Introduction
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Fig 1: Crossrail route and connections[/caption]
Crossrail is delivering a new east-west railway across London, to be named the Elizabeth line when it is opened. It will provide a high-frequency, high capacity service linking 40 stations over 118km, from Reading and Heathrow in the west to Shenfield and Abbey Wood in the east.
It increases central London’s rail capacity by 10 per cent and creates new routes into and through the city, giving 1.5 million additional people access to central London within 45 minutes. Outside the centre of London, the railway runs on the surface within existing railway corridors, but within the central area of the city the railway is below ground, running in 42km of new tunnels linking eight new underground stations.
One of the main purposes of Crossrail is to improve interconnectivity - it interchanges with most of the London Underground lines and it provides alternative, or improved, access to London’s six airports (Fig 1).
Development history
Crossrail has had a long history of development. In 1989, the Central London Rail Study proposed an east-west route (now 'Crossrail') as well as a southwest–northeast route known as the Chelsea-Hackney Line (now ‘Crossrail 2’) and, in 1991, a private bill was submitted to parliament for Crossrail by London Underground and British Rail.
Shortly after the practical completion of the full detailed design in 1994, the bill was rejected, but the route between Paddington and Liverpool Street was safeguarded. The project was resurrected in 2001 when Cross London Rail Links (now Crossrail Ltd (CRL)), was formed by the UK Strategic Rail Authority and Transport for London, to develop and promote the scheme.
In 2005, the hybrid bill required to construct and operate the railway was laid before parliament and the bill received royal assent in 2008 as the Crossrail Act; CRL was appointed as lead nominated undertaker for the delivery of the railway.
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Fig 2: Crossrail funding sources (courtesy Crossrail Ltd)[/caption]
CRL then appointed Network Rail as its delivery partner for the surface routes. Shortly after the receipt of royal assent, Transport for London and the Department of Transport signed the Crossrail Sponsors' Agreement committing them to finance the project, alongside contributions from Network Rail, BAA plc (now Heathrow Airport Limited) and the City of London.
Detailed design commenced in 2008 and, following an active value management process, the project survived the government’s September comprehensive spending review in September 2010. The initial construction works started in 2009 at Canary Wharf station, but the main construction works only commenced in 2010.
The project is due to open progressively from December 2018 through to the end of 2019 – 30 years after the publication of the Central London Rail Study. More detail on the development of the project from option selection through to royal assent may be found in Bennett (2017a & 2017b).
Funding
Crossrail has a funding envelope of £14.8 billion. The sources of funding are complex and more information can be found in Buck (2017). Fig 2 gives an overview of the sources of funding, with London business and future passenger revenues contributing approximately two-thirds of the funding and the rest coming from central government.
Central section detailed design
The central section design frameworks were tendered in 2008 to cover all aspects of the central section (with the exception of Canary Wharf and Woolwich stations, which were delivered through agreements with developers). There were seven categories:
• Tunnels and shafts
• Central stations
• Portals
• Rail systems
• Communications and control systems
• Architectural component design
• Materials and workmanship specification
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Fig 3: AECOM’s design commissions on Crossrail[/caption]
A total of 12 design companies were appointed to one or more of the framework categories in February 2009, using the NEC 3 Contract form. Of these, nine companies were successful in the mini competitions for design packages throughout 2009. CRL put significant emphasis on quality and people in the tenders.
AECOM was fortunate enough to be appointed to design two of the six stations procured under the framework, Paddington and Farringdon, as well as a number of other significant contracts (Fig 3).
These two stations are quite different: Paddington is constructed in a cut-and-cover box under the road adjacent to the mainline station, whereas Farringdon is a mined tunnel station under Smithfield market with entrance boxes at each end. Paterson (2013) gives a comparison between the two stations. The remainder of this paper will focus on the design and optimisation of Paddington station.
Paddington station design and optimisation
Located to the southwest of the existing mainline station, underneath Eastbourne Terrace and Departures Road, the Crossrail station at Paddington will connect the new railway with four London Underground lines, a myriad of bus routes and the mainline railway station.
