Pile structural deformation using intrumented test pile with distributed fibre optic sensor

Abstract

The distributed fibre optic sensor (DFOS) technology adopted in this study is based on Brillouin scattering sensing technology known as Brillouin Optical Time Domain Analysis (BOTDA). BOTDA is a well-established technology for various civil engineering applications, but the study of its application in the instrumented pile load test is still very limited. Here, the study considers the instrumental static axial top-loaded, bi-directional loaded, and laterally loaded in bored pile and static axial top-loaded in the precast reinforced concrete pile. The study also focuses on the pile structural deformation measurement technique, anomaly detection, and interpretation of instrumented test piles with DFOS. The DFOS strain sensing system was first calibrated in the laboratory and then installed in the full-scale on-site control specimen to compare and verify the instrumented data with conventional instrumentation such as Vibrating Wire Strain Gauge (VWSG) and Telltale Extensometer. Subsequently, the DFOS was used in seven (7) full-scale instrumented pile load tests. DFOS via BOTDA technology had successfully measured continuous strain profile. With the continuous strain profile in the axially loaded instrumented test pile, DFOS is capable of measuring the pile structural deformation of the entire pile length. If there are any imperfections in bored piles, such as shaft bulging, cold joints in concrete, intrusion of foreign matter, and improper toe formation due to contamination of concrete, it can be detected through anomaly measurement along the continuous strain profile. By eliminating those measurement anomalies, misinterpreting load transfer curves and pile geotechnical behaviour through continuous strain profiles can be minimised. The measurements were further verified by numerical analysis in RATZ software, the pile integrity test (proof coring test and unconfined compression strength test), and visual inspection. A new installation technique and configuration of DFOS had been established in instrumented precast reinforced concrete (RC) piles and laterally loaded instrumented bored piles. The entire pile length deformation in long slender RC piles, including pile joints, was successfully measured and interpreted with DFOS measurement. In the laterally load instrumented test piles, the interpreted lateral movement via DFOS was found to be in good agreement with conventional sensor measurement. In addition, the pile defect detected through anomaly measurement was further verified with a low-strain pile integrity test. In conclusion, the DFOS via BOTDA technology is successfully implemented in various instrumented test piles. Continuous pile structural deformation measurement and anomaly detection improve the reliability of instrumented test pile analysis. This technology will reform the current practice on various types of instrumented test piles and provide a better understanding or comprehensive interpretation of pile structural and geotechnical behaviour.