Crossrail is a new east-west railway under construction beneath London, comprising upgrades to 90km of existing surface railway and 21km of new twin bore tunnels along its route. The project includes upgrades to 30 existing stations and construction of 10 new stations in London.
A joint Atkins/Arup team undertook one of the largest and most important elements of the Crossrail project - the detailed design of the major twin tunnels to be bored beneath central London.
The tunnels are 6.2m internal diameter, at depths of up to 37m. Eight TBMs were used to construct the tunnels - six Earth Pressure Balance Machines (EPB) and two Slurry Machines - to cater for differing ground conditions. The tunnels are lined with 250,000 Precast Concrete Segments. Tapered tunnel rings with 8 segments were designed to accommodate the minimum track alignment radius. The majority of the tunnel segments are steel fibre reinforced and include poly-propylene fibre for fire protection. Segment joints are designed with locating dowels to facilitate high quality build ensuring good waterproofing performance and long term durability. Curved geometry on the segment joints is included to improve performance and protect the segment. There are a total of 18 cross passages along the bored tunnels length, constructed in a variety of linings, including insitu concrete, pre cast SGI and sprayed concrete.
Some of the most challenging conditions were faced on the central tunnels which pass beneath the two major financial districts, the City of London and Canary Wharf, where water bearing sand lenses were encountered. EPB TBMs were driven in parallel with station constructions which required TBM arrival, transit and re-launch systems to be developed.
One of the biggest challenges of the tunnelling works under central London was protection of the vast network of existing third party utilities, tunnels, structures, railways and historical buildings. Atkins' engineers seconded into the Crossrail project have been responsible for the damage assessment and the design of mitigation to protect third party assets from settlement induced damage. The assessments were undertaken in accordance with a robust phased approach to ensure consistency with focus placed on the most sensitive structures.
Timely resolution of concerns with stakeholders and third parties was key to securing “letters of no objection” to allow construction to progress. The extensive third party asset information assembled during the design phase was carefully managed and organised, allowing it to be integrated as a key data set into the Crossrail’s BIM system.
Over 3,400 buildings, 640 structures (including London Underground tunnels and other rail assets) and up to 14,000 utilities (gas, sewer and water mains) have been assessed to date. As part of the project management team, our engineers continue to advise the client on all issues relating to third party asset protection, accounting for the actual works being undertaken and the ground response on site.
Atkins was also awarded the architectural component contract for all the stations and associated structures. This award covers the design and specification of interior elements such as the wall and ceiling finishes, cladding systems, seating, lighting and wayfinding.
Upon completion the line is expected to carry 72,000 passengers per hour through 40 stations travelling from Reading and Heathrow in the west, to Shenfield and Abbey Wood in the east.
