Extrapolation of Unit Side Resistance in Drilled Shaft Foundations Based on Bi-Directional Load Test Data-Paper

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The study evaluates whether three established pile-capacity extrapolation methods can predict unit side resistance in drilled shafts from bi-directional load-test data, which often do not fully mobilize shaft capacity.<br /><br />Researchers reviewed 149 test records and assembled a complete dataset of 116 projects, covering shafts 2–9.8 ft in diameter and 13.2–250.5 ft long. They digitized the data and derived 590 unit side-resistance curves for intervals between strain gauges. Curves were classified as fully mobilized and asymptotic (A), approaching asymptotic behavior (B), or approximately linear (C). The analysis focused on Criterion A curves.<br /><br />The Hansen 80%, Chin–Kondner, and Decourt methods were applied to the final 50% of each measured displacement record. Predicted side-resistance curves were compared with measured values using root mean square error (RMSE). The Chin–Kondner and Decourt methods produced identical results, while Hansen 80% showed reasonable agreement with them. Overall, the three methods generally estimated the measured resistance of the fully mobilized curves well.<br /><br />The findings suggest that these techniques may help interpret drilled-shaft resistance when full mobilization is impractical or too costly to achieve in a load test. However, the study’s conclusions are limited to fully mobilized curves; performance on partially mobilized or linear curves remains untested. The authors propose examining other extrapolation starting points, applying the methods to Criteria B and C, and investigating whether shaft diameter affects unit side resistance.

Keywords

drilled shafts

unit side resistance

pile-capacity extrapolation

bi-directional load test

Hansen 80%

Chin-Kondner method

Decourt method

root mean square error

fully mobilized curves

asymptotic behavior