Showing papers on "Discontinuity (geotechnical engineering) published in 2009"

Advanced Geostructural Survey Methods Applied to Rock Mass Characterization

Abstract: The location and orientation of rock discontinuities, which are traditionally obtained from geological surveys with obvious drawbacks (safety, rock face accessibility, etc.), may also be derived from a detailed and accurate photogrammetric or laser scanning survey. Selecting from the point cloud determined on the rock face a set of points distributed on a particular discontinuity, location, dip, and dip direction can be computed from the least-squares estimate of the plane interpolating the set of points. Likewise, the normal vector to the surface may be computed from an interpolation or approximation of the surface by appropriate functions. To become a real alternative (both in terms of productivity as well as accuracy) to a traditional survey, interactive or automated software tools are necessary, to allow the efficient selection of the point sets on the discontinuities or the interpretation of the normal vector pattern. After introducing the two best technologies available today for data acquisition and their performance, the paper presents an approach, based on the random sample consensus (RANSAC) procedure, to the segmentation of the point cloud into subsets, each made of points measured on a discontinuity plane of the rock face. For each subset, the plane’s equations coefficients are first determined by robust estimation and then refined by least-squares estimation after outlier removal. The segmentation algorithm has been implemented in RockScan, a software tool developed to facilitate the interaction with the point cloud in the identification of the discontinuities; rather than using the three-dimensional (3D) data, selection of regions of interest is performed on oriented images of the rock face. Finally, application of RockScan to four different test sites is discussed and results presented. The sites differ in size (from tens to hundreds of meters), rock surface characteristics, and the technology used to produce the point cloud (in three cases photogrammetry, in the fourth laser scanning), giving the opportunity to test the methodology in different contexts. In the first and in the fourth site an extensive traditional survey has been performed, providing reference data to validate the RockScan results.

Numerical simulation of dynamic fracture using finite elements with embedded discontinuities

Abstract: This paper presents the extension of some finite elements with embedded strong discontinuities to the fully transient range with the focus on dynamic fracture. Cracks and shear bands are modeled in this setting as discontinuities of the displacement field, the so-called strong discontinuities, propagating through the continuum. These discontinuities are embedded into the finite elements through the proper enhancement of the discrete strain field of the element. General elements, like displacement or assumed strain based elements, can be considered in this framework, capturing sharply the kinematics of the discontinuity for all these cases. The local character of the enhancement (local in the sense of defined at the element level, independently for each element) allows the static condensation of the different local parameters considered in the definition of the displacement jumps. All these features lead to an efficient formulation for the modeling of fracture in solids, very easily incorporated in an existing general finite element code due to its modularity. We investigate in this paper the use of this finite element formulation for the special challenges that the dynamic range leads to. Specifically, we consider the modeling of failure mode transitions in ductile materials and crack branching in brittle solids. To illustrate the performance of the proposed formulation, we present a series of numerical simulations of these cases with detailed comparisons with experimental and other numerical results reported in the literature. We conclude that these finite element methods handle well these dynamic problems, still maintaining the aforementioned features of computational efficiency and modularity.

Dynamic Fracture of Shells Subjected to Impulsive Loads

Abstract: lation or in the vicinity of the crack tip 14. In related work, Armero and Ehrlich 15 used embedded discontinuity elements to model hinge lines in plates. The development of a fracture criterion that is computationally efficient and is easily applied in terms of available data poses a significant difficulty. Fracture criteria for quasibrittle materials, such as aluminum, are usually expressed in terms of the critical maximum principal tensile strain. However, in low order finite element models solved by explicit time integration, the maximum principal tensile strain tends to be quite noisy, so that crack paths computed by direct application of such a criterion tend to be erratic and do not conform to experimentally observed crack paths. Here, we propose a nonlocal form of a strain-based fracture criterion. The nonlocal form is obtained by a kernel-weighted average over a sector in front of the crack tip. In addition, we describe a combination of this kernel-weighted average with an angular component that can be used to indicate crack branching. The methodology is applied to the fracture of shell experiments performed by Chao and Shepherd 16. Although these experiments are very interesting, they do not provide enough experimental data for a validation of the methodology. Nevertheless, we show that the method is able to reproduce the change in failure mode that occurs for longer notches as compared with shorter notches and that the overall final configuration agrees reasonably well with that observed in the experiments.

