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The concrete challenge of Canada Water

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The engineering challenge that faced the design and construction teams on Canada Water Station has tested their skill and ingenuity, and has set new standards of excellence. Jim Paterson of Robert Benaim & Associates discusses the civil and concrete engineering aspects of Canada Water Station with David Bennett

Jim Paterson is technical director of Robert Benaim & Associates and was project director for Benaim-Works jv on Canada Water jle Station

David Bennett is a technical consultant specialising in architectural concrete

Canada Water is one of six new stations to be built for the Jubilee Line Extension, one of the most important additions to the London Underground network in the past 25 years. The jle forms a chain of 12 stations that run from Green Park to Stratford via the London Docklands.

Canada Water Station is bisected by two underground lines, the operating line of the East London Line - ell for short - which was some 11m below ground level running north-south across the project site, and undermined for 22m of its length by the new jle line, which runs 8m below it, in an east-west direction. Two large chasms had to be excavated through the Thames gravel, the Woolwich and Reading Beds and into the Thanet Sands, and it was also necessary to deal with a perched water table in the Thames gravel and a second one under Artesian pressure in the Thanet Sands.

The jle 'gorge' that was excavated was 150m long, 23m wide and 22m deep, while the ell slot at right angles to it was 130m long, 13m deep and tapered from 32m to 20m in width, cradling an active tunnel. The void created in the ground was large enough to swallow St Paul's Cathedral whole and resulted in 120,000 m3 of excavation spoil. 33,000m3 of concrete was poured back into the hole to build the new station structure within this enormous burrow.

'To compound the problem of finding the simplest and most economic cut- and-cover construction operation, the excavation was sandwiched between a pair of tower blocks at one end and a dock at the other,' says Jim Paterson of Robert Benaim & Associates, project director for the joint-venture design team Benaim-Works jv. One side of the excavation ran close to the disused Canada Water Docks and the other came close to the foundations of two existing 22-storey tower blocks. In addition, the roof slab had to carry a roadway and the interlinking bus station, while the box structure of the jle section had to be capable of supporting a nine-storey air- rights building in the future.

For both the deeper jle excavation and the ell excavation, a secant-pile wall enclosure was chosen, sleeving the top 8m through made ground and the water-bearing gravel layer. The piles were bored to 24m under bentonite for the jle section, and were fully reinforced and designed as propped cantilevers. Prior to commencing the piling works, the water table across the site was lowered by deep-well dewatering into the Thanet Sands. Two series of observation wells were taken, one just into the Thames gravel, and the other going into the Thanet Sands to monitor the draw-down of the Artesian water table. The secant-piled wall for the jle box section was formed by interlocking hard and soft piles of 900mm and 750mm diameter respectively, which toe into the Thanet Sands. Tubular steel struts nominally 1m in diameter were placed across the width of the excavation to brace the piled wall excavation. Each strut was capable of taking a thrust of 1000 tonnes.

'Geotechnical monitoring of the structure, the ground movement and effect on the tower blocks, during excavation and after de-watering,' says Paterson, 'revealed that there was negligible movement.' Ground heave in the tunnel excavation was not a problem as the base excavation went through the Woolwich and Reading Beds and into the Thanet Sands.

Standardisation with innovation

Pile construction started on the northern end of the site, enclosing the excavation boundary to the ell section and the deeper jle section. The shallower secant-pile wall for the ell box was supported at the top by ground anchors. The existing brick tunnel structure of the ell was carefully excavated and ties installed to carry lateral thrusts. The active tunnel was revealed for the entire length of the box before it was broken out. 'We had a six-month closure period for the East London Line, during which time we were able to break out the existing tunnel structure, form the new base structure, relay the track bed and build the permanent bridging structure over the jle line,' adds Paterson.

Specially made inverted precast T beams, supplied by Tarmac, were used to form the base of the new box structure, saving on excavation depth and speeding up construction. The T beams were infilled with in-situ concrete to form a solid base slab. Where the ell bridged the jle line, plunge columns were installed into temporary piles to allow construction of the roof over the ell, in advance of the excavations for the lower jle platform and tracks.

Following this, the excavation was taken down for the jle platform and track, with either two or three rows of tubular struts, according to the depth of excavation, supporting the piled wall. The in-situ-concrete inverted arch of the base slab was then formed within the excavation at a depth of 22m. The arch design reduces the depth of the construction and excavation, and the cost of both permanent and temporary works, while transferring the uplift forces acting on the structure very efficiently. 'We won the design contract because of our design ideas,' says Paterson. 'The savings in the depth of excavation and cost that accrued were considerable from the inverted arch design for the base, compared with a conventional flat- slab base and rectangular box.' Flotation of the box structure was resisted by the self weight of the structure, aided by tension piles anchored into the underlying chalk and by the soil resistance mobilised by extending the arch base slab beyond the retaining wall line. 'It is much more cost- effective to use tension piles rather than adding mass concrete, to counter a flotation risk,' says Paterson.

