the visco-elastic response of the joint. Two models for wave transmission were examined with the experimental data. The results show that the velocity discontinuity model which includes the effect of viscosity is in better agreement with the experimental observations for both P- and S-waves under dry conditions than the displacement discontinuity model. This suggests that at the real asperity contacts, the solid material behaves visco-elastically to transmission waves in the high frequency range. A possible mechanism for the visco-elastic behavior is plastic flow at the asperities caused by high local pressure.

Applications of the boundary element and dislocation density methods in plane crack problems

Abstract: Crack modelling using the dislocation density method has become an increasingly popular technique for plane crack problems and allows the fast, efficient determination of stress intensity factors. The range of geometries which may be solved by the basic technique is, however, somewhat limited and this paper outlines a hybrid formulation combining the dislocation density technique with constant displacement discontinuity boundary elements to satisfy a range of far boundary conditions. The method described can analyse configurations with an arbitrary number of buried or surface breaking cracks and can be implemented on a personal computer to provide an efficient tool for crack analysis.

Experience with the Application of Modern Rock Classifications in Coal Mine Roadways

Abstract: Publisher Summary Most rock formations in the earth's crust contain major and minor weakness planes such as faults, slips, joints and bedding planes. Rock classifications provide a useful index for quantifying these weakness or discontinuity planes and for defining a rock mass as distinct from a laboratory small-scale sample of rock. This chapter presents the experiences of applying the Council for Scientific and Industrial Research (CSIR), Norwegian Geotechnical Institute (NGI), and Central Mining Research Station classifications to stability and support of rectangular coal mine roadways and roadway intersections. One factor that sometimes becomes important in roadway or tunnel support is rock weathering. This is only obliquely considered by the CSIR system under condition of joints—weathered wall rock—and under the NGI system as rock swelling because of the chemical action of water. The graphical relation between stand-up time and excavation span given in the CSIR classification is somewhat inconvenient to use in coal mine roadways.

Determination of the ‘Design Block’ for Tunnel Supports in Highly Jointed Rock

Abstract: Publisher Summary This chapter presents the determination of design block for tunnel supports in highly jointed rock mass. For an underground excavation through a competent and discontinuous rock mass, a preferred block failure can be expected with particular boundary joint orientations and half-space combinations. This block can be found analytically by using the rock mass discontinuity distributions, the rock mass spacing distributions, and the procedures and methods of block theory. The three major factors that contribute to the block failure are the frequency of its boundary planes in the rock mass, the shape of its joint pyramid, and the instability of its joint pyramid. Block theory is established as a relevant and powerful tool for assessing the influence of discontinuities and tunnel geometry on design factors. The design block for an opening can be found prior to actual construction on the basis of the geological mapping or exploration data at hand. A great variability in joint orientations can be handled by introducing more joint sets to the system.