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.