Snow Hill development glides into action
One of the UK’s largest city centre schemes under construction outside of London has recently undergone preparation works with the completion of the piling contact led by geotechnical specialist, Bachy Soletanche Limited (BSL). The project, at Snow Hill in Birmingham, is a 96,000sq metre mixed use development set in the heart of Birmingham’s business district, featuring two towers that contain residential apartments and a five star hotel.
The piling works contract, worth in of excess of £1.5million, commenced in July of this year and required the installation of 220 linear metres of anchored contiguous piled wall. The wall consisted of 241 No.750mm diameter piles, drilled to an average depth of 17metres. These piles are anchored back with 70 No. temporary ground anchorages to form the perimeter retaining wall which will be excavated to create a five-storey basement car park.
The ground conditions were an important factor when BSL designed the retaining wall solution for the project. The ground profile is made ground, sand and sandstone varying from depths of 2metres below ground level to 13.5metres. As a result, BSL designed a piled wall solution for the client, utilising two separate piling techniques, as Steve Mallinson, contracts manager for Bachy Soletanche Limited commented:
“Some parts of the wall had to be drilled using a cased Large Diameter Auger (LDA) rotary bored piling method, while other parts of the wall have utilised the more cost effective and timely, Continuous Flight Auger (CFA) method.
This is a result of extensive probing that we carried out on the development, and due to the length of the rock sockets required, it has been necessary to use a combination of both CFA and LDA to form the contiguous piled wall.”
Alongside the contiguous piled wall, BSL has also constructed 10No. 1200mm diameter LDA plunge column piles and 14No. 750mm diameter bearing piles.
In addition to the piling works, BSL had to undertake the coring out of obstructions, such as the reinforced concrete slabs that formed part of the historic system of subways and underpasses on the site. These obstructions could not have been dug out simply by an excavator, due to the sites location on a major traffic junction; and piles located right on the site boundary had to be cored out with piling rigs to ensure the adjacent roads and services were not undermined. The history of the site, city centre location and the sharing of site access with another contractor from an adjacent site have made logistics problematic.
Steve Mallinson continued:
“As two