Showing papers on "Discontinuity (geotechnical engineering) published in 2014"

Changes in Seismic Anisotropy Shed Light on the Nature of the Gutenberg Discontinuity

Abstract: The boundary between the lithosphere and asthenosphere is associated with a plate-wide high seismic velocity “lid” overlying lowered velocities, consistent with thermal models. Seismic body waves also intermittently detect a sharp velocity reduction at similar depths, the Gutenberg (G) discontinuity, which cannot be explained by temperature alone. We compared an anisotropic tomography model with detections of the G to evaluate their context and relation to the lithosphere-asthenosphere boundary (LAB). We find that the G is primarily associated with vertical changes in azimuthal anisotropy and lies above a thermally controlled LAB, implying the two are not equivalent interfaces. The origin of the G is a result of frozen-in lithospheric structures, regional compositional variations of the mantle, or dynamically perturbed LAB.

Discrete modeling of rock joints with a smooth-joint contact model

Abstract: Structural defects such as joints or faults are inherent to almost any rock mass. In many situations those defects have a major impact on slope stability as they can control the possible failure mechanisms. Having a good estimate of their strength then becomes crucial. The roughness of a structure is a major contributor to its strength through two different aspects, i.e. the morphology of the surface (or the shape) and the strength of the asperities (related to the strength of the rock). In the current state of practice, roughness is assessed through idealized descriptions (Patton strength criterion) or through empirical parameters (Barton JRC). In both cases, the multi-dimensionality of the roughness is ignored. In this study, we propose to take advantage of the latest developments in numerical techniques. With 3D photogrammetry and/or laser mapping, practitioners have access to the real morphology of an exposed structure. The derived triangulated surface was introduced into the DEM (discrete element method) code PFC3D to create a synthetic rock joint. The interaction between particles on either side of the discontinuity was described by a smooth-joint model (SJM), hence suppressing the artificial roughness introduced by the particle discretization. Shear tests were then performed on the synthetic rock joint. A good correspondence between strengths predicted by the model and strengths derived from well-established techniques was obtained for the first time. Amongst the benefits of the methodology is the possibility offered by the model to be used in a quantitative way for shear strength estimates, to reproduce the progressive degradation of the asperities upon shearing and to analyze structures of different scales without introducing any empirical relation.

Midcrustal discontinuities and the assembly of the Himalayan midcrust

Abstract: Detailed quartz lattice preferred orientation (LPO) data define two structural discontinuities in the exhumed high-grade metamorphic core of the Himalaya exposed in the upper Tama Kosi region of east central Nepal The structures are marked by abrupt breaks in a general trend of up structural section increasing quartz LPO-defined deformation temperatures Deformation associated with the upper structural discontinuity, which occurs within sillimanite grade rocks, is postpeak metamorphism in both the hanging wall and the footwall New geochronologic data constrain the timing of metamorphism in the hanging wall of the upper discontinuity to between 24 and 16 Ma, indistinguishable from previously published ages for the footwall Movement across this structure represents Early Miocene strain localization and thickening in the Himalayan midcrust Movement across the lower discontinuity, which occurs between staurolite and kyanite grade rocks, appears to be synmetamorphic with material in its footwall at approximately 10 Ma, but postpeak metamorphism for material in its hanging wall This movement is interpreted to reflect the underplating and incorporation of material into the metamorphic core The recognition of two thrust-sense discontinuities in the exhumed Himalayan core in the Tama Kosi region is consistent with other similar structures recognized along the Himalaya The widespread nature of these structures reinforces that they are important to our understanding of the evolution of the kinematics of large, hot orogens

Modeling progressive failures in rock slopes with non‐persistent joints using the numerical manifold method

Abstract: Rock slope failure is a complex process that usually involves both opening/sliding along pre-existing discontinuities as well as fracturing of intact rock bridges. Discontinuity persistence is an important factor governing rock slope instabilities. However, traditional slope failure analysis assumes persistent discontinuities, and rock slope fails along a predefined persistent continuous potential failure surface because of the limitations of the analysis tools. This paper proposes the numerical manifold method (NMM) incorporated with a Mohr–Coulomb criterion-based fracturing algorithm to simulate the progressive failure of rock slopes with non-persistent joints. Detailed fracturing algorithm is first presented. Then, the NMM enabling fracturing is calibrated through simulating an edge-cracked plate and the Brazilian test. Lastly, the developed code is applied to investigate the failure process of rock slopes involving non-persistent joints. Numerical results indicate that the proposed method can capture the opening/sliding along existing discontinuities, the fracturing in intact rock bridges and the final kinematic release. Progressive slope failure is well exhibited. Copyright © 2013 John Wiley & Sons, Ltd.

