Data report: preliminary assessment of Pleistocene sediment strength in the Ursa Basin (Gulf of Mexico continental slope) from triaxial and ring shear test data

We report the preliminary results of a triaxial and ring shear study on clay-rich, fine-grained Pleistocene sediments cored in Ursa Basin, Gulf of Mexico continental slope. Specimens from Integrated Ocean Drilling Program (IODP) Expedition 308 Sites U1322 and U1324 document friction coefficients in the range of 0.13–0.31, with internal angles of friction of ~7.4°–17.2° in ring shear experiments. At intermediate (7.624 MPa) to high (15.237 MPa) overburden pressure, the majority of the samples tested show velocity weakening, whereas lower overburden pressures do not give a clear trend regarding velocity weakening or strengthening of the samples. In consolidated-undrained triaxial tests, peak shear stresses observed are between 27 and 140 kPa, with the strongest sample by far coming from a core catcher section. We suspect that this is an effect of fabric changes induced during hydraulic piston coring. One sample coming from the base of a mass transport deposit at Site U1322 is the weakest one tested. Young’s moduli calculated range from 2 to 17.4 kPa. Stress paths indicate slight overconsolidation of the samples, which is in line with the information gained from preconsolidation stresses in other studies. Permeability determined from consolidation data is in the range of 10–16 to 10–17 m2, and hydraulic conductivity is around 10–9 to 10–10 ms–1. Grain density of the tested samples is around 2.7 g/cm3, and water content ranges from 18.3% to 30.7%.