Design and construction of Fisher Street crossover cavern on Crossrail contract C300/C410, London

Summary

Creating a complex sprayed concrete lined (SCL) crossover between the Crossrail running tunnels at Fisher Street presented the project team with many interesting challenges, constructing 15m wide caverns beneath sensitive London buildings but without the provision for Compensation Grouting.

The paper describes these challenges and how they were successfully overcome by an integrated design, construction and supervision team comprising BAM Ferrovial Kier (BFK) Joint Venture, Donaldson Associates, IGT (Austrian partner of DAL) and Mott MacDonald.

Geometric complexity was a key driver; rail- and running tunnel alignment, geology and face stability considerations dictated the use of a sidewall drift with constantly flaring profile. The crossover caverns featured large diameter TBM pilot tunnels passing through them, and the geometry was further complicated by the presence of two niches and two cross-passages within the crossover structure.

Reinforcement and joint detailing, and excavation sequencing required careful consideration to ensure that the design was buildable to the required quality standards and minimised ground movements. Construction of the temporary sidewall posed specific challenges, particularly the phasing of the demolition of the temporary wall and the forming of the joints.

Unlike the majority of the tunnels on C300/C410, the crossover was constructed with the top heading in the London Clay and the bench/invert within the Lambeth Group. Systematic probing, combined with targeted depressurisation drilling, was used to minimise the risk of encountering water-bearing sand channels within the Lambeth Group.

The project team used 3D printing and 3D visualisation to ensure correct alignment between the designer’s requirements and the Contractor’s surveying systems. The paper offers recommendations to enable BIM to be exploited to its full potential in the future, to ensure that designer’s CAD models can be directly utilised in the field both for construction and logistics planning.

The paper examines the successes of the project, and highlights a number of useful €œlessons learned€ which may benefit future similar projects.