Shear banding of colloidal glasses: observation of a dynamic first-order transition.

Abstract: We demonstrate that application of an increasing shear field on a glass leads to an intriguing dynamic first-order transition in analogy with equilibrium transitions. By following the particle dynamics as a function of the driving field in a colloidal glass, we identify a critical shear rate upon which the diffusion time scale of the glass exhibits a sudden discontinuity. Using a new dynamic order parameter, we show that this discontinuity is analogous to a first-order transition, in which the applied stress acts as the conjugate field on the system’s dynamic evolution. These results offer new perspectives to comprehend the generic shear-banding instability of a wide range of amorphous materials.

Numerical experiments of fracture-induced velocity and attenuation anisotropy

Abstract: SUMMARY FracturesarecommonintheEarth’scrustduetodifferentfactors,forinstance,tectonicstresses and natural or artificial hydraulic fracturing caused by a pressurized fluid. A dense set of fractures behaves as an effective long-wavelength anisotropic medium, leading to azimuthally varying velocity and attenuation of seismic waves. Effective in this case means that the predominant wavelength is much longer than the fracture spacing. Here, fractures are represented by surface discontinuities in the displacement u and particle velocity v as [κ ·u + η ·v], where the brackets denote the discontinuity across the surface, κ is a fracture stiffness and η is a fracture viscosity. We consider an isotropic background medium, where a set of fractures are embedded. There exists an analytical solution—with five stiffness components—for equispaced plane fractures and an homogeneous background medium. The theory predicts that the equivalent medium is transversely isotropic and viscoelastic. We then perform harmonic numerical experiments to compute the stiffness components as a function of frequency, by using a Galerkin finite-element procedure, and obtain the complex velocities of the medium as a function of frequency and propagation direction, which provide the phase velocities, energy velocities (wavefronts) and quality factors. The algorithm is tested with the analytical solution and then used to obtain the stiffness components for general heterogeneous cases, where fractal variations of the fracture compliances and background stiffnesses are considered.

The effect of surface roughness and mixed-mode loading on the stiffness ratio κx/κz for fractures

Abstract: The characterization of fractures using elastic waves requires a parameter that captures the physical properties of a fracture. Many theoretical and numerical approaches for wave propagation in fractured media use normal and shear fracture specific stiffness to represent the complexity of fracture topology as it deforms under stress. Most effective medium approaches assume that the normal and shear fracture specific stiffness are equal, yielding a shear-to-normal specific stiffness ratio of one. Yet several experimental studies show that this ratio can vary from zero to three. We conducted a series of experiments to determine the stiffness ratio for fractures with different surface roughness subjected to mixed-mode loading conditions. Specimens containing a single fracture were subjected to either normal loading or combined normal and shear loading during ultrasonic measurements of transmitted and reflected P- and S-waves. Theoretical analysis based on the displacement discontinuity theory shows, ...

Analogue modeling of positive inversion tectonics along differently oriented pre-thrusting normal faults: An application to the Central-Northern Apennines of Italy

Abstract: Inversion tectonics represent a key process in many orogens worldwide. The related mechanisms of fault reactivation and the effects of an articulated preshortening setting on thrust and fold development are still challenging questions. Modes and geometries of inversion have been the object of several analogue models. In this work, we analyzed the influence of an articulated high-angle preexisting discontinuity in the development of thrusts using sandbox modeling. The model geometry is based on the architecture of the major faults in the Central-Northern Apennines of Italy, where differently oriented Mesozoic–Cenozoic inherited extensional structures are clearly detectable and display contrasting styles of positive inversion tectonics. Quartz-sand is the analogue material adopted to model Mesozoic–Cenozoic sedimentary successions, and glass microbeads represent preexisting fault rocks. The geometry of the segmented preexisting structure is composed of two segments with the same dip (∼60°): one oblique and another orthogonal to the shortening direction. Our results show that different styles of positive inversion tectonics can coexist and that the obliquity angle between inherited structures and the shortening direction is a leading factor controlling the degree of inversion: The oblique segment of the discontinuity exhibits a complete reactivation, whereas along the orthogonal segment, shortcut is the prevalent mechanism. The oblique element, moreover, represents a cross-strike discontinuity that guides the localization and curved geometry of the thrusts, compartmentalizing the deformation. Our findings can be applied to fold-and-thrust belts characterized by the presence of cross-strike discontinuities.

