INTRODUCTION

Gas hydrates are gas-water compounds formed under high pressures and low temperatures. Gas hydrates consist of a gas molecule, most commonly methane, enclosed within a crystal lattice of water. The resulting compound is an icelike solid that dissociates into gaseous form when exposed to low pressures and high temperatures. Conditions favorable to the formation of gas hydrates exist beneath the seafloor in a region called the gas hydrate stability zone (GHSZ). The free gas zone (FGZ), beneath the GHSZ, is where the geothermal gradient causes the temperature to exceed that required for gas hydrate stability.

Ocean Drilling Program (ODP) Leg 204, a 2-month-long cruise to study the biogeochemical factors controlling the distribution and concentration of gas hydrates in an accretionary margin—specifically, Hydrate Ridge, Cascadia continental margin—began in July 2002. This region is located off the coast of Oregon in the Pacific Northwest, USA. ODP Site 1244 is located in 890 m of water, ~3 km northeast of the southern summit of Hydrate Ridge (Tréhu, Bohrmann, Rack, Torres, et al., 2003). Three-dimensional seismic data have shown that the bottom-simulating reflector (BSR) occurs at a depth of ~125 meters below seafloor (mbsf) at this location (Tréhu, Bohrmann, Rack, Torres, et al., 2003). Eight whole-core samples totaling 8.5 ft obtained from Hydrate Ridge were brought to the Massachusetts Institute of Technology (MIT; Cambridge, Massachusetts, USA) geotechnical laboratory to perform experiments with the following goals in mind:

1. To determine the consolidation and strength properties of the soil, such as the compression and recompression indexes, hydraulic conductivity, undrained strength, friction angle, and modulus.
2. To determine the stress history profile at the site, keeping in mind that Hydrate Ridge is located in an accretionary margin setting and may be under passive loading.
3. To estimate the stress history and normalized soil engineering properties (SHANSEP) parameters for the site as a means of determining the strength profile.
4. To obtain the input parameters for a complex soil model, specifically, the MIT E-3 soil model developed by Whittle (1987) using the "best-fit" parameter method proposed by Korchaiyapruk (2000).

Consolidation properties were determined from results of constant rate of strain consolidation (CRSC) tests on both intact and resedimented samples. Strength properties were measured from the results of Ko-consolidated undrained (CKoU) triaxial tests.

The eight whole-core samples were obtained from Holes 1244B and 1244C at various depths. Table T1 gives a list of the samples provided by ODP to MIT for laboratory tests. Sections 204-1244B-1H-4, 3H-3, 4H-6, and 6H-8 and 204-1244C-8H-7, 9H-5, and 13H-3 were located within the GHSZ. The sample from Section 204-1244C-17H-3 was located below the BSR in the FGZ. Table T2 gives an overview of all the tests performed.