Design of Farringdon Elizabeth Line Station
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Structural engineering for the Elizabeth line Farringdon station thestructuralengineer.org Design of Farringdon Elizabeth line station Ghanshyam Kumar David Sharples NOTATION BEng, MTech, MEng, CEng, MIStructE BSc (Hons), CEng, MICE AOD above ordnance datum TfL Engineering (formerly Structural Technical Director, AECOM, London, UK BIM Building Information Associate, AECOM, London, UK) Modelling CAD computer-aided design Synopsis ETH East Ticket Hall Farringdon is one of eight new underground stations being built in central London LU London Underground for the Elizabeth line and will be one of the key interchange stations on the new mATD meters above tunnel datum (AOD +100m) line. Upon completion, over 140 trains per hour will pass through the Farringdon OSD oversite development interchange, making it one of Britain’s busiest stations. With Thameslink, SCL sprayed concrete lining Elizabeth line and London Underground services, it will be a key link in bringing SH-W1 circular shaft (West passengers from outer London to the business hubs in the City and Canary Wharf. Ticket Hall) SH-W2 rectangular shaft (West The station will also provide direct rail links to three of London’s fi ve airports. Ticket Hall) Farringdon Elizabeth line station comprises two platform tunnels, each 245m SH-W3 escalator shaft (West long, between new ticket halls over 300m apart. Each ticket hall has been Ticket Hall) SH-E3 trapezoidal shaft (East designed to accommodate future oversite developments. Ticket Hall) This paper discusses the structural engineering challenges encountered during TBM tunnel boring machine design and construction of the two ticket halls on constrained sites surrounded by WTH West Ticket Hall existing transport infrastructure, utilities and historic buildings. Figure 1 Site of Farringdon Elizabeth line station, showing West and East Ticket Halls, Thameslink ticket hall and Barbican London Underground (LU) station 26 July 2018 | TheStructuralEngineer TTSE78_26-34_Farringdon.inddSE78_26-34_Farringdon.indd 2626 220/06/20180/06/2018 117:147:14 Structural engineering for the Elizabeth line thestructuralengineer.org Farringdon station Figure 2 Existing structures and listed buildings around West Ticket Hall (WTH), including eastbound (EB) and westbound (WB) platform tunnels and Thameslink (TLP) Figure 3 ticket hall East Ticket Hall site at time of advanced works Introduction (near Canning Town station) in the east and The new Farringdon Thameslink station ticket Farringdon Elizabeth line station is located in two from Royal Oak just outside Paddington hall adjoins the eastern boundary. an area of London with a rich history. Former in the west. The design and construction A masonry retaining wall on the buildings on the site were demolished to phasing of the station, therefore, had to Farringdon Road boundary dates back to allow construction of the ticket halls, shafts, make provision to accommodate their arrival, the development of the area as a goods etc. to take place. Both the East Ticket Hall decommissioning and removal/disposal. depot in the late 19th century. To the north, and West Ticket Hall sites are surrounded Designing and constructing a new the former Cowcross Street bridge structure by roads, railways and structures, with the underground station in this highly constrained had previously been infi lled with mass original ground level typically 4–6m below the location presented signifi cant challenges concrete supported on piled foundations existing road level. These sites have a long and opportunities. The fi nal design has been and fronted with a masonry skin. The history of railway-related activities, including driven by effi ciency, physical constraints, former Caxton House building to the south, railway maintenance workshops, goods yard buildability and construction phasing, as well another 1960s development, has a basement operations, refuelling and storage of trains. as the need to provide a functional station at approximately the same level as the The two sites are roughly rectangular and appropriate to its location and historical basement of the demolished Cardinal Tower their overall dimensions are approx. 70m × setting. This paper presents an overview of building. 