Semiautomated fault interpretation based on seismic attributes

Abstract: Three-dimensional fault interpretation is a time-consuming and tedious task. Huge efforts have been invested in attempts to accelerate this procedure. We present a novel workflow to perform semiautomated fault illumination that uses a discontinuity attribute as input and provides labeled fault surfaces as output. The procedure is modeled after a biometric algorithm to recognize capillary vein patterns in human fingers. First, a coherence or discontinuity volume is converted to binary form indicating possible fault locations. This binary volume is then skeletonized to produce a suite of fault sticks. Finally, the fault sticks are grouped to construct fault surfaces using a classic triangulation method. The processing in the first two steps is applied time slice by time slice, thereby minimizing the influence of staircase artifacts seen in discontinuity volumes. We illustrate this technique by applying it to a seismic volume acquired over the Netherlands Sector of the North Sea Basin and find that the proposed strategy can produce highly precise fault surfaces.

Non-destructive evaluation of cracks in massive concrete using normal dc resistivity logging

Abstract: Discontinuities are one of the most harmful damage to the durability of concrete structures. Currents approaches are limited to assessments of surface damages, even if non-destructive methods appear to be effective for the detection and the location of cracks. That is why, these methods might be unsuitable for investigation of massive concrete body as dams and locks. Present works aim at the presentation of new results with electrical resistivity to study damages within concrete structures. In this article, the electrical resistivity focused on the study of cracks and discontinuities (concrete joints, interfaces…) in massive concrete structures by measurements done in preexisting boreholes. The used array is the normal dc resistivity logging. The study presents numerical and experimental results. First, modeling allows correcting experimental data. Then, development shows the sensitivity relatively to the discontinuity characteristics (aperture and the resistivity contrast between the discontinuity and the concrete). The tests on a real structure are carried out to define the methodology for on-site measurement. Results support the modeling.

A multiscale framework for localizing microstructures towards the onset of macroscopic discontinuity

Abstract: This paper presents a multiscale computational homogenization model for the post localization behavior of a macroscale domain crossed by a cohesive discontinuity emanating from microstructural damage. The stress---strain and the cohesive macroscopic responses are obtained incorporating the underlying microstructure, in which the damage evolution results in the formation of a strain localization band. The macro structural kinematics entails a discontinuous displacement field and a non-uniform deformation field across the discontinuity. Novel scale transitions are formulated to provide a consistent coupling to the continuous microscale kinematics. From the solution of the micromechanical boundary value problem, the macroscale stress responses at both sides of the discontinuity are recovered, providing automatically the cohesive tractions at the interface. The effective displacement jump and deformation field discontinuity are derived from the same microscale analysis. This contribution focusses on scale transition relations and on the solution procedure at the microlevel; the highlights of the approach are demonstrated on microscale numerical examples. Coupled two-scale solution strategy will be presented in a subsequent paper.

A ubiquitous low‐velocity layer at the base of the mantle transition zone

Abstract: Global stacks of receiver functions clearly exhibit the upper mantle stratification. Besides the most prominent seismic discontinuities, such as the Moho and the 410 and 660 km discontinuities, a negative discontinuity is detected at a depth of ~600 km, indicating a low-velocity layer at the base of the mantle transition zone. The slant-slack technique helps to identify the primary conversions from the multiple reverberations. Presence of the negative 600 km discontinuity underneath both continent and ocean island stations, where the crustal thickness significantly differs, also precludes the possible cause of crustal reverberations. We conclude that the negative 600 km discontinuity could be a global feature, possibly resulted from accumulation of ancient subducted oceanic crust. The X-discontinuity at ~300 km depth is also observed in our global stacks, which can be explained by the coesite-stishovite phase transformation.

