Effects of Stress on Failure Behavior of Shallow, Marine Muds from the Northern Gulf of Mexico

Abstract:

Direct simple shear (DSS) experiments on mud samples from 4.3-13.4 meters below sea floor (mbsf) in the Ursa Basin of the northern Gulf of Mexico document how stress level impacts soil strength and pore pressure genesis during failure. As burial depth increases from 7.3 to 13.4 mbsf, cohesion decreases from 12.3 to 6.5 kPa and internal friction angle increases from 18° to 21°. For the same depth increase, peak shear strength increases from 30 to 63 kPa. For a specimen from 11.75 mbsf, an increase in maximum consolidation stress from 45 to 179 kPa results in an increase in the shear-induced pore pressure from 29 to 150 kPa. The normalized shear strength at peak shear strength, however, decreases from 0.37 to 0.25 over this consolidation stress range. Our results document that compaction induces a positive feedback on pore pressure genesis. This feedback suggests an increase in failure potential during burial at shallow depth. To further understand the physical controls on this behavior, we complete DSS experiments on resedimented samples to erase stress history and sediment fabric. For the resedimented samples, cohesion is 3.6 kPa and internal friction angle is 24°. As maximum consolidation stress increases from 40 to 254 kPa on resedimented samples, peak shear strength increases from 14 to 91 kPa and shear-induced pore pressure increases from 22 to 203 kPa. The normalized shear strength at peak shear strength, however, decreases from 0.32 to 0.25. These results show resedimented samples have similar strength and failure behavior to intact samples. By constraining pore pressure and strength in comparison to initial stress state, we gain better insight on slope failure dynamics. Therefore, this study may provide constraints on soil strength of shallow sediments and pore pressure genesis during failure of shallow marine sediments. The impact of compaction stress on soil strength and pore pressure genesis during shear failure can be included in slope failure and risk models to better constrain failure conditions and potential.