Guidelines to minimize the occurrence of mega-wedge falls in rock caverns

Abstract Over the last decade, large rock caverns have been developed for storing vast volumes of hydrocarbons and in particular crude oil. In this context, large rock caverns have light support made of grouted steel or fibreglass rock bolts. Reinforcing cables are strictly prohibited to avoid leak paths for the product. Therefore, the risk of mega-wedge occurrence is high since the size of the potential mega-wedges is proportional to the cavern size. Some geometrical situations are more detrimental than others and the possibility of a local or extensive collapse be very high, including in rock masses which can be ranked as good and above. Such an apparent paradox could have been detrimental to the concept of large caverns because mega-wedges are extremely difficult to stabilize once discovered because rock bolts are generally too short to stabilize them. Anticipation is therefore the key. First, we clarify the term mega-wedge and then, analyse the various possibilities of occurrence in large caverns, as well as classical tunnels. Guidelines are proposed to identify whether the conditions are met for experiencing mega-wedge failure at hand. Two main geological structures, shear fractures and smooth-persistent-planar-spaced (SPPS) joints, are favourable for mega-wedge formation. These two fracture types are analysed and criteria are given to ensure a quick and efficient determination procedure. The field approach is synthesised by a decision chart, to be used at site, during the excavation works.</jats:p

Phenomenological behavior of rock salt: On the influence of laboratory conditions on the dilatancy onset

International audienc

Avaliação de soldagem de aço estrutural através do Ruído Magnético de Barkhausen Evaluation of carbon steel welded plates with Magnetic Barkhausen Noise

Este trabalho apresenta resultados de avaliação de juntas soldadas de aço estrutural ASTM A36 por meio do Ruído Magnético de Barkhausen (RMB). A soldagem foi feita em chapas de 6 mm de espessura com preparação de chanfro em V, com um e dois passes. Foram feitas a caracterização de microestructuras e microdurezas através da junta soldada, e na superfície foram feitas medições de RMB. Os sinais de RMB foram analisados com o valor máximo, uma média de valores acima de 80% do pico máximo, o rms e o rms do envelope, dos quais foram obtidas curvas da variação com respeito ao centro do cordão e também foram obtidos mapas superficiais. Na caracterização microestructural obteve-se que as amostras de um e dois passes apresentaram tamanhos da zona afetada pelo calor (ZAC) diferente. As microdurezas mostraram que no limite de fusão tem-se a maior dureza enquanto que no fim da ZAC tem-se a menor dureza. O RMB mostrou que no limite de fusão tem o menor valor, enquanto que no fim da ZAC tem o maior valor. Este estúdio mostrou que as mudanças na microestrutura influenciam em todos os parâmetros analisados, sendo que a junta soldada ficou melhor representada pelo valor rms e rms do envelope do RMB.<br>This paper shows results for the evaluation of ASTM A36 carbon steel welded joints by Magnetic Barkhausen Noise (MBN). V-groove shape welded samples were made in a 6 mm thickness plates, with one-pass and two-pass. Microstructural and microhardness characterization were made on a transversal section of welds. In the surface was made measurement of MBN. The signal of MBN were analyzed with the maximum value, threshold of 80% above of maximum, the root mean square (rms), the root mean square of the profile, and these parameters were plotted in function of the distance to center bead. Surface maps were obtained, too. The microstructural characterization identified different heat affected zone (HAZ) sizes for one-pass and two-pass. In addition, changes on microhardness showed which higger hardness was happened in the fusion boundary whereas lower microhardness happened in the HAZ end. With MBN monitoring were verified that the lower value happened in the fusion boundary, whereas the MBN higher value happened in the HAZ end. The study showed that changes in the microstructure influence all analyzed parameters. The welded joint are best represented by the root mean square (rms), the root mean square of the profile