Robert Bird Group (RBG) worked with Dragados as the Civil and Structural Engineers for London Underground’s Bank Station Capacity Upgrade project.
Bank and Monument stations form the third busiest interchange on the network with 96,000 passengers using the station during the morning peak period. The project increased capacity at Bank Station by approximately 40%, principally the Northern line and interchange routes to the Central line and Docklands Light Railway (DLR), with the aim of reducing journey times and congestion and improving safety.
The project works include:
- A new railway tunnel and platform for the Northern line, reducing interchange times and creating space for passengers
- Step-free access from Cannon Street to the Northern line and DLR platforms
- Direct routes within the station between Northern line, DLR, and the Central line via a new 100m link tunnel with moving walkways
- Two new lifts and 12 new escalators
- A new station entrance which includes a combination of raft slabs, piled shoring walls and vertical shafts cascading downwards from Cannon Street, providing improved passenger access to the Northern line platforms and to the DLR some 35m below street level
Key technical challenges
RBG started in 2014 with a scope including the compliance design, detailed design, and construction phase services for:
- All permanent reinforced concrete and steelwork for the new Cannon Street station entrance
- Development of construction sequence and over-site development interfaces for the new Cannon Street station entrance (top-down deep basement)
- Conceptual and detailed design of the temporary support truss enabling top-down construction without plunge columns
- Permanent RC structures in tunnel inverts for platforms, concourse, escalators and moving walkways at Northern line, Central line, Central line link, and DLR levels
- Arthur Street shaft permanent structures and structural works associated with the relocation of plant
- Arthur Street construction access shaft temporary works
- Bank Station and Monument Station enabling works including modification, strengthening, and assessment of existing structures
- Stage 3 building damage assessments for movement-sensitive structures
- Highly constrained work sites with onerous ground movement limits
- Presence of numerous existing underground assets imposing significant constraints on pile design and construction
- Difficulty implementing traditional top-down construction where plunge piles would clash with existing tunnels and new shaft
- Limited space for site logistics and material & plant storage
- Working within a 120-year-old existing station that must at all times remain operational with full capacity
- A significant number of existing underground services around the Arthur Street construction access shaft
- Irregular geometry of station box including the resulting tensile stresses in waterproof concrete elements
- Co-ordination with congested MEP service routes, 4 banks of new triple escalators, lifts, and bespoke architectural solutions
- Tunnelling directly below two buildings, including through several of their loaded under-reamed end-bearing piles, without causing significant damage to the building’s structural fabric or sensitive façades
Working within the framework of London Underground’s Innovative Contractor Engagement procurement model, RBG collaborated closely with Dragados and their specialist design consultants and tier two contractors Byrne Brothers and Keltbray. This enabled RBG to deliver a bespoke design fully coordinated not only with other design disciplines, but also the construction techniques, methodologies and construction sequences agreed upon at an early stage in design with the contractors who would be executing the works.
RBG developed an innovative solution which enabled plunge columns to be eliminated, temporarily suspending 4 levels of top-down basement slabs from a steel truss. This eliminated the detrimental programme and health and safety impacts of having plunge columns within the low-level 20m deep sprayed concrete lining (SCL) shaft. The truss includes approximately 100 tonnes of steelwork and a jacking system which is used to compensate for the complex response to incremental load application as the slabs are progressively constructed and excavation advances below.
Integrating the design of a logistics platform with the permanent works helped to improve site safety by eliminating complex vehicle movements and easing congestion on the City of London Road network from an early stage in the construction programme. The ground-level slab and associated vertical and foundation structures were re-designed to accommodate construction vehicles and act as a ground-level working platform from which top-down excavation and muck away could be undertaken.
The adoption of advanced analysis techniques for the reinforcement design of the 1.5m thick raft slab saving >30% reinforcement compared with typical codified approaches.
In collaboration with Dr Sauer & Partners (DSP), RBG developed an analysis approach which maximised the benefits of DSPs’ sophisticated ground model with RBG’s complex and detailed above-ground structural model. It was decided that two separate computational models with a bespoke automated routine for iteration and co-ordination would capture the complex soil-structure interaction, and provide the best outcomes compared with a single model in a simplified software.
£565 million GBP
London Underground & Dragados
Robert Bird Group Services:
Structural Engineering Design Services with ATS