Gas hydrates can form more or less at the same time as seafloor sediment. They can have the effect of significantly stiffening and strengthening deep-ocean sediments. Subsequent increases in situ temperature or decreases in pressure may trigger hydrate dissociation, leading to large reductions in the strength and stiffness of the sediment and possible seafloor instability. Gas hydrate dissociation not only removes cementing. It also releases freshwater and significant amounts of trapped gas that are dependent on multiple factors such as type of sediment, available pore space, hydrate morphology, and hydrate saturation. The presence of pock marks in areas of known seabed instability suggests that hydrate dissociation may have been a factor in triggering failure at these locations. Having reviewed the mechanisms by which the strength and stiffness of seabed sediment may be changed during dissociation, this paper reports the results of laboratory testing to evaluate the effects of loss of hydrate cement on strength and stiffness, for a range of sand-sized materials with differing particle size, specific surface area, and particle shape, using a laboratory gas hydrate triaxial apparatus. The results suggest that both the strength and the stiffness of hydrate-cemented granular materials are affected significantly by the specific surface available for hydrate cementation and, to a certain extent, by the particle shape. Uniform coarse granular sediments of lower specific surface area can suffer significant loss of stiffness and strength upon hydrate dissociation, changing the sediment from dilative to contractive. Finer-grained sediments appear less affected by dissociation. Song et al. (2010) found the compressive strength of solid methane hydrate at À5°C to be 0.9 MPa under a 1-MPa confining pressure. For hydrate formed from ice/methane mixtures, at À5, À10, and À20°C, under back pressures of 2.5, 5, and 10 MPa, Yu et al. (2011) measured unconfined compressive strengths of between 1.2 and 3 MPa. It can be seen from the above that solid hydrate has the strength of ice, similar to that of a weak MADHUSUDHAN ET AL. 65
This paper presents the results of a series of hollow cylinder tests carried out to investigate the role of drainage conditions on the response of railway track foundation materials during cyclic loading. Three sand-clay mixes were tested. It was found that, below a certain cyclic shear stress threshold, and depending on the drainage conditions, changes in principal stress direction should not adversely affect the cyclic stability of a railway foundation. However, significant stiffness degradation and failure may occur if this cyclic shear stress threshold is exceeded. The cyclic shear stress threshold increased with moderate additions of clay per unit volume of sand, and reduced significantly when specimen drainage was prevented. For the materials tested, the cyclic shear stress threshold in free-to-drain conditions was generally similar to the cyclic shear stress in the soil immediately below a 0·3 m deep ballast bed, but comfortably greater than the cyclic stress at a depth of 1 m below the sleeper base. In undrained conditions, the cyclic shear stress threshold was generally similar to the cyclic shear stress at a depth of 1 m below the sleeper base. This has implications for the suitability of such materials for railway track foundations.
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