Les glissements de terrain argileux affectent de nombreux versants à travers le monde et menacent régulièrement les activités humaines dans les zones urbanisées montagneuses. Ces glissements sont caractérisés par des cinématiques souvent lentes mais ils peuvent brutalement se liquéfier et accélérer de manière imprévisible. Cette transition solide-liquide a été étudiée sur les argiles de la région du Trièves (Alpes Françaises) à l'aide d'études rhéologiques. Elles ont montré le caractère de fluide à seuil thixotrope avec une bifurcation de viscosité importante lors de la fluidification pouvant expliquer le caractère catastrophique de l'accélération observée sur le terrain. Cette perte de rigidité du matériau peut être observée par une chute de la vitesse des ondes de cisaillement (Vs). Des études réalisées en parallèle à la fois sur un modèle analogique de plan incliné et sur le terrain (glissement de Pont-Bourquin, Suisse) ont permis d'observer une chute de Vs précédent à cette fluidification montrant ainsi que Vs pourrait être un bon proxy pour la surveillance des instabilités de terrain argileux. / Landslides affect many clay slopes in the world and regularly threaten people in urban areas mountainous. These landslides are characterized by a slow velocity but they may suddenly liquefy and accelerate unexpectedly. The solid-liquid transition on the clay has been studied of Trièves region (French Alps) using rheological experiments. They have shown the yield stress thixotropic behavior with a viscosity bifurcation which can explain the catastrophic fluidization observed in the field. This loss of material stiffness can be followed by a drop in the shear wave velocity (Vs). Inclined plane test and field experiments (Pont-Bourquin landslides in Switzerland) have both shown a precursor drop of Vs indicating that it could be a good proxy for monitoring unstable clay slope.

abstract: Laterally-loaded short rigid drilled shaft foundations are the primary foundation used within the electric power transmission line industry. Performance of these laterally loaded foundations is dependent on modulus of the subsurface, which is directly measured by the Pressuremeter (PMT). The PMT test provides the lateral shear modulus at intermediate strains, an equivalent elastic modulus for lateral loading, which mimics the reaction of transmission line foundations within the elastic range of motion. The PMT test, however, is expensive to conduct and rarely performed. Correlations of PMT to blow counts and other index properties have been developed but these correlations have high variability and may result in unconservative foundation design. Variability in correlations is due, in part, because difference of the direction of the applied load and strain level between the correlated properties and the PMT. The geophysical shear wave velocity (S-wave velocity) as measured through refraction microtremor (ReMi) methods can be used as a measure of the small strain, shear modulus in the lateral direction. In theory, the intermediate strain modulus of the PMT is proportional to the small strain modulus of S-wave velocity. A correlation between intermediate strain and low strain moduli is developed here, based on geophysical surveys conducted at fourteen previous PMT testing locations throughout the Sonoran Desert of central Arizona. Additionally, seasonal variability in S-wave velocity of unsaturated soils is explored and impacts are identified for the use of the PMT correlation in transmission line foundation design. / Dissertation/Thesis / Doctoral Dissertation Civil, Environmental and Sustainable Engineering 2018

