The gardens will be most densely planted over the piers, where the structure will be widest and capable of carrying the greatest vertical load. © Arup
Engineers develop a design that is both durable and aesthetically pleasing to achieve a one-of-a-kind bridge.
January 14, 2014—A new bridge that will cross the River Thames in London will be more than a pedestrian link between the city’s contemporary theater district to the south and its traditional professional services district to the north. It will also be an expansive garden—complete with trees, flowering plants, shrubs, and meandering pathways—where people can stroll and relax as the river flows below. The first purpose-built bridge of its kind, the project presents a unique set of design and construction challenges.
The Garden Bridge, as the project is known, will be located between Blackfriars Bridge and Waterloo Bridge and will traverse the River Thames to connect the areas of South Bank and Temple in central London. The 360 m long structure will have a total area of 6,000 m2 and will boast expansive gardens and winding brick pathways. The bridge’s approximately 180 m long main span will be 12 m high at its center to provide sufficient clearance for the barges and other vessels that frequently travel the river. The bridge will gradually slope from its midpoint toward the riverbanks, where stairs and elevators will lead visitors into the respective neighborhoods on either side.
The 360 m long Garden Bridge across the River Thames in Central
London will be the first purpose-built bridge of its kind. © Arup
British actress and activist Joanna Lumley conceived the idea for the bridge and approached Arup, an international engineering firm headquartered in London, and designer Thomas Heatherwick, the founder of the London-based design firm Heatherwick Studio and designer of many notable projects, including the 2012 London Olympics cauldron, about making it a reality. Mike Glover, Ph.D., the project manager for Arup, says Heatherwick is the perfect designer for such a bridge because he is known for creating structures that are both functional and artistic. “In the U.K., there is an increasing trend towards urban architecture [that] doesn’t just serve a functional purpose but is a design object in its own right,” Glover says. “The Copper Box and the ArcelorMittel Orbit are examples of great designs that came out of the Olympics and that reinforce this vision.” As a result, Glover says, “Now is the perfect time to build this bridge.” (Read “Iconic Tower Offers London, Olympics Venue Views” on Civil Engineering online.)
The bridge will have two in-water piers that are designed to suggest trees rising out of the river, their branches (the superstructure) extending toward the center of the river and to the riverbanks. The piers will be founded on piles measuring more than 2 m in diameter and descending approximately 45 m into the dense Thanet Sand beneath the riverbed. Foundation strength is a key factor in carrying the large vertical loads that will be imposed on the structure as a result of the gardens’ soil, trees, and plants. The foundations will be formed using jack-up rigs like those typically used in the offshore industry. “The way we get increased foundation strength is by injecting grout through the base of the piles and into the sand,” Glover explains. “It’s a technique that we have been closely developing in London, and it’s one of the reasons we can build so tall in London now, because that particular pile type has been tried and tested.”
The piers will be exposed concrete up to the water line, and above that, the concrete will be encased in copper-nickel, giving it a bronze appearance while also providing durability. The superstructure will have a skin of roll-bonded copper-nickel/carbon steel plates. “Bonded plates are often used in the petrochemical industry when you’re looking for a high-strength but also corrosion-resistant material. However, it is not often used in the construction industry,” Glover says. “But we’ve decided to use this particular technology to give the heavy load-bearing steel bridge its copper color without stinting the need for strength.”
Stairs and elevators will provide access to and from the bridge on
either side of the River Thames. © Arup
The bonded plates will be attached to the bridge’s frame, which comprises a series of steel radial trusses. Like the ribs of a ship, the trusses will accommodate a varying geometry along the length of the structure. “The geometry of the bridge is extremely fluid,” Glover says. “Therefore we needed to use a design that’s much more akin to a ship’s construction.” A layer of concrete over the bonded plates and trusses will protect the structure from the roots of the trees and plants within the gardens. “The gardens will be themed from one end of the bridge to the other and densely planted,” Glover says. “Some plantings will be annual, others will be shrublike, and others will be literally trees.” The gardens will be most densely planted over the piers, where the structure will be widest and capable of carrying the greatest load. Dan Pearson Studio, a London-based landscape architecture firm, is working closely with Arup and Heatherwick Studio to design the gardens.
The Garden Bridge Trust was formed in November to raise funds to build the bridge and will maintain the structure upon its completion. Construction is scheduled to begin in early 2015 and to be completed in late 2017. The bridge’s opening will be determined by the planting season. Glover says he expects the bridge to become a destination for London residents and visitors alike, much like the Millennium Bridge, an Arup project that opened across the River Thames in 2000. “We’ve been amazed at people wanting to visit the Millennium Bridge, just to walk across it,” Glover says. “But this bridge’s main purpose is to be a signature landmark attraction, not just for the U.K. but worldwide.”