Estimating pile shaft capacities by use of direct CPT/CPTu-based methods for continuous flight auger piles in silts and sands

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Date
2023-08-10
Authors
Machibroda, Patrick
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Abstract

Cone penetrometer (CPT) and piezometric cone (CPTu) test methods are widely used to estimate pile shaft resistances with depth, but these methods are typically derived from empirical correlations to reference load tests. They may not be applicable to all geological conditions and pile types. Thus, developing correlations for specific geographic areas and pile types can be beneficial. In this study, the accuracy and precision of five applicable direct CPT/CPTu methods were evaluated for continuous flight auger (CFA) piles in predominantly silt and sand soils of the Condie aquifer near Regina, Saskatchewan. Six instrumented CFA test piles were evaluated through a combination of CPTu testing and instrumented static axial load testing to determine reference shaft capacities. The direct CPT/CPTu method determined shaft capacity estimates were compared to refence shaft capacities, and the relative performances of the methods were evaluated using statistical and non-statistical quantitative evaluation criteria. The best performing methods (Modified Unicone and KTRI) were then calibrated by minimizing the square root of the residual sum of squares between the predicted and measured shaft capacities. Once calibrated, the methods were re-evaluated using the initial criteria. It was found that all methods overestimated the unit shaft resistance in the upper 1.5 m, but the calibrated KTRI method provided the best overall fit. The calibrated Modified Unicone method was found to be slightly less accurate but more conservative in its predictions of shaft capacity. These findings can be used to improve pile design in the Condie aquifer region and other areas with similar geological conditions.

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Geotechnical, Engineering, Design, Piles
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