A London, England-based team of engineers and consultants has come up with an innovative piled raft foundation solution for construction sites too tight to fit a piling rig — a method that could be used in urban areas around the world.
Walsh has overcome a number of challenges with a 33-storey, 454-unit student accommodation building at London’s Vauxhall, including a small 1,000 square-metre site next to a Network Rail viaduct and several other existing buildings and a tight building schedule. Those factors meant it wasn’t possible to undertake a pile load test, reducing the design factor of safety. Without the test, Walsh could only work with 1200mm piles — but a rig to drive them in was too large for the site.
In working with geotechnical consultants, Walsh came up with a solution — reducing the foundation loads by creating a lean concrete frame supported by a pile raft, using high and deep level ground support capacities to limit deflections.
“Work on 5 Miles St. has certainly been challenging with the tight construction programme, small site and sensitive settlement requirements,” says Walsh director Andy Stanford. “We have, however, undertaken a number of similar projects with Urbanest and Balfour Beatty who are always very supportive of us pushing the boundaries and using innovative solutions and technologies like the piled raft we adopted for this scheme”.
The core that Walsh is designing will be slip formed. Drainage was placed on top to shorten design time, allowing foundation construction to begin prior to the drainage scheme being fixed. Water attenuation pipes were used instead of the typical attenuation tanks.
Walsh, appointed as full-service structural and civil engineers, has undertaken the Network Rail liaison and negotiated all necessary technical approvals, including approval for all permanent structures and major temporary works systems, such as the slip form rig and a RMD screen that was incorporated within the piled raft as the collapse radius of the crane fell within Network Rail operational land.
Construction of the student accommodation building at 5 Miles St. began July 2016 and is expected to wrap up at the end of 2017.