United States Department of Transportation
i
Improving Design Phase Evaluations for High Pile Rebound Soils with an Emphasis on SPT Testing: Final Report
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2019-10-30
[PDF-6.31 MB]
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Edition:Covered Final Report, April 2017 – October 2019
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Abstract:Pile rebound, which is a complex phenomenon that occurs when large diameter displacement piles are driven into very fine sands with silts and clays, causes serious construction delays and cost overruns. It has been a problem many contractors have faced for well over a decade. Piles driven into thick layers within both the Hawthorn and Alum Bluff Group of soils have caused the problem. An evaluation of pile rebound related to SPT N values and CPT profiles was undertaken to validate the High Pile Rebound Level 1 Decision Tree developed under FDOT Contract BDV 28 977-01. The database was increased from 15 piles at eight sites throughout central, north and northwest Florida, to 37 piles at eight additional sites from this same region plus locations in southwestern Florida. Rebound levels ranged from 0.38 to 2.63-inches based on pile driving analyzer data with inspector sets. A thorough evaluation of the SPT/CPT and PDA produced an updated Level I Decision Tree based on 0.5-inches of pile rebound. A key improvement in this analysis was to eliminate what was termed “early driving” data from the SPT and CPT results. Early driving data was defined as any data within the predrilling elevations and any pile driving conducted up to fuel setting 2 on the hammer. This new analysis produced three additional changes to the Level I Decision Tree. The revised data more clearly produced results that showed poorly graded sands (SP) would not produce rebound above 0.5-inches. Clear dilative soil tendencies were shown for rebound soils based on N1(60) values over 30 and Seed et al. 1985 recommendations based on soil liquefaction. The USCS classifications most likely to produce rebound in excess of 0.5-inches are SM and ML/MH, indicating that silt content of these very fine sands is critical to rebound. CPTu soil behavior type charts were shown to possibly be useful a a “screening-tool” to identify rebound, but must be used in conjunction with other techniques.
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