The first construction phase of any major project is to install piled foundations from ground level. Where the St Giles Circus development differs is in its sheer scale, the purpose of the piling and its proximity to the historic buildings on Denmark Street, affectionately known ‘Tin Pan Alley’.
Whereas piled foundations are normally installed to keep buildings up, at St Giles Circus the piled foundations are being installed to help pin down the Crossrail tunnel that runs directly beneath the site. To achieve this the piles have to be precisely located either side of the tunnel and must be drilled to the bottom of the London Clay in order to pick up the required tensile capacity. The location for the piles unfortunately is straight through a ‘Smithy’ building, a former blacksmiths shop or forge dating back to the early 18th century. The ‘Smithy’ is a simple square plan masonry building with a timber roof and for its type and use is considered a rare survivor from this period. The ‘Smithy’ also sits adjacent to No.26 and 27 Denmark Street, which are much older buildings dating to the 1680’s, survivors from before the Great Fire of London.
A number of options were considered for the piles, from using fewer large diameter piles and specialist restricted access piling rigs, to building transfer slabs at depth using tunnels dug between piles and cast with reinforced concrete. Each option had its disadvantages, the risk to the adjacent historic buildings, the safety and practicality of construction or the complexity of solution to hold down Crossrail. The chosen solution in many ways was the simplest - temporarily move the Smithy!
Following careful surveying, design and planning, the ‘Smithy’ was transferred in-situ on to a reinforced concrete slab support, ready for a basement to be built beneath it. This required the careful repair and propping of the building as it had to be stooled up and supported whilst the reinforced concrete slab was cast under half of the building at a time. Once cast, the slab was tightly dry packed to the underside of the ‘Smithy’ walls to provide a strong and stable base to the building. Bracing and lifting points were cast into the reinforced concrete slab and a propping system was erected to fully support and tie all of the external walls back to the newly cast slab.
With all this complete, the building was ready to be moved and on Wednesday 31st August, with one of the largest mobile cranes in the UK erected in place, the ‘Smithy’ was successfully lifted from its location and moved 25m across the site to its temporary new home.
With the move safely completed the piling can now begin unhindered, enabling the best engineering solution for securing the Crossrail tunnel to be undertaken, with less risk to the adjacent historic buildings as more conventional and smaller diameter piling will be used. Once the piling has been completed, the ‘Smithy’ will be lifted again and placed back in to its original position with the development being built around and under the ‘Smithy’.
John Bailiss, St Giles Circus Project Leader and Associate