Mechanical behaviors of SD and CFA piles using BOTDA-based fiber optic sensor system: A comparative field test study

Screw displacement (SD) pile and continuous flight auger (CFA) pile are two similar deep foundation technologies, and new developed SD piles are used for the first time in Anhui Province, China. In order to investigate the differences of their mechanical behaviors under the load, field static load tests were performed. The distributed strains along the full length of piles were measured using fiber optic sensor (FOS) system based on Brillouin optical time domain analysis (BOTDA) technology. The axial fore, shaft resistance and tip resistance were calculated using the measured axial strains according to the stress calculation method. Thereby, the load transfer mechanism and bearing capacity of SD and CFA piles were compared. The test results indicate that FOS system is an alternative method to measure the mechanical behaviors of the pile. When the ultimate load exerted on the pile top is reached, the pile tip force of SD pile is about 7.8% of the total load, which means that SD pile works as an end bearing friction pile. The ultimate bearing capacity of SD pile is 66.7% higher than that of CFA pile in this case study. The change of stress state of soil element induced by pile installation shows that the densification and shear strength improvement of surrounding soil is the main reason why the bearing behaviors of SD pile is better than that of CFA pile.