EVALUATING SOIL PROPERTIES AND CYCLIC LOADING EFFECTS OF FLORIDA’S CLAYEY SILTY FINE SAND SOILS ASSOCIATED WITH HIGH PILE REBOUND
Omar, Ali Fadil M.
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Throughout the state of Florida, pile rebound has been encountered while driving large displacement piles. During periods of excessive rebound, the piles could not achieve further penetration and could require an excessive number of blows to advance the pile into the ground. This phenomenon affecting pile driving is called high pile rebound producing construction delays, higher costs; and may lead to pile damage. A comprehensive experimental program was performed involving the characterization and identifications of the typical soils properties associated with high pile rebound during pile installation. The research focuses on the determination of the relationships between high and no-rebound soils to yield a clearer understanding of the soil behavior associated with rebounding piles. Based on available geotechnical data, 15 prestressed concrete test piles (PCP) at six sites in Florida were chosen for this study. These PCP high displacement piles were driven using single-acting diesel hammers. Test borings with minimally disturbed thin-walled tube samples, were conducted in addition to in situ data from standard penetration test (SPT) to determine soil parameters used to identify of any correlations exist based on these soil parameters. A total of 44 undisturbed samples were evaluated, 25 of these samples were retrieved at depths where the pile produced rebound greater than 0.5 inches, while 19 samples were retrieved at depth where the pile experienced no-rebound define as (< 0.25 inches). High rebound soils were characterized as either cohesionless and cohesive soils. Relationships between high rebound soils and soil properties were evaluated; and the following differences were observed: a) The silt content of cohesionless soils may be an effective index for predicting high pile rebound. The average silt content in high rebound soils is more than twice as high as for no-rebound soils, while in the cohesive soils, the average silt content in high rebound soils decreases to less than half in comparison to the no-rebound soils. Moreover, the transition point at which the influence of the fines and silt content on high pile rebound become significant is found to be around 27 % fine and 20 % silt content. Cohesinless soils with fines and silt content greater than the transition point are more likely to produce high rebound soils. b) High pile rebound in cohesionless soils is insensitive to clay content, while in cohesive soil it is found to be an effective index for identifying high pile rebound piles. Cohesive soils with clay contents above about 37 % shows the greatest potential for high pile rebound. c) Casagrande’s plasticity chart shows that all high rebound cohesive soils were inorganic highly plastic clays with LL greater than 50 % and plotted above the A-line d) Casagrande’s plasticity chart can be used an effective index for predicting high pile rebound. Based on the finding obtained from the soil response to cyclic loading, high rebound soils were likely to have higher resistance to develop larger axial strain than no rebound soils. During the cyclic loading, no-rebound soils at the same cyclic loading rate develop axial strain more rapidly than high rebound soils. Therefore, most of the no rebound soils failed at 15% strain, while high rebound soils failed mostly at strain less 5% range. High rebound soils produced lower pore water pressure ratios (∆u/σɜ') than no rebound soils, clearly indicating that the high rebound soils take longer to produce increases in pore pressures than the no rebound soils.