Esta dissertação apresenta o método de análise espectral de ondas superficiais (SASW) para a obtenção das variações do módulo cisalhante (Gmáx) com a profundidade, no domínio das deformações muito pequenas (abaixo de 0,001%). O SASW é um método sísmico in situ, não destrutivo, baseado na geração e detecção de ondas Rayleigh e na natureza dispersiva desta onda. Pela aplicação de um impacto na superfície do solo e detecção da onda em vários pontos, através de dois receptores, é construída uma curva de dispersão (velocidade de fase versus comprimento de onda). Esta curva de dispersão é, então, invertida. A inversão é um processo analítico para a reconstrução do perfil de velocidade de onda de cisalhamento (VS), partindo-se da curva de dispersão experimental de campo. O módulo de cisalhamento máximo de cada camada é facilmente obtido a partir do perfil de VS. No conteúdo teórico da dissertação discutem-se propriedades dinâmicas dos solos e descrevem-se as equações que dominam a propagação das ondas elásticas, tanto em meios homogêneos como em meios estratificados. A metodologia desenvolvida para a obtenção das curvas de dispersão, através da realização de ensaios SASW, apresenta os resultados obtidos em ensaios realizados na Cidade Universitária em São Paulo, sendo esses resultados comparados com estimativas feitas a partir de correlações baseadas em ensaios SPT existentes. Essas comparações permitem concluir que a metodologia SASW é uma boa alternativa para a determinação do perfil de rigidez (Gmáx) do solo, concordando com o nível de deformação envolvido nos ensaios. São desenvolvidos estudos de sensibilidade do método para verificar a influência na mudança dos parâmetros assumidos (peso específico, coeficiente de Poisson e espessuras das camadas) no processo de redução de dados (inversão) sobre o perfil final de VS, concluindo-se que o parâmetro que apresenta maior influência é o coeficiente de Poisson. / This dissertation presents the spectral-analysis-of-surface-waves (SASW) method as a tool for obtaining the variations in the modulus shear (Gmax) with depth in the field of very small strains (below 0,001%). The SASW method is a nondestructive in situ seismic method, based on the generation and measurement of Rayleigh wave and on its dispersive characteristic nature. Throughout the implementation of an impact on the soil surface and the detection of the wave at various points by two receptors a dispersion curve is constructed (phase velocity versus wave-length). This dispersion curve is then inverted. Inversion is an analytical process for reconstructing the shear wave velocity profile from the experimental field. The shear modulus of each layer is readily obtained from the shear wave velocity profile. The theoretical content of the dissertation presents dynamic properties of the soils and is described in the equations that dominate the propagation of elastic waves, both in homogeneous media and in stratified media. The methodology developed to obtain the dispersion curves through the implementation of SASW test is defined, and results from tests carried out at the University Campus in São Paulo are presented and compared with values obtained from correlations based on SPT tests. These comparisons indicate that the SASW method is a good alternative to determine the profile of stiffness (Gmax) of the soil, agreeing with the level of deformation involved in the tests. Studies on the methods sensitivity are developed to verify the influence on the changing of the parameters given (natural unit weight, Poisson coefficient and thickness of layers) in reduction of data (inversion) on the final profile of VS. The conclusion is that the Poisson coefficient is the parameter with greater influence.

South Africa, as one of the world’s largest gold producing countries, also generates large amounts of tailings. These tailings are disposed in tailings dams, which pose great threat to the environment in the case of failure, in particular, liquefaction. In order to evaluate the potential of liquefaction, the void ratio of the tailings is required and is often impossible to obtain. Seismic methods allow an indirect method to estimate void ratio of in situ deposits of which tailings are examples of. Currently, the use of seismic methods to estimate void ratio of tailings rely on shear wave velocity – void ratio relationships derived for sands. It is thus uncertain whether this relationship holds for gold tailings, which is classified as a sandy silt or silt. The measurement of shear wave velocity of tailings is done in the laboratory using a triaxial apparatus modified to accommodate bender element. Shear wave velocities are measured using wide square pulses and continuous sinusoidal waves. The results show that there is a near linear relationship between void ratio and shear wave velocity normalized against effective stress. The position of this relationship lies below the previously published results for sands. Shear wave velocity of gold tailings is more sensitive to changes in effective stress than changes in void ratio or over-consolidation ratio. Furthermore, using phase sensitive detection of continuous waves, we can conclude that shear wave velocity of gold tailings is also frequency dependent. / Dissertation (MEng)--University of Pretoria, 2006. / Civil Engineering / MEng / Unrestricted

