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Island life: Crossrail Wallasea Wild Coast Project

Three million tonnes of spoil from the Crossrail project is being used to create a wetland bird reserve in Essex in a landmark partnership with the RSPB.

On a small peninsula, just to the north east of Southend on Sea, the £14.8bn Crossrail project is having a very different impact on the landscape from the one that it is delivering roughly 80km to the west under the streets of central London.

As eight tunnel boring machines (TBMs) have created 42km of tunnels under the capital, 3M tonnes of the spoil they have excavated has been transported to Wallasea Island in Essex to turn this quiet area of farmland into a wetland bird reserve. 

Wallasea Island breach

Aerial shot showing Wallasea Island on breach day

In a landmark partnership with the Royal Society for the Protection of Birds (RSPB), Crossrail has burnished its sustainability credentials by using the spoil to landscape a 172ha section of the peninsula. A key moment in the project arrived earlier this month when the sea wall was breached in three places to allow the rising tide to create a series of new creeks, salt marshes and pools that will provide an inviting home for migratory birds and a new line of defence against tidal surges.

The £12M project, designed by Aecom and RSPB’s marine designer APR Mer, is being delivered by contractor Bam Nuttall. The construction firm’s project manager Dave Bugden explains that the works will return the land to its natural state.

“The land is lower than the surrounding river levels,” he says. “It was reclaimed in the 1950s by excavating the soke dyke around the perimeter and using that material to create a new sea wall.

“About 10 years ago they gave back some of the land to the RSPB and Natural England and they breached an area to take it back to how it was before they reclaimed it in the 50s.”

Before the project began, 30,000 birds lived on Allfleet’s Marsh, this small 100-hectare saltmarsh at the north eastern edge of the peninsula. The RSPB has now purchased the remaining land from Wallasea Farms and plans to return the whole of the 670-hectare site into salt marshes in a series of stages.

Wallasea Island

Illustration showing how the work is divided into five areas or ‘cells’

The area is bounded to the north by the River Crouch, to the south east by the River Roach and to the west by Paglesham Pool and the narrow Paglesham Creek. In the first stage of the project, the 3M.t of spoil from Crossrail was used to raise and landscape a further tranche of the easternmost tip of the peninsula – an area known as “cell 1” by the contracting team. At the same time, a new 2.5km sea wall was created at the western edge of this new raised area and approximately 1M.t of material was won on site by excavating lagoons and features in two further areas known as “cell 5” and “cell 3” to the north of the site (see diagram).

Further down the line, spoil from projects like Crossrail2 and Thames Tideway will be used to fill a further two cells to the south of the peninsula – cells two and four – to complete the job. Most recently, however, the focus has been on cell 1 in preparation for the July breach.

“This first phase was designed to allow 1.2M.m3 of water per tide into site and back out again,” says Bugden. “We created a series of creeks and channels and pools that are able to hold water and let that water out to make it right for different types of migratory birds.”

The spoil that has been used on the project has taken a circuitous route to the site in Essex. Earth from the central and eastern tunnels was taken by lorry to a transfer station by the Thames at Barking, while earth from the western tunnels was taken by rail to a similar transfer station further east at Northfleet. In these two locations material was loaded onto 2,000t capacity freight ships and transported to a pontoon on the northernmost side of Wallasea Island adjacent to the River Crouch.

Wallasea Island

Aerial image showing the loading pontoon and conveyor system at the north of the peninsula

“The pontoon can take two boats simultaneously,” says Bugden. “You unload the boats simultaneously onto a conveyor system, combined, and that’s 800m onto the land, 24/7 for two and a half years.

“From there we moved the material out to the field using traditional dumpers – I think we had a maximum of 28 dumpers, six excavators and five blades.”  

To ensure that the earth was distributed in the correct locations, Bugden reveals that all of the blades and excavators were guided by ground positioning satellites. The stacker was also fully automated to continue to load material onto a store capable of holding 16,000t of material, even when nobody was working on site at nights and weekends.

Conveying the spoil

The process sounds relatively straightforward in explanation but, as it transpired, the varying consistency of the clays and soils arriving on the freight ships from east London complicated their conveyance.

“The problem we had when we first started was that we struggled to maximise the efficiency of the unloading facility,” says Bugden. “The heavy tunnelled clay would just clog [the conveyor] up. Whatever it was in that material, whether it had been kneaded, or the additives that they had used, it was such that the heavy clay clogged up the conveyor system.”