Discontinuity structure of the mantle transition zone beneath the North China Craton from receiver function migration

Abstract: [1] A better understanding of the significant Phanerozoic tectonic reactivation and destruction of the North China Craton (NCC) demands a detailed knowledge of the deep structural features of the region. We applied the wave equation-based poststack migration technique to a combined receiver function data set from more than 250 broadband seismic stations to construct the structural image of the mantle transition zone beneath the NCC. Our imaging results reveal a relatively simple and flat 410-km discontinuity but a structurally complicated 660-km discontinuity beneath the region. Double discontinuities and a 30-km depression of the 660-km discontinuity are observed locally in the southern part of the eastern NCC, in contrast to the smoothly varying structure to the north and in the central and western parts of the craton. Distinctly rapid variations in both the 660-km discontinuity structure and mantle transition zone thickness were found across the northsouth gravity lineament (NSGL) near the boundary between the eastern and central NCC, which probably reflects different thermal and probably chemical properties on the two sides of the NSGL. These differences are possibly associated with the Pacific slab, which is imaged tomographically as a flat-lying structure in the mantle transition zone under the region east of the NSGL. The structural variation in the deep upper mantle appears to coincide with the sudden changes in surface topography, gravity field, and crustal and lithospheric structures as well, indicating that the two domains may have tectonically deformed differently throughout the whole upper mantle during the Phanerozoic cratonic destruction. The mantle transition zone on the eastern side of the NSGL is up to 30 40 km thicker than the global average; this thickness and the complex structure of the 660-km discontinuity in this region may reflect the strong influence that the deep subduction and stagnancy of the Pacific slab, and its possible sporadic penetration into the lower mantle, have had on mantle dynamics and lithospheric reactivation in the eastern NCC since the Mesozoic time. On the other hand, the less variable structure and normal-to-thin mantle transition zone imaged beneath the central and western NCC may indicate that the IndiaEurasia collision has had a relatively weak effect on the Cenozoic tectonics of these regions.

Mantle transition zone structure along a profile in the SW Pacific: thermal and compositional variations

Abstract: SUMMARY Events from the South Pacific, recorded in the Tien Shan region are studied with a migration method, to measure discontinuity depth along a profile between the Mariana and Molucca Sea subduction zones. Deflections of the upper-mantle discontinuities within and between the subduction zones are investigated using PP precursors filtered to periods of 3 to 10 s. We find P-wave reflections near the 410, 520 and 660 km discontinuities. The 410 km discontinuity is elevated in the subduction region and near the predicted depth (410 km) in the average velocity region. In some cases, we find negative polarities of the reflection from the 410 km discontinuity, which may indicate water-induced melting above the olivine to wadsleyite transition. The 520 km discontinuity is shallow between the two subduction zones, and deepens towards the Mariana backarc region: this discontinuity topography may be due to detection of both the wadsleyite to ringwoodite transition in a depleted downwelling (elevated region) and the formation of calcium-perovskite in a more fertile upwelling (depressed region). The 660 km discontinuity is split in the Halmahera slab with one reflection each near 600 and 700 km depth, consistent with a cold sinking slab and detection of phase transitions for both garnet and olivine to perovskite, respectively. In the region between the subduction areas, we find a downward deflection of the 660 km discontinuity with short length-scale (100– 300 km) undulations that may reflect compositional variation.

Localization of shear along a lithospheric strength discontinuity: Application of a continuous deformation model to the boundary between Tibet and the Tarim Basin

Abstract: [1] A marked contrast in strength (or viscosity) within a continuously deforming zone can lead to concentration of shear strain in the weaker material adjacent to the boundary between them, but localization comparable to the width of the Altyn Tagh shear zone requires an additional weakening process. During numerical experiments on a thin viscous sheet indented by a rigid object, a shear zone develops adjacent to a strong region mimicking the Tarim Basin, when the boundary between the weak and strong regions is oblique to the orientation of convergence. The width of this shear zone narrows with increased strain, and for comparable penetration by the indenter, the strain is more concentrated for larger values of n, the exponent that relates strain rate to a power of stress, and for smaller values of the Argand number Ar, a measure of buoyancy-induced stress relative to viscous stress. Increasing concentration of shear occurs as the indentation develops without weakening because of a change in material properties. Additional localization develops with the inclusion of strain-dependent weakening associated with, for instance, a temperature increase due to shear heating. For such localization to scale to the width of the Altyn Tagh fault zone of Tibet, the initial temperature near the Moho must be relatively low (∼600°C), and a large value of n (∼10) is required. This suggests that deformation there is described by a high-strength flow law, such as that proposed by Evans and Goetze (1979), in which the lithosphere would deform approximately plastically.

