Onde sismique

Résumé

thumb|Différentes ondes sismiques.|alt= Les ondes sismiques, ou ondes élastiques, sont des mouvements vibratoires qui se propagent à travers un milieu matériel et peuvent le modifier irréversiblement si leur amplitude est suffisante. Elles sont engendrées par un événement initial, généralement un séisme. L'impulsion de départ déplace les atomes du milieu, qui en poussent d'autres avant de reprendre leur place, ces déplacements oscillatoires se propageant ensuite de proche en proche. Un séisme émet des ondes sismiques dans toutes les directions. La physique des milieux élastiques est régie par l'équation de Navier, qui implique l'existence de deux grands types d'ondes, mises en évidence expérimentalement : les ondes de volume qui traversent la Terre et les ondes de surface (ondes L) qui se propagent dans une couche d'épaisseur limitée sous la surface terrestre. Les ondes de volume se subdivisent par ailleurs en ondes longitudinales (ondes P) et transversales (ondes S). Sur les enregis

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https://fr.wikipedia.org/wiki/Onde_sismique

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Seismic Rehabilitation of Deficient Steel Braced Frames with Conventional and Innovative Retrofit Techniques

Valériane Matthey

Steel concentrically braced frames (CBFs) as lateral load resisting system are common in steel construction, for their high strength and architectural versatility. However, the brace connection details have to be well designed to ensure a ductility of the frame under seismic excitation. This Master Thesis studies the behavior of a steel CBF, located in Sendai (Japan). It experienced significant damage at brace connections after the 2011 Tohoku-Oki Earthquake in Japan. They are mainly due to the eccentricity created by the single-lap gusset plates along the long duration of the imposed ground motion hazard. The seismic response of the CBF is assessed by means of nonlinear analysis with the performance-based approach according to ASCE/SEI 41-13 (2014). As the braces do not meet any performance criteria, seismic retrofit is required. Three retrofit solutions are investigated and compared. Two of them are considered as conventional solutions. They consist to replace the braces and gusset plates, once with new Chevron braces and once with new X braces. The new braces are designed with the Capacity Design rules. The third solution modifies the CBF into a Rocking Braced Frame (RBF). This innovative high-performance system allows the column bases to uplift in order to dissipate the seismic energy and localize the damages in easily replaceable fuses to protect the primary members. The retrofit works are mainly located at the column bases. Dynamic analyses of the initial frame and its retrofits are computed for a seismic input recorded during the 2011 Tohoku-Oki Earthquake. The peak storey drift ratio (SDR), residual SDR and story shear forces are compared. The new Chevron CBF has the worst performance of the three retrofits. New X-bracing CBF and the RBF show a similar performance, the RBF being slightly better.

2020

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Water-induced strength reduction is one of the most critical causes of rock engineering disasters. Understanding the influence of water on the fracture toughness of rocks is necessary for rock fracture mechanics and rock engineering applications such as mining, tunneling, and the exploitation of deep geothermal reservoirs. However, only a few studies have been conducted to understand the effect of water on fracture properties such as fracture toughness. In this thesis, the effect of water on fracture toughness and fracture energy in several sandstones was investigated. First, several preliminary characterization tests were performed on all samples: P and S-wave velocity, porosity analysis, X-ray diffraction method, and thin section microscopic observation. CCNBD method was used to measure Mode I fracture toughness under dry, saturated and immerged conditions. Results show a decrease in fracture toughness and fracture energy after saturation and a higher decrease for immerged samples. The toughness reduction was larger for the most porous rocks and the one that contained a high percentage of phyllosilicates (clay minerals). We conclude that fracture toughness in sandstone is strongly affected by saturation, and mineralogy of the rock.

2020

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Ambient seismic noise monitoring of a clay landslide: Toward failure prediction

Michel Jaboyedoff, Clément Michoud

Given that clay-rich landslides may become mobilized, leading to rapid mass movements (earthflows and debris flows), they pose critical problems in risk management worldwide. The most widely proposed mechanism leading to such flow-like movements is the increase in water pore pressure in the sliding mass, generating partial or complete liquefaction. This solid-to-liquid transition results in a dramatic reduction of mechanical rigidity in the liquefied zones, which could be detected by monitoring shear wave velocity variations. With this purpose in mind, the ambient seismic noise correlation technique has been applied to measure the variation in the seismic surface wave velocity in the Pont Bourquin landslide (Swiss Alps). This small but active composite earthslide-earthflow was equipped with continuously recording seismic sensors during spring and summer 2010. An earthslide of a few thousand cubic meters was triggered in mid-August 2010, after a rainy period. This article shows that the seismic velocity of the sliding material, measured from daily noise correlograms, decreased continuously and rapidly for several days prior to the catastrophic event. From a spectral analysis of the velocity decrease, it was possible to determine the location of the change at the base of the sliding layer. These results demonstrate that ambient seismic noise can be used to detect rigidity variations before failure and could potentially be used to predict landslides.

2012

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