Evaluation of disturbance induced on soft offshore sediments by two types of gravity piston coring techniques

Sample disturbance is still a key issue in offshore investigations, especially when logistic and financial limitations do not allow the use of drilling equipment. This paper 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 non-clayey fraction and a non-negligible sandy fraction. 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 several volcaniclastic levels present 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 equivocal. In fact, methods for assessing sample disturbance have demonstrated to bear limited effectiveness when applied to soils with relatively low clay content and significant overconsolidation as it is the case of the studied sediments.