Axial capacity of piles supported on intermediate geomaterials

Abstract

Pile foundations used to support bridges and other structures are designed and installed to sustain axial and lateral loads without failing in bearing capacity and without undergoing excessive movements. The axial load-carrying capacity of a driven pile is derived from friction or adhesion along the pile shaft and by compressive resistance at the pile tip. There are well established analytical methods for evaluating pile capacity and for predicting pile driving characteristics for cohesive soil, cohesionless soil, and rock. However, past experience indicates these methods may not be reliable for piles driven into intermediate geomaterials (IGMs), which often exhibit a wide array of properties with characteristics ranging from stiff or hard soil to soft weathered rock. Methods to determine the axial capacity, driving resistance, and long-term resistance of piles driven into intermediate geomaterials are not well established. Nine projects, in which piles were driven into IGMs, from the Montana Department of Transportation were analyzed. Each project contained information from CAPWAP dynamic analyses, construction records, and design reports. The purpose of any analyses, of the nine projects, was to better predict the behavior of piles in IGMs. IGMs were divided into two broad types, cohesive and cohesionless. The computer program DRIVEN is often used to predict the axial capacities of piles; however, in IGMs the design method is unreliable. Attempts were made to determine trends within the available data. Normalized resistances for shaft and toe capacities did yield slight correlations of shaft resistance to pile length in IGMs. Iterative solutions using DRIVEN to match the CAPWAP ultimate capacity did not provide meaningful trends or correlations. Slight modification of MDT's original DRIVEN inputs was required in most cases to match the CAPWAP ultimate capacity. Because no meaningful trends were found from analysis, other capacity calculation methods were used to determine other methods that accurately predict pile capacity within IGMs. The Washington Department of Transportation Gates formula is the most accurate method of those attempted. More research is required for further analysis of piles in IGMs.