Residual Force and Downdrag Impacts on Static Loading Tests and Design of CFA Piles

Abstract

Construction of a three-storey school complex over compressible silt and silty clay followed by sand was affected by fill placement required to reach finished grade. Pile foundations were selected to mitigate excessive settlement caused by the building loads and downdrag resulting from the fill. Static loading tests on instrumented piles were performed to confirm the design. The back-analyses of the tests revealed presence of residual force and were adjusted for the final design by applying interactive response of force and movement. The analysis results showed that the initial design of the foundations was suitable. Mistakes in assessing live load and drag force, when applying the results of the static loading tests, and a misinterpretation of the building code led to the conclusion that the piles, as initially designed, would need to be substantially lengthened. Final assessment showed that lengthening was not necessary.