Structural engineering for the Elizabeth line Canary Wharf station and oversite development thestructuralengineer.org Design of Canary Wharf Elizabeth line station and Crossrail Place oversite development Tim Worsfold Michael Bryant John Crack MEng, ACGI, CEng, MICE BSc, CEng, MICE BSc, CEng, MICE, MIStructE, MSM, MBA Associate Director, Arup, London, UK Operations Executive, Canary Wharf Group, Associate Director, Canary Wharf Group, London, UK London, UK Marriott NOTATION Hotel ETFE ethylene tetrafl uoroethylene West India West Quay DLR MEWP mobile elevating work platform North Dock OSD oversite development Station Box Upper Bank Street Bank Upper Docklands Light Railway Light Docklands Bank of America HSBC KPMG Financial Figure 1 Conduct W Overall plan showing proximity of Canary Wharf Authority station to major neighbouring buildings Figure 2 S View from West India Quay station of almost- complete station and retail superstructure, One Canada Square showing proximity of tall offi ce buildings ARUP Introduction Canary Wharf was the fi rst station on the Elizabeth line to be constructed, and the fi rst to be let as a design-and-build contract, with developer Canary Wharf Group. Innovative design and construction techniques enabled the station box to be completed four months ahead of the development programme. Construction of the Crossrail Place retail and leisure oversite development (OSD) proceeded concurrently with that of station. The OSD included a number of features aimed at increasing future fl exibility for the developer and tenants. A timber gridshell roof completes the development, partially covering a large roof garden that is open to the public. The OSD opened in May 2015, nearly four years ahead of the planned station opening. Canary Wharf station The station site is located on the north side of the Isle of Dogs in the London Borough of Tower Hamlets, and is within the West India Dock (Figure 1). The dock was decommissioned in the 1970s and is no longer ARUP 78 July 2018 | TheStructuralEngineer TTSE78_78-83_CanarySE78_78-83_Canary Wharf.inddWharf.indd 7878 220/06/20180/06/2018 117:327:32 Structural engineering for the Elizabeth line thestructuralengineer.org Canary Wharf station and oversite development Figure 5 S Level –4 being prepared for reinforcement placement and concreting. This is fi rst top-down level, formed by moling beneath Level –3 slab above Figure 3 N Giken silent piling rig in operation. Rig sits on already-completed piles ARUP ARUP in use as a commercial port. The station box schemes to construct the station box within a bottom of the tubular piles and the whole is 260m long and 25–30m wide. The dock drained coff er dam without the need to import pile was then reinforced and concreted: the water is 9m deep and the station base slab is material to fi ll or partially fi ll the dock. detailed analysis, construction methodology approx. 18m below dock bed level. The station Relative to the reference design, the depth and monitoring is explained in Yeow et al1. box sits within 10m of four existing offi ce of the station box was reduced to minimise The southern station retaining wall was buildings which are up to 40 storeys high impact on the adjacent buildings and the formed by a line of 1180mm diameter hard- (Figure 2). centreline was relocated to mimise eff ects on fi rm secant piles at 875mm centres from the Docklands Light Railway viaduct the dock bed. This saved a year on the Station box design (Fig. 1). The developer and its technical construction programme from Crossrail’s The developer was a leading advocate for advisers worked to reduce the cost of the original proposal. To allow for fl exibility the introduction of the southeast spur of station while maintaining functionality: this in the future OSD, there are 105 tension the Elizabeth line, which included a station included reducing the length of the station piles of 1200mm diameter along the at Canary Wharf. The developer and box to approx. 260m; the width of the box station; these are spread across the base designer worked strenuously to minimise was also reduced over the central section. slab footprint and are for the permanent the environmental impact of the station Access to future developments to the north groundwater case only. In addition, 30 construction on the Canary Wharf area and and existing developments to the south was plunge-column piles of 2100mm diameter maximise the value the project would bring. explored. The space above the box was also facilitated the top-down construction. The early reference designs developed developed as a retail area. by Crossrail Ltd had a station box well over Station structure 300m long and involved fully or partially fi lling Station confi guration The station structure is a mixture of in situ the dock. The north, east and west retaining walls reinforced concrete and precast concrete With over 20 years of local knowledge in were formed from 310 Giken tubular piles of (Figure 