Showing papers on "Discontinuity (geotechnical engineering) published in 1986"

Fluid interaction and element mobility in the development of ultramylonites

Abstract: Shear zones are often characterized by the presence of mylonitic fabrics. The textural development of such fabrics is enhanced by the presence of a fluid phase. In a single specimen of rock from the Brevard fault zone in North Carolina, we can demonstrate the development of ultramylonite domain by focused fluid flow. The ultramylonite interface retains a sharp textural and chemical discontinuity; this suggests that solute transport was dominantly parallel to the tectonic layering. Major chemical changes between the ultramylonite zone and the protolith include losses of Si(>2, Na 2 0, and K 2 0 and gain of CaO, FeO, and HjO in the high-strain domain. INTRODUCTION During the tectono-thermal evolution of mountain belts, zones of high strain are often recorded by the development of mylonites. Although mylonites and subcategories within mylonites (Sibson, 1977) can be generated by numerous well-documented physical processes, their chemi-cal and isotopic evolution is much more complex. Mechanical processes often associated with the development of high-strain zones include micro-fracturing, dilatancy, grain-boundary sliding, and dislocation glide. These properties are strongly dependent on the modal mineralogy (bulk compo-sition), heterogeneity of the rock, and the temperature and pressure con-ditions during deformation. Such variations in the mechanics of deforma-tion have led to the more common classification of shear zones as either ductile or brittle (Sibson, 1977; Ramsay, 1980). However, most shear zones are characterized by the availability of fluids that cause a drastic change in the style of deformation by altering the behavior and assem-blage of minerals (hydration reactions) and the rates of chemical and mechanical processes during progressive deformation. The presence of a fluid phase during deformation enhances solution transport, microfractur-ing, and recrystallization Suc.h chemically active zones generate mineral assemblages that record the metamorphic history of the mylonites, al-though it is not unusual to find minerals that persist through such an event. These relict minerals, if recognized, can identify equilibrium vs. nonequilibrium assemblages observed in shear zones. Clearly, the pres-ence of relict minerals would have a substantial impact on the trace-element and isotopic signature of the rock. To more carefully document the behavior of elements in a strain zone that has abundant fluid, we have chosen to study a transect across the Brevard fault zone, near Rosman, North Carolina. Within this tran-sect, one particular outcrop provided a single large specimen that over a distance of-25 cm showed the entire strain-gradient fabric from proto-mylonite to ultramylonite (see Sibson, 1977, for mylonite series) ob-served by us as discontinuous patches in the longer (15-km) transect. We report petrographic, textural, and chemical data from this block and pro-vide a physical model to explain these data.

Determination of Shear Failure Envelope in Rock Masses

Abstract: In this work, an analytic expression has been developed that enables the shear strength to be determined using the empirical failure criterion developed by Hoek and Brown. Applying this new hypothesis, along with elasticity theory and basic mathematical concepts for obtaining the failure envelope of a family of failure circles, the shear strength has been determined to be such that τα=f(σα,σc,m,s), where σα=the normal stress on the discontinuity, σc the uniaxial compressive strength of the intact rock and the parameters m and s are constants dependent upon the properties of the rock. An expression of this type will help us develop new methods of analysis of slope stability and support of galleries, design of anchor support systems in rock masses, estimation of the skin friction of poor quality rock in driven pile foundations, as well as in other innumerable related areas of geotechnology.

The 400-km seismic discontinuity and the proportion of olivine in the Earth's upper mantle

Abstract: The 400-km seismic discontinuity has traditionally been ascribed to the isochemical transformation of α-olivine to the β-modified-spinel structure in a mantle of peridotitic bulk composition1–6. It has recently been proposed7,8 that the observed seismic velocity increase at 400km depth is too abrupt and too small to result from a phase change in olivine but instead requires that the transition zone be chemically distinct in bulk composition from the uppermost mantle. By requiring phase relations in the Mg2SiO4-Fe2SiO4 system to be internally consistent thermodynamically, we find that the α–β transition in olivine of mantle (Mg0.9Fe0.1)2SiO4 composition is extremely sharp, occurring over a depth interval (isothermal) of ∼6 km. The magnitude of the predicted velocity increase is in agreement with that observed seismically9,10 if the transition zone is composed of ∼60–70% olivine. Thus, our results indicate that seismic velocities across the 400-km discontinuity are consistent with a transition zone of homogeneous peridotitic composition and do not require chemical stratification.

