“EVALUATION AND DEVELOPMENT
OF CPT BASED PILE DESIGN IN NEBRASKA SOILS”

Advisor: Dr. Chung Song

Cone penetration testing (CPT) is a well established geotechnical subsurface investigation technique commonly used for site characterization and soil classification. The CPT gives real time end resistance, side friction, and pore pressure readings. Axially loaded piles also share these two resistance mechanisms, suggesting the cone can be considered similar to a miniature pile. This study focused on evaluating eight CPT methods prediction of pile bearing capacity. The Nebraska Department of Roads (NDOR) conducts dynamic load tests (PDA) of driven pile to verify pile capacity for bridge foundations. 91 comparisons of CPT logs and PDA data were evaluated. CPT prediction methods were assessed based on prediction ratio and statistical performance. Controlling bearing mechanism was identified as a key influence in method accuracy. Subsequently, piles were identified as end bearing or skin friction pile for further method analysis. The CPT methods were calibrated to maximize accuracy for Nebraska’s regional soil conditions. A numeric modeling study was also conducted to investigate cone vs. pile behavior. The study found cone influence depth for end resistance about 10D, while pile influence depth ranged from 1-3 times the diameter. Relative sensitivity to over and underlying soft/hard layers was also identified. Most importantly, computational modeling confirmed qb/qc factors in accordance with or slightly higher than the empirical methods. Bearing capacity was most accurately predicted by modified Prince & Wardle method for end bearing pile, while a modified Philipponnat’s method gave the best prediction for friction pile.