Design phase identification of high pile rebound soils : final report
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Design phase identification of high pile rebound soils : final report

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  • English

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      Final report; April 28, 2008-February 28, 2010
    • Abstract:
      An engineering problem has occurred when installing displacement piles in certain soils. During driving, piles are rebounding excessively during each hammer blow, causing delay and as a result may not achieve the required design capacities. Piles driven at numerous locations have recorded rebound values well over 1 inch per blow. The research objective was to determine geotechnical testing protocol to help engineers anticipate high rebound. There are high pile rebound sites throughout North America. This problem typically occurred when displacement piles driven with single acting hammers, encountered silts and clays, in medium dense or stiff soils. Computer models indicated that soil quake and pile rebound were high. Withi Florida, a geologic layer known as the Hawthorn Group was encountered when high pile rebound occurred. Testing was conducted at three sites; two in the Orlando area and a third in the Florida Panhandle. Field tests included Standards Penetration Borings to produce N-values, Pocket Penetrometer tests to produce unconfined compressive strengths, Cone Penetrometer soundings to produce point bearings, sleeve friction and pore water pressures, Pencel Pressuremeter tests to produce in situ stress-drain data and Dilatometer testing to produce elastic moduli. Lab testing included natural moisture contents, grain size and hydrometer analyese, Atterberg limits, permeability and consolidated undrained triaxial testing. Orlando area test results combined with Pile Driving Anlayzer (PDA) data, revealed one high pile rebound zone through which the piles were able to be driven over a lower zone which pprevented pile penetration indicating a zone of influence effect on these displacement piles. SPT N-values plotted versus elevation data indicated a large change in N-values when high pile rebound occurred. Within the Central Florida sites, N-values increased from six to seven pile diamteres into the rebound zone, while excessive rebound changed into pile bouncing when penetration was prevented from 7.5 to 9 pile diameters into the rebound zone. These changes also corresponded to the upper elevations reported for the Hawthorn group. SPT N values increased to over 50 blows per foot at about the same elevations that the displacement piles were no longer able to achieve penetration, termed bouncing. The silt content increased to over 18 percent at the elevations the prevented pile penetration. The pocket penetrometer uncomfined compression results increased to 1.9 tsf (182 kPa) at about this same elevation. CPT tip resistance values increased to over 65 tsf(6,234 kPa), while sleeve friction values increased to over 1.1 tsf(106kPa). The CPT data produced negative pore water pressures in the soils overlying the rebound zone, which increased to positive values in excess of 100 psi (700kPa) for all three sites, again at about the bouncing elevations. These variations in pore water pressures in combination with the increased stiffness and high silt contents in saturated soils, could be the geotechnical conditions that would produce high pile rebound. Large changes between the overlying no rebound and rebound zones data, determined from PDA data were, also identified based on all of the testing data. The results were reported as ratios between the rebound zones divided by the overlying no rebound zone. The grain size with hydrometer data showed that the silt content increased by a factor of 1.9, the pocket penetrometer unconfined compression data increased by a factor of about 2.6 in the rebound zone, the CPT point and friction data increased by a factor of about 3.8 and 4,.4 respectively and the raw N-values increased by a factor of about 3.7.
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