Evaluating the Direct CPT-Based Design Methods for Estimating the End Bearing and Side Resistance of Instrumented Test Piles

The in-situ cone penetration test (CPT) has been widely used for site characterization, evaluation of soil properties, and other geotechnical engineering applications. This study presents the evaluation of 8 direct CPT-based methods for separately estimating the end bearing and side resistance of 14 instrumented precast prestressed concrete (PPC) test piles. The PPC piles were instrumented with strain gauges placed in pairs at different depths along the piles. The strain gauge measurements were used to calculate the measured end bearing, measured side resistance, and the distribution of unit side resistances along the piles. Different statistical evaluation criteria were considered in this study. The results showed that the LCPC and Philipponnat methods are the most accurate for estimating the total ultimate pile capacity. Separating the end bearing and side resistance of piles reveals that the 8 pile-CPT methods provided a good estimation of the measured side resistance, but a significantly less accurate estimation of the measured end bearing. The pile-CPT methods showed fair estimation of the measured accumulated side resistance with notable variation between methods and test piles. When separating the unit side resistance for sand and clay layers, the pile-CPT methods showed poor accuracy in estimating the side resistance in sand layers, while close estimation was noticed in clay layers. However, the Schmertmann and ERTC3 methods were demonstrated to be the best in estimating the unit side resistance in clay and sand layers, respectively.