Diatomaceous soils are comprised of the silica frustules of diatom microalgae that are present in marine and lacustrine environments throughout the world. Owing to their unique origin, diatomaceous soils are typically characterized by high intraparticle porosity, complex particle shapes, and uniform mineralogy, causing them to exhibit atypical physical and engineering behaviors. A substantial deposit of diatoma-ceous silt was observed during site exploration for construction of the Buck Creek Bridge on OR140 near Klamath Falls, OR, USA. A comprehensive laboratory and in situ testing program indicated that the diatomaceous soil possessed“non-textbook”engineering properties. Specifically, tested samples had high liquid limits (≈100％-140％) with natural water contents at or near the liquid limit. Geologically, the soil is expected to be normally consolidated, yet high apparent overconsolidation ratios (OCR) (≈15-40) were observed both in oedometric consolidation tests and through cone penetration test (CPT) corre-lations. Standard penetration test (SPT) results show a corrected standard penetration resistance consistent with a medium-dense sand (i.e. (N1)60≈25). CPT results include corrected tip resistances (qt) of approximately 7-10 MPa and excess pore pressures (u2) of up to 4 MPa. In CPT dissipation tests, pore water pressures (PWPs) returned to hydrostatic pressure in less than 1 h. In this work, we synthesize these seemingly disparate material properties in an attempt to infer appropriate engineering properties for the diatomaceous deposit at the Buck Creek Bridge and attempt to provide insight into the underlying reasons for the observed behavior.