Geotechnical characterization of sediments from Hydrate Ridge, Cascadia Continental Margin

Abstract

Eight whole core sediment samples were obtained from ODP Site 1244, Hydrate Ridge, Cascadia Continental Margin with the goal of understanding the stress history, consolidation behavior and strength characteristics of the soil. A series of Constant Rate of Strain Consolidation (CRSC) and Ko- Consolidated Undrained triaxial (CKoU) tests were performed in order to study the behavior. In addition, Atterberg limits and x-ray diffraction were performed in order to better classify the material. One of the key issues regarding this soil is the level of disturbance imparted during sampling and transportation. Evidence of the disturbance are cracks and voids in x-rays, as well as highly rounded compression curves. Established soil quality criteria have shown the soil to have poor quality. A new criterion comparing the initial loading to the reload cycle shows that soil quality varies, but has no pattern with depth. Nonetheless, highly disturbed samples and trimmings from previous tests were resedimented to produce better quality specimens for consolidation and strength testing. Conventional application of the strain energy method yielded high preconsolidation pressures that indicate the soil is normal to overconsolidated (1<OCR<8). An alternative method used to estimate the pre-consolidation stress based on extrapolation of the virgin consolidation curve to the in-situ void ratio predicts that samples shallower than 33 mbsf are near normally consolidated (OCR-1.2) whereas deeper sediments are underconsolidated (OCR<1). However, analysis of the stress path history from a horizontally-oriented CRSC sample gives evidence that the in-situ horizontal effective stress is greater than the vertical effective stress.

(cont.) This analysis provides an upper and lower bound factor that is applied to the strain energy preconsolidation pressure. The result is a reduced preconsolidation pressure that indicates the soil is underconsolidated (0.2<OCR<0.8). The in-situ hydraulic conductivity (ko) is found to vary between 1.5x10-7 to 3x10-8 cm/s with no trend with depth. The compression ratio (Cc) ranges from 0.473 to 0.704 and is fairly constant up to a depth of 79 mbsf, after which, Cc decreases. The triaxial tests have revealed that the site may be divided into two layers, with the shallow layer extending up to 20.3 mbsf and the deep layer extending below 20.3 mbsf. The resedimented specimens exhibited behavior similar to the specimens from the shallow layer. The average normalized undrained strength for the shallow and deep layers are 0.35 and 0.31 respectively. The average friction angle in triaxial compression for the shallow layer is 36⁰ and 33⁰ for the deep layer. The laboratory test results were used to determine the SHANSEP parameters of the soil, which, when combined with the stress history at the site, gives the strength profile of the site. Finally, the input parameters for the MIT E-3 soil model were estimated, which will allow further study of the behavior of the soil.