L'élastographie par résonance magnétique (ERM) est une technique d'imagerie, reconnue comme une méthode pertinente pour la caractérisation mécanique des tissus humains in vivo. Celle-ci représente un intérêt fondamental en diagnostic clinique car le développement d'un processus pathologique s'accompagne la plupart du temps d'altérations des propriétés mécaniques des tissus atteints. L'ERM consiste à enregistrer le champ de déplacement induit au passage d'une onde de cisaillement généré dans le milieu étudié. Les paramètres mécaniques comme la vitesse, v, et les modules de viscoélasticité de cisaillement, G' et G'', peuvent être cartographiés. La quantification des paramètres mécaniques dépend à la fois de la fréquence mécanique, fexc, de la taille de voxel, a, de l'amplitude des champs de déplacement induits, A, de l'amplitude du rotationnel du champ de déplacement, q, des erreurs de mesure, ΔA et Δq, donc du rapport signal à bruit, RSB, et enfin de la méthode de reconstruction. En inversant les équations différentielles du champ de déplacement acquis selon les trois dimensions de l'espace, ces paramètres ont été considérés pour déterminer la précision et l'exactitude des modules mécaniques obtenus et établir les conditions de validité de l'ERM. Dans cette thèse, nous avons tout d'abord considéré A et A/ΔA afin de définir un premier seuil de validité pour l'ERM. Nous avons étudié l'influence de ces deux paramètres sur un fantôme hétérogène dans un appareil IRM 1,5 T avec deux types d'antennes. Dans une première étude, les champs de déplacement ont été acquis en fonction de A en utilisant deux séquences écho de spin (RFE) et écho de gradient (FFE) sensibilisées au mouvement pour une taille de voxel isotrope de 1 mm. Dans une seconde étude, ils ont été acquis en RFE en fonction de A pour trois résolutions spatiales différentes. Ces études ont révélé l'existence d'un seuil en A/ΔA au-delà duquel les paramètres extraits (G', G'') atteignent un plateau et l'ERM est fiable. Nous avons ensuite considéré le nombre de voxel par longueur d'onde, λ/a, comme paramètre déterminant des conditions de validité de l'ERM et nous avons caractérisé la qualité des données acquises par le rapport q/Δq. Sur des simulaitons dans un milieu élastique, homogène et isotrope avec un RSB variant entre 5 et 30, la précision et l'exactitude des mesures se sont avérées optimales pour 6 à 9 voxels par longueur d'onde. Nous avons reproduit expérimentalement à 2 kHz les conditions des siimulations sur un fantôme de PVA. Les champs de déplacement ont été acquis à 11,7 T en utilisant une séquence RFE sensibilisée au mouvement pour des résolutions spatiales de 150 μm à 300 μm afin de balayer le rapport λ/a de 1 à 20. Les résultats expérimentaux confirment pleinement les prédictions de la simulation. La vitesse de cisaillement diminue et tend vers la vitesse de référence attendue lorsque l'acquisition est réalisée dans le domaine optimal, à savoir ici lorsque a est inférieure ou égale à 200 μm. En outre la dispersion de la vitesse est réduite dans le domaine optimal et des estimations plus précises des paramètres mécaniques ont pu être déduites. Cette thèse montre d'une part que la précision et l'exactitude de l'ERM sont optimales lorsque les acquisitions sont réalisées ou traitées pour un domaine d'échantillonnage de la longueur d'onde déterminé par le RSB. Elle montre d'autre part que la comparaison des résultats obtenus doit être menée dans une gamme similaire de q/Δq. La prise en compte des conditions de validité de l'ERM, déterminées par les rapports λ/a et q/Δq, conduit à une mesure quantitative effective des paramètres mécaniques. Il est ainsi possible d'envisager un diagnostic clinique pertinent au sein d'un même organe, d'un même sujet, entre sujets ou au cours du temps. / Magnetic Resonance Elastography (MRE) is a imaging technique, recognized as a pertinent method for the mechanical characterization of human tissue in vivo. It offersa particular interest in clinical diagnosis because the development of a pathological process is often accompanied by modifications of the mechanical properties of diseased tissues. MRE consists of recording, along the three spatial dimensions, the displacement field induced by the propagation of a shear wave generated by excitation of the investigated tissue. Mechanical parameters such as shear wave velocity, v, and shear moduli, G' and G'', can then be mapped. The quantification of the mechanical parameters depends on the frequency of the mechanical excitation, fexc, the spatial resolution, a, the amplitude of the induced displacement field, A and the amplitude of the curl field displacement, q, with associated measurement errors, ΔA and Δq, (related to the signal-to-noise ratio, SNR) and finally the reconstruction method. All these parameters were considered to determine the precision and the accuracy of the estimated mechanical moduli and to establish the conditions of validity of MRE following the inversion of the differential equations of the displacement field. In this work, first A and A/ΔA were considered to define a validity threshold for MRE. The influence of A and A/ΔA was studied on a heterogeneous phantom acquired using a 1.5 T MRI with two different types of coils. In a first study, the displacement fields were acquired as a function of A using motion-sensitized spin-echo (REF) and gradient-echo (FFE) sequences for an isotropic spatial resolution of 1 mm. In a second study, the displacement field was acquired as a function of A using RFE for three different spatial resolutions. These studies revealed the existence of a threshold in A/ΔA beyond which the extracted parameters (G', G'') reach a plateau and the MRE is reliable. Then the number of voxels per wavelength, λ/a was considered as a parameter determining the conditions of validity of MRE. This parameter was studied according to the quality of the acquired data characterized by the ratio q//Δq. Simulations were carried in a homogeneous and isotropic elastic medium with a SNR between 5 and 30. The accuracy and the precision of the measurements were found optimal for 6 to 9 voxels per wavelength. The simulation conditions were experimentally reproduced at 2 kHz on a home-made polyvinyl alcohol phantom. The displacement fields were acquired at 11.7 T using a motion-sensitized RFE sequence with spatial resolutions ranging from 150 μm to 300 μm in order to obtain a λ/a ratio ranging from 1 to 20. The experimental results fully confirm the predictions of the simulation. The shear wave velocity decreases with λ/a. It tends towards the expected reference value when the acquisition is performed in the optimal condition, namely here when a is less than or equal to 200 μm. In addition, the standard deviation of the shear wave velocity is reduced for the optimal conditions. Therefore, accurate estimation of mechanical parameters could be deduced. This thesis first demonstrates that the precision and accuracy of MRE are optimal when the acquisitions are performed or processed for a certain wavelength sampling range determined by the SNR. We also showed that for fair comparison of the results, MRE must be carried out in a similar range of q/Δq. Taking into account the conditions of validity of MRE, determined by the ratios λ/a and q/Δq, leads to an effective quantitative measurement of the mechanical parameters making it possible to establish a relevant clinical diagnosis within the same organ, the same subject, between subjects or over time.

