Axial capacity of piles supported on intermediate geomaterials

The natural variability of intermediate geomaterials (IGMs) exacerbates uncertainties in deep foundation design and may ultimately increase construction costs. This study was undertaken to investigate the suitability of conventional pile capacity formulations to predict the axial capacity of piles driven into IGM formations. Data from nine Montana Department of Transportation bridge projects were collected, compiled, and analyzed. Axial pile analyses were conducted using a variety of existing methods and computer programs, including: DRIVEN, GRLWEAP, FHWA Gates driving formula, WSDOT Gates driving formula, and an empirical method used by the Colorado Department of Transportation. The results of the analyses were compared to pile capacities determined using PDA measurements obtained during pile driving and wave equation analyses conducted using the CAPWAP program. The capacity comparisons clearly demonstrated the inherent variability of pile resistance in IGMs. Most of the projects exhibited considerable variation between predicted capacities calculated using DRIVEN and measured CAPWAP capacities. For example, five of the six restrike analyses were over predicted using DRIVEN, one by as much as 580%. The majority of shaft capacity predictions for cohesionless IGMs were less than the measured CAPWAP capacities; the worse case was a 400% under prediction (a factor of 5). Toe capacity predictions were also quite variable and random, with no discernible trends. This study indicates that traditional semiempirical methods developed for soil may yield unreliable predictions for piles driven into IGM deposits. The computed results may have little to no correlation with CAPWAP capacities measured during pile installation. Currently CAPWAP capacity determinations during pile driving or static load tests represent the only reliable methods for determining the capacity of piles driven into IGM formations.