Frictional convergence at coastlines

Abstract: The coastline generally represents a marked discontinuity in surface roughness. The resulting mechanical forcing leads to a secondary circulation in the boundary layer, and consequently to a vertical motion field that may have a strong influence on the weather in the coastal zone. In potentially unstable air masses, frictional convergence may cause a more-or-less stationary zone of heavy shower activity, for example. In this paper, we present a calculation of secondary flow patterns forced at a roughness discontinuity, as a function of the geostrophic wind. A neutral boundary layer is studied, with homogeneous conditions along the coastline (i.e., we study the circulation in a plane perpendicular to the coastline). The closure scheme for the turbulent mixing of momentum is based on a calculation of the eddy kinetic energy density (the I,E closure). The equations are solved on a grid stretched in both the vertical and horizontal directions, with largest resolution at the roughness discontinuity. Various relaxation techniques are combined to give an efficient calculation of the stationary flow fields. Upward motion turns out to be most pronounced when the geostrophic wind makes a small (˜20“) angle with the coastline (in a clockwise direction), and not when the geostrophic wind is perpendicular to the coastline, as is sometimes mentioned. The asymmetry relative to the normal to the coastline is due to the Coriolis acceleration, and not a nonlinear effect Nevertheless, nonlinear effects are important, because they create a tendency to frontogenesis in the case mentioned above. This is shown by a comparison of the linear and nonlinear solution. We finally present an example of heavy shower activity in the coastal zone of Belgium and The Netherlands, which is apparently due to frictional uplift and frontogenesis in a maritime polar air mass hitting the coastline at the critical angle referred to above. DOI: 10.1111/j.1600-0870.1986.tb00473.x

EVIDENCE OF A LOWER MANTLE SHEAR VELOCITY DISCONTINUITY IN S AND sS PHASES

Abstract: Teleseismic recordings of direct S and sS body wave phases, and their core-reflected counterparts ScS and sScS, from intermediate and deep focus earthquakes are used to analyze the lowermost mantle shear velocity structure beneath Alaska. A model with a 2.75 % shear velocity increase 280 km above the core-mantle boundary accurately matches waveform complexi- ties in both the S and sS wavetrains. Variations in source depth produce systematic shifts in the timing of the triplication arrivals between the S and sS travel time branches that are readily ob- served in long-period WWSSN tangential component recordings. The systematic range- and depth-dependence of the observed shifts are well-predicted by the discontinuity model, and preclude explanations of the waveform complexity as resulting from multi- ple ruptures at the source, receiver reverberations, or near-source scattering from slab structure. Accurate models of the seismic velocity structure in the lower- most 200 km of the mantle (D" region) are essential for calcula- tions of the thermal history and dynamics of the core and mantle. Several recent investigations of seismic body waves have indi- cated the presence of velocity stratification above the core-mantle boundary (Lay and Helmberger, 1983; Wright et al., 1985), while models of long-wavelength lateral velocity variations in the lower mantle indicate strong heterogeneities in the D" region (Dziewon- ski, 1984; Clayton and Comer, 1983). Current thermal models indicate a large temperature contrast of about 1000 o between the core and mantle, requiring a thermal boundary layer at the base of the mantle (Jeanloz and Richter, 1979; Loper, 1984). This is consistent with the general decrease in velocity gradients observed in the lowermost mantle (Doornbos, 1983; Doornbos et al., 1986). The combined effects of stratification, lateral hetero- geneity, and decreased velocity gradients complicate interpreta- tion of seismic data sampling the D" region. It is particularly important to resolve whether velocity discontinuities are actually present within D", since compositional stratification would strongly affect the thermal boundary layer dynamics, as well as the interpretation of the lateral velocity variations. This paper presents an analysis of a combination of seismic body waves that is particularly sensitive to the presence of a velocity discontinuity in D". Lay and Helmberger (1983) found evidence for a 2.75 % shear velocity discontinuity about 280 km above the core in S waves from intermediate and deep focus earthquakes. The long-period tangential component seismograms they analyzed show wave- form complexities in the distance range 75 o to 95 o that were attributed to a triplication produced by an abrupt velocity in- crease. This interpretation was criticized by Schlittenhardt et al. (1985), who argue that such a discontinuity is incompatible with

