HR: 0830h
AN: OS51C-0873    [PDF]
TI: Consolidation Characteristics of Hydrate Saturated Sediments from ODP Site 1244, Hydrate Ridge, Cascadia Continental Margin
AU: * Tan, B B
EM: brain@mit.edu
AF: The Massachusetts Institute of Technology, Department of Civil and Environmental Engineering, Cambridge, MA 02139 United States
AU: Germaine, J T
EM: jgermaine@mit.edu
AF: The Massachusetts Institute of Technology, Department of Civil and Environmental Engineering, Cambridge, MA 02139 United States
AU: Flemings, P B
EM: flemings@geosc.psu.edu
AF: The Pennsylvania State University, Department of Geosciences, University Park, PA 16802 United States
AU: Goldberg, D
EM: goldberg@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, 61 Route 9W, Palisades, NY 10964 United States
AU: Janik, A
EM: ajanik@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, 61 Route 9W, Palisades, NY 10964 United States
AU: ODP Leg 204 Scientific Party, .
EM:
AF: Ocean Drilling Program, Texas A&M University, College Station, TX 77845 United States
AB: A geotechnical investigation to measure consolidation and strength properties was performed on piston core sediment samples from ODP Site 1244, Hydrate Ridge, Cascadia Continental Margin. Eight whole core sections taken between 5.7 meters below sea floor (mbsf) to 136 mbsf were used for consolidation testing. The deepest sample was from the free gas zone beneath the gas hydrate zone. Constant Rate of Strain Consolidation (CRSC) tests were conducted on both intact and remolded samples. The CRSC results are used to evaluate the preconsolidation pressure, hydraulic conductivity, and compressibility parameters of the soil (e.g. compression ratio, recompression ratio). The preconsolidation pressure is usually determined by the Casagrande method or the strain energy method. The Casagrande method is a graphical method of determining the preconsolidation pressure that relies heavily on good quality samples to produce accurate results. The strain energy method is also a graphical method, but uses a plot of the work per unit volume vs. effective stress and has been found to be less subjective. Both techniques give comparable results and rely on the existence of a sharp transition between the elastic and plastic loading domain. The observed compression curves are rounded and do not have a clear transition between the overconsolidated and normally consolidated state. Furthermore, the strains at this transition stress are high compared to typical on-shore clay samples. Unload-reload cycles were performed during testing in order to compare the initial loading to subsequent unload-reloading. The resulting comparisons show that the initial loading slope is much softer than the unload-reload slopes. All of these characteristics indicate that the samples are of poor quality. As a result, the estimation of the preconsolidation pressure is obscured and contains a significant amount of uncertainty. Application of the strain energy method yield high preconsolidation pressures that indicate the soil is normally to slightly overconsolidated (1$<$OCR$<$2). Because of the uncertainty in these OCR values, an alternative method was used to estimate the pre-consolidation stress based on the in-situ void ratio and the extrapolation of the virgin consolidation curve. This method predicts that samples shallower than 33 mbsf are near normally consolidated (OCR~1.2) whereas deeper sediments are underconsolidated (OCR$<$1). The in-situ hydraulic conductivity was found by extrapolating the log-hydraulic conductivity vs. void ratio curves back to the in-situ void ratios. The in-situ hydraulic conductivity is found to vary between 9x10-6 to 8x10-7 cm/s with no trend with depth. The compression ratio (Cc) ranges from 0.473 to 0.704 with an average of 0.600. Cc is fairly constant up to a depth of 79 mbsf, after which, Cc decreases. The recompression ratio (Cr) ranges from 0.015 to 0.027 with an average of 0.021. Cr is constant throughout the depth.
DE: 1615 Biogeochemical processes (4805)
DE: 1829 Groundwater hydrology
DE: 8105 Continental margins and sedimentary basins
SC: Ocean Sciences [OS]
MN: 2003 Fall Meeting