Nature’s Way

Wallasea Island, currently surrounded by sea walls, sits on the River Crouch opposite Burnham-on-Crouch

Material excavated from the Crossrail scheme across London will be reused to create one of Europe’s largest nature reserves. Stephen Cousins reports.

Humans are not the only commuters likely to benefit from the Crossrail link. The 4.5 million tonnes of material excavated from its tunnels will be transported to Wallasea Island in Essex to create the largest bird habitat of its type in Europe.

The Wallasea Island Wild Coast Project is the most ambitious habitat recreation and engineering scheme yet undertaken by the Royal Society for the Protection of Birds (RSPB). The project will reinstate 1,500 acres of ancient wetland landscape, including mudflats, saltmarsh, lagoons and pasture, and is designed to combat the increased risk of coastal flooding and compensate for the loss of tidal habitats elsewhere in England.

When the project is completed, by 2019, it is expected to attract a host of wildlife, including rare saltwater fish and migratory birds. Roughly half of the 10 million tonnes of chalk, clay, sand and gravel needed to create this habitat will be excavated by eight giant tunnel boring machines (TBMs) as they plough their way beneath the capital to create 21km of twin-bore tunnels. The project involves breaching the existing sea walls, then raising the low-lying habitat behind them.

Pairing the UK’s largest infrastructure project with its biggest nature project might seem daunting, but it couldn’t have come at a better time, says Rob Paris, head of sustainability at Crossrail. “When the Crossrail project gained Royal Assent in 2008 we started looking for sites to beneficially reuse our excavated material,” he says. “The timing with Wallasea Island was perfect: they needed huge amounts of material, the majority of ours is clean and suitable for placement without decontamination and we had easy access to sustainable transport, via train and ship, to move the material to site.”

Excavation of the 6.4km-long western tunnels, which run from Royal Oak in west London to Farringdon in east London, started in May. This material is being moved through the tunnel by conveyor to the entrance at Royal Oak, where it is placed into temporary storage hoppers.Freight trains then move the earth to Northfleet in Kent, where it is transferred to ships and transported along the Thames Estuary to Southend–on-Sea, then north to Wallasea Island, situated on the River Crouch opposite Burnham-on-Crouch.

Earth will also be taken from the two eastern tunnels. Work on the longest is due to commence this winter and will extend 8.3km from Farringdon station, out past Canary Wharf to the tunnel entrance at Limmo Peninsula. Material moved through this entrance will be transported by lorry to nearby Instone Wharf and Docklands Wharf and then by barge to Wallasea.

A conveyor is installed to transport material from ships at Wallasea

Crossrail’s involvement with the Wild Coast Project forms an important part of its environmental management strategy, which commits it to recover at least 95% of clean excavated materials and 90% of demolition and construction waste. Around two-thirds of the total excavated material will go to Wallasea.

Reducing carbon emissions during transport was a big consideration, says Paris. “As a rail project we are very fortunate we can load the materials directly onto trains. In total 85% of the material will be moved either by rail or water,” he says.

Apart from the tunnels, several shafts are being dug along the central section of the western tunnel to create sub-surface stations. Since tunnel boring machines haven’t yet reached this zone, the excavated materials will be loaded onto lorries for transport to Docklands Wharf. Paris says that Crossrail’s biggest challenge will be to avoid bottlenecks at the tunnel entrances as incoming deliveries of rebar and concrete needed to construct the tunnel clash with the removal of excavated materials.

The contract to ship and place the excavated material was won by partners BAM Nuttall and Van Oord UK, which are also responsible for building related infrastructure, including a newly completed jetty at Northfleet, designed to accommodate two 2,500-tonne ships side by side, plus a new jetty, pontoons and a conveyor system designed to transport the material across the marsh at the Wallasea end, which is currently under construction. Concrete manufacturer Lafarge has also reinstated the 2.25km rail freight link running from the North Kent main line to the Northfleet site.

As excavation from the western tunnel continues apace, the first of more than 2,000 ship deliveries to Wallasea is scheduled for later this summer. The material will be used to raise land and create hillocks and dips through which seawater will ebb and flow, attracting saltwater fish such as herring and flounder, and saltwater plants such as sea lavender and samphire.

It’s also hoped the calorie-rich saltmarsh, mudflats and other habitats will attract species of rare birds. All of which should make the cost of a season ticket a little easier to bear.