A STUDY OF THE PHYSICAL PROPERTIES AND STRUCTURAL DEVELOPMENT OF CLAYS IN THE SHALLOW METHANE HYDRATE FIELDS

Abstract

The physical and consolidation properties of sediments were investigated using borehole cores collected from the Sakata Knoll, where shallow methane hydrate fields exist, and from nearby sites where methane hydrate is not present. The soil particle density, grain size distribution, liquid limit, plastic limit, and plasticity index were similar at all depths. The void ratio change with depth was approximately e=3 near the seafloor, decreased to approximately e =2 at approximately 20 meters below the seafloor, and remained almost unchanged until approximately 56 meters below the seafloor. This void ratio change was found to be similar to that of the seafloor in the Joetsu Basin, which has shallow methane hydrate fields. The degree of soil structure development was examined by performing incremental loading consolidation tests on the collected core samples. Structural coefficient A values indicated that structure developed with increasing depth in both cores, but the degree of structural development was small regardless of whether the cores were in the Methane hydrate bearing area or not.