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The hydraulic piston coring device (HPC-15) allows recovery of deep ocean sediments with minimal disturbance. The device was used during Leg 72 of the Deep Sea Drilling Project (DSDP) aboard the Glomar Challenger. Core samples were recovered from bore holes in the Rio Grande Rise in the southwest Atlantic Ocean. Relatively undisturbed sediment cores were obtained from Holes 515A, 516, 517, and 518. The results of shipboard physical property measurements and on-shore geotechnical laboratory tests on these cores are presented in this chapter. A limited number of 0.3 m cores were obtained and used in a series of geotechnical tests, including one-dimensional consolidation, direct shear, Atterburg limit, particle size analysis, and specific gravity tests. Throughout the testing program, attention was focused on assessment of sample disturbance associated with the HPC-15 coring device. The HPC-15 device limits sample disturbance reasonably well in terrigenous muds (clays). However, sample disturbance associated with coring calcareous sediments (nannofossil-foraminifer oozes) is severe. The noncohesive, granular behavior of the calcareous sediments is vulnerable to severe disturbance, because of the design of the sampling head on the device at the time of Leg 72. A number of modifications to the sampling head design are recommended and discussed in this chapter. The modifications will improve sample quality for testing purposes and provide longer unbroken core samples by reducing friction between the sediment column and the sampling tool. INTRODUCTION Until recently, the Deep Sea Drilling Project (DSDP) was equipped to recover only rotary core samples from the upper 200 m of an ocean sediment profile. This technique produced some disturbance in recovered core samples (Demars and Nacci, 1978). A number of studies have addressed physical property measurements of these relatively disturbed samples, including Boyce (1974), Hamilton (1976), Bennett and Keller (1973), and Demars and Nacci (1978). The development of the hydraulic piston coring device (HPC-15) has brought the opportunity to recover relatively undisturbed core samples. Since late 1978, the HPC-15 device has permitted recovery of continuous, undisturbed samples from sediment profiles exceeding sub-bottom depths of 50 m. This device has provided specimens for sedimentologists, paleontologists, and physical properties specialists to examine uniformly continuous sediment profiles with moderate compression of the sediment fabric. The compression is attributed to the effects of advancing the tool into the sediment column. Both laboratory and shipboard geotechnical testing can be used to evaluate the relative disturbance of sediment cores obtained from the HPC-15 device. Schmertman (1955) and Suva (1974) discussed the influence of Barker, P. F., Carlson, R. L., Johnson, D. A., et al., Init. Repts. DSDP, 72: Washington (U.S. Govt. Printing Office). sample disturbance on the behavior of the one-dimensional consolidation tests. Use of HPC-15 in the sampling program of Leg 72 provided sediment for assessing the effectiveness of this tool in reducing sample disturbance. Results of this evaluation, along with recommendations for redesign of HPC-15, are contained in this chapter. HPC PROCESS, HANDLING, AND LABORATORY PROGRAM To obtain high quality sediment cores, DSDP engineers designed and implemented the hydraulic piston coring HPC-15 device (see Fig. 1). The HPC-15 has been in operation since December 1978, Leg 64. Cores are taken by injecting a 4.4 m (15 ft.) long sampling tube by hydraulic pressure into the sediment at a high velocity, and then withdrawing the sample with a wire line through the drill string to the ship's deck. The HPC-15 recovers relatively undisturbed 63.5 mm diameter core. By repetitive coring downward in the same hole, relatively undisturbed continuous cores are recovered. The HPC-15 system was designed with limits for total penetration below the seafloor of 200 m. Maximum shear strength of recovered material is 188.1 kN/m (1.92 kg/ cm) as reported by Storms (1979). Coring is severely limited when the shear strength consistently exceeds 98.0 kN/m (1.00 kg/cm). The design of the sampling head restricts full penetration when stiffer sediments are encountered. Once the core barrel reaches the ship's deck, the core liner filled with sediment is removed from the core bar537 W. H. WALTON, D. A. SANGREY, S. A. MILLER Its Overshot Top sub Outer body cap Shear pin

Geotechnical Engineering Characterization of Hydraulically Piston-Cored Deep Ocean Sediments