Distributed optical pressure and temperature sensors

Abstract: Disclosed herein is a carrier for an optical fiber having a plurality of optical sensors located thereon. The carrier has a test section comprising a cavity and at least one geometric discontinuity, wherein in response to a pressure applied to the test section, a stress concentration is formed proximate to the geometric discontinuity, and wherein the optical sensor is adhered to at least a part of the geometric discontinuity. The cavity may be filled with a liquid or a gel. A temperature optical sensor may also be provided adjacent to the pressure optical sensor.

A broad 660-km discontinuity beneath northeast China revealed by dense regional seismic networks in China

Abstract: [1] We examined the P wave velocity structure around the 660 km discontinuity at the tip of the subducting Pacific slab beneath northeastern China by forward modeling waveform triplication data. A total of 742 broadband seismograms were recorded by dense regional seismic networks in China from a deep earthquake that occurred near the border of east Russia and northeast China, providing an unprecedented density of ray coverage near the front edge of the subducting Pacific slab. Multiple P waves were observed on single seismograms in the distance range of ∼14°–29°. The P wave triplication shows the following two features: (1) the direct arrival traveling above the 660 km discontinuity (AB branch) extends as far as ∼29°, approximately 6° further than the prediction from velocity model of International Association of Seismology and the Physics of the earth's interior (iasp91); (2) the refracted wave propagating through the lower mantle (CD branch) appears at a distance a few degrees greater than that of the iasp91 synthetics. Forward waveform modeling suggests that they are best explained by a high-velocity transition zone underlain by a ∼50–70 km thick 660 km discontinuity. The broadened discontinuity is likely caused by multiple phase transitions associated with the dissolutions of olivine and garnet components.

Mantle anomalies beneath southern Africa: evidence from seismic S and P receiver functions

Abstract: SUMMARY To understand deep structure and processes beneath southern Africa, we apply the doublestacking version of the S receiver function (SRF) technique to the recordings of the South African Seismic Experiment. In this technique the wavefields of S and SKS are separated by space–time filtration, and the receiver functions are constructed separately for the S and SKS seismic phases. The results are consistent with those obtained from the SRFs of the permanent stations BOSA and LBTB. Evidence for a reduced S velocity atop the 410-km discontinuity is present in both SRFs and P receiver functions: the S velocity contrast at the 410-km discontinuity is ∼40 per cent larger than the norm, and there are observations of S350p and P350s seismic phases from the negative 350-km discontinuity beneath the Kaapvaal craton. The S350p and P350s phases display a dependence on the azimuth, which can be caused by anisotropy in the layer atop the 350-km discontinuity. This dependence is consistent with observations of shear wave splitting in SKS. There are observations of the S450p phase from the 450-km negative discontinuity in the transition zone. The most anomalous transition zone is found close to the region where the Kalahari craton was located in the Mesozoic. Lateral variations of the S velocity are found beneath the 660-km discontinuity. Teleseismic S and P traveltime residuals with respect to IASP91 model are evaluated from the traveltimes of the P410s and Pp410s seismic phases. In the uppermost mantle these residuals as well as S410p traveltimes require a reduced VP/VS ratio (around 1.75 ± 0.01 versus 1.8 in IASP91), an effect of the depletion of the mantle lithosphere in basaltic material. In the models obtained by a joint inversion of the receiver functions and the teleseismic traveltime residuals the low velocity zone (LVZ) with the onset of low velocity at a depth of ∼140 ± 20 km is present at most locations. The minimum S velocity in the LVZ is ∼4.5 km s −1 . The LVZ in our models is consistent with the S velocity, VP/VS velocity ratio and depth range of high-temperature lerzholites in the mantle xenoliths from southern Africa. Our most intriguing finding is a very low quality factor QS (on the order of a few tens) in the upper mantle. We interpret high attenuation, the LVZ and the low S velocity atop the 410-km discontinuity as the effects of plume-like phenomena in the upper mantle.

