Determination of Loading Capacities for Bi-directional Pile Load Tests Based on Actual Load Test Results

A bi-directional pile load test (PLT) is regarded as the most reliable method for verifying the design capacity of a large-diameter drilled shaft. The loading capacity is often improperly set while conducting this test, leading to inadequate verification of appropriate design capacities for large-diameter drilled shafts. This problem necessitates a new, rational method for estimating the loading capacity for bi-directional PLTs. In this study, results of numerous bi-directional PLTs conducted by different researchers were analyzed for their failure patterns, load increasing ratios, loading capacity increasing ratios, and sufficiency ratios of the design load. The results indicate that most failure patterns involved a lack of loading capacity. In our assessment, the load increasing and loading capacity increasing ratios were less than 2, confirming that the maximum equivalent test load is not always twice the total bi-directional load. Hence, it is difficult to verify the design capacity using the current planned loading capacity. An analysis of the sufficiency ratios of the design load revealed that 18.6 % of the test results did not satisfy the requirement. To eliminate the uncertainties in verifying the design load, the one-directional loading capacity should be at least 2.5 times the design load.