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 (z100%e140%) 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) (z15e40) 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 z 25).  CPT results include corrected tip resistances (qt) of approximately 7e10 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.

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