Capacity and Load Movement of a CFA Pile: A Prediction Event: Fid-Bau Portal

Capacity and Load Movement of a CFA Pile: A Prediction Event

Fellenius, Bengt H.

A static loading test on a 406 mm diameter, strain-gage instrumented, 18.5 m long CFA pile presented an opportunity for arranging an impromptu prediction event. Several people known to be interested in deep foundation challenges were invited to submit a prediction of capacity and load-movement response of the pile ahead of the testing. Although the window of time was a mere week, no fewer than 41 persons responded, representing 15 countries and most continents. The soil profile at the site consisted of a transported and redeposited silty sandy medium plastic, very stiff, ablation clay till containing lenses of sand and gravel categorized by SPT N indices and low water content. Concern for safety of the test arrangement forced the static loading test to be aborted when the applied load was 1,790 kN before any tendency toward an ultimate resistance had developed. However, the extrapolated load-movement curve was used as reference to the predicted load-movement curves. Strain-gage instrumentation indicated a low E-modulus commensurate with the mere 13-day wait time between constructing and testing the pile. The records also indicated that only about 500 kN of the maximum applied load had reached the pile toe when the test was aborted. The paper presents the results of the test, the approach to determining the pile modulus from the strain-gage data, the load distribution, and extrapolated pile-head load-movement curves for reference to the predicted curves. The predicted values of pile capacity ranged from a low of 830 kN through a high of 3,600 kN with an average of 1,920 kN. Thirty-six of the predictions included the pile head movement at the predicted capacity and the movements ranged from 8 mm through 220 mm with an average of 35 mm. Clearly, the predictors' definitions of pile capacity differed. The compilation of the predictions presented in the following include bell curve distributions of submitted capacities and movements, showing that the widths of two standard deviations, thus encompassing two-thirds of all values, for capacity and movement were 1,090 kN and 2,760 kN and 9 mm through 61 mm, respectively.