32m and 65m × 48m respectively. the design and considers the constraints that The River Fleet, which originally fl owed The new station stretches the length of infl uenced both the design and construction. near the site, now fl ows in a culvert directly the neighbouring Grade II* listed Smithfi eld beneath Farringdon Road. A culverted branch Market: from its West Ticket Hall immediately Existing infrastructure of the sewer (the St John’s branch), which to the west of, and integrated with, the West Ticket Hall was diverted as part of the Thameslink recently constructed Farringdon Thameslink The West Ticket Hall occupies the site of the Project, crosses the southeast corner of ticket hall; to its East Ticket Hall adjacent to former 1960s Cardinal Tower and is located the site. Figure 2 shows the surrounding Barbican London Underground station to the southwest of the existing Farringdon infrastructure and the listed buildings as per (Figure 1). London Underground station. It is bounded by a register maintained by Historic England During construction, Farringdon was also Cowcross Street to the north and Farringdon (previously known as English Heritage). the termination point for four tunnel drives Road to the west. Demolition of Caxton using four tunnel boring machines (TBMs): House, which formed the southern boundary East Ticket Hall two commenced their journey from Limmo of the site, assisted construction activities. The East Ticket Hall site is located at the TheStructuralEngineer | July 2018 27 TTSE78_26-34_Farringdon.inddSE78_26-34_Farringdon.indd 2727 220/06/20180/06/2018 117:147:14 Structural engineering for the Elizabeth line Farringdon station thestructuralengineer.org west end of Barbican London Underground station. It is an island site bounded to the north by Charterhouse Street, to the east by Hayne Street, to the south by Long Lane and to the west by Lindsey Street. The London Underground’s Metropolitan, Hammersmith and City, and Circle lines run east–west Fi gure 4 The jack arches and steelwork deck • Geological long through the north of section through spanning the London Underground and westbound the site in a cut- tunnel Moorgate spur lines support Smithfi eld and-cover tunnel House and a row of single-storey buildings, approx. 10m below as well as a section of the street itself. street level. The The site was also occupied by Lindsey roof of this tunnel Street House, a concrete-framed structure is formed of jack- dating from the 1960s built on stilts founded arch construction. at track sidings level. The remaining buildings The recently around the site occupy the higher-level area decommissioned behind the large retaining wall and date back Thameslink lines to Moorgate (the Moorgate Sidings originally used by Smithfi eld Market to the 19th and mid-20th centuries, with spur) run just to the south in the same cutting extend under Lindsey Street and occupied single basements below surrounding street and were partially covered by a steelwork part of the East Ticket Hall site to the south level. The East Ticket Hall site at the time of deck at general street level. of the Moorgate spur tracks. A large masonry advanced works is shown in Figure 3. Masonry arch bridges carry Hayne retaining wall previously cut through the site, Street and Lindsey Street over the London forming the change in level between the Ground conditions Underground and Moorgate spur lines. track/sidings and street level. The general geology at Farringdon is typical of the London Basin, comprising made ground, alluvium (localised only), river terrace deposits, London clay, Lambeth Group, Thanet sands a) West b) East Ticket Hall Ticket Hall and chalk1. However, the ground around the site is very Fi gure 5 complex and heavily faulted, with London Design pore water pressure clay overlying Lambeth Group and Thanet for West Ticket sands (Figure 4). The layer of London clay Hall and East Ticket Hall is relatively thinner over the West Ticket Hall site compared to the East Ticket Hall site. Although the site is under-drained at depth, there are perched water tables and water-bearing gravels which create particular problems for tunnelling activities. The distribution of alluvium is restricted to the infi ll of the former River Fleet and any associated tributaries over the western part of the site. There are two aquifers at Farringdon1: a shallow aquifer within the river terrace deposits and alluvium; and a deep aquifer within the chalk and the Thanet sands formation and Upnor formation of the Lambeth Group. The two aquifers are separated by the low-permeability London clay and low- permeability units of the Lambeth Group. The shallow and deep aquifers are both water- 28 July 2018 | TheStructuralEngineer TSE78_26-34_Farringdon.indd 28 21/06/2018 14:17 Structural engineering for the Elizabeth line thestructuralengineer.org Farringdon station Fi gure 6 • Platforms and passages (pink) were "THE OVERALL TUNNELLING formed using SCL STRATEGY AND techniques CONSTRUCTION PROGRAMME EVOLVED OVER THE DESIGN PERIOD" bearing. The water table in the deep aquifer is between 60 and 62mATD (meters above tunnel datum). The site geology, geotechnical risks posed by the heavily faulted ground, and Fi gure 7 presence of water-bearing gravel channels • SCL enlargement of eastbound TBM that could collapse