Evaluation of disturbance induced on soft offshore sediments by different sampling techniques

Sample disturbance is still a key point in offshore investigations, especially when logistic and financial limitations do not allow the use of drilling equipment controlled on board. In fact many geotechnical, geological and geophysical analyses require knowledge of physical and mechanical properties, e.g. undrained shear strength, strength parameters in effective stresses, stiffness and damping parameters. These parameters are affected by soil disturbance induced by the sampling procedure, which in most offshore projects is particularly significant. This presentation focuses on the comparison between the disturbance induced by a conventional free fall piston corer (FF) and a modified piston corer (AD) equipped with a velocity control (Angel Descent method). Twin core samples were retrieved in two successions of pelitic sediments with a prevailing nonclayey fraction and a non-negligible sandy fraction. Seafloor sampling was performed in 2014 during the SAOS (Stability Assessment of an Open Slope) cruise onboard R/V Urania (CNR) within the national research project RITMARE. The cruise was jointly organized by the IAMC (now ISMAR) and IGAG institutes of the CNR and by OGS, in the South Tyrrhenian Sea (Southeastern Italy). At three of the ten coring locations during the SAOS cruise, sampling was duplicated using a standard piston corer and a piston corer employing the Angel Descent® method described in Magagnoli (2017). Two pair of cores were collected in the unfailed sedimentary section just up-slope of the Licosa Landslide crown scarp in about 250 m water depth (Sammartini et al., 2018). One pair of cores was collected from an intra-slope basin in 673 m water depth. Comparison was based on different acquisition, physical and mechanical parameters ranging from accelerometer data to magnetic susceptibility logs and geotechnical parameters from laboratory investigations, including oedometer compression tests and cyclic simple shear tests. Accelerometer data highlighted the sharp reduction in velocity obtained for AD samples. Magnetic susceptibility logs, characterized by a pattern of peaks induced by volcaniclastic levels interspersed in the succession, indicated that the AD method significantly reduces core shortening. Among geotechnical investigations, cyclic shear tests provided small-strain shear moduli always higher in AD samples, whilst the response of oedometer compression tests was non-univocal, being influenced by the prevailing non-clayey fraction, as it has been pointed out in the very recent technical literature