Behaviour of offshore soils containing gas bubbles

Abstract

An extensive programme of research into the influence of undissolved gas bubbles on the behaviour of fine-grained onshore soils is reviewed. The programme has been based on the development of a laboratory technique for the preparation of reconstituted soil samples containing a uniform distribution of gas bubbles. The structure of these samples is similar to that observed in sediment recovered from the sea bed, and consists of large gas-filled cavities surrounded by a matrix of saturated soil. It is found that surface tension effects limit the difference between gas pressure and pore water pressure, and that the overall void size is effectively a function of the strength of the matrix, so that changes in void volume may be modelled by cavity expansion and contraction in an ideal plastic medium, leading to limits on the difference between gas pressure and mean total stress. A new parameter, operative stress, is shown to influence both the consolidation and the strength of these gassy soils. Thus, during consolidation, the gas volume is controlled by the total stress and the water volume by the operative stress; the undrained shear strength may be increased or decreased by the presence of the gas, depending on the specific values of total stress and operative stress. The operative stress for a gassy soil may therefore be seen as being analogous in its definition (total stress minus pore water pressure) to the effective stress for a saturated soil, but different from the effective stress in that it does not, on its own, control the strains and strength. Gas bubbles are shown to have a major influence on the acoustic behaviour of fine-grained offshore soils. L'article passe en revue un programme de grande envergure pour étudier l'influence exercée par des bulles de gaz non-dissoutes sur le comportement des sols à grains fins en mer. Le programme a été basé sur le développement d'une technique de laboratoire pour la préparation d'échantillons de sols reconstitués comprenant une distribution uniforme de bulles de gaz. La structure de ces échantillons ressemble à celle observée dans des sédiments obtenus au fond de la mer, comprenant de grandes cavités remplies de gaz entourées d'une matrice de sol saturé. Il a été trouvé que des effets de tension superficielle limitent la différence entre la pression du gaz et la pression de l'eau interstitielle et aussi que la valeur totale des vides dépend effectivement de la résistance de la matrice, de sorte que des changements dans le volume des vides peuvent être modelesés par l'expansion et la contraction des cavités dans un milieu idéal plastique, conduisant à des limites sur la différence entre la pression du gaz et la contrainte totale moyenne. On démontre la façon dont cette différence représente un nouveau paramètre (contrainte effective) qui influence à la fois la consolidation et la résistance de ces sols gazeux. Par exemple, pendant la consolidation le volume de gaz est contrôlé par la contrainte totale, tandis que le volume d'eau est contrôlé par cette contrainte effective. Selon les valeurs spécifiques de la contrainte totale et de la contrainte effective la résistance au cisaillement non-drainé peut être augmentée ou diminuée par la présence du gaz. On peut alors considérer que cette définition de la contrainte effective dans le cas d'un sol gazeux (contrainte totale moins pression d'eau interstitielle) est analogue à la contrainte effective pour un sol saturé mais diffère de celle-ci en ce qu'elle seul ne contrôle pas les déformations ni la résistance. On démontre que les bulles de gaz exercent une influence très importante sur le comportement acoustique des sols à grains fins en mer.

Microbubble transport in water‐saturated porous media

“…Using Darcy's law gives, v bf 5jgðq w 2q mix Þ=l mix [Falta et al, 1989], where j is intrinsic permeability and l mix is the effective dynamic viscosity of the mixture. According to the work of Brown [2000] and Mari e [1987] and given that the gas volume fraction in the experiments was below 3%, a range comparable with natural conditions observed [Mogoll on et al, 2011;Sills et al, 1991], the effective viscosity of water-bubble mixture changed little from the water viscosity (details in supporting information), i.e., l mix % l w . As q g << q w , the characteristic velocity (v bf ) of the plume front can be approximated as v bf 5cq w gj=l w .…”

Modelling acoustic scattering, sound speed, and attenuation in gassy soft marine sediments

“…To address the linear bubble dynamics problem, the resonance frequency and the linear damping terms will be derived from equation (2). Solutions to the linear problem can be found by assuming small amplitude pulsations ≪ 1 about the bubble equilibrium radius 0 , i.e.…”

“…Firstly, for understanding the stability of the sediment with respect to both natural disturbances, such as undersea landslip, and manmade processes, such as piling for civil engineering projects [1][2][3]. Secondly, for assessment of the gas reserves for fuel and climate risk [4][5][6][7].…”

Degradation of flood embankments – Results of observation of the destruction mechanism and comparison with a numerical model

“…[13] proposed a conceptual model of soil containing large gas bubbles. The number of papers on the topic is limited, probably due to the complexity of these phenomena and the difficulty in testing them by experimental methods [1,5,[11][12][13][14][15][16][17][18]. An important work on the appearance of open discontinuities is Shin and Santamarina [10], which describes the different types of cracks caused by water or gas.…”