Description: Marine diatomaceous sediments are common along the polar belts and equator, but very little is known about their effect on sediment geotechnical properties and slope stability. Consolidation state analysis is frequently applied to derive past maximum overburden stress, to quantify over- or underconsolidation, or to infer excess pore water pressure, all relevant to assess risk of slope failure. Diatoms significantly alter geotechnical and other fundamental engineering properties usually observed in organic or inorganic sediment. The consolidation state of diatomaceous sediments is ambiguously discussed because geological evidence and laboratory data do not always correspond. A literature review revealed a near systematic overconsolidation of shallow diatomaceous sediments (〈 100 mbsf) and normal or underconsolidation in deeper sediment sequences. One-dimensional compression tests are carried out on material sampled during the R/V POLARSTERN cruise ANT XXIX/4 to a landslide-prone area of the South Sandwich Trench, and on generic clayey-silt - diatomaceous earth sample mixtures. Results indicate that diatoms alter geotechnical properties to an extent that in situ stress conditions may not well be inferred from common consolidation state analysis. Undrained vane shear strength underestimates the in situ undrained shear strength and leads to underestimated normalized undrained shear strength ratios. Enhanced secondary compression with overburden and diatom content leads to a natural curvature of consolidation lines, the latter occasionally falsely interpreted as preconsolidation stress. The observations are furthermore dependent on the predominant diatom order. Moreover, inverse trends of porosity are not necessarily related to excess pore water pressure, but solely to a gradual increase of diatoms with depth.

Description: The upper shelf of the landslide-prone Ligurian Margin (Western Mediterranean Sea) off Nice well-known for the 1979 Airport Landslide is a natural laboratory to study preconditioning factors and trigger mechanisms for submarine landslides. For this study low-stress ring shear experiments have been carried out on a variety of sediments from 〉50 gravity cores to characterise the velocity-dependent frictional behaviour. Mean values of the peak coefficient of friction vary from 0.46 for clay-dominated samples (53 % clay, 46 % silt, 1 %) sand up to 0.76 for coarse-grained sediments (26 % clay, 57 % silt, 17 % sand). The majority of the sediments tested show velocity strengthening regardless of the grain size distribution. For clayey sediments the peak and residual cohesive strength increases with increasing normal stress, with values from 1.3 to 10.6 kPa and up to 25 % of all strength supported by cohesive forces in the shallowmost samples. A pseudo-static slope stability analysis reveals that the different lithologies (even clay-rich material with clay content 〉=50 %) tested are stable up to slope angles 〈26° under quasi-drained conditions.

Description: Submarine groundwater discharge (SGD), the flow of fresh and saline groundwater from the seabed into the coastal ocean, has been intensively investigated in the recent years. This research has usually been restricted to shallow water and intertidal areas, whereas knowledge about groundwater seepage in deeper water is mainly limited to point sources from karstic aquifers. In this study we observed submarine groundwater seepage and a subterranean estuary in sediments at water depths of 20-44 m located within the Ligurian Margin, western Mediterranean Sea. Here, a catastrophic submarine landslide occurred near the Nice airport (French Ligurian coast) in the fall of 1979 after a period of heavy rainfall. During two research cruises, gravity cores were recovered in and around the area of the landslide scar. Pore water samples collected from sediment cores indicated sediments containing freshwater within the landslide scar. Pore water profiles of selected ions, such as chloride, ammonium, manganese, sulfate and barium were used to assess transport and reaction processes within the sediment. A 1-dimensional transport model indicates in most cores upward pore water velocities of 2.3-8.8 cm/yr. This study shows that submarine groundwater seepage along the French Mediterranean coastline can occur at water depths reaching 44 m.