To solve the problem, the construction team went about trying to improve the unloading facility. A series of augers was purchased in Ireland and placed at the intersections of the conveyor to prevent blockages. A decision was also taken to mix half the spoil from the Barking vessel with half of the material from the Northfleet vessel.

Kentucky Fried Chicken

“The material that went into the Barking vessel had been dug by machines; it hadn’t been tunnelled so it was a much drier, more granular material,” says Bugden. “It could have gone one of two ways: it could have either been like Kentucky Fried Chicken, just coating this heavy clay, or it could have broken it up – but it broke it up and we were able to convey it.”

Once this problem was resolved, the speed with which the earth could be conveyed nearly doubled. Bugden explains that the first million tonnes of earth took 62 weeks to move but after the conveyor had been modified, the second million tonnes took 32 weeks.

Before any of the earth could be redistributed and used to landscape cell 1, Bam had the delicate task of relocating some of the existing wildlife in the area. Around 8,900 reptiles and roughly 380 water voles were trapped and, in the case of the reptiles, transferred to a hibernaculum.

Landscaping

Then the process of landscaping and raising the level of cell 1 could begin. Bugden explains that the 3M.t of Crossrail spoil, plus 1M.t exported from cells 3 and 5 has raised the level of this area by an average of 1.4m.

A large proportion of the spoil was used to create the new sea wall which will hold back the tide now that cell 1 has been breached. This was constructed on a geomembrane and built in stages to allow pool water to dissipate.

“We had to build up to 3m above the existing level and it was prescribed that [after building each layer] we leave it for 12 months to allow the pool water to dissipate,” says Bugden. “But it [dissipated] a bit quicker, so we were able to then put another metre on.”

At 4.8m, the new sea wall is 800mm higher than the old one to allow for some long term settlement, but also for the greater tidal surges concomitant with climate change. It consists entirely of a clay bund, is 33m wide at the bottom and will require maintenance to regulate it at the required height.

Wallasea Island

However the real challenge was how to breach the existing sea wall in three places simultaneously to allow the sea to enter cell 1. This had to be coordinated with a low neap tide on 11 July with little margin for error.

“Neap [tides] don’t go as high and don’t go as low, so at that not very high [neap] tide, we started chasing the tide down and we cut the sea wall in three places simultaneously,” he says.

The contracting team only had one opportunity to get this right: if they failed to cut any of the three walls correctly at the first attempt, the rising tide would have returned and completely jeopardised the success of the project. “You can’t do two and not do one,” says Bugden. “You’ve got to do three, so if there was an incident on one, a breakdown for example, they’d all stop.”

The process was complicated by the fact that an area of low salted mud flats sat immediately outside the sea wall which would have prevented water from flowing through the sea walls once they had been breached. Amphibious excavators were brought in from the Netherlands to cut channels varying in length from 50m to 140m out from the sea wall, each generating between 15,000m3 and 45,000m3 of soft material.

(Click on the image above to view video)

Bugden describes the breach week as a “chasing of the tide”. Armed with the knowledge that the high tide levels were dropping as the neap tide approached, the project team aimed to take as much material out of the sea wall as possible to leave the minimum excavation on breach day. Checking the weather at all times, they always left at least half a metre of sea wall above high tide to allow for any surges. 

“Each breach was 170m wide at the top of the sea wall, tapering down to a 20m wide channel that is 6m below top of sea wall,” says Bugden.

During breach week, the sea wall was reduced from 7,000m3 to 3,000m3, and, starting at 8am on breach day, the final 3,000m3 was removed simultaneously on each of the three breaches.

“The initial three hours involved reducing the height of the sea wall as the tide dropped. As the tide fell, the sea wall was removed and the 2.5m deep channel was excavated in the poorest of ground conditions,” says Bugden.

“The plant was able to overcome this and all three breaches were completed as the tide rose and the first tidal waters passed through the breach. The plant vacated the working area in good time for the expectant local public to witness the high tide flood the site successfully.”

The next phases

Wallasea Island

Bam Nutall has constructed a series of inlets and outlet structures to regulate water flow between the different cells

One million tonnes of the earth used to landscape cell 1 came from a newly excavated series of lagoons in cells 3 and 5 of the peninsula. Now that cell 1 is complete, cell 3 requires further imported material to complete the earthworks. This will be placed around the perimeter, connecting the area to cells 1,2 and 4.

As part of the first phase of the project, the Bam Nuttall team also constructed a series of inlet and outlet structures to regulate water flow between all of the different cells when they are complete. Some of these areas will remain dry until further breaches take place. Ultimately, further breaches are planned to the south of the peninsula to allow water into cells 2 and 4. 

 

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