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High Flyers

Foundations work for the UK’s first urban cable car is under way in east London. Claire Symes visited the site to find out more about the scheme.

London’s last project to create a gondola-based tourist attraction resulted in the London Eye - a development whose popularity has lasted well beyond the planned year-long opening.

But the capital’s next scheme adds something more.

The London cable car - which is being sponsored by Emirates and will be called the Emirates Air Line - will give users spectacular views and provide a permanent new river crossing too.

Transport for London (TfL) describes the new link between the O2 and the Royal Victoria Dock as a “travel experience”, but with a five-minute journey time for the 1.1km route, it could offer a good commuter link.

“The aim of the scheme is to help with the regeneration of that part of London”

Mace is the lead contractor on the scheme, but the main focus on site is currently the piling work, which is being undertaken by a joint venture of Bachy Soletanche and Red7Marine.

The project is being undertaken by Mace under a fixed price design, build and operate contract.

“It is effectively a civils project that includes cable car equipment,” says Mace construction executive Nic Hamlin.

Mace is partnering with Doppelmayr to supply the cable car components for the project.

According to TfL, Mace’s contract for the building work is valued at £45M, while the three-year contract to operate the facility is priced at £5.5M. Emirates’ 10-year sponsorship deal will contribute £36M to the scheme.

“The aim of the scheme is to help with the regeneration of that part of London,” says a TfL spokesperson.

The project has five separate construction sites - the south station is located at North Greenwich close to the O2 arena and the north station hovers over the Royal Victoria Dock close to the Excel exhibition centre.

The other construction sites are for the south tower within the tidal reaches of the Thames, the north tower and the north interim tower, which will carry the route over the Docklands Light Railway’s (DLR’s) elevated track.

Mace broke ground on the first of the five sites in May and took possession of the final site - the location of the interim north tower - on the day of GE’s visit in September.

Bachy Soletanche is a preferred supplier for Mace and the piling contractor’s joint venture with Red7Marine for the overwater work on the scheme made it the best option for the contractor.

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In addition to the piling work for the stations and main towers, Bachy and Red7 are also undertaking piling for the two compression towers at the front of each station and for ship impact protection piles around the bases of the south tower and the north station.

Site investigations

Before any piling work could get under way at each of the sites, Bachy had to undertake probing for unexploded ordinance.

“Fortunately nothing was found,” says Bachy Soletanche business development manager Paul Hodgson.

The positions of the piles also had to take account of future plans to develop the Silvertown road tunnel and ground has been safe-guarded to ensure the cable car system does not impact on this.

Site investigations for the project were undertaken by Mott MacDonald as part of the planning for the scheme, which included working with project architect Wilkinson Eyre.

According to Hamlin, the main challenges are building the river crossing on made ground and the loading placed on the substructures.

“The ground at the location of the south station had been partially remediated when the area was redeveloped for the O2,” explains Hamlin. “But this was only carried out to 2m below ground level and then sealed.

To work below this we had to get a permit from Greenwich Peninsula Regeneration which operates the area.”

With this consent in place, ground was excavated to 8m and removed from site for testing and classification for final disposal.

“As far as we know, these are the largest piles that have used polymer so far”

When GE visited the site, piling work on the two stations was complete and pile caps and beams were about to be built.

Red7’s jack-up barges off the southern bank of the Thames were in position installing cased piles and Bachy was working on installing the rotary piles at the interim north station.

Work on piling for the north tower is expected to start soon.

As Hamlin points out, the difference in made ground at the five sites was a major challenge.

In addition to the contamination at the south station site, Bachy also had to contend with the location of the north interim tower being located within a backfilled lock that once served the Royal Victoria Dock.

“The amount of made ground did influence the choice of rotary as the piling technique,” says Hodgson.

Although the depth of made ground varies between the five sites, with the maximum depth recorded at 14m at the location of the north intermediate tower, the geology below comprises London Clay over Harwich Clay and Gravel before progressing into the Lambeth Sands and Clays over the Thanet Sands and Chalk.

Bachy used a Casagrande R625 piling rig at the south station to install the 750mm piles to a depth of 40m into the Thanet Sand.

The station and compression tower will be supported by 43 piles.

Piling at the north station was carried out by Red7 through an 18m long permanent casing to take the 750mm diameter piles through the bottom of the dock.

The 31 piles to support the station and compression tower were drilled to 45m below water using a Bauer BG24 piling rig mounted on a Haven Seareach spud leg barge.

WEA_682_LCC_South_Tower

The BG24 rig is now working from Red7’s Haven Seaway jack-up barge to install the four 1,800mm diameter piles for the south tower, with additional jack-up barges providing support by carrying lifting equipment and auxiliary plant.

Piles for the south tower are being installed to a maximum depth of 51m with a 24m long permanent casing socket into the lower layers of the Thanet Sand or chalk.

The piles for the south tower are being augured using a polymer-based drilling fluid rather than the conventional bentonite.

“The polymer is more environmentally friendly,” says Hodgson. “As far as we know, these are the largest piles that have used polymer so far.” In addition to the environmental benefits, Hodgson also says that the mixing equipment for the polymer is more compact.

Another Bauer rig - this time a BG26/36 - is also being used by Bachy for the north intermediate tower which will be supported by a grid of nine 900mm diameter piles drilled 45m below ground level into the Thanet Sand.

The main north tower will be supported by 40m long, 1,050mm diameter piles, also founded in the Thanet Sand.

Piling work is due to be completed in February next year.

Hodgson describes progress on site so far as “good” and added that the overwater skills of Red7 have been critical to the work going so well.

Tower construction

Given the cable car’s close proximity to London City Airport, the challenge is to get the gondolas high enough to clear shipping on the Thames, but low enough not to interfere with the flight path into London City airport.

“The north and south tower will be 84m tall and give the gondolas a clearance of 55m above high water level on the Thames,” says Hamlin.

“The helicopter will carry a light cable across the route and this will be used to gradually thread larger and larger cables through until the final 50mm diameter cable is in position”

Mace is expecting to start work on the installation of the steel towers from November, starting with the north intermediate tower.

“With the location of the tower close to the DLR, it is likely that we will need to carry out the erection work there with a track possession,” says Hamlin.

Although Hamlin adds that each tower will take nine days to erect, weld and bolt into position, it will take longer to assemble the parts on site and prepare them for lifting.

Station building construction will also get under way soon and will be built on a concrete ground floor slab over the pile caps and beams.

Once the towers are in position, the next milestone will be positioning the cables in spring next year.

This is set to be an interesting event - Mace plans to use a helicopter to thread the cable across the Thames.

“Doppelmayr use helicopters on Alpine cable cars to ‘string’ the cable through the towers,” explains Hamlin. “The helicopter will carry a light cable across the route and this will be used to gradually thread larger and larger cables through until the final 50mm diameter cable is in position.”

What is remarkable about the project is the speed at which it went from the planning phase to work starting on site.

The link has been championed by London major Boris Johnson and the scheme is expected to be completed next summer.

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