United States Department of Transportation
i
Improving design phase evaluations for high pile rebound sites : final report.
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2016-05-31
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[PDF-62.58 MB]
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Edition:Final report
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Abstract:A testing program performed to help determine typical soils properties encountered during pile installation when high rebound
occurs produced a decision matrix for geotechnical engineers. High pile rebound (HPR) occurred at numerous sites in Florida.
Samples from standard penetration test (SPT) borings and thin-walled tube sample borings were used in addition to cone
penetrometer with pore pressure (CPTu) data to determine soil properties trends.
Relationships between rebound and (a) SPT blow counts (N), (b) CPTu pore water pressure, and (c) fines content (FC) from
previous studies were evaluated. Based on a large number of data points, weak correlations exist between inspector’s rebound based
and N, with rebound decreasing as N increased. There could be a relationship between rebound and FC up to about 35%; however,
beyond this threshold, there is no clear relationship. A weak correlation exists between CPTu pore water pressure and rebound.
FDOT Specification 455-5.10.3 based 0.25-inch rebound criterion was originally used but produced inclusive
comparisons; therefore 0.5 inch rebound was used to yield these results. Grain size data show that rebound may be a
function of certain grain sizes, implying that engineers could inexpensively locate HPR soils. The dry unit weights of the cohesive
HPR soils are much lower than expected, with many values being less than the density of water. Although there was no difference in
the Unified Soils Classification System or American Association of State Highway and Transportation Officials Classifications
between HPR or non-HPR soils, as both classified as SM or A-4/A-2-4, the following differences were observed: (a) The average silt
content for the HPR soils is more than twice as high as non-HPR soils, while both D30 and D60 are three times higher in the HPR soils
than in the non-HPR soils; (b) the Atterberg limits of the HPR cohesive soils produced an average plastic index nearly twice that of
the soils that displayed low to non-HPR problems; (c) the presence of silts significantly affects HPR; (d) clay content of cohesionless
soils may be an effective index for predicting HPR; and (e) the Atterberg limits PI and clay content clearly showed differences
between HPR and non-HPR soils. FC in the 30 to 40 % range could be an indicator of rebound greater than 0.5 inches. Sands with
fines from 12 to 50% showed the greatest rebound potential.
Permeability of HPR soils was one or two orders of magnitude lower than the non-HPR soils. Cyclic triaxial testing indicated
that HPR soils are much more resilient than non-HPR soils. HPR and non-HPR soils plotted in somewhat distinct regions on soil
behavior type charts. The rebound soils plotted as cemented silty fine sand with trace phosphate and shell or as cemented clayey fine
sand with fines. Rebound soils are dilative while non-rebound soils are contractive.
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