Implications of gas content for predicting the stability of submarine slopes

Abstract

The behavior of gas-laden, soft submarine soils subjected to changes in mean normal and shearing stresses is discussed. Information developed for partially saturated soils is extended to soft sediments. Calculations indicating that gas-laden submarine soils generally have degrees of saturation in situ that exceed ~ 90% are presented. Therefore, it is suggested that insignificant error is introduced in predicting the effective stresses of soft sediments using the standard effective stress equation and neglecting the pore-gas pressure.The presence of gas is shown to permit volume changes of soft sediments under wave loadings. The compressibility of the gaswater pore fluid is quantified. The pore-pressure response, related to the ratio of the compressibility of the pore fluid and soil structure, is shown to be similar to that of fully saturated soils. The relevance of "undrained" shipboard tests to the prediction of slope stability is discussed. It is concluded that the presence of gas leads to undrained strengths, as measured on recovered samples, which are lower than those that occur in situ. The use of these measured strengths in stability calculations leads to conservative predictions of submarine slope stability.

Submarine Slope Failures in Gassy Soils

“…The occurrence of gassy sediments has been reported in all of the world's oceans at any depth by many researchers: North Sea (Hovland, 1993), Baltic Sea (Soderberg and Floden, 1992), Irish Sea (Taylor, 1992), Gulf of Mexico, Alaska (Esring and Kirby, 1977;Bryant and Roemer, 1983;MacDonald et al, 1990), Mississippi River Delta front (Whelan et al, 1978) and the marine sediments off Hong Kong (Premchitt et al, 1992). High concentrations of gas have been observed in the pore fluid of the Fraser River Delta sediments in British Columbia, .…”

“…Appropriate model preparation techniques and control and measurements systems were developed for that test. Offshore soft sediments contain undissolved gas bubbles of carbon dioxide, hydrogen sulfide, ethane but mostly methane as noted by Esrig et al (1977) & Jones et al (1986. Ethylene was selected as the dissolved gas with water as the pore fluid to provide the necessary control of degree of sand saturation.…”

3D Modeling of Wave-Seabed-Pipeline in Marine Environments

“…On the other hand, the seabed material is reported to be highly saturated with the degree of saturation ranging from about 0.90 to 1.00 as pointed out in Esrig and Kirby [13]. In such nearly saturated condition, the relation between total and effective normal stresses of soil can be expressed, in any direction such as i-, as:…”

“…Therefore, stresses and strains in solid skeleton are related on the basis of Hooke's law. That is: (13) in which, G is the shear modulus of soil; μ is the Poisson's ratio; u, v and w are soil displacements in x-, y-and z-directions, respectively; and is the volumetric soil strain defined as:…”

Submarine Slope Stability of a Fjord Delta: Bella Coola, British Columbia

“…At saturation levels the gas enters the bubble phase and may act to increase pore pressures in the sediment (Esrig and Kirby, 1977). In areas affected by large tidal ranges, such as Bella Coola (3.9 m mean range, 5.9 m large range), excess pore pressures can be produced as the tide falls.…”