Dynamic motion of the bow shock and the magnetopause observed by THEMIS spacecraft

Abstract: [1] We present an observational study of the dynamic motion of the bow shock and the magnetopause and suggest that the dynamic motion of the bow shock is due to the interaction of an interplanetary shock with the Earth's bow shock. THEMIS B spacecraft crossed the magnetopause, a discontinuity and the bow shock successively in 5 min during its outbound journey on 10 July 2007. Following THEMIS B, THEMIS C, D, E and A consecutively crossed the magnetopause and the discontinuity but not the bow shock. Timing analysis shows that the magnetopause and the discontinuity were moving earthward with speeds of ∼47 km/s and ∼90 km/s, respectively. There is a trend that the discontinuity decelerates as it propagates toward the magnetopause. We suggest that the dynamic motion and the discontinuity are results of the interaction of a weak (MA = 1.4) interplanetary shock with the Earth's bow shock. After the interaction, the transmitted interplanetary shock took the form of a discontinuity where total magnetic field and density increase and the temperature decreases. The rotation of the magnetic field across this discontinuity was similar to that of the interplanetary shock. The expected fast shock ahead of the discontinuity for shock-shock interaction was not observed.

Kinematics of Overhanging Slopes in Discontinuous Rock

Abstract: The kinematics of overhanging rock slopes and the mechanical constraints associated with this specific slope geometry were studied. Investigation of the problem began with a generalized rigid body analysis and was followed by a numerical discontinuous deformation analysis, both of which were performed in two dimensions. It was found that eccentric loading and hence the development of tensile stresses at the base of overhanging rock slopes control their stability. Global slope instability, which is typically manifested in a forward rotation failure mode, may ensue if a through-going vertical discontinuity, typically referred to as “tension crack,” transects the slope at the back. The transition from stable to unstable configurations depends on the distance between the tension crack and the toe of the slope. On the basis of the analysis, a simple threefold stability classification—stable, conditionally stable, and unstable—is proposed. In addition, geometrical guidelines, based on standard field mapping dat...

The enigmatic San Gorgonio Pass

Abstract: The largest discontinuity along the San Andreas fault occurs in the San Gorgonio Pass (SGP) region of Southern California. Here, the San Bernardino and Coachella Valley strands (from the northwest and southeast, respectively) disaggregate into a family of irregular and discontinuous right-lateral,

Deformation and evolution of solar wind discontinuities through their interactions with the Earth's bow shock

Abstract: [1] The present study examines the interaction of solar wind discontinuities with the Earth's bow shock, using multipoint observations in the magnetosheath by Time History of Events and Macroscale Interactions During Substorms (THEMIS), Cluster, and Double Star TC1 We focus on the deformation and evolution of two discontinuities observed on 21 June 2007, one of which involves a density increase and a magnetic field decrease, while the other is accompanied by a density decrease and a magnetic field increase In the magnetosheath, the discontinuities are deformed into a concave shape; that is, the normal is inclined toward dusk (dawn) on the dawnside (duskside) The density-increase (-decrease) discontinuity is being compressed (expanded) as it propagates in the magnetosheath We conclude that the compression (expansion) is due to antisunward (sunward) motion of the bow shock which is initiated or enhanced by the impact of the discontinuity on the bow shock The steepening of Bz reversal followed by an overshoot of the total magnetic field, which appears at the trailing edge of the density-decrease discontinuity, is also discussed

Stressed state of pipeline in zones of soil local fracture

Abstract: A deformation model of underground pipeline under complicated geotechnical conditions of soil local fracture is proposed. The effect of displacement discontinuities and rotations of foundation blocks on the stressed state and limit equilibrium of pipe has been studied.

Towards the treatment of boundary conditions for global crack path tracking in three-dimensional brittle fracture

Abstract: The aim of the present paper is a systematic elaboration of a three dimensional finite element analysis tool for discontinuous fracture in brittle solids. Brittle or quasi-brittle fracture usually occurs when a material reaches the limit of its strength and no plastic deformation has been observed prior to failure. In the present approach, this kind of failure is characterized by three sets of governing equations: (i) the elastic bulk problem and (ii) the cohesive interface problem regarding the solid deformation field and (iii) the crack tracking problem concerning the crack kinematics. This manuscript describes a unique modular tool set for the coupled set of nonlinear equations. We focus in particular on the boundary conditions for the crack tracking problem of this analysis tool. We critically discuss important implementation details: (i) the choice of crack onset boundary conditions for the additional global field, (ii) the numerical integration, (iii) the modeling of geometrically exact crack tips for cohesive fracture, and (iv) the post-processing procedure for the discontinuity visualization. The potential of the method to simulate brittle fracture is demonstrated by qualitative and quantitative comparisons with experiments from the literature as well as by common benchmark problems.