Geotechnical, microscopic and mineralogical characterization of surficial sediments nearshore the southern coast of Brazil

Many important maritime projects depend on the correct characterization of surficial sediments (e.g. gas and water pipelines; dredging works; transport of sediments, and water-soil-structure interaction). However, the sampling and characterization of such sediments are very challenging due to the environmental conditions. Furthermore, the investigation often consists only of a basic granulometric analysis. In this context, this paper presents a geotechnical, mineralogical, and microscopy characterization of an offshore area on the southern coast of Brazil. Surficial samples were collected from the seabed, in water depths between 13.0 and 91.0 metres. Then, they were submitted to basic geotechnical characterization tests (size distribution by sieving and sedimentation, Atterberg limits, and basic physical properties), mineralogical analysis by X-ray diffraction tests (XRD), microscopy investigation by scanning electronic microscopy (SEM) and shear vane tests. Based on the state and index parameters of the sediments, as well as the undrained shear resistance, three main sets of sediments have been identified: group I (sands), group II (clayey sands), and group III (mixtures of sand, silt, and clay). The spatial distribution of sediments was also inferred, reflecting different deposition environments and bottom dynamics. The clay mineral quantification through XRD revealed that the predominance of 2:1 clay minerals is related to the higher plasticity found in some samples. Moreover, the presence of diatoms could explain the difference in porosity in the samples, resulting in a higher plasticity and lower shear strength.