Failure of a barrette as revealed in an O-cell test

Abstract

A 1.5 × 2.8 m, 44.5 m deep barrette was constructed in weathered sedimentary rock of the Jurong formation in Singapore with an O-cell assembly of two O-cells installed at 33.5 m depth, 11 m above the barrette toe, chosen presuming that resistance above and below the cell level would balance. When the O-cell test was performed, a proof test at the end of the construction work, the shaft resistance for the section above the O-cell level was surprisingly low; the shaft failed at a load of only about 3 MN. In a re-test, at the maximum O-cell load of 12.5 MN, the upward movement exceeded 100 mm. The barrette was then tested in a conventional head-down test, which first (second test stage) was with the O-cell vented to validate the results of the O-cell test. In a third stage, the opening between the O-cell plates was grouted in order to transfer load across the O-cell level to mobilize the shaft resistance along the lower section of the barrette and the toe resistance. The results of Stage 1 O-cell upward test and Stage 2 head-down test on the pile with free-draining O-cell agreed well, as did the Stage 1 equivalent head-down test and Stage 3 head-down test. Stage 3 test confirmed a 12 MN working load, downgraded from the original 33 MN value. The test results are discussed and referenced to a numerical (FEM) analysis. © 2012 American Society of Civil Engineers.