Measurement of dissolved and occluded gas pressures during loading and unloading of loose gassy sands

A Khurana

In the proceedings of: GeoVancouver 2016: 69th Canadian Geotechnical Conference

Session: FUNDAMENTALS - V Soil & Rock Mechanics

ABSTRACT: Soils with relatively large dissolved gas concentrations within the pore fluid have the potential to initiate and propagate soil liquefaction if unloading occurs. Gassy sands commonly occur in offshore and coastal regions, and submarine and offshore slope failures are often inevitable in such soils. The ratio of the pore water pressure and the total dissolved gas pressure (PTDG) controls if and when free gas is likely to form during unloading, which affects the compressibility of the pore fluid and the integrity of the overall soil matrix. Previously, PTDG has not been measured, but rather has been estimated based on differences between the pore water and capillary pressures, which may result in inaccuracies and incorrect model predictions. Here, we measure both total dissolved gas pressure and pore water pressure in gassy sand to investigate the response to unloading. A laboratory program was designed to accommodate unloading scenarios using Ottawa sand within a triaxial cell. The paper demonstrates the relationship between measured total dissolved gas pressure and pore water pressure response associated with unloading.

RÉSUMÉ: Les sols avec des concentrations relativement importantes de gaz dissous dans le fluide des pores ont le potentiel pour initier et propager la liquéfaction du sol si le déchargement se produit. sables grisouteuses se produisent fréquemment dans les régions offshore et côtières et sous-marin et des ruptures de talus en mer sont souvent inévitables dans de tels sols. Le rapport de la pression de l'eau interstitielle et la pression totale de gaz dissous (PTDG) contrôle si et lorsque le gaz libre est susceptible de se former pendant le déchargement, ce qui affecte la compressibilité du fluide des pores et l'intégrité de la matrice globale du sol. Auparavant, PTDG n'a pas été mesurée, mais a été estimée en fonction des différences entre l'eau des pores et des pressions capillaires, ce qui peut entraîner des inexactitudes et des prédictions des modèles incorrects. Ici, nous mesurons à la fois la pression totale de gaz dissous et la pression interstitielle de l'eau dans le sable gassy pour étudier la réponse au déchargement. Un programme de laboratoire a été conçu pour accueillir des scénarios de déchargement à l'aide de sable Ottawa dans une cellule triaxiale. L'article démontre la relation entre la pression de gaz totale mesurée dissous et réponse de la pression de l'eau interstitielle associée à déchargement. Index Terms Gas pressure, gassy soil, soil behavior. I. Introduction The presence of even a small amount of gas contained within the pore water of a soil has the potential to change its behavior drastically, where the shear strength and the volume change are most affected. Gas is known to be most detrimental during undrained unloading, when gas exsolution and expansion occur. It is vital to broaden our understanding of gassy soils because of their potential to destabilize the ground surface and overlying infrastructure. When the soil is unloaded, possibly due to tidal drawdown, excavations or erosion, then gas exsolution and expansion may occur causing the pore pressures to increase and the effective stress to decrease. The net effect of which is loss of the supporting shear strength.

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Cite this article:
A Khurana (2016) Measurement of dissolved and occluded gas pressures during loading and unloading of loose gassy sands in GEO2016. Ottawa, Ontario: Canadian Geotechnical Society.

@article{3962_0808144712,author = A Khurana,title = Measurement of dissolved and occluded gas pressures during loading and unloading of loose gassy sands,year = 2016}