Stabilisation of soil pillar adjacent to Eastern Ticket Hall at Bond Street Crossrail Station

Summary

Sprayed concrete lining (SCL) tunnelling alongside the secant pile walls forming the Eastern Ticket Hall (ETH) box at Bond Street station left a narrow pillar of clay, less than 1m wide in places, between the piles and the tunnel lining. Since the tunnelling was to occur after excavation of the box, the stability of this soil pillar during excavation was identified as a concern. Analysis undertaken as part of a due diligence review of the lining design indicated that there was indeed a high risk of overstressing in this area with potential for a collapse mechanism to develop during SCL excavation.
This paper describes this identification and modelling of the ground stability risk, and also the selection and design process that was employed in the development of a detailed temporary works scheme to mitigate that risk. The difficulties associated with the implementation of that design in a highly constrained location are also discussed. A significant degree of collaboration between the construction and design teams enabled amendments to be made to alleviate some construction difficulties without compromising the effectiveness of the soil stabilisation scheme.
Ultimately it was decided to use a grouted pipe arch at the breakout from the box and, where a pipe arch was no longer feasible due to the tunnel geometry, an innovative solution to create a mattress of reinforced soil above the tunnel was developed using overlapping arrays of grouted spiles installed from the shaft.
The works undertaken to stabilise the soil pillar adjacent to the ETH at Bond Street exemplify the successful use of an integrated approach to design and construction to mitigate potential ground instability during SCL excavation.