Investigation of the O-cell test conversion coefficient in extremely soft rock conditions based on energy conversion

The O-cell test is a widely used method for assessing pile bearing capacity, with the accuracy of its conclusions significantly influenced by the conversion coefficient, γ. However, the determination of γ in rock geological conditions, particularly in extremely soft rock, remains underexplored. This study, based on the Xuanji high-speed railway project, conducts static load tests on pile foundations using both the O-cell test method and the anchor pile method on test piles of the identical dimensions. These tests were designed to evaluate the bearing capacity of piles in extremely soft rock conditions. A power function fitting model for pile side friction resistance at varying depths is proposed, revealing that the measured and fitted values are approximately 50%–67% higher than the recommended friction resistance values in geological prospecting. Using the energy method, a calculation model for γ under extremely soft rock conditions is developed. Results indicate that γ decreases gradually and converges to 0.6 as the load increases, with the measured and theoretical average value of γ being approximately 0.79 and 0.76, respectively. Verification with data from other projects demonstrates that the model is also applicable to conventional geological conditions, such as rock, sand, and clay. Ultimately, considering construction safety, a γ value of 0.9 is recommended for extremely soft rock geological conditions.