5 Broadgate is a new twelve storey office development located in the City of London adjacent to Liverpool Street Station. The new development will take the place of two existing eight storey office buildings and will include the construction of a two level basement. Initially the foundations of the new development were designed to comprise of six hand dug under ream piles 18.5m to 22.5m deep, but a new scheme was put forward by the contractor replacing the under ream piles with 136 nos. 305mm diameter micro-piles arranged in six groups. This new foundation scheme was an improvement on the former in terms of sustainability, safety and speed of construction.
GCG was commissioned to undertake pile capacity and pile settlement analyses to assess the adequacy of the alternative foundation scheme. The ultimate limit state analyses examined the loading distribution within the foundation system accounting for loading eccentricity, various load combinations, and individual and pile group capacities. Serviceability limit analyses/ checks were also performed for key construction stages and for the long term conditions. The results showed that the new foundation scheme would produce small and tolerable total and differential settlements. Site micro-pile testing, carried out to verify the design, were instrumented with vibrating wire strain gauges and fibre optic cables – a first in micro-pile testing – which revealed the load distribution and the average shaft friction along the pile shaft. The results verified the novel alternative scheme proposed.