Geomechanical behaviour of gassy soils and implications for submarine slope stability: a literature analysis

Abstract

Submarine slope failures pose a direct threat to seafloor installations and coastal communities. Here, we evaluate the influence of free gas on the soil's shear strength and submarine slope failures in areas with gassy soils based on an extensive literature review. We identify two potential destabilization mechanisms: gas bubbles in the pore space lead to a reduced shear strength of the soil and/or gas induces excess pore pressures that consequently reduce the effective stress in the soil. Our evaluation of the reported mechanical and hydraulic behaviour of gassy sediments indicates that the unfavourable impact of entrapped gas on a soil's shearing resistance is not sufficient to trigger large-scale slope failures. Liquefaction failure due to high gas pressures is, however, a viable scenario in coarse-grained soils. Transferring the gas influence on the soil mechanical behaviour to constitutive models is identified as the most important prerequisite for a successful future analysis of slope stability.

Revised Storegga Slide reconstruction reveals two major submarine landslides 12,000 years apart

“…1 and 2e). The presence of free gas has the potential to reduce the effective stress in marine sediments and may contribute to a reduction of slope stability 22 . Peak sedimentation rates greater than 12 m/kyrs during the LGM caused excess pore pressure 23 , which would have the potential to precondition slope failures 24,25 .…”

Assessing Spatio‐Temporal Variability of Free Gas in Surficial Cohesive Sediments Using Tidal Pressure Fluctuations

“…Through an extensive literature review Kaminski et al (2020) point out the mechanisms by which gas exsolution and subsequent accumulation of bubbles can affect the stability of submarine slopes. The results of the present study add to the rare in situ evidences that free gas migration and accumulation in shallow, fine-grained sediments or peat are associated with transient overpressures (Acharya et al, 2016;Bennett et al, 1996;Tjelta et al, 2007).…”

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Recognition of the widespread occurrence of gas in shallow marine sediments has sustained interest for its significance in offshore engineering activities, slope stability, and climate change (Egger et al, 2018;Fleischer et al, 2001;Kaminski et al, 2020;Sills & Wheeler, 1992). On continental shelves, methane is commonly produced by the biogenic degradation of buried organic matter (Fleisher et al, 2001;Mogollón et al, 2011).

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Confined Compressibility of Fine-Grained Marine Sediments with Cavities after Complete Dissociation of Noduled Natural Gas Hydrates

“…The compression of marine sediments closed to a rigid boundary (e.g., suction bucket, offshore pile, drilling wellbore) or far from the boundaries of a large area are laterally confined, and the laterally confined consolidation behavior of clayey silt with large voids occupied by gas bubbles is controlled by the compressibility and drainage capacity of pore fluids [22]. A double compressibility model [23] has been proposed to describe the consolidation process, and the total compressibility can be summarized by introducing two types of deformation mechanism [24]: one is the compression and dissolution of free gas induced by loading alterations, plus deformation of the soil matrix around gas voids due to local shearing; the other one is the soil matrix compression after drainage caused by changes in the operative stress. The deformation mechanism indicates that gas type, solubility, temperature, and stress state are crucial factors controlling the overall compressibility.…”