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Geotechnical property results from Deep Sea Drilling Project Hole 576A sediments represent the most complete data set ever compiled for a continuously depositional, sedimentary section that spans 65 m.y. Geotechnical analyses included index-property and sediment characterization, and consolidation, permeability, and triaxial testing. Excellent agreement exists between test data generated in this multilaboratory effort. 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. INTRODUCTION The constraints of standard coring techniques have limited most studies of the geotechnical properties of marine sediments to samples recovered from less than 15 m sub-bottom. Many of these studies have tried to relate variations in geotechnical properties to variations in geological processes. In many instances, however, the relative geological complexity of the area or an insufficient geotechnical data base have precluded the development of such relationships. At Deep Sea Drilling Project (DSDP) Site 576, we were able to evaluate these relationships for a fairly simple, continuously depositional, pelagic clay environment whose geological history is well substantiated over the last 65 m.y. Site 576 was triple cored (Holes 576, 576A, 576B) to get enough sediment samples for detailed sedimentological, stratigraphic, geochemical, and geotechnical analyses. At the request of the Marine Geological Consortium, Hole 576A was continuously cored and preserved in the full-round for detailed shore-based geotechnical property analyses. These analyses were intended to evaluate 1 Heath, G. R., Burckle, L. H., et al., Init. Repts. DSDP, 86: Washington (U.S. Govt. Printing Office). ^ The Marine Geotechnical Consortium for Deep Sea Drilling Project Leg 86 consisted of the University of Colorado (CU), Boulder, Colorado; Humboldt State University (HSU), Arcata, California; Institute of Oceanographic Sciences (IOS), Wormley, Godalming, United Kingdom; Oregon State University (OSU), Corvallis, Oregon; San Diego State University (SDSU), San Diego, California; Sandia National Laboratories (SNL), Albuquerque, New Mexico; Texas A&M University (TAMU), College Station, Texas; University of Rhode Island (URI), Narragansett, Rhode Island; and United States Geological Survey (USGS), Denver, Colorado. The following persons contributed to this study. CU: Robert Schiffman, Andrew Dean; HSU: Ronald Chaney, J. Suehanek, B. Vega, T. McFarland; IOS: Peter J. Schultheiss, David Gunn; OSU: George Keller; SDSU: Iraj Noorany, M. Faghihi; SNL: Les Shephard, Joel Lipkin, Jean Kinker; TAMU: William Bryant, Elliot Taylor, Mark Johns; URI: Armand Silva, William Levy, Ricci Siciliano, Kathleen Dadey; USGS: Harold Olsen. the interrelationships between the geotechnical properties and the geological processes of an abyssal hill environment, specifically as they pertain to variations in the sediment stress history, consolidation-permeability relationships, and shear strength. GEOLOGIC SETTING Site 576 is located east of Shatsky Rise in the western North Pacific red clay province (Fig. 1) very near magnetic Anomaly Ml3, which indicates a basement age of 129 m.y. (Site 576 chapter, this volume). Regional seafloor morphology and near-surface seismic profile characteristics have been mapped and discussed by Damuth et al. (1983). The location of Site 576 is characterized by a smooth to gently rolling seafloor. Seafloor bathymetry gradually deepens from southwest to northeast at a gradient of approximately 1:1000, reflecting an abyssal hill oceanic basement covered by pelagic sediments. Water depth at Site 576 is 6127 m. The sedimentary section of Hole 576A can be divided into three general lithologic units on the basis of macroscopic descriptions and smear-slide analyses (Site 576 chapter, this volume; Fig. 2). Unit I consists of pelagic, brown clay extending to 55 m sub-bottom that can be divided into two subunits. Subunit IA (0 to 28 m sub-bottom) is a pelagic, yellowish brown, burrowed clay. The abundance of burrows and the silt-sized fraction generally decrease down through Subunit IA. Subunit IB (28 to 55 m sub-bottom) is an unburrowed, homogeneous, dark brown, pelagic clay generally containing less than 20% silt. The contact between these subunits is gradational over several meters. Unit II is composed of interbedded carbonate oozes and brown pelagic clays (55 to

Geotechnical Properties of Northwest Pacific Pelagic Clays: Deep Sea Drilling Project Leg 86, Hole 576A