Compression behaviour of hydrate bearing carbonate sand - fines mixtures

Abstract

High gas hydrate content has been found in fine-grained sediments containing substantial amount of foraminifera in the South China sea. One of the possible hydrate accumulation habits is filling the intra- particle voids in the foraminifera. To understand the effects of this hydrate accumulation habit on the compression behaviour of the fine-grained sediment, two series of isotropic compression tests were conducted. Due to high intra-particle porosity, carbonate sand (CS) was mixed with the fines to mimic the hydrate formation in the intra-particle voids in the laboratory prepared soil specimens. The compression test results revealed that the mixtures of fines with as high as 40% CS content can exhibit the transitional behaviour such that non-convergent compression lines are observed at the high stress level. It is evident that breakage of CS grains is negligible in these mixtures. Hence, the initial fabrics are not erased under the high stress level resulting in non-convergent compression lines. The compression curves of the hydrate bearing CS-fines mixtures can be classified into three different stress regimes. There is no significant difference in the compressibility of the soil mixtures with and without hydrate in the low stress regime. As the stress increases further reaching the medium stress regime, the hydrate reduces the compressibility of the soil mixtures with increasing hydrate saturation. Upon reaching the high stress regime, the bond breakage at the inter-particle contacts becomes significant leading to the convergence of compression curves between the hydrate bearing and host soil mixtures. It is also found that a new effective hydrate saturation, representing the amount of hydrate in the inter-particle voids, is better correlated to the compressibility of the hydrate bearing soil mixtures.