Gas hydrates are icelike compounds composed of water and methane gas in very compact form. There is substantial evidence from case histories that links gas hydrate dissociation to submarine slope failures and other geohazards. Theoretical analyses have also shown that upon dissociation gas hydrates will cause an increase in fluid pressure and a reduction in effective stress and thus result in loss of the soil strength. This paper presents a preliminary quantification of the effects of gas hydrate dissociation through development of a pore-pressure model that was incorporated into one-and two-dimensional slope stability analyses. The ensuing numerical study investigated submarine slope stability through parametric studies and application to two important case histories and found that dissociation of even small amounts of hydrate can have a significant destabilizing effect. Yet whether gas hydrate dissociation can alone cause large-scale slope failures has still to be demonstrated as there are often many destabilizing processes; however, this research highlights the importance of assessing the effects of gas hydrate dissociation on the behaviour of submarine slopes. Résumé :Les hydrates gazeux sont des composés comprenant de l'eau et du gaz de méthane dans une forme très compacte. D'après des histoires de cas, il y a une évidence substantielle d'un lien entre la dissociation de l'hydrate gazeux et les ruptures de talus sous-marins et autres risques géotechniques. Des analyses théoriques ont aussi montré que lors de la dissociation, les hydrates gazeux vont produire un accroissement de la pression du liquide et une réduction de la contrainte effective, et vont ainsi résulter en une perte de résistance du sol. Cet article présente une quantification préliminaire des effets de la dissociation des hydrates gazeux au moyen du développement d'un modèle de pression interstitielle qui a été incorporé dans des analyses de stabilité de talus à une et deux dimensions. L'étude numérique qui s'en est suivie a examiné la stabilité des talus sous-marins au moyen d'études paramétriques et de l'application de deux importantes histoires de cas, et a montré que la dissociation même de faibles quantités d'hydrate peut par ellemême avoir un effet déstabilisant important. Encore que de savoir si la dissociation des hydrates gazeux peut par ellemême causer des ruptures de talus reste à démontrer parce qu'il y a souvent plusieurs processus déstabilisants; cependant, cette recherche met en lumière l'importance d'évaluer les effets de la dissociation de l'hydrate gazeux sur le comportement des talus sous-marins.Mots-clés : hydrates gazeux, stabilité de talus, marin, offshore, gaz de méthane, instabilité.[Traduit par la Rédaction] Nixon and Grozic 325

No abstract

No abstract

In recent years there has been an increasing interest in production of methane gas from hydrate-bearing sediments, located below the permafrost in arctic regions and offshore within the continental margins. In order to simulate the geomechanical response of the hydrate accumulation during gas production, comprehensive evaluation of the sediments' properties is imperative. This paper presents an analysis of the mechanical properties of methane-hydrate-bearing sediments determined through numerical simulation of drained triaxial compression tests on three different sand types. The adjustment of the numerical to the experimental results was performed for the entire stress-strain curves and therefore enables a good understanding of the material constitutive relations. New constitutive relations are suggested for the hydrate-related properties. An optimization process was used, finding separately the soil skeleton-related coefficients and the hydrate-related coefficients. The values of the obtained coefficients associated with the hydrate were found to have minor deviations from each other, for the three examined sand types. By separating the soil skeleton and the hydrate-related response, this paper suggests a prediction method for the mechanical response of hydrate-bearing sands.

Stress history is usually determined by interpreting the results of oedometer tests to obtain the preconsolidation stress using traditional methods such as those proposed by Casagrande and Janbu. Both these methods are based on the assumption that the soil experiences a change in stiffness, from a stiff response to a soft response, close to the preconsolidation stress. Disturbed soils, however, especially overconsolidated clays of low plasticity, exhibit a soft response at low stresses, thereby making the interpretation of oedometer tests using traditional methods extremely difficult. Using data mainly from the Haltenbanken area, offshore Norway, a study was undertaken to investigate the various methods and testing procedures available to determining stress history. Oedometer verification tests were also performed on reconstituted specimens to compare interpreted values with the actual applied preconsolidation stress. This paper describes the results of the study by illustrating the variation in the interpretation methods and testing procedures used to determine preconsolidation stress.Key words: preconsolidation stress, yield stress, overconsolidated, low plastic clays, oedometer test, stress history.