Abstract

The HS2 Bromford Tunnel intermediate shaft has an external diameter of 21.9m and a depth to formation of 47.1m below ground surface. The shaft is formed with a 63.1m deep 1.5m thick diaphragm wall primary lining, with a cast in situ secondary lining. The shaft works started in spring 2022, and formation level was reached in late 2023. Situated at the base of the Tame valley, the shaft excavation is predominately in the Mercia Mudstone, and is understood to be the largest and deepest excavation of its type in the Birmingham area. Instrumentation and monitoring played a vital role in managing ground risks during the diaphragm wall installation and bulk excavation. The paper covers: An overview of the shaft works and the subsequent tunnelling in the area; The geotechnical risks and parameters to be monitored; The specification, installation and performance aspects of the instrumentation, including the embedded SAAs and Vibrating Wire Piezometers; The design and performance of the specialists’ subcontractors dewatering system and excavation groundwater pressure behaviour during construction; Ground movements and third-party asset monitoring during diaphragm wall installation and excavation; diaphragm wall deflections during excavation - a comparison between design model calculations and actual monitored movement. As the shaft was constructed largely in advance of the main tunnelling works, some of the potential benefits from the experience and field observations made during the shaft works have been realised during the tunnelling.