Monitoring of shaft excavations in clay

Abstract

Field observations of infrastructure performance, during and following construction, are vital for efficient design of infrastructure in subsequent projects. Fibre optic sensing technology offers considerable benefits because it provides a continuous measurement profile along its length whereas conventional monitoring instrumentation, such as strain gauges, only offer discrete measurements. This paper describes two case studies in which fibre optic sensors were used to monitor the bending of heavily reinforced diaphragm walls installed during the construction of the Elizabeth Line (formerly called Crossrail), in London, UK. The first case study was a 30 m diameter, 44 m deep circular shaft constructed to launch the tunnel boring machines. The second case study was an almost rectangular ventilation and access shaft, 65 m in length, 16 m to 18 m wide and up to 36 m deep. The fibre optic sensing system was secured to the reinforcement cages of the diaphragm wall panels to measure mechanical and thermal strains during excavation. This allowed the effect of temperature changes to be accounted for. Field measurements were taken at regular intervals during excavation of both shafts. The two case study shafts, details of the fibre optic sensing system and some of the strain measurements are presented in this paper. The measurements demonstrate the effectiveness of fibre optics for structural monitoring of diaphragm wall shaft linings. They also confirm that fibre optic monitoring has an important role to play in performance-based design - in optimising and improving future designs.