Geotechnical properties of pelagic clays at DSDP Hole 86-576A

Sediment characterization and index-property data delineate three well-defined units that conform with major lithologic divisions. Within Subunit IA these data appear inconsistent with accepted geotechnical relationships, showing water content, plasticity, and grain size increasing over similar intervals. These unusual relationships are attributed, in part, to silt-sized bonded clay aggregates.

Within Subunit IB geotechnical properties become more uniform, and a marked increase in X-ray amorphous material occurs below 28 m sub-bottom. A pronounced change in the geotechnical behavior of Hole 576A sediments occurs in Unit II and corresponds to the presence of calcareous oozes. Consolidation test results show overconsolidated sediments in the upper 28 m and normally consolidated sediments between 28 and 55 m sub-bottom.

Mechanisms to explain the state of overconsolidation that are consistent with the geological history of the area include cementation and interparticle bonding, the release of high hydrostatic pressures during core recovery, and artifacts resulting from the consolidation test process. Additionally, the relevance of the preconsolidation stress determined using consolidation test results may be limited. Variations in the shear strength gradient with depth are consistent with the consolidation test results.

The gradient for the overconsolidated interval (0 and 28 m sub-bottom) is significantly higher than between 28 and 55 m sub-bottom, where the sediments become normally consolidated.