Urbanest Student Accommodation’s £42 million project at 5 Miles Street has seen CGL and Walsh overcome a number of challenges including a very small site, sensitive settlement requirements and a tight construction programme. The project was shortlisted for the 2017 Ground Engineering Award – UK Project with Geotechnical Value up to £1m.
The combination of the restricted 1000m2 triangular site next to a Network Rail viaduct and several existing buildings, plus the deadline to commence work, meant that it was not possible to undertake a pile load test to reduce the design factor of safety. Without such a test, the contractor could only achieve the loads by using 1200mm piles, but the rig would have been too big for the site. The project had reached a hiatus and was potentially at risk.
CGL and Walsh took a proactive approach to reduce the foundation loads by proposing a pile raft solution, utilising high and deep level ground support capacities to limit deflections supporting the 33 storey, 454 unit building.
The structure relies on the innovative use of the contiguous piled wall to provide both a retaining wall – for the excavation to form a raft on the gravels underlying the made ground and to provide significant load carrying capacity around the perimeter of the structure, further reducing the piles required under the building. Analysis of the load split between the walls and the bearing piles was carried out using 3D (Finite Element) FE software which also assessed the soil-structure interaction to optimise the pile layout and control settlements.
Below ground challenges included potential for scour features in the area. With no time for further site investigation Walsh and CGL worked together with the contractor and piling subcontractors so piling in the potential scour feature was prioritised, consequently if one was found there was sufficient time to adjust the design.
The judges remarked our entry “showed detailed consideration of geotechnical issues related to the design.”
Construction on 5 Miles Street started in May 2016 and is due to finish early summer 2018.