Always up for a challenge, some of the most complex structural engineering projects we take on are inner-city subterranean developments.  Acting as temporary works engineers to construct a 9 metre deep basement on this constrained site in central London, our work at Mount Row, Mayfair, has been one of the most rewarding.

The proposed building included a gallery at first basement level and a plant room at second basement level. Upper storeys were formed of an reinforced concrete frame for residential use. The site is over the running tunnels of the Jubilee Line. To minimise vibration within the basement a ‘box-in-a-box’ arrangement was proposed with acoustic bearings surrounding the internal box.

We developed the design using our BIM expertise with the structural steelwork drawn out in Revit and analysed using compatible 3D structural analysis software. With other consultants also working in 3D, exchanging models, as well as drawings, helped to coordinate our designs. We also dissected the model to formulate our sequence of construction drawings, showing the 14 key stages of construction in full 3D.

The new basement stretched across the full extent of the site leaving no room for welfare, storage or other site activities. With no less than seven different party wall interfaces, including the Grosvenor Estate and the Crown Estate, we developed a sequence of construction and designed the temporary works to maintain support to the surrounding ground and buildings throughout the works. This meant regular contact with the permanent works engineer and two further monitoring engineers acting for neighbouring properties, as well as continuous discussions with Contractor Forcia to make sure the phasing of the temporary works were coordinated with all other site activities. This included installation of the tower crane, and providing temporary decks for site offices and welfare.

One of the key ideas of the proposals was to extend the central alignment of piles, designed to support the lowest basement level raft foundation, up towards ground level to act as columns as the basement is excavated. This provided a central line of support for temporary horizontal propping, reducing the weight of steel that would otherwise be needed if props were to span the full width of the excavation.

The box-in-a-box design required careful thought on how temporary works were connected to the outer box at excavation progressed, and then to the inner box as construction raised back up to ground level.

The basement has now been successfully installed with ground movements occurring within the anticipated values. Another successful London basement project for our London studio.