Showing papers on "Discontinuity (geotechnical engineering) published in 1983"

Strength of jointed rock masses

Abstract: Jointed rock masses comprise interlocking angular par- ticles or blocks of hard brittle material separated by discontinuity surfaces which may or may not be coated with weaker materials. The strength of such rock masses depends on the strength of the intact pieces and on their freedom of movement which, in turn, depends on the number, orientation, spacing and shear strength of the discontinuities. A complete understanding of this problem presents formidable theoretical and experi- mental problems and, hence, simplifying assumptions are required in order to provide a reasonable basis for estimating the strength of jointed rock masses for engineering design purposes. This Paper summarizes some of the basic information upon which such simplifying assumptions can be made. A simple empirical failure criterion is presented and its application in engineering design is illustrated by means of a number of practical examples. Des masses jointives de rochers comprennent des particules angulaires enchevetrees ou des ...

Phase transitions and mantle discontinuities

Abstract: Density and elasticity data are consistent with, but do not require, a uniform upper- and lower-mantle composition. Such data cannot at present resolve the small changes in physical properties that would be required to keep distinct mantle reservoirs dynamically separated. Alternatively, the issue of chemical stratification in the mantle can be addressed by analyzing the details of the phase equilibria of the appropriate minerals. In order to do this we construct an internally consistent pressure calibration scale which is based in part on the coesite-stishovite transition pressure Ptr(GPa) = 7.45(±0.3) + 2.1(±0.3) × 10−3T(°C) according to our reanalysis of the available data. We conclude that a discontinuous reaction occurs in the olivine component (α+γ = α+β) of the mantle at the conditions of the 400-km seismological discontinuity; however, no discontinuous reactions corresponding to the 670-km discontinuity have yet been identified. The only reaction observed in diamond cell experiments at approximately the pressures existing at 670-km depth, the breakdown of γ-spinel to form a silicate perovskite assemblage, appears not to satisfy the observed sharpness of this discontinuity. Thus it may be necessary to invoke either a univariant reaction that has not yet been observed experimentally or a chemical discontinuity at this depth. As the mantle is likely to be at temperatures higher than those of the experiments (estimated to be near 1000°C), additional univariant reactions involving the silicate ilmenite or garnet phases that are predicted to occur may be significant. One possible interpretation is that the mantle is of uniform composition and that such (hypothetical) reactions produce the 670-km discontinuity. This would imply that the phase assemblages so far studied in high-pressure-temperature experiments are not those occurring in the mantle. Alternatively, the mantle could be chemically stratified.

A lower mantle S-wave triplication and the shear velocity structure of D"

Abstract: Summary. A lower mantle S-wave triplication detected with short- and longperiod WWSSN and CSN recordings indicates a substantial shear velocity discontinuity near 280 km above the core-mantle boundary. The triplication can be observed in rotated SH seismograms from intermediate and deep focus events throughout the distance range from 70" to 95". Three distinct source region-receiver array combinations that have been investigated in detail demonstrate consistent travel time and relative amplitude behaviour of the triplication, with slight systematic shifts in the triplication indicating up to 40km variations in the depth of the discontinuity. Modelling of the observations with synthetic seismograms produced with the Cagniard de Hoop and reflectivity methods constrains the shear velocity increase to be 2.75 k 0.25 per cent, comparable to upper mantle discontinuities. Short-period observations indicate that the velocity increase may be a sharp first-order discontinuity, or may extend over a transition zone no more than 50 km thick. The shear velocity gradient below the discontinuity, within the D" layer, is not well-constrained by the SH data, but slightly positive or near zero velocity gradients are consistent with the long-period amplitude ratios of ScSH/SH.

Reflection properties of phase transition and compositional change models of the 670-km discontinuity

Abstract: Seismic observations of the amplitude ratio amplitude P′670P′/amplitude P′P′ suggest a reflection coefficient R ∼ 7–13% for reflections from the underside of the 670-km discontinuity. This can be compared with calculated reflection coefficients to constrain the nature of the mantle transition region. Velocity and density profiles through the transition region were calculated for olivine, pyroxene, garnet, and olivine + garnet model mineral assemblages. For models with identical upper and lower mantle compositions, the computed reflection coefficients from the phase transitions pertinent to the 670-km discontinuity are less than 0.5%. These phase transitions produce their maximum R if they are effectively discontinuous, which gives R ∼ 2–3%, except for the garnet to perovskite transformation which gives R ∼ 8%. In general, given realistic impedance contrasts across a phase transition, the change in material properties must occur over a region less than 3 km thick for R to be of the order of 5%. A discontinuous change in chemical composition at 670-km depth, coupled with a change in phase, can also produce a reflection coefficient of the observed magnitude: R ∼ 2–3% for a first-order change in phase at 670 km with different iron concentrations on each side of the discontinuity, and R ∼ 4–6% for cases with a discontinuous increase in silica content. Based on our analysis, the thermal and chemical boundary layers which would occur across such a chemical discontinuity would probably have negligible effects on the reflection coefficient. Thus although limited by considerable uncertainties, the mineralogical and seismic data suggest a two-layered model of the mantle with a discontinuous change in both phase and composition at 670 km.