The retaining walls of the deeper box section were nominally 1200mm, reducing to 400mm thick where it joined the retaining wall line of the ell box. The walls of the jle box were formed within the excavation and the secant-pile wall coffer dam; while along the west boundary of the ell, the secant pile wall is integrated into the permanent works by cutting back the face of the piles and casting a skin wall.

Although the cross-section of the deep jle box varied along its length and is punctured by deep access shafts at each end and a large opening for the station in the roof, a great deal of standardisation was designed into its construction. A stiff, upstand beam runs down the spine of the base slab to help distribute the varying loads acting on the base and to reduced any differential settlement. The upstand beam and adjacent compartment walls also form the service enclosures under the platform that house the cable trays, bundles of heavy electrical wiring, the sprinkler mains and other essential systems that are needed to operate and run the station. For the platforms, the common access areas, ticket hall and roof, a flat slab was adopted to give maximum headroom. This slab spans the width of the box to act as a horizontal prop to the retaining wall. The roof slab over the main concourse area was 750mm deep, to support heavy vehicle loading from the proposed bus station above it.

Top-down construction at the west end of the site was adopted to minimise foundation settlements and any ground movement to the tower blocks nearby. 'It also allowed us to cast the roof slab in this location in advance of the excavation,' says Paterson, 'keeping noise breakout and disturbance to the occupants in the tower blocks to a minimum.'

Reinforcement tonnage for the box structure was kept to a minimum by rigorous analysis of the different loading regimes and the combinations of axial force and bending moment acting on the structure. 'We were able to justify a reinforcement saving of 20 per cent compared to a conventional design, where it is usual to assume that the worst bending moment and the maximum axial force act together, and to design for it,' says Paterson. 'Careful analysis of all the loading conditions shows that this approach is more efficient.'

Quality control

Generally the concrete for the external retaining walls, the inverted- arch base slab and any part of the structure in contact with the ground was required to meet a durability standard defined by a maximum oxygen diffusion rate of 1x10-8m2/sec and chloride ingress of not more than 1x10- 12m2/sec. Generally the concrete supplied was a pump mix, with a cementitious content of 400 kg/m3 for a 10mm coarse aggregate, made up of 280kg of opc and 120kg of pfa. This met the durability compliance and strength grade of 40N that was specified. Admixtures were used to make the concrete workable, without increasing the water content above a 0.45 water-cement ratio. Where a nominal 20mm coarse aggregate was used, the cementitious content of the pump mix reduced to 370kg/m3.

The large 1.5m-diameter columns with their flared heads, run the length of the jle platform and rise 18m to support the ticket-hall floor and roof slab above. The smooth concrete finish was achieved using metal forms that had been grit-blasted to take the shine off the metal face. The metal face was then lightly coated with a high-performance release agent and any surplus removed with a cloth. The release agent was left to dry before the shutters were positioned around the reinforcement cage.

Placing and handling procedures were routine, with care taken to direct the concrete away from the form face and prevent it bouncing off rebar on the way down the formwork. Once the formwork was struck, the concrete was protected and then lightly rubbed down before handover. Construction joints were carefully detailed to avoid any risk of grout loss or honeycombing. The retaining walls were cast in sequence, with pour lengths of generally 10m and pour height of 4m, and continuously reinforced at the bay joints with the provision of water bars.

A feature of the jle stations is the glass wall barriers along the platform edge that prevent people, paper, perishables and combustibles from getting on to the track. Any fire or smoke in the track corridor will now be contained within this space and not spread into the station, with smoke hoods and extract ducts ventilating any build-up of smoke.

The jle interchange with both the East London Line and the new bus station will open up new travel links into the City and West End for local residents. The cascading banks of escalators taking commuters to and from the platforms are easily followed and are visible from many corners of the station over its entire length. Large openings formed in the roof of the concrete box and at concourse level allow daylight to flood into the station to reach deep down onto the platforms of the jle. Up to 6700 passengers an hour are expected to use the station during rush hours.

Canada Water jle station has demonstrated the civil engineering enterprise of the design and construction team, and the adaptability and robustness of concrete - with a choice of in-situ and precast options, top-down, secant-piling, circular-column and flat-slab construction - responding to the rigours of creating a new underground interchange, with the minimum of disruption.

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