Any new River Thames crossing in the heart of London faces formidable regulatory constraints.
The normally inviolable visual restrictions of London mayor Boris Johnson’s London View Management Framework, coupled with the Port of London Authority’s insistence on minimum clearances over the main navigation channels significantly limit the range of structural alternatives available to design teams.
In the case of the £175M Garden Bridge project, however, there was an additional challenge - ensuring the long term survival of the extensive planting that is intended to give the crossing its unique appeal. Among the criticisms the project has attracted is the claim that the lush planting depicted on the promotional material was a fantasy, and could never be achieved in reality.
But project promoter the Garden Bridge Trust has consulted closely with the Royal Horticultural Society and appointed the notable garden designer Dan Pearson of Dan Pearson Studios to come up with appropriate planting strategy. He has a track record with large scale horticultural projects, .
Garden Bridge Trust programme director Anthony Marley says one of the main concerns was the potentially destructive effect of high winds, which can be gusty in central London.
“On many projects, new trees and shrubs are confined in individual planters, which gives no opportunity for the roots to spread out and get a firm grip on the soil,” he explains.
“We’ve adopted a very different approach.”
In plan the bridge flares out to 30m wide over its two piers. The main pedestrian footways follow the outer profile, leaving two linked central oval shapes for planting.
Over the piers the planting medium - layers of topsoil, subsoil, tree sand and drainage material - will be 1.8m deep.
All this will sit on a steel deck extending across the entire bridge, set above the substructure. “This will allow all the root systems to intermesh, making all the planting much more wind resistant,” Marley says.
“And although some of the 270 trees will be 5m tall when planted, nearly all will be multi-stemmed specimens, and so will be even more wind resistant. For extra security we will be using tethers and stakes where appropriate.”
“What we are planning is effectively a sustainable drainage system”
Anthony Marley, Garden Bridge Trust
Pearson’s planting plan is designed to reflect the history of both banks of the Thames by the bridge site. On the North bank, the ancient Inns of Court have a long established horticultural tradition, which led to the choice of garden varieties of figs, apples, camellias, maples and magnolias, as well as climbing plants such as wisteria, jasmine and vines.
Herbaceous perennials and flowering bulbs will add seasonal colour. There will be a more natural planting at the southern end of the bridge, inspired by the marshland that existed on the South Bank until well into the 18th century.
So native trees such as birch, alder, hawthorn and willow, along with wild honeysuckle and dog roses will feature here.
Tenders are already out for the landscaping and topsoil, which must meet the strict requirements of consultant Arup’s soil scientists. Collection of the actual plants will begin next spring.
“They will then be planted out elsewhere in a location where the exposure conditions mirror those we expect on the bridge,” says Marley. “Those that flourish will be transplanted onto the bridge sometime in early 2018.”
Dense plantings like those proposed require irrigation and fertiliser. Water and nutrients have to percolate freely through the layers of planting medium. The water-holding capacity of the medium has to be optimised, but stagnation and anaerobic soil conditions must be avoided.
This requires effective drainage. “What we are planning is effectively a sustainable drainage system,” says Marley. “All paving will be porous, allowing rainwater to percolate into the ground where it will be picked up by the subsoil drainage system.”
The crucial water levels in the subsoil will be monitored by a series of wells ranged across the full length of the bed. French drains and dams will manage the release of rainwater among the subsoil levels. A zonal irrigation system will keep the planting green during periods of prolonged drought.
Any excess water will finally drain into the Thames via a gargoyle on each pier. For the proposed South Bank Building, however, there will be a rainwater harvesting and a storage system, for toilet flushing.
Whoever wins the landscaping contract will have to maintain the planting for five years. During this time the Trust will employ two full time gardeners and an executive gardener. Maintenance and management of the entire crossing will remain the responsibility of the Trust. It will also be responsible for raising the necessary funding, currently estimated at £3.5M annually.
Those competing for the £175M Garden Bridge contract will be expected to put forward their own proposals for certain aspects of the design and construction of the 366m long crossing.
“The cupro-nickel exoskeleton is fixed and can’t be changed,” explains Garden Bridge Trust programme director Anthony Marley.
“It was chosen because it combines a visually appealing colour and texture with maintenance-free protection against the river environment.
“But we will consider proposals on the articulation of the superstructure, and on the construction of the pier foundations, for example.
“The outline design is not mandatory and is open to optimisation.”
Design of the foundations is already finalised, however. It is proposed that each pier will sit on six, 2.5m diameter bored and base grouted piles going down 56m into the Thanet sands.
Installing these out in a heavily tidal river swarming with marine traffic is never going to be straightforward.
No fewer than four main navigation channels pass through the site, with the three main channels between the bridge’s two piers. The Port of London Authority insists that an overhead clearance of between 8m and 10m has to be maintained at all times above these channels.
“Obviously there are navigational risks associated with cofferdams in the river and these have been assessed,” says Marley. “But we’re expecting each tenderer to put forward their own individual proposals for this phase.”
Bam Nuttall, Spanish contractor Dragados, and the French-Italian joint venture of Bouygues and Cimolai will have to wait until the autumn for the actual contract award (NCE 19 December 2014).
All being well, particularly on the funding front, work could start as early as January 2016.