Monitoring Axial Force Development in a Super-Long Pile during Construction Using BOFDA and Data Interpretation Approaches: A Case Study

Long-term monitoring data for super-long piles are scarce and valuable. This paper reports axial strain measurements of a cast-in-place large-diameter pile embedded 76.7 m into a "weathered trench" of granite in Nanshan District, Shenzhen, China, using BOFDA monitoring technology. An approach based on the load-transfer method to interpret data is proposed, in which the axial load at the pile head and the shear behavior at the pile-soil interface can be analyzed. Results show that these data can well reflect the increase in axial strain as the number of floors built increases, although there is deviation related to fiber cable bending due to the installation and compaction of concrete, and the complex loading condition at the pile head. Sensitivity analysis of parameters disclosed that the friction angle between the soil and the pile was approximately 10 degrees for the cast-in-place pile monitored in this study, which is approximately one third of the interface friction angle, considering the slurry cake effect. The average axial force exerted on the pile head induced by building one floor ranged from 116.00 kN to 297.43 kN; this increased with the number of floors built and the total loads of the superstructure. This implies that the raft carried a large portion of the structural load during the early construction stage; piles gradually carried a major portion of the increased load due to continuous construction. The overall mobilized percentage of skin friction was approximately 40.8% when 40 floors were built, and the pile had the potential to carry more axial load.