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Although extensive experience exists with performing and interpreting cone penetration tests (CPT) in sands and clays, relatively little experience exists with performing and interpreting cone penetration tests in silty soils (that is, soils ranging in gradation from very silty sands to sandy silts, silts, and clayey silts). In the fall of 1981, over 365 m of cone data were obtained at four offshore California sites. At these sites, the subsurface materials were predominantly of late Pleistocene geologic age, and bracketed the range of soil gradations from very sandy silt to clayey silt.
This experience indicates that because of their intermediate permeability, silt deposits frequently exhibit partial drainage (that is, development of undissipated pore pressures) during cone penetration testing. Thus, the direct quantitative evaluation of material properties using procedures developed for the behavior of drained sand or the undrained behavior of clay was not appropriate. By comparing the CPT data with laboratory strength and permeability data, we were able to determine the primary factors affecting CPT data in silts. The analyses indicate that the permeability/drainage effects are of primary importance and that strength, density, and overburden effects are of secondary importance. The range of soil permeabilities that result in partial drainage during cone penetration testing are interpreted.
silts, penetration tests, permeability, cone penetration tests, partial drainage
Engineer manager, McClelland Engineers, Inc., Ventura, CA
Senior civil engineer, Chevron U.S.A., Inc., Concord, CA