Civil Engineering Proceedings of the Institution of Civil Engineers Volume 170 Issue CE5 Civil Engineering 170 May 2017 Issue CE5 Pages 47–56 http://dx.doi.org/10.1680/jcien.16.00011 Crossrail project: a deep-mined station Paper 1600011 on the Elizabeth line, London Received 04/04/2016 Accepted 30/09/2016 Keywords: tunnels & tunnelling/geotechnical St. John, Barker, Frost and Harris engineering/project management.
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Crossrail project: a deep-mined station on the Elizabeth line, London Adrian St. John BEng, Eur Ing, FICE, CEng Stephen Frost BSc, Eur Ing, FICE, CEng, MCIHT Chief Engineer, Kier Infrastructure, part of BFK JV, London, UK C134 Lead Engineer (Civil), Arup Atkins JV, London, UK John Barker BSc, Eur Ing, MICE, CEng David Harris BSc, MSc, DIC, CEng, MICE, CGeol, FGS Manager of Engineering (London Underground stations), Aecom on C121 Package Manager, Mott MacDonald, Croydon, UK behalf of Crossrail, London, UK
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This paper describes some of the key challenges and lessons learned in creating a deep-mined, sprayed concrete- lined station shell for London’s new Elizabeth line. Tottenham Court Road station was one of five such stations on the £14·8 billion Crossrail project to deliver a new 118 km east–west railway across the UK capital by 2019. Creation of two 240 m long platform tunnels in a dense, urban environment required the development of unique design and construction solutions, including ground improvement and safeguarding measures to mitigate ground movement. Strong collaboration and effective technical and construction coordination across the many asset interfaces were key factors in achieving a successful outcome.
1. Introduction esign contract ey 1 1 0 12 The £14·8 billion Crossrail project to deliver the new 118 km 121 Elizabeth line east–west railway across London, UK includes the 1 12 construction of eight new deep underground stations in the central 122 London section. Delivering a new world-class railway, which 100 will increase London’s rail capacity by approximately 10%, has 1 0 presented numerous challenges. 1 The new stations will provide bright, spacious underground spaces to facilitate high volumes of passenger movements across central London. All stations are required to make provision for trains up to 240 m long, thus being compatible with the longest trains in use on the UK conventional rail network. However, it was not practical to acquire work sites of this length in central London and this led to a typical station configuration comprising a shaft at each end of the station with platforms and connecting passageways 0 m 2 constructed by mining techniques. The shafts provided worksites and accommodated ticket halls, lifts and escalators, ventilation Figure 1. Cross-section of deep-mined platform tunnel illustrating and station plant. Typically, an over-site development was to be complexity of design contract scope and interfaces incorporated above the ticket halls. Following extensive passenger modelling studies, a minimum useable platform width of 4·5 m was established. Platform screen doors were to be provided and these, together with EU technical west, both terminating at Farringdon, thus permitting spoil removal specifications for interoperability and a 25 kV overhead line by way of barge and rail, respectively. equipment traction system, led to determination of a minimum The timing of the tunnel drives was crucial. Initial studies focused internal platform tunnel diameter of 10·6 m. The only practical on a sequence whereby the platform tunnels would be constructed technique for achieving this sort of dimension on five of the eight first using a temporary logistics access shaft located at one of the deep stations was considered to be mining using sprayed concrete ticket hall sites. The tunnel-boring machines would then be pulled linings (Figure 1). The 6·2 m inner diameter running tunnels in through a completed platform tunnel. The disadvantage of this this location were constructed conventionally using tunnel-boring approach was that the station shaft permanent works construction machines. could not progress effectively until the tunnel-boring machine drives The tunnelling strategy proposed long tunnel-boring machine were complete, because the shaft could not be safely constructed drives from Limmo in the east and from Westbourne Park in the while also providing temporary tunnelling logistics.
