A marine clay found within the development area of a gas field offshore east Africa was tested in the laboratory to measure its geotechnical properties. That work found the marine clay to exhibit abnormally high shear strength at shallow depth below the seafloor primarily due to overconsolidation from erosion and aging. This marine clay also exhibits relatively high values of carbonate content, but its mechanical response is not controlled by it. Given that the gas field is located within an area of moderate seismicity, a cyclic testing program was implemented. Laboratory tests were conducted to measure shear strength degradation and cyclic softening. Consistent with findings in geotechnical literature, results indicate that 1) higher applied cyclic stress ratio (CSR) causes specimens to reach similar strain levels after fewer cycles, 2) the CSR vs. N curve is relatively flat, 3) increasing overconsolidation ratio causes an increase in the cyclic strength of the soil (more cycles required to reach similar strain), and 4) specimens tested at a higher frequency exhibit higher cyclic strength than specimens tested at a lower frequency. Most cyclic tests were followed by either a monotonic direct simple shear test to measure post-cyclic undrained strength, sustained shear stress to measure creep, or reconsolidation to the initial vertical effective stress. The results of these tests are also described herein.
Rodolfo B. Sancio and Logan C. Brant
Keywords: Marine clay, offshore east Africa, cyclic shear strength
Publisher: DFI