Monitoring of a Peanut-shaped TBM Launching Shaft Excavation using Fibre Optics and Remote Sensing Techniques

The trial application of fibre optics and remote sensing techniques for monitoring a peanut-shaped tunnel boring machine (TBM) launching shaft in the Trunk Road T2 and Cha Kwo Ling Tunnel project has recently been completed. This is the first time in Hong Kong that these techniques are deployed to systematically monitor the entire excavation process of the peanut-shaped shaft. In particular, distributed fibre optic sensing (DFOS) technique based on optical frequency domain reflectometry (OFDR) was used to capture the continuous profiling of the strain measurement by fibre optics installed in the diaphragm wall panels, thus enabling the development of hoop strain to be revealed. To facilitate data interpretation, the excavation process was regularly recorded by the handheld light detection and ranging (LiDAR) scanning technique. This paper reports the background and key findings of the monitoring work as well as the results of the data analysis. The monitoring work provides valuable field data, which could not be easily obtained on site in the past. The data may be of use for numerical back-analysis to better understand the behaviour of shaft excavation. Insights gained in this study could also be useful to future design and construction of similar excavation works.