Working with engineers Arup, they plan to use a method of “top-down” construction borrowed from big towers in the City (where both space and time for construction are at a premium) to liberate the subterranean booty with minimal disruption. Dividing the site into segments of around 100 metres squared to allow the park to be used while construction continues, they will first scrape back the topsoil before drilling great concrete columns into the ground, then cast a concrete slab on top and return the soil, landscaping the park with raised berms and pathways. The gravel can then be excavated over time and extracted via access ramps with a concrete batching plant on site for the project’s own construction.
“It’s usually expensive to build top-down,” says Carmody, “but here we’re able to make savings because the structure is made out of the very stuff we’re digging on site.”
Once excavation is complete in around 15 years time, it will leave behind a 180,000 sq metre warehouse space, almost twice the size of the largest logistics shed in the country. The income from this space will be used to pay for the upkeep of the park, as agreed in the planning permission. A final use for this vast basement has yet to be determined, but it could host everything from long-term museum and archive storage, to logistics space and potentially even amenities like a theatre, gym or pool for the nearby school in its 10-metre deep cavern.
The park itself, which at 110 acres will be around the size of St James’s and Green Park combined, has been designed by landscape practice Vogt. In the tradition of London’s royal parks, it will be bisected by a 1km-long tree-lined avenue and further divided into pockets by raised berms and drainage channels, marking the column grid below ground. Developed in consultation with Sport England, it will also host a mixture of pitches for football, hockey and cricket, with the berms creating more intimate outdoor “rooms” and providing acoustic buffers so players can hear the referee’s whistle.