The Multichannel Analysis of Surface Waves (MASW) method traditionally uses an array of collinear vertical geophones to measure seismic wave propagation velocity at discrete points along the ground surface. Distributed fiber optic sensors (FOS) measure the average longitudinal strain over discrete lengths (i.e., zones) of a buried fiber optic cable. Such strain measurements can be used to assess ground motion and thus analyzed with the MASW method. To evaluate the feasibility of using FOS strain measurements in the MASW method, field experiments were conducted with both FOS and surface vertical geophones. Synthetic seismograms were also used to compare FOS to vertical and horizontal geophones and investigate the effect of installation depth and sensor type. Through the MASW method, shear wave (Vs) profiles from the FOS showed comparable results to those obtained with the geophones and achieved the same degree of uncertainty from the non-uniqueness of the MASW inversion process.

Characterization and assessment of liquefaction potential of gravelly soil in a reliable cost-effective manner has always been a great challenge for the geotechnical engineers. The typical laboratory investigation techniques have proven to be ineffective for characterizing gravelly soil due to the cost and difficulty of extracting undisturbed sample from gravelly deposits. The traditional in-situ tests like SPT or CPT are not very suitable for gravelly soil because of interference with large size gravel particles which can artificially increase the penetration resistance. The Becker Penetration Test, well known for gravelly soil characterization, is cost-prohibitive for routine projects and is not available in most of the world. The Chinese dynamic cone penetration test (DPT) with a larger diameter probe compared to the SPT or CPT, can be economically performed with conventional drilling equipment. Besides the penetration testing, in-situ measurement of shear wave velocity (Vs) is another alternative of characterizing gravel liquefaction. Probabilistic liquefaction triggering curves were developed by performing both DPT and shear wave velocity test at the Chengdu Plain of China where massive gravel liquefaction took place during 2008 Wenchuan earthquake. These curves have significant uncertainty as they were developed from a single event database. As a part of this study, both DPT and Vs tests have been performed at various sites around the world where gravelly soil did or did not liquefy in various past earthquakes. These newly collected data have been added to the existing Chinese dataset to form a large database on gravel liquefaction case histories for both DPT and Vs. Based on this larger database, new magnitude dependent probabilistic liquefaction triggering procedures have been developed for both DPT and Vs. The larger database has significantly improved the triggering curves by reducing the spread and constraining the curves at both the higher and lower end. New Magnitude Scaling Factor (MSF) curves have been developed for both DPT and Vs which were found to be consistent with existing MSF curves. Further, an instructive comparison has been drawn between the performance of CPT and newly developed DPT triggering procedure the liquefaction potential of gravelly deposits CentrePort in Wellington. Results showed that both DPT and CPT performed reasonably well in liquefaction assessment of the gravelly fill. However, the CPT-based CRR profiles contain intermittent spikes due to the interaction with gravel particles whereas the DPT resistance appear to be relatively smooth. Similar comparison has been presented between the DPT and BPT in performing liquefaction assessment of gravelly soil at the Borah Peak sites in Idaho. It is found that both DPT and BPT successfully evaluate the liquefaction potential of the loose critical layers but the medium dense to dense layers are identified as non-liquefiable by the DPT whereas the same deposits are identified as liquefiable by the BPT. Lastly, an investigation has been carried out to observe the effect of hydraulic conductivity and in-situ drainage on the liquefaction triggering in gravelly soils based on field data along with a group of numerical analyses. It is found that the hydraulic conductivity of gravelly soil reduces with sand content which eventually may cause liquefaction during earthquake shaking. Low permeability cap layer may also impede the drainage path to generate excess pore pressure to trigger liquefaction in the gravelly strata.

