An Assessment of the Overconsolidation and Post-Sedimentation Structure of Marine Clay from Offshore Northern Mozambique-Paper

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The study investigated why marine clay offshore northern Mozambique exhibits unusually high undrained shear strength, sometimes exceeding 100 kPa at the seafloor. Researchers assessed whether the strength results from overconsolidation caused by natural processes or from post-sedimentation effects such as cementation or leaching.<br /><br />The assessment used 118 one-dimensional consolidation tests on intact specimens and six matched pairs of intact and remolded specimens. Yield stress ratios (YSRs) exceeded 10 near the seafloor and generally ranged from 2 to 5 at depths of 10–40 m, declining to about 2 at depths of 40 m or more. Pre-overburden pressure (POP) was typically 100–350 kPa. Canyon samples generally had higher and more variable YSRs than samples from areas between canyons, consistent with erosion having removed overlying sediment.<br /><br />Remolded specimens followed the intrinsic compression line, while intact-specimen compression curves generally fell between the intrinsic and sedimentation compression lines. The tests showed no characteristic evidence of cementation or other post-sedimentation structure. Higher carbonate contents affected compression behavior at high stresses, likely due to crushing of biogenic calcite particles, but did not indicate cementation.<br /><br />Taken together, the stress-history measurements and compression curves suggest that the clay’s overconsolidated behavior is principally attributable to natural mechanical consolidation, followed by erosion and aging—not to cementation or leaching. These findings help explain the clay’s high strength and inform offshore foundation and slope-stability design.

Keywords

marine clay

offshore Mozambique

undrained shear strength

overconsolidation

yield stress ratio

pre-overburden pressure

consolidation tests

cementation

erosion

carbonate content