47 Civil Engineering Crossrail project: a deep-mined station Volume 170 Issue CE5 on the Elizabeth line, London St. John, Barker, Frost and Harris
In the case of Tottenham Court Road, which was towards the end of the western tunnelling drive, this created a long delay to ford treet the station programme, but was considered, on balance, to be the optimal solution in the context of undertaker Crossrail Limited’s entral line wider procurement and programme strategy. This key disadvantage was only finally resolved in 2011 when the contracts for both bored entre (C300) and mined (C410) tunnels were won by the same contractor oint – BFK, a joint venture of BAM, Ferrovial and Kier. The contractor Northern oho uare proposed an alternative early tunnelling sequence, whereby line the platform tunnels would be constructed after the passage of the tunnel-boring machines using an enlargement technique. haring ross oad This reduced the programme by 7 months, produced savings of m £80 million, reduced settlement by 30% and reduced the number of lorry movements by 50 000 (St. John et al., 2015b). It was still necessary to provide access to the tunnel zones Figure 2. Tottenham Court Road station plan view before Crossrail – Westbourne Park is to the left, Farringdon is to the right before the tunnel-boring machine arrival to undertake early sprayed concrete lining works. Adopting a ‘bottom-up sequence’ for the Tottehham Court Road western ticket hall meant that it was possible to complete the station foundations, excavate the shaft and construct the base slab well before the arrival of the tunnel-boring entral line machine. Construction of the station shaft foundations was awarded as a separate contract and the excavation, including the propping astern tic et hall design, was included in BFK’s scope to reduce construction oslett ard o interfaces. On completion of the tunnelling, the shaft was to be ford treet entre oint tic et esternhall handed over to a station fit-out contractor, who would remove the ast ound props, construct the station internal structure and complete the design and installation of the mechanical, electrical and plumbing elements and the architectural finishes. This paper focuses on the construction of the Tottenham Court est ound Road station shafts and tunnels. The alignment at this location runs parallel to London Underground’s Central line and crosses m Northern line the Northern line tunnels with minimal clearance. A total of 120 buildings including one grade I and 25 grade II listed structures Figure 3. Tottenham Court Road station plan view after Crossrail were within a predicted settlement zone of 1 mm and there were a large number of other sensitive utilities in the area. Extensive measures were taken to protect these surrounding structures, and these are described more fully in Section 5. estern entrance estern ean treet o vent to er entre 2. Station configuration oint ast ound platform tunnel Tottenham Court Road station is situated at the east end of entral line Oxford Street and will provide access to shopping, office and residential space. The station is situated in the south-west quadrant of the junction formed by the intersection of Oxford Street and Charing Cross Road (Figures 2, 3 and 4). The layout of the station was dictated by the horizontal and vertical tunnel alignment, the need to avoid piled foundations est ound platform to the east of Tottenham Court Road and, accordingly, the tunnel limits of deviation defined by the Crossrail Act (Li et al., 2014). astern vent to er These prescribed limits in plan and profile forced the eastbound platform tunnel to follow a curved alignment. The eastern end of astern entrance oslett ard o the Crossrail station was integrated with London Underground’s ondon nderground major upgrade of the existing underground station, which was also Northern line upgrade tic et hall authorised as part of the Crossrail Act and was under construction unning tunnels at the same time as the Crossrail project. It was agreed between London Underground and Crossrail Figure 4. Three-dimensional view of Tottenham Court Road that it was not practical to have two contractors working in close station – the complex arrangement around tunnel AP8 is circled in proximity, so the construction of the eastern shaft of the Crossrail the foreground station, known as Goslett Yard box, was entrusted to London
48 Civil Engineering Crossrail project: a deep-mined station Volume 170 Issue CE5 on the Elizabeth line, London St. John, Barker, Frost and Harris
Underground and was delivered by its station upgrade contractor. introduced as part of BFK’s bid-winning solution (St. John et al., The new London Underground ticket hall serves both Crossrail and 2015b). In addition to the western ticket hall acting as a sprayed London Underground passengers. concrete lining access shaft, the accelerated excavation of the The station provides a major interchange between Crossrail and western ticket hall also ensured that the lowest raft construction the Northern line. However, the close spacing of the Northern line was in place prior to the transit of the eastbound tunnel-boring platform tunnels meant that London Underground could not include machine (Figure 6). escalators between them, which resulted in a stepped interchange Both subsurface box structures have ventilation towers rising from between London Underground and Crossrail. The design therefore them, with the western ticket hall rising six storeys and the Goslett sought to minimise the height of the interchange, resulting in Yard box five storeys. These provide stability to over-site development minimal clearances as the Crossrail running tunnels passed over structures built over the station structures. The western over-site the Northern line, which remained live. The eastbound tunnel development structure is seven-storey residential while the eastern had a nominal clearance of 600 mm while it threaded through over-site development structure is a nine-storey theatre and office. the triangular opening formed by the south face of the new London Underground ticket hall and the underside of the London tage a tage Underground and Crossrail escalators (BBC2, 2016) (Figure 5). 21 00 10 The western ticket hall located at Dean Street contains a single escalator bank running from street to platform level. The site is bisected by Fareham Street, which was closed during construction, and was to be relocated further north in order to provide viable development plots to both north and south. The ticket hall is located in the north block and the station entrance was moved into Dean Street, thus preserving Oxford Street as a retail frontage. tage c tage d Third-party assets constrained the design of the foundations. 00 10 00 10 London Underground’s Central and Northern lines dictated the proximity of foundations, while deep subsurface BT telecommunications tunnels and other utility structures dictated both footprints and construction techniques. Fareham Street had major BT infrastructure at depth, which was diverted around the southern perimeter of the site as part of the advanced works. Both station boxes were formed by diaphragm wall panels. tage e tage f The eastern Goslett Yard box was connected to the neighbouring 00 10 00 10 London Underground upgrade ticket hall using secant piling and was excavated in a top-down sequence. The western ticket hall box was constructed using bottom-up techniques with temporary propping to expedite excavation of the box, allowing its use as a construction site and access shaft to service the sprayed concrete lining tunnelling works. The approach supported the revised tunnel drive programme tage g tage h strategy known as ‘tunnel-boring machine first’, which was 22 22