Comparison of geotechnical properties from large-diameter long cores and borings in deep water Gulf of Mexico
Large-diameter long piston cores (Jumbo Piston Corer, JPC) and Large-diameter Gravity Cores (LGC) were taken immediately adjacent to previously drilled geotechnical borings at three floating platform sites: Auger, Jolliet, and Marlin. This task was included as part of a more comprehensive NSF program on seabed processes in the deep water Gulf of Mexico. Sediment properties measured included bulk density, magnetic susceptibility, compression wave velocity, vane shear strength, and unconsolidated-undrained triaxial strength. A comprehensive geotechnical-testing program confirms the samples are high quality and shear strengths within the 63-ft core depth were comparable to the results of tests on the geotechnical borings. The exception occurred when gassy deposits were encountered. The use of the LGC and Multi-Sensor Core Logger (MSCL) in conjunction with the JPC proved to be valuable in assessing the quality and continuity of the piston cores. At the Auger and Marlin sites, there was good agreement between the sediment properties obtained from the borings and cores over the cored depth of 63 ft. At the Jolliet site, the values of strength obtained from the core in the upper 10 to 20-ft. were considerably higher than those obtained from the nearby boring. With modifications, the long coring system can be extended to take 100-ft samples. The use of large-diameter piston and gravity cores can provide an economical alternative to traditional borings for the design of shallow foundations for subsea completions, pipelines, suction caissons, and identification of geohazards.