Engineering Properties of Pile Rebound Soils Based on Cone Penetration Testing
High pile rebound (HPR) has been identified by Florida Department of Transportation (FDOT) to occur during the installation of square prestressed concrete piles at many sites in Florida. Significant pile rebound values of up to 1.5 inch/blow were measured resulting in increased blow counts. Pile refusal is a common occurrence when blow count exceeds 240 blow/ft; leading to pile redesign and economic consequences. The overall objective of this research is to identify the engineering properties of soil deposits which may cause HPR and develop improved correlations that may be used to predict HPR during the design process. Seven sites were studied in this research. Pile driving analyzer (PDA) data was used to identify the rebound zones. Cone penetration tests (CPT) and Standard penetration tests (SPT) were conducted near the associated test piles. The SPT data was used to develop soil profile for each site. The CPT data was used to estimate profiles of engineering soil properties. An existing correlation between the CPT pore pressure and pile rebound was evaluated and improved. High CPT pore pressures measured at the rebound zones were found to correlate linearly with pile rebound. Using the CPT the rebound soils were classified as dense silty sands and highly overconsolidated or cemented silty clays. These soils are dilative under shear loading increasing the shear strength of the surrounding soil and the pile skin friction. As a result higher blow counts are required to reach pile penetration. The HPR soils have very low permeability; therefore, high compressioninduced pore pressures may be generated near the pile tip during driving. These pore pressures at the pile tip may provide upward forces leading to rebound. The SPT data showed that cemented silty fine sand (SM) and clayey fine sand (SC) with trace phosphate and shell with fines content of 25 % to 40 % were found in the rebound zones. The CPT data superimposed on soil behavior type (SBT) charts provides an engineering method to predict pile rebound soils.