Abstract: Self-balanced loading test (also called O-cell test) has been widely used in the testing of pile foundation with the benefits of solving many difficult problems not to be resolved by the static load test. Determining the value of conversion coefficient (the ratio of downward shaft resistance to the upward shaft resistance) is crucial issue to the calculation of bearing capacity. On the other hand, because the universal existence of clay soil in China, the study of conversion coefficient in clayed area is required. Thus, based on the project of Taizhou bay bridge, the value of conversion coefficient of self-balanced loading test is studied by field tests. Three cast-in-situ piles, 1.5 m in diameter and 83.5 m in length in clay soil are loaded by self-balanced loading method firstly, and then by kentledge loading method. In order to eliminate the effect between these tests, kentledge loading test was carried out 4 month after self-balanced loading test. At each soil layers, strain gauges are installed in each pile to obtain the value of shaft resistance. The value of shaft resistance of each layer from self-balanced loading test will be compared with one from kentledge loading test. The values of conversion coefficient are obtained. Also, the compared results from the other two similar projects are collected. From above researches, the ratio of downward shaft resistance and the upward one in Taizhou bay bridge is rational ranged from 0.7 to 0.8; and the ratio from other two different projects also proved this result.

Key words: cast-in-situ pile, self-balanced loading test, kentledge loading test, conversion coefficient