Boundary element methods in solid mechanics: With applications in rock mechanics and geological engineering

Abstract: Review of linear elasticity introduction to boundary element methods the fictitious stress method the displacement discontinuity methods the direct boundary integral methods improvements and extensions applications in rock mechanics and geological engineering.

Rotational discontinuities and the structure of the magnetopause

Abstract: Symmetric and asymmetric rotational discontinuities are studied by means of a one-dimensional computer simulation and by single-particle trajectory calculations. The numerical simulations show the symmetric rotation to be stable for both ion and electron senses of rotation with a thickness of the order of a few ion gyroradii when the rotation angle of the tangential field is 180 deg or less. Larger rotation angles tend to be unstable. In an expansive discontinuity, when the magnetic field on the downstream side of the discontinuity is larger, an expanding transition layer separating the highfield from a low-field region develops on the downstream side, and a symmetric rotational discontinuity forms at the upstream edge. The implication of these results for magnetopause structure and energy flow through the magnetopause is described.

Engineering Behaviour of Rocks

Abstract: 1 Engineering Description of Rocks.- 1.1 Rock testing.- 1.2 Uniaxial or unconfined strength.- 1.3 Empirical field and laboratory tests.- 1.4 Porosity and permeability.- 1.5 Discontinuous rock.- 2 Stress and Strain.- 2.1 Stress at a point.- 2.2 Pore pressure and effective stress.- 2.3 Strain at a point.- 2.4 Representation of stress and strain.- 2.5 Relation between stress and strain.- 2.6 Geostatic stresses.- 2.7 Measurement of in situ stress.- 3 Rock Deformation.- 3.1 Rock tests in compression.- 3.2 Rock deformation in compression.- 3.3 Mechanics of microfracture.- 3.4 Rock macrofracture.- 3.5 The complete rock deformation curve.- 4 Rock Strength and Yield.- 4.1 Rock strength criteria.- 4.2 Yield criteria.- 4.3 The critical state concept.- 4.4 Triaxial testing.- 4.5 Axial and volumetric strain data.- 4.6 The Hvorslev surface in rocks.- 5 Time Dependency.- 5.1 Creep strain.- 5.2 Phenomenological models of creep.- 5.3 Time-dependent deformation.- 5.4 Time-dependent strength reduction.- 5.5 Cyclic loading.- 5.6 Rapid loading.- 6 Discontinuities in Rock Masses.- 6.1 Discontinuity measurement.- 6.2 Discontinuity orientation data.- 6.3 Shear resistance of a rock containing a discontinuity.- 6.4 Shear resistance of a discontinuity.- 6.5 A critical state model for rock discontinuity strength.- 6.6 Measurement of discontinuity shear resistance.- 7 Behaviour of Rock Masses.- 7.1 Discontinuity frequency.- 7.2 Rock mass classification systems.- 7.3 Rock mass strength criterion.- 7.4 The relevance of rock mass strength.- References.- Author Index.

Fault Type Predictions from Stress Distributions on Planetary Surfaces: Importance of Fault Initiation Depth

Abstract: The prediction of fault type on planetary surfaces from model stresses calculated at depth is discussed. These fault-type predictions yield different faults than those predicted using the surface criteria commonly employed in geophysical models. For elastic-plate flexure models of mascon loading on the moon, stresses calculated at the surface predict the occurrence of strike-slip faulting at the radial distance where grabens are found. Normal faults bounding lunar grabens and thrust faults responsible for wrinkle ridges are analyzed. It is found that the former initiate at the mechanical discontinuity that separates the breccia of the megaregolith from in situ fractured rock and that the latter initiate at the mechanical discontinuity between basalt layers and the underlying basin floor. The difference between elastic constants for the outer few kilometers of brecciated megaregolith and the underlying lunar lithosphere are evaluated. Superposing nonisotropic stresses resulting from the weight of overburden to the depth of the relevant mechanical discontinuity yield stresses that predict wrinkle ridges in the basin centers and grabens outside the basin margin, and eliminate the predicted zone of strike-slip faults.