The effects of discontinuity surface roughness on the shear strength of weathered granite joints

Abstract: Surface roughness is one of the most important parameters governing the shear strength of rock discontinuities. Roughness types may vary based on genesis, physico-mechanical, and mineralogical properties of rocks. In this study, granite samples representing three different weathering degrees were selected to evaluate the effects of surface roughness and weathering degree on shear strength. To this aim, we determined the profile roughness coefficient (PRC), profile roughness angle (PRA), and joint roughness coefficient (JRC) for the selected fresh and weathered granite joint samples. Values of PRC were in the range of about 1.043–1.073, and PRA and JRC varied in the ranges of 16.67–21.45 and 12–18, respectively. Weathering led to the increment of joint surface roughness of the selected granitic joints due to the higher resistance of quartz crystals in the weathered matrix. However, the increment in surface roughness did not result in an increase in the shear strength. On the contrary, the shear strength of discontinuities dramatically decreased.

Numerical simulation of crack propagation in layered formations

Abstract: In the present study, a boundary element method based on the higher order displacement discontinuity formulation is presented to solve the general problem of hydraulic fracture propagation in layered formations. Displacement collocation technique is employed to model the higher order displacement variation along the crack and the special crack tip element near its ends. The hydraulic fracture propagation and its interaction with the layer interface in non-homogenous rock materials are studied by the proposed semi-analytical (hybridized boundary element-boundary collocation) method. The maximum tangential stress criterion (or σ-criterion) of fracture mechanics considering different elastic constants (Young modulus and Poisson’s ratio) is used to obtain the fracture path. The fracture propagation from stiff to soft and soft to stiff media for cracks having different inclination angles is modeled, and the effects of elastic constants on the hydraulic fracture propagation is studied. The results show that if the hydraulic fracture originates in the stiffer layer, its capability to cross the layer increases and is vice versa for the softer material. The comparison of the results gained from the numerical method with those in the literature show a good performance of the method in the case of propagation of hydraulic fracture in layered formations.

A review of equivalent pre-crack sizes in aluminium alloy 7050-T7451

Abstract: This paper reviews some analyses of quantitative fractography measurements of the fatigue fracture surfaces of 7050 aluminium alloy specimens along with relevant fatigue crack information including crack initiating discontinuity size and type. These data were used to assess whether surface finish or applied stress level has any effect on the estimated effective crack initiating discontinuity size, namely the equivalent pre-crack size (EPS). The statistical distributions for the EPSs of the following initiating discontinuity types were examined: chemically etched pits, glass bead peening damage, mechanical damage, inclusions and porosity. The EPSs at various percentile levels for these types were determined on the basis of the samples considered. Finally, the correlation between measured initiating discontinuity depth and EPS was investigated, and good correlation was found in the case of mechanical damage. The purpose of conducting these analyses was to gain a better understanding of the parameters governing the fatigue crack-like effect of discontinuities to facilitate the better prediction of fatigue lives.

Investigating the Evolution of Rock Discontinuity Asperity Degradation and Void Space Morphology under Direct Shear

Abstract: Rock mass discontinuities represent planes of relative weakness and enhanced hydraulic conductivity and, thus, have a substantial influence on the hydro-mechanical behaviour of the overall rock mass. While the shearing of rock mass discontinuities has been extensively studied in the past, there remains uncertainty surrounding the mechanisms by which surface asperities deform and degrade during shear and how this degradation influences the aperture distribution. Although prior studies have attempted to investigate asperity failure mechanisms, they have been hampered by the lack of appropriate visualization and modelling tools. In particular, until recently it was not possible to observe asperity damage without physically separating the joint specimen or explicitly modelling the development of damage during a direct shear test. Over the last decade, micro X-ray Computed Tomography (μCT) has emerged as an ideal tool to nondestructively characterize fractures and damage in geomaterials. Over this same period, hybrid continuum/discontiuum modelling techniques, capable of explicitly modelling fracture and fragmentation have been developed and applied to rock mechanics problems. However, to date, there has been limited application of these technologies to the study of rock discontinuities subjected to shearing. The overall goal set forth in this thesis was to combine the use of these two technologies to develop an improved understanding and confirm empirical assumptions regarding the evolution of asperity degradation and fracture geometry as a result of shearing. The adopted experimental approach involved creating a series of replicated discontinuity specimens that were then subjected to varying shear displacements under different normal loading conditions. Subsequently, μCT imagery of the specimens was acquired and an image processing and analysis procedure was developed to quantitatively evaluate changes in asperity damage and fracture geometry as a function of shear displacement and applied normal load. Through the use of numerical modelling based on the hybrid Finite-Discrete element method (FDEM), the experimentally observed shearing process was then recreated numerically to glean further insight into the shearing process and the different failure mechanisms involved. Ultimately, this work has led to an improved understanding discontinuity morphology related to shearing, the failure mechanics of individual asperities, and the limitations of using a 2D FDEM approach to model discontinuity shearing.

