Assessment of LRFD Resistance Factors for Auger Cast-in-Place Piles in South Florida Limestone Using Conventional Design Methods and Measuring While Drilling

The focus of this paper is the assessment of load and resistance factor design (LRFD) resistance factors (ϕ) for auger cast-in-place (ACIP) piles constructed in South Florida limestone using conventional design methods and measuring while drilling (MWD) compared to fully mobilized segments of load-tested ACIP piles. The paper covers three different methods of LRFD assessment, which includes the first order second moment (FOSM) method, a revised version of the FOSM method that provides similar results to the first order reliability method, and Monte Carlo simulations with 50,000 trials per simulation. Additionally, each LRFD method considered six different reliability indices (𝛽) ranging from 2 to 4. The conventional design methods included four different Standard penetration test (SPT) design methods using SPT data collected throughout the site; and seven different design methods using unconfined compression strength (qu) data acquired from laboratory testing of recovered rock core specimens collected at the site. In total, this amounted to 198 LRFD resistance factor assessments for ACIP piles in South Florida limestone using conventional design methodology. For the MWD assessments, two approaches were considered. The first approach used MWD data collected during ACIP production pile drilling to simulate MWD borings performed within 100 ft from mobilized load test locations. This allowed the investigation to quantify the effects of spatial variability over short distances when sufficient strength data are collected within the vicinity of ACIP piles. The second approach used only MWD data collected in the footprint of mobilized ACIP piles to show the benefits of eliminating spatial variability from pile capacity estimates, leaving only the method error as a source of uncertainty. The first MWD approach considered four qu design equations, and the second approach considered three MWD side shear methods. In total, this amounted to 126 MWD LRFD resistance factor assessments and 324 LRFD assessments when combined with the conventional design methods. LRFD ϕ vs. 𝛽 curves were developed for each LRFD approach and design method to provide guidance in future south Florida ACIP pile designs that utilize SPT, conventional rock coring, and/or MWD data. From the research, it was found that methods that employed SPT data produced the lowest ϕ values. Conversely, methods that employed MWD data resulted in the highest ϕ values due to the excellent correlation that exists between specific energy obtained from MWD, unconfined compression strength, side shear strength, and pile capacity. In conclusion, it was determined that MWD is viable for ACIP pile design and construction QA/QC purposes.