Geotechnical Site Investigation Over 3500 Meters Depth

This paper aims to present a survey executed in 3000 meters depth, to support the design of subsea structures in exploration blocks located in Sergipe/Alagoas basin, Brazil, with geotechnical work made over 3500 meters depth, using a Piston Corer for sampling and a gravity CPT for testing. The vessel used for this project was R/V Seward Johnson, a Research/Survey vessel. The Piston Corer used for the survey, to work in depths above 3000 meters, was configured with a core head weighing 1,500 kg, with a maximum length of the sampling tube of 12 m and a 3.5 meters free fall. The samples were recovered in a 102 mm inner diameter plastic PVC (liner) coating within the 112 mm outer diameter steel tube. In addition to being stationary, the Piston Corer have a piston immobilizer, that protects the sample from disruptive forces during pull-out. The gravity CPT that was used in this campaign was coupled on Piston Corer structure, to function in the pressure of deep water. For this Project, gCPT tests was conducted to verify the soil resistance and Piston Corer samples collected, to analyze tactile-visual Classification, determination of moisture content, determination of apparent specific weight, evaluation of the content of Calcium carbonate (CaCO3) and determination of Undrained Shear Strength (Su) by pocket penetometer test, torvane test and laboratory mini vane test. Through the piston core sampling and tactile visual classification, it was possible to define the soil content in the area, consisted of normally consolidated clays. This geotechnical survey has few locations to make a more accurate assessment of what may have occurred geologically in this area. Despite few evidence, analyzing the Piston Core samples onshore, along with gCPT results, the layers of deposition of the sediment were identified, with the general soil composition of the area being described as light brown clayed SILT with fine sand and shell fragments in the superficial sediments and dark gray silty CLAY with fine sand in the deeper sediments. The data produced from this campaign shows the possibility of geotechnical studies in areas of deep water, over 3500 meters depth, with reliable results. In order to work in a deep area, there were some alterations needed in the equipment, such as assembling the CPT in Piston Corer structure, and using a Piston immobilizer on the piston corer sampler. The improved equipment provided good quality data of the region.