Skanska Costain STRABAG JV (SCSJV) is responsible for the detailed design and construction of Phase One of the UK's high-speed railway project known as High Speed Two (HS2). The project involves building a route of approximately 16 miles, including two 13-mile tunnels in London. To ensure precise tunnelling, extensive checks were conducted before excavation using six 9.8m diameter tunnel boring machines (TBMs) and a 6.2m diameter TBM for a logistics tunnel in Old Oak Common.

The accuracy of the TBMs' alignment was a crucial concern due to the potential cost of rectifying any deviations. To mitigate this risk, SCSJV conducted 3D connection checks between control stations before commencing excavation, ensuring alignment in both horizontal and vertical planes. Additionally, a unique rooftop traverse method was employed to verify the accuracy of the tunnel route. Over an 8km stretch, nine high-rise buildings were traversed, covering horizontal distances ranging from 500m to 5km. Three campaigns were undertaken, involving 15 SCSJV Engineering Surveyors per campaign, to ensure precise measurements and mitigate safety risks.

By implementing the rooftop traverse method, SCSJV minimized the number of setups compared to a street-level traverse, reducing potential errors. The successful results of the rooftop traverse instilled confidence in the project team that the TBMs would accurately follow the designated route, achieving precise breakthroughs and ensuring the project's overall success.

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