No description available.

Subsurface volume loss takes place in many geotechnical situations, and it is inherently accompanied by complex stress and displacement fields that may influence the performance of engineered geosystems. This research is a deformation-centered analysis, it depends on soil compressibility and it is implemented using finite elements. Soil stiffness plays a central role in predicting ground deformation. First, an enhanced Terzaghi’s soil compressibility model is proposed to satisfy asymptotic conditions at low and high stress levels with a small number of physically meaningful parameters. Then, the difference between small and large strain stiffness is explored using published small and large-strain stress-strain data. Typically, emphasis is placed on the laboratory-measured stiffness or compressibility; however, there are pronounced differences between laboratory measurements and field values, in part due to seating effects that prevail in small-thickness oedometer specimens. Many geosystems are subjected to repetitive loads; volumetric strains induced by drained repetitive ko-loads are experimentally investigated to identify shakedown and associated terminal density. The finite element numerical simulation environment is used to explore the effect of localized subsurface mass loss on free-surface deformation and shallow foundations settlement and bearing capacity. A stress relaxation module is developed to reproduce the change in stress associated to dissolution features and soft zone formation. The comprehensive parametric study is summarized in terms of dimensionless ratios that can be readily used for engineering applications. Field settlement data gathered at the Savannah River Site SRS are back-analyzed to compare measured values with predictions based on in situ shear wave velocity and strain-dependent stiffness reduction. The calibrated model is used to estimate additional settlements due to the pre-existing cavities, new cavities, and potential seismic events during the design life of the facility.

In this study, ambient noise wavefield was used for the first time to image spatial and temporal upper crustal seismic structures in NW-Bohemia/Vogtland region. The data come from 111 stations and were collected from continuous recordings of the permanent station networks of Germany and Czech Academy of Sciences as well as temporary stations of the BOHEMA and PASSEQ experiments. Rayleigh and Love waves travelling between each station-pair are extracted by cross-correlating long time series of ambient noise data recorded at the stations. Group velocity dispersion curves are obtained by time-frequency analysis of cross-correlation functions between 0.1 and 1 Hz, and are tomographically inverted to provide 2-D group velocity maps. At shorter periods Rayleigh wave group velocity maps are in good agreement with surface geology where low velocity anomalies appear along Mariánské Lázně Fault and Eger rift. A low velocity zone is observed at the northern edge of Mariánské Lázně Fault which shifts slightly to the south with increasing period and correlates well with the main focal zone of the earthquake swarms at 5 s period. We invert the 2-D group velocity maps into a 3-D shear wave velocity model. In this step Love waves were excluded from further analysis because of their high level of misfit to modelled dispersion curves. Horizontal and vertical sections through the model reveal a clear low velocity zone above the Nový Kostel seismic focal zone which narrows towards the top of the seismic activity and ends above the shallowest hypocenters at 7 km depth. We investigate temporal variation of seismic velocity within and around the Nový Kostel associated with 2008 and 2011 earthquake swarms by employing Passive Image Interferometry method using 7 continuous seismograms recorded by the WEBNET network. The results reveals stable seismic velocities without a clear post seismic velocity change during earthquake swarms in the Nový Kostel area.