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

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The case study examines the design of continuous-flight-auger (CFA) piles for a three-storey school built on compressible soils at a site where 3–5 m of fill had been placed. The fill induced ground settlement and potential downdrag, so instrumented static loading tests were conducted on three 22.7 m piles.<br /><br />Back-analysis of the test data identified residual forces caused by ongoing soil settlement between pile installation and testing. Ignoring these forces overestimated shaft resistance and drag force. After accounting for residual force and analyzing the interaction between pile forces and movements, the authors derived more representative shaft-resistance values and assessed long-term settlement.<br /><br />The study emphasizes that drag force and live load should not be added together when determining the maximum axial force in a pile: live load is resisted near the pile head and does not increase the maximum force at the neutral plane. It also distinguishes serviceability assessment of long-term settlement from strength and ultimate-limit-state checks.<br /><br />The original design incorrectly combined sustained load, live load, and drag force, and misinterpreted a building-code requirement by excluding pile resistance above the neutral plane. These errors suggested that piles needed to be lengthened by about 35%. The corrected analysis showed that the original pile lengths were adequate and that predicted foundation settlement was below the project’s 35 mm limit. Correct interpretation of test results and code provisions therefore avoided unnecessary construction costs and delays.

Keywords

continuous-flight-auger piles

CFA piles

settlement analysis

drag force

downdrag

residual force

shaft resistance

axial load

pile design

school foundation