We explore the relation of stress history and failure mechanics of sediments from the Ursa Basin, Gulf of Mexico using direct simple shear experiments. We consolidated specimens to different preconsolidation stresses via load-increment consolidation; then sheared each specimen under undrained conditions to determine shear strength, shear-induced pore pressure, and their responses to stress history.
The stress history influences sediments properties before failure, and may impact failure dynamics. By shearing sediments with different initial porosity or different initial stress, we study how pre-failure conditions influence failure behavior (e.g., contractional or dilational). This can help us define if failing sediments are prone to contract and accelerate, to creep, or to dilate and stop failing.
Preliminary results on mud samples from the Ursa Basin indicate that as burial depth increases from 430 to 1340 cm below sea floor (bsf) the cohesion decreases from 18 to 6.5 kPa and internal friction angle increases from 10° to 21°. The preconsolidation stress for these specimens ranges from 16 to 42 kPa and experimentally derived peak shear strengths are 30–63 kPa. To investigate stress history, we studied three specimens from 1315–1340 cm bsf that were consolidated to different maximum effective vertical stresses or 52, 104, and 208 kPa. The specimens show an increase in shear-induced pore pressures from 38 to 73 to 180 kPa. The normalized undrained shear strengths are 0.29, 0.27, and 0.23 at maximum shear. This indicates that the normalized undrained shear strength decreases with vertical consolidation stress, whereas the maximum shear-induced pore pressure increases with vertical consolidation stress. Together these parameters may influence the depth at which failure occurs and the post-failure dynamics of these mud-rich sediments, but additional work is needed.
All these data are part of the geotechnical characterization of shallow, marine sediments from the northern Gulf of Mexico. These types of data will be used to test and calibrate sediment failure models which are vital for subseafloor geohazard analyses and offshore infrastructure development. Our study may lead to new assessment techniques of submarine landslide risks and provide new approaches to mitigate risk.