Crossrail – dig it out

Materials World magazine
1 Jan 2018

Crossrail’s boring machines excavated 42km of tunnels for the new train line that will run through the south east of England in 2019, but what happened to the excavated material? Ines Nastali investigates. 

With the beginning of the new year a new form of transport for southern England edges closer towards full operation. Crossrail, called the Elizabeth line after Queen Elizabeth II, will see newly designed trains enter into service from London Paddington to Heathrow Airport in May this year – in addition to the ones that already operate between London Liverpool Street and Shenfield, Essex – and will also make use of the new central tunnels at the end of the year for the first time.

Construction of the route generated over seven million tonnes of material, Crossrail states, adding that over 90% of it has been reused. ‘We will be judged on how we’ve done it, how we delivered the project. That will be our legacy, especially because we are spending public money,’ says Andrew Wolstenholme, Crossrail’s Chief Executive Officer. He spoke at the Policy Forum for London in November, giving an update on delivering Crossrail and the next steps for follow up project Crossrail 2.

Planning ahead

The first step towards sustainability was to ensure that the infrastructure surrounding the construction sites allowed seamless removal of the excavated material and secondly, to find appropriate disposal sites – 10 designated landfill sites were created in the counties surrounding London. A waste hierarchy was created, to keep the generation of waste as low as possible and to reuse material within the project where possible. 

Crossrail managed to transport 80% of every tonne per km by rail or water – and avoided putting too much additional traffic on the already busy London roads when working on the 10 station construction sites. This was during the planning phase back in 2008, and the Crossrail team wasn’t the only one about to start work on a large project. The Royal Society for the Protection of Birds (RSPB) was working on a conservation effort at Wallasea Island, Essex, to protect the Thames estuary and its animal inhabitants from rising sea levels and coastal erosion, and was therefore in need of material to reshape the geography of the island and to create habitats for the animals. This coincidence turned out to be an ideal match. ‘As an island, it promoted the use of water transport and Wallasea could take over four million tonnes of excavated material,’ Crossrail states in The Transport and Beneficial Reuse of Crossrail Excavated Material technical report. 

Material delivery

Due to issues arising during the delivery, the amount of excavated material was reduced to three million tonnes. One problem was the conveyer belt that transported the material from port to site. Lorna Mellings, Crossrail’s Environmental Assurance Manager and one of the authors of the report, explains, ‘When the tunnelling material (mainly London clay) was on the ship from Northfleet to Wallasea, some of the material became more cohesive, so on reaching Wallasea, it would come out in large lumps, be dropped into the hopper but couldn’t be broken down.’ As a result, the conveyor became blocked. The solution was to mix the clay with granular materials and amend the mechanical parts of the conveyor to enable it to more easily process the material. 

Before delivery could start in 2012, Crossrail had to look at the excavated material – the main western and eastern tunnels drove predominantly through clay, the Thames tunnel through chalk – and how Wallasea could use it. ’The delivered material consisted mainly of relict estuarine mixed sediments and marine deposits, which are eminently suitable for the creation of intertidal habitats,’ RSPB states.

The ecotoxicity potential for the aquatic environment also had to be assessed. Crossrail had to ensure that any additives and polymers used during the excavation were risk assessed. ‘Polymers are used to facilitate the handling of material when being processed through the tunnel boring machine and as they are entrained within the material itself, there was a need to assess what impact, if any, they may have on the aquatic environment when placed at Wallasea Island,’ Mellings explains.

‘Wallasea could not accept chalk as it was unsuitable for habitat creation and the moisture content of the excavated material, which is extracted as a slurry, was far too high for it to be shipped in any case,’ she adds. ‘Instead, the slurry was processed through a plant on site to remove the water and the resulting material was transported by road to Pitsea, where it was used as a capping layer on the landfill site to create rare chalk wildflower meadow habitats, another RSPB nature reserve as well as to Kingsnorth where it was used in land raise to allow for construction of a commercial park.’ Crossrail also delivered gravel to the conservation site until March 2015.

Moreover, local birds needed protection. ‘Water voles are known to inhabit most of Wallasea Island’s ditches and as the construction programme at the island included extensive reshaping of the existing water course, many of the voles were rehomed. The work was undertaken over a period of two years and completed in 2014 with over 150 water voles living in the area being moved nearby,’ Mellings states, adding, ‘Over 8,200 common lizards and 40 adders were also relocated.’ 

A number of main and outlier badger setts were located in the area and an artificial sett created in another suitable location on the island.Further reuse of materials included the creation of a golf complex, while old landfills used the deliveries as capping.

More material needed

It’s not only Crossrail that delivered material to help create a nature reserve. There is another project currently underway in London, which has dug up London clay, terrace gravels, Thanet sands and dredged material, and was therefore in need of a recycling plan – the expansion of London Underground’s Northern Line, due to be finished in 2020. This is the first major extension of the Tube since the Jubilee line in the late 1990s, Transport for London (TfL) states. The excavated material comes from tunnelling works, which were completed in November 2017. 

‘The dug-up earth is being used in farming,’ London Mayor Sadiq Khan says. Commenting on the completion he adds, ‘Around 680,000 tonnes of material will be excavated over the project’s lifetime – equivalent to the weight of two Empire State Buildings. More than 90% of it is being transported out of London by river and much will be used to create arable farmland, minimising the impact on the environment.’

Using the water ways around the capital has turned out to be the preferred option for TfL. ‘We try to transport as much excavated material as possible by barge as it takes many thousands of lorries off the road,’ Michael Tarrega, a spokesperson for TfL, tells Materials World

‘It is then transferred to Goshems Farm in Tilbury, Essex. Here, the material is used as part of a major land restoration programme,’ Tarrega adds. ‘All the excavated material from our tunnelling was transported in this way. Over the whole project, 92% of excavated material will be taken by barge to Goshems Farm.’

Going further

With Crossrail nearing completion, a second project aiming at connecting the south of England with the northeast region of London has been proposed. Above-mentioned Crossrail 2 is ‘currently awaiting the government’s decision to go ahead,’ says Michèle Dix, Managing Director for Crossrail 2, also managed by TfL. 

In order to obtain approval, Crossrail 2 is outlining its strategy in a consultation paper. This includes which new stations to build, where tunnels need to be excavated and what happens to the material. ‘Where possible, we would remove excavated material along the tunnels, rather than taking it out on the surface through station worksites and using vehicles to remove it,’ an environmental impact report states. 

While Crossrail 2 is a separate project, Crossrail shared lessons learned with the sister project to improve sustainability. The report adds, ‘As a result of this proposed approach, the number of waste movements from a typical Crossrail 2 station could be half of those similar projects in the past. We will be considering options for productive reuse of the excavated material in our next phase of design work.’