Mantle transition zone structure beneath India and Western China from migration of PP and SS precursors

Abstract: We investigate the seismic structure of the upper-mantle and mantle transition zone beneath India and Western China using PP and SS underside reflections off seismic discontinuities, which arrive as precursors to the PP and SS arrival. We use high-resolution array seismic techniques to identify precursory energy and to map lateral variations of discontinuity depths. We find deep reflections off the 410 km discontinuity (P410P and S410S) beneath Tibet, Western China and India at depths of 410–440 km and elevated underside reflections of the 410 km discontinuity at 370–390 km depth beneath the Tien Shan region and Eastern Himalayas. These reflections likely correspond to the olivine to wadsleyite phase transition. The 410 km discontinuity appears to deepen in Central and Northern Tibet. We also find reflections off the 660 km discontinuity beneath Northern China at depths between 660 and 700 km (P660P and S660S) which could be attributed to the mineral transformation of ringwoodite to magnesiowuestite and perovskite. These observations could be consistent with the presence of cold material in the middle and lower part of the mantle transition zone in this region. We also find a deeper reflector between 700 and 740 km depth beneath Tibet which cannot be explained by a depressed 660 km discontinuity. This structure could, however, be explained by the segregation of oceanic crust and the formation of a neutrally buoyant garnet-rich layer beneath the mantle transition zone, due to subduction of oceanic crust of the Tethys Ocean. For several combinations of sources and receivers we do not detect arrivals of P660P and S660S although similar combinations of sources and receivers give well-developed P660P and S660S arrivals. Our thermodynamic modelling of seismic structure for a range of compositions and mantle geotherms shows that non-observations of P660P and S660S arrivals could be caused by the dependence of underside reflection coefficients on the incidence angle of the incoming seismic waves. Apart from reflections off the 410 and 660 km discontinuities, we observe intermittent reflectors at 300 and 520 km depth. The discontinuity structure of the study region likely reflects lateral thermal and chemical variations in the upper-mantle and mantle transition zone connected to past and present subduction and mantle convection processes.

Failure modes of shales and their implications for natural and man-made fracture assemblages

Abstract: Shale is one of the most common rock types, with a rich and complex variety of failure structures in the Earth’s continental crust. In this paper, a synopsis of these structures including joints, pressure-solution seams and cleavages, faults, and shear bands is presented. First, two main categories, sharp and diffuse structures, each of which has subclasses based on its displacement discontinuity type including shear, compaction, and dilation are defined. Then, natural field examples are provided for each class as well as complex structural assemblages that include more than one type of failure-mode structures. Finally, the significances of these assemblages in terms of how older structures may influence later natural and man-made fractures and how they may interact in terms of fluid and gas flow are briefly discussed.

Wave and fracture propagation in continuum and faulted rock masses: distinct element modeling

Abstract: In this study, to investigate the dynamic fracture mechanism of blast-induced fracturing of rock mass around a blasthoe, two-dimensional (2D) distinct element method was used. The dynamic stresses, material status, and velocity vectors are shown to investigate rock mass failure subjected to blast load. On the other hand, rock masses consist of intact rock and discontinuities such as faults, joints, and bedding planes. The presence of such discontinuities in rock masses dominates the response of jointed rock masses to static and dynamic loading. This paper focuses on the effect of fault orientation on dynamic fracturing of rock mass. In order to investigate the effect of faults on dynamic fracturing of rock mass, a numerical simulation was conducted. The two-dimensional (2D) distinct element code was used to simulate the effect of a fault on rock failure and stress distribution through the rock mass due to blast wave propagation. The blast loading history was calculated using analytical method and was applied to the blasthole walls. Accordingly, the interaction of explosive energy transferred to the rock mass from the blasthole was examined as a function of distance to the fault plane. A Mohr–Coulomb material model was used for host rock to allow for plastic failure calculations. The study can be divided in two parts: firstly, surface blast and secondly, underground blast. The conducted numerical study describes the role of fault in blasting in a qualitative manner. On the other hand, a free-face boundary was considered as a common blast operation which is conducted in surface mining. In second part, blast was modeled in underground operations at presence of in situ stresses. The results in such areas showed predominant effect of pre-stresses as well as fault orientation on fracture propagation.