Region-specific calibration of resistance factors for use with static and wave equation analyses of driven piles

Abstract

In this study, resistance factors for the wave equation analysis of piles (WEAP) and a commonly used static analysis (SA) method are calibrated for use with piles driven in the Puget Sound Lowlands. Resistance factors are calibrated using a database of dynamic pile load test data with 95 piles monitored at the end-of-drive (EOD) condition and 94 piles monitored at the beginning-of-restrike (BOR) condition. Capacities are estimated using stress wave measurements collected with the pile driving analyzer (PDA) and interpreted using the CAse Pile Wave Analysis Program (CAPWAP) signal-matching procedure. The accuracy and uncertainty associated with the selected SA method was evaluated, along with WEAP bearing graph analyses for two alternative approaches for the estimate of per cent shaft resistance: (1) that computed from CAPWAP, and (2) that computed using the selected SA method. In general, the WEAP-based estimates of capacity were relatively accurate on average, with coefficients of variation ranging from 26 to 42%, whereas the selected SA method produced coefficient of variations as high as 85%. When compared to the AASHTO (2014, LRFD bridge design specifications. 7th edn. Washington, DC: AASHTO) recommended resistance factors that are based on a national database, the region-specific resistance factors based on WEAP at EOD increased by about 50% to 0.67 and 0.65 for the CAWAP and SA-based per cent shaft resistances, respectively. The resistance factors calibrated for the WEAP capacity estimates at the BOR condition were 0.53 and 0.46, lower than at EOD in part because of the transformation error associated with the use of dynamic loading tests for a long-term static capacity. Owing to its larger variability, the resistance factors calibrated for the selected SA method were significantly lower. The study described in this paper illustrates the benefit of using a high-quality, region-specific database for calibration of resistance factors.