A diminishing stabilizer? Studies on the influence of ice and gas hydrates on the geo-mechanical properties of sediments.

Natural gas hydrates form at elevated pressure and low temperatures in the presence of sufficient quantities of gas and water and have therefore been discovered on all continental margins and in permafrost regions. In the marine hydrate-bearing sediments, gas hydrates, depending on their content, can transform a loose sediment into a consolidated rock with a strongly increased strength. In permafrost regions the hydrate stability zone can extent deep into the ice-bearing permafrost and, therefore, both, ice and hydrate can consolidate the sediment. However, the strength of methane hydrate is much higher than that of ice, which behaves much more ductile. Consequently, the resulting strength of a sediment, containing both components, strongly depends on the ice to hydrate ratio. Conversely, the decomposition of natural gas hydrates in marine or permafrost sediments leads to a reduction in the mechanical strength of the host sediment. In addition, the release of gas can create overpressure in the pore spaces, reducing the effective stress and leading to instabilities in the sediment structure.

Since both continental margins and permafrost regions are used by humans for various activities that largely depend on the mechanical stability of the sediments, knowledge of the main factors and processes that determine the stability of weakly consolidated sediments is crucial. Both the thawing of ice and the decomposition of gas hydrates in permafrost soils lead to a change in the geo-mechanical properties of the host sediment. The residual and peak shear strengths of ice- and hydrate-bearing sediments were investigated using a ring shear cell developed at the GFZ. Based on literature data and our results, we discuss the dependence of the geo-mechanical properties of sediments on ice and hydrate saturation and the possible consequences if their proportion diminishes.