The marine clay found offshore northern Mozambique was investigated for the development of a proposed natural gas field. The marine clay was found to exhibit an unusually large undrained shear strength at shallow depths, in some instances exceeded 100 kPa at the seafloor, a response initially thought to be caused by either overconsolidation or cementation. The post-sedimentation structure of the marine clay was assessed to determine if cementation was present and the most likely mechanisms contributing to the overconsolidation. A series of one-dimensional consolidation tests were conducted on intact and remolded specimens. All intact specimens tested from the study area were found to be overconsolidated, typically with moderately high yield stress ratios (YSRs) (2 < YSR < 5) at depths between 10 and 40 meters (m) below the seafloor; even higher ratios were encountered in the upper 10 m. Collectively, the high YSR and pre-overburden pressure (POP) measured within the deeper canyons, the absence of an "S" shaped virgin consolidation curve, POP values generally consistent within each exploration, and compression curves for intact specimens generally between the intrinsic compression line and sedimentation compression lines, provide evidence suggesting that the overconsolidation of the northern Mozambique offshore marine clay is principally the result of natural sedimentation due to mechanical consolidation with erosion and aging and not the result of post-sedimentation structure, such as cementation or leaching.
Logan C. Brant and Rodolfo B. Sancio
Keywords: sedimentation structure, intrinsic compression, marine clay, offshore northern Mozambique
Publisher: DFI