4). The station features an off set the design and construction of more than 1214mm diameter, installed using two Giken central line of columns which extend through 30 buildings in Canary Wharf, the developer silent pilers (Figure 3). Reinforced concrete and designer were able to develop several piles were formed by boring through the Figure 4 Example of below-ground fl oor plan S Tunnel Station MEP Station In-situ spine beam Retail core ventilation fans plant rooms escalators Retail escalators Station floor slab (in-situ flat slab / Retail floor slab (pre-cast beams/planks) Station floor slab (in-situ flat slab / ARUP solid slab on beams) solid slab on beams) TheStructuralEngineer | July 2018 79 TTSE78_78-83_CanarySE78_78-83_Canary Wharf.inddWharf.indd 7979 220/06/20180/06/2018 117:327:32 Structural engineering for the Elizabeth line Canary Wharf station and oversite development thestructuralengineer.org the building and are founded on the base slab at Level –6. These columns support a central Park +1 Level beam running the full length of the station. SSL 117.100 At Level –4 this beam is designed to span twice its normal span under accidental load Ground 0 Level conditions, in case a column below should SSL 111.250 be lost due to a train derailment. Longitudinal beams also run along the north and south Promenade -1 Level sides of each fl oor. SSL 105.900 Level –3 was the fi rst level to be cast as Normal Highest Water Level 104.300 part of the top-down sequence, and a beam Middle Concourse -2 Level and slab fl oor structure was adopted, with SSL 100.950 moling holes incorporated for excavation access below. Level –4 is a solid slab Lower Concourse -3 Level SSL 95.800 spanning onto the central spine beam Approx Dock Bed Level 95.000 (Figure 5). At Levels –2 and –1, connecting the three Ticket Hall Slab -4 Level longitudinal beams are precast beams at SSL 88.950 approx. 1.5m centres. Precast lattice planks span between the beams. An in situ concrete topping connects the precast elements Platform -5 Level and completes each fl oor diaphragm. SSL 81.950 This arrangement reduced the amount of Base Slab -6 Level SSL 79.600 formwork needed. Figure 6 Typical cross-section Level –1 completes the box structure, through building with horizontal forces from earth and water showing principal fl oor levels and pressure being resolved through this level. ARUP ground conditions In some ways, the building can be thought of as a concrete ship, with the Level –1 to –6 substructure being the ‘hull’ and Levels a particular OSD scheme or, for that matter, 0 to +3 being the ‘superstructure’ (Figure Relationship between station and OSD any scheme at all. 6). Without the superstructure, the high The development agreement between The station design and construction surrounding water levels and relatively large Canary Wharf Group, the Secretary of State therefore proceeded on the basis of: air volumes inside the substructure would and Crossrail required Canary Wharf to only station entrances and vent shafts mean that tension piles are needed to hold construct the station within a stated time being constructed above water level; below- down the box in the dock. frame. However, it also allowed the developer water retail areas being mothballed; various The longitudinal elevation of the station the freedom to develop the OSD at any concepts being developed for what to shows that, in common with tunnelled/mined time to suit market conditions. The long place between the entrances; structure and Elizabeth line stations, the station areas programme duration for Crossrail, coupled services being arranged to enable the OSD are confi ned to the platform spaces, plant with the market uncertainty that followed the at an unknown time in the future rooms at each end, and station entrances. 2008 fi nancial crisis, meant that a solution the OSD being developed concurrently with Considerable space is available below water had to be developed whereby the station the station box (a retail scheme received level for retail at Levels –2 and –3, as well as design and construction could proceed planning consent from the London Borough at Level –1 between the entrances (Figure 7). without Canary Wharf having to commit to of Tower Hamlets). The station box structure featured concrete buttress walls up to 1.7m long at regular intervals along each longitudinal wall, upon which the superstructure columns are supported. During design development of the OSD, and after the station structure was substantially complete, the location of these columns was adjusted to enable up to 30% more retail fl oor area to be accommodated. This revised scheme required a revised planning application to be submitted; therefore, the station structure and starter bars at Level –1 were confi gured and Figure 7 positioned to enable any of the ‘no OSD’, Longitudinal section through station and retail development ‘original OSD’ and ‘revised OSD’ schemes CANARY WHARF GROUP WHARF CANARY to proceed.