Limit equilibrium conditions for deep sea sediments and infinite plane failure analysis, offshore Gabon

Abstract

Deep offshore geological surveys have been conducted on the Gabon continental shelf. A geological description of the area explored reveals the presence of seepage features and gas bubbles in the upper 50 m of sediments. Seafloor instabilities also occurred at quite low slope gradients. A laboratory study of the mechanical behavior of the sampled sediments helped verify that the ratio between undrained shear strength and preconsolidation pressure is very high at low confining pressures in terms of classical empirical regressions used mostly by geotechnicians. These experiments also lead to a mathematical model of undrained shear strength versus depth profile for some saturated and normally consolidated sediments. Hypotheses are developed concerning the undrained behavior of underconsolidated sediments. The eventuality of seepage forces in sediments leads to the calibration of safety factors under drained and undrained conditions. It is established that static infinite plane failure cannot be triggered at low slopes if the sediments are normally consolidated. Graphs are finally constructed in order to visualize the impact of water excess pore pressure on superficial sediments stability. A realistic physical interpretation is also given.