Variability of the Harang discontinuity as observed by the Chatanika radar and the IMS Alaska magnetometer chain

Abstract: The relative location of ionospheric parameters and ground magnetic perturbations in the vicinity of the Harang discontinuity is examined. For this purpose, we use combined observations of the Chatanika incoherent scatter radar and of the IMS Alaska meridian chain of magnetometers, which were continuously measuring the crucial ionospheric parameters near the Harang discontinuity when a sizeable substorm took place. The Harang discontinuity, as defined by the reversal of the north-south electric field, is found to be located at 1-2° poleward of the discontinuity identified by the conventional method of using ground magnetic perturbations. It is also found that the relative location of auroral conductivity enhancements is quite variable, depending perhaps upon substorm time. During relatively quiet times and the early stage of a substorm, a conductivity enhancement occurs only on the poleward side of the Harang discontinuity defined by the electric field reversal, whereas the enhancement surges equatorward extending beyond the discontinuity during the maximum to recovery phase of a substorm.

Physics and mathematics of heat and mass transfer in polymers

Abstract: Heat and mass transfer phenomena may induce structural changes in polymeric materials; the kinetics of such changes are then coupled with those of the transport processes. Two categories of phenomena are considered: bulk structural changes taking place gradually throughout the polymers, and abrupt changes taking place at a surface of morphological discontinuity. For both categories both the physics and the mathematics are discussed in some detail. The content of this paper is largely a review of recent results, though some of the points presented are original ones.

Labour market discontinuity and spatial development

Abstract: role of labour as a prime influence upon the locational decisions of industrialists. This work seems to be divisible into two strands. The first presents capital as, in principle, quite indifferent to abandoning often substantial concentrations of usable labour power to seek out more profitable locations elsewhere (Walker and Storper, 1981; Storper and Walker, 1983). A second strand accepts this point to some extent, but holds that the pools of labour left when traditional industries

Lower Viruan discontinuity surfaces in central Sweden

Abstract: Near the base of the Viru Series (Middle Ordovician) in Vastergotland, south-central Sweden, discontinuity surfaces in the limestones mark breaks in sedimentation. The morphology of these surfaces has been studied in Hallekis Quarry at Kinnekulle and Gullhogen Quarry at Billingen, with particular emphasis on lateral variations. Four main morphological types of discontinuity surface are distinguished. One has furrows arranged in a polygonal pattern which may represent mudcracks. The basal Viruan discontinuity surface in Gullh[obar]gen Quarry shows laminated structures resembling stromatolites. At H[abar]llekis, the basal Viruan discontinuity surface has sac-shaped cavities up to 10 cm deep, produced by boring organisms. At Billingen, the amount of boring by hard-bottom endofauna was low and the discontinuity surfaces were mostly associated with high energy conditions, as indicated by conglomerates, blankets of oolites, and erosional phenomena. Stromatolites and mudcracks suggest shallow water cond...

The shear-layer structure in a rotating fluid near a differentially rotating sidewall

Abstract: This paper describes the flow of a homogeneous fluid contained in a rapidly rotating cylinder. The upper part of the cylinder rotates slightly faster, giving rise to a discontinuity in the sidewall velocity. The Stewartson-layer structure arising at the sidewall is essentially affected by this discontinuity. In contrast with previously studied problems, the E ¼ layer ( E is the Ekman number) is unable to perform the matching of the interior flow to the sidewall. It is shown that this matching is carried out partially by the E ¼ layer and partially by the E 1/3 layer, the latter accounting for the jump discontinuity. This paper also presents an analytical description of the flow in the singularity region near the sidewall discontinuity.

Influence Of Shear Velocity On Rock Joint Strength

Abstract: The influence of the rate of shear displacement on the frictional resistance of rock discontinuities was examined by testing jointed samples of three rock types in a dynamic direct shear machine. The influence of rock type, joint roughness, apparent area of contact, normal stress level and gouge infilling were investigated. The experiments consistently showed that the frictional resistance is dependent on the slip velocity. This rate-dependency was observed within a certain rate of shear velocities below and above which the frictional resistance was essentially independent of the rate of shear displacement. A model based on mechanical instabilities is proposed to explain the observed behaviour. The number of the covering abstract of the congress is TRIS no. 385148. (Author/TRRL)

Skin effect antennas

Abstract: A microwave device in which resonance at a first microwave frequency is determined by the dimensions of a discontinuity defined by conductive material, such as slot antennas, cavity-backed antennas and transmission line stubs. In one aspect of the invention, the thickness of the conductive material in a bounded portion of the conductive material adjacent the discontinuity is equal to the skin depth of the conductive material at the first microwave frequency to thereby effectively redimension the discontinuity at a lower second microwave frequency to cause the device to also be resonant at the second frequency. In a separate aspect of the invention, the thickness of the conductive material in a bounded portion of the conductive material adjacent the discontinuity varies continuously from the skin depth of the conductive material at the first frequency at the discontinuity to the skin depth of the conductive material at a lower second microwave frequency at the boundary of said portion to thereby effectively redimension the discontinuity in a frequency band extending between the first and second frequencies to cause the device to be resonant over such frequency band.