Region-Specific Calibration of Resistance Factors for Use With Static and Wave Equation Analyses of Driven Piles-Article

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The study calibrates resistance factors for driven piles in the Puget Sound Lowlands using a region-specific database of dynamic load tests: 95 piles assessed at end of drive (EOD) and 94 at beginning of restrike (BOR). Measured capacities were derived from pile driving analyzer data interpreted with CAPWAP. The authors compared two wave equation analysis (WEAP) approaches—using shaft-resistance proportions from CAPWAP or from a static analysis method—with the FHWA-recommended static analysis (FHWA-SA).<br /><br />At EOD, the WEAP methods were accurate on average and showed relatively low variability: mean bias values were 0.99 and 0.98, with coefficients of variation (COVs) of 26% and 27%. At BOR, their mean biases were 1.04 and 1.07, with COVs of 34% and 42%. In contrast, FHWA-SA had much greater variability: COVs of 85% at EOD and 65% at BOR, although it was conservative on average at BOR.<br /><br />For a target reliability index of 2.33 and a typical dead-to-live load ratio of 3, calibrated WEAP resistance factors were 0.67 and 0.65 at EOD, falling to 0.53 and 0.46 at BOR. The corresponding FHWA-SA factors were 0.20 at EOD and 0.36 at BOR. The lower BOR WEAP factors partly reflect uncertainty in converting dynamic test results to long-term static capacity.<br /><br />The EOD WEAP factors are about 50% higher than factors based on national databases. The authors attribute this improvement to the quality and regional consistency of the data and construction practices. They conclude that reliable, region-specific calibration can improve design reliability and economy, but caution against applying these factors outside the Puget Sound Lowlands.

Keywords

Puget Sound Lowlands

resistance factors

driven piles

dynamic load tests

CAPWAP

wave equation analysis

FHWA static analysis

pile driving analyzer

reliability calibration

pile foundations