Displacement of tunnels from a basement excavation in London

A site in central London called One New Change is currently being redeveloped. The new development has a basement that extends to approximately 14 m below ground level. Demolition of the building that formerly occupied the site, excavation of the new basement and construction of the new development have caused unloading and subsequent reloading of the ground. This change of loading has resulted in displacements occurring to the nearby London Underground Central Line tunnels that run parallel to the northern boundary of the site. The crowns of the tunnels vary from approximately 20 to 25 m below ground level. The piles and retaining wall for the proposed works are located at least 7 m clear distance to the south of the tunnel closer to the site boundary. The results of analyses carried out to assess the effects on the tunnels indicated that the imposed distortions caused by the proposed works were within tolerable limits. Monitoring was proposed to check that ground and structural movements were within the range of calculated values, and contingencies were identified in the event of agreed trigger levels being exceeded. Displacements measured during the demolition, excavation and construction phase of the project are presented. Conclusions are drawn from comparing the measured displacements with the tunnel deformation calculations.