Influence of the donor depth on the determination of the band discontinuity of isotype heterojunctions by the capacitance‐voltage technique

Abstract: Capacitance‐voltage measurements have been simulated to find the conduction‐band discontinuity in an n‐type GaAs‐Al0.3Ga0.7As heterojunction. In the calculations the partial ionization of the donor level in the (Al,Ga)As is taken into account. It is found that exact knowledge of the donor depth is a prerequisite in order to infer the conduction‐band discontinuity from capacitance‐voltage measurements. For a dopant concentration of 3×1016 cm−3 on either side of the heterojunction the same apparent carrier profile is obtained with a conduction‐band discontinuity equaling 65% of the total band‐gap difference and zero donor depth as with 75% discontinuity and 100 meV donor depth. It is also shown that compensation influences the apparent carrier profile. Taking literature values for the donor depth in Al0.3Ga0.7As the discontinuity of the conduction band is estimated to be 70% of the total band‐gap difference.

First analyses of broadband records on the Geoscope Network: Potential for detailed studies of mantle discontinuities

Abstract: Examples of the first analyses of broadband GEOSCOPE data at station SSB (France) show that these data will make possible a detailed study of the deep mantle discontinuities. Analysis of the P-coda of earthquakes using specific methods suggests the existence of P to S conversions at 670 km depth beneath SSB for earthquakes with northern to eastern azimuths; a comparison with results from the NARS and Grafenberg arrays indicate that the 670 km discontinuity beneath western Europe is highly heterogeneous or horizontally discontinuous. A frequency analysis reveals that the relevant discontinuity is not very sharp ( ∽ 20 to 70 km thick). Besides, a sharp ( < 5 km thick) discontinuity at 900 km depth is observed beneath the Mid-Atlantic Ridge from P reflections, giving PP precursors, for the strong March 1985 Chilean earthquake, but no 670 km discontinuity seems to be present beneath the Ridge.

Stem waves along a depth discontinuity

Abstract: The nonlinear Schrodinger equation derived by Liu and Tsay (1984) is used to investigate the forward scattering of the second-order Stokes waves by a depth discontinuity. It is shown that, due to nonlinearity, stem waves develop along a line caustic. The structure of the present stem waves is similar to that found by Yue and Mei (1980) along the wall of a thin wedge.

A transient three-dimensional analysis of non-uniform dislocation distribution growth by climb and glide over non-planar surfaces

Abstract: As a basis for obtaining insight into both plastic flow described in terms of dislocation motion and dynamic crack extension, a transient 3D analysis of the non-uniform growth of dislocation distributions by climb and glide over largely arbitrary non-planar surfaces is considered. An exact solution for the case of an unbounded, isotropic, homogeneous, linearly elastic solid is obtained in vector form. It is found that information about essential distribution and surface properties are contained in the solution in a symmetric tensor. This tensor arises as a generalized consequence of the body-force equivalent representation of dislocations in elastic continua. The solution is also found to have two components: one component depends on the velocity discontinuity induced across the surface, the other depends on the displacement discontinuity at the moving boundary of the distribution and the speed of the boundary. Two examples are then considered to illustrate the utility of the solution.

Correlation of preliminary and explosion-induced cracking with the comminution characteristics of brittle rock

Abstract: A scheme is proposed for reducing the global grain size composition of fragments in the sequential explosion of a series of charges in a real rock mass to the preliminary fracturing and the local fracturing induced by each individual charge in the given cracked rock; within the framework of the approach here developed (relating the fragment formation and the fracturing), definitions are given for the concept of a comminution zone in an isotropic monolith and its mean radius and also for the explosive comminution zone in rock with isotropic fracturing and its mean radius. It is shown how the correlation between fragment formation and fracturing explains the existence of the comminution discontinuity in a real rock mass.

