At 163m high Principal Tower will be one of London’s tallest residential buildings with space for 301 apartments, with views stretching from Canary Wharf to St Paul’s Cathedral and the Houses of Parliament.
The neighbouring 600,000 sq ft landmark office space, which is also part of the Principal Place development, is occupied by Amazon and is a bridge to the creative industries just to the north of the financial buzz of the city, in Shoreditch.
Both buildings are fronted by a half-acre piazza, also constructed over the railway and surrounded by cafes and stores. It’s a new city hub for the future and, when Crossrail opens, will be roughly 10 minutes from the capital’s other financial centre in Canary Wharf.
Buildings have been erected over railways before, many times, but the really tricky part was that almost half of the Principal Tower’s foundation had to leave space below the structure in a protected corridor for a potential further two tracks in and out of Liverpool Street station – known as the 8 track corridor.
At the same time, the architect Foster & Partners did not want the conventional, visible, massive arches or A-frames that are the usual options for bridging rail lines. For Principal Tower it was felt that no one would understand the link to the railway usually made by exposed structural elements, so a primary objective was to develop a design that appeared to be built on solid ground.
Our solution had to be a hidden gem.
We devised a design that involved forming the sides of the protected rail corridor out of 1.5m diameter piles and reinforced concrete capping beams 6m deep, effectively creating a tunnel.