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Fig 4: RIBA C – Preliminary design[/caption]
The existing infrastructure is rich in engineering and architectural heritage: Brunel’s mainline station opened in 1854 and was followed in 1863 by the western terminus of the Metropolitan Railway, the first underground railway in the world which, coincidentally, connected Paddington and Farringdon mainline stations.
Building a new underground station for Crossrail in this location therefore presents significant challenges and opportunities both for design and construction. The final design has been driven by efficiency, buildability, physical constraints, functionality, passenger experience and the need to provide a station appropriate to this location and historical setting.
The primary physical constraints are the buildings which surround the new station and, in particular, Brunel’s Grade 1 listed mainline station. Excavation of a large hole in the ground adjacent to mainline station required special measures to limit any damage to the structure.
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Fig 5: Final design[/caption]
Constructed using top-down methods
The preliminary design was taken to RIBA stage C and handed over to AECOM for further design in August 2009. This scheme located the station box underneath Eastbourne Terrace, to be constructed using top-down methods.
The main station box itself was 264m long, 25m wide, 26m deep, with four permanent slabs propping diaphragm wall panels both during construction and in the permanent case.
In the central portion of the station, inclined concrete struts between the main slabs (in back of house areas) formed a ‘structural truss’, transferring vertical loads to the perimeter walls and removing the need for columns at platform level (Fig. 4).
Long voids ran along the box centre line through each of the upper three slabs, creating a ‘light well’ from street level to platform, housing escalators and supplying daylight to the lowest areas of the station.
The main entrance was within the existing mainline station and required the relocation of an existing underground toilet block and of an electricity substation which provided power to the mainline station and other railway assets – substantial and disruptive works to the existing station.
While maintaining approximately the overall dimensions of the main underground station box, AECOM, together with its sub-consultant architect, Weston Williamson, and Crossrail, redesigned the internal layout of the station. In particular, the entrance was taken out of the existing station and passengers now enter through the roof of the new station (Fig. 6, main image, top right).
The change in the entrance, together with several other changes, resulted in a number of substantial improvements:
• Improved passenger comfort by reducing the time spent, and distance travelled, underground;
• Improved passenger experience as the roof entrance and canopy create a grand ‘atrium’. Passengers entering the station will enjoy direct line of sight from street level to the gate lines below, while those ascending from the platforms will emerge into a large, airy, naturally lit space (Figs. 7 and 8);
• Columns are introduced in the central region of the station box, removing the inclined struts between slabs and therefore providing more space back of house for equipment and circulation;
• Less disruption to the mainline station during construction as no excavation is required for the entrance within the existing concourse;
• No entrance tunnel now required, so demolition of the toilets and substation are no longer required;
• Substantial cost savings as a result of all of the above points.
The significant improvement in the design demonstrates the benefits that can be achieved when clients and designers work together collaboratively, in a co-located office with aligned and incentivised goals.
References
Buck, M. Crossrail project: finance, funding and value capture for London’s Elizabeth line. Proc Inst. Civil Eng., Civil Engineering, Vol 170, Nov. 2017, pp15-22.
Bennett, S. Crossrail project to deliver London’s Elizabeth Line: from options to parliamentary bill. Proc Inst. Civil Eng., Civil Engineering, Vol 170, Nov. 2017, pp3-9.
Bennett, S. Crossrail project to deliver London’s Elizabeth Line: the parliamentary bill process. Proc Inst. Civil Eng., Civil Engineering, Vol 170, Nov. 2017, pp10-14.
Patterson, J & Raiss M. Development of two different station designs – a comparison of Paddington and Farringdon Stations. Crossrail Project: Infrastructure design and Construction, published by the Institution of Civil Engineers.
Scantlebury, R., Brennan, G. & Raiss, M. Design of the new Crossrail station, Paddington, London. The Structural Engineer, January 2014.
Author: Dr Mark Raiss, engineering director, civil infrastructure, Europe, Middle East, India & Africa, AECOM