Evaluation of Design Parameters (α and β) for Analysis and Design of Piles on Soft Clays

This paper presents the analyses of 12 prestressed concrete instrumented test piles (TPs) that were driven in different bridge construction projects of Louisiana as part of a load testing program. The 12 TPs were driven mainly in cohesive soils. Detailed soil characterizations including both laboratory and in situ tests were conducted to determine the different soil properties. The TPs were instrumented with vibrating wire strain gauges to be able to calculate the distribution of skin friction and end-bearing capacities separately. Static load tests and dynamic load tests were conducted on each TP after end of driving to calculate the ultimate load capacity. Fifty six soil layers exhibiting clayey soil behavior and sandy soil behavior dominated the remaining 15 soil layers. The total stress parameter (alpha) and the effective stress parameter (beta) were back-calculated for each soil layer. The alpha values ranged from 0.22 to 1.80, and the beta values ranged from 0.11 to 0.83. Regression analyses were performed on 42 soil layers (75% data) to develop an empirical model to predict alpha as a function of undrained shear strength. The remaining 25% of the data were used to verify the developed model, and good agreement was observed between the measured and predicted values.