Cyclic Response of an East African Marine Clay-Paper

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The study evaluated the cyclic behavior of overconsolidated marine clay from a gas-field development offshore east Africa, where earthquake-induced softening could affect seabed slope stability and foundation performance. Although the clay has substantial carbonate content, its mechanical behavior was broadly similar to that of other marine clays.<br /><br />Researchers conducted 23 stress-controlled cyclic direct simple shear tests on samples recovered about 15–20 m below the seafloor. Specimens were recompressed and tested at an effective vertical stress of 100 kPa. Cyclic stress ratios ranged from 0.15 to 0.98, with tests varying in cycle count, loading frequency, overconsolidation ratio (OCR), and initial shear stress. Most tests used a frequency of 1 Hz; selected tests used 0.1 Hz.<br /><br />Results showed that higher cyclic stress ratios produced comparable strain levels in fewer cycles, while the relationship between stress ratio and cycles to a given strain was relatively flat. Higher OCR increased cyclic strength, as did higher loading frequency. The clay’s cyclic response was consistent with findings for other clays, despite its high carbonate content. Applying an initial shear stress representative of sloping ground reduced cyclic resistance by about 20% at shear strains above 2%.<br /><br />Post-cyclic tests found that undrained strength generally declined as cyclic shear strain increased. Strength was usually near its pre-cyclic level when cyclic strains remained below about 3%; greater strains could cause substantial strength loss. Creep and reconsolidation tests showed modest deformation, but did not establish clear relationships between deformation and prior cyclic loading conditions.<br /><br />The findings provide laboratory data for assessing seismic effects on offshore infrastructure and emphasize the importance of accounting for stress history, loading frequency, initial shear stress, and cyclic strain.

Keywords

overconsolidated marine clay

cyclic direct simple shear

cyclic stress ratio

earthquake-induced softening

seabed slope stability

offshore gas-field development

overconsolidation ratio

undrained strength

initial shear stress

cyclic shear strain