Investigation of Wave Focusing over a Parabolic Step

Abstract: We report here upon an exploratory investigation of wave focusing over a bottom step which has a parabolic shape in the horizontal plane. Although building of a large-scale reflector can be a very costly project, the idea of using a parabolic reflector--either in the form of a wall or a step discontinuity at the bottom-- will be presented along with small-scale experimental results to test the feasibility of such systems. Some theoretical considerations for analytical description will be discussed.

Method and sensor for torque measurement

Abstract: The invention describes a contactless, inductively coupled leakage flux torque sensor and a method for measuring the torque by determining the change in the leakage flux at a point of discontinuity on the surface as a result of a twisting strain. Two identical, mirror-image points of discontinuity are formed on a ferromagnetic part. The points of discontinuity can occur in groups of slots or ferromagnetic projections on the surface of the end face of the ferromagnetic part. As a result of twisting strain on the part, the points of discontinuity change their shape or size or are displaced, thus causing a change in the leakage flux. The change in the leakage flux is detected by two coils connected in a differential arrangement. A modulator/demodulator circuit guarantees the determination of the torque as a function of the changed flux.

Proposal of a New Crack Model Considering the Discontinuity in the Cracked Plane and Its Application to the Evaluation of Crack Parameter

Abstract: A new crack model which enables us to evaluate crack parameters such as crack energy density and its distribution, COD and COA under arbitrary load history is proposed and the availability of the proposed model is demonstrated through finite element analyses of elasto-plastic crack. The contents are as follows ; (1) A crack model considering the discontinuity in the cracked plane is introduced and the constitutive equation for a plane with discontinuity is formulated on as elasto-plastic case. (2) The finite element formulation of the model is carried out by introducing a plane element. (3) An elasto-plastic crack expressed by the proposed model under monotonic loading is analyzed by finite element method and the availability of the model is verified through the evaluation of crack energy density.

Influence of step discontinuity in porous plate temperature in free convection with transpiration

Abstract: A simple finite difference scheme is used to solve the second order, simultaneous, non-linear partial differential equations with a nonhomogeneous boundary condition, to study the influence of a step discontinuity in the surface temperature on heat transfer from a vertical porous plate in free convection with uniform transpiration The numerical procedure is described in detail and results are presented forPr =072 in the blowing parameter range, - 15 < ©1< 15, for non-dimensional temperature range of 025 to 20

The non-uniform motion of arbitrary dislocation distributions by climb and by glide along non-planar paths

Abstract: A s a basis for obtaining insight into both plastic flow in terms of dislocation motion and dynamic crack extension, the general problem of non-uniform motion of largely arbitrary dislocation distributions by climb and by glide along non-planar paths is considered. An exact solution is found in two forms: one form, vectorial in nature, shows that the essential distribution and path properties are contained in a symmetric tensor. The other form, consisting of complex functions, shows that the solution involves the inner product of the displacement discontinuity vector and complex vectors whose components normal and tangential to the path contour are related through tangent angle derivatives. Both forms illustrate that the solution has two components, one arising from the velocity discontinuity along the contour, the other arising from the displacement discontinuity at its edge and the edge speed.

Investigation of Numerical Model for Crack Initiation in Filament-Wound Composites under Lateral Compressive Loading

Abstract: The structure of filament-wound composite is similar to that of unidirectional reinforcement materials. Therefore, two unique types of fracture appear in filament-wound composite under lateral load as in the case of unidirectional reinforcement materials. One of them is microscopic fracture which leads to a decrease in stiffness of filament-wound composite, and the other is macroscopic fracture due to interlamina delamination which causes large-scale dropping of lateral compressive load. Previously, the microscopic fracture had been investigated by using the elastic-plastic analysis.In this paper, the numerical model was improved in order to investigate the behaviours of both microscopic and macroscopic fracture.It was found that the numerical model considering the discontinuity of displacement at the node points and the Tresca's equivalent stress as the fracture criterion was in good agreement with the experiment results.

Some restrictions on lehigh-purdue concrete plasticity model

Abstract: For the Lehigh-Purdue concrete plasticity model, there seems to be certain limitations on the choice of material properties to ensure the convexity of the limiting surfaces describing the model. A set of inequalities is derived, the satisfaction of which ensures that the initial discontinuous, subsequent loading and failure surfaces have the desired shape. An example is given to show that the model fails for a feasible choice of material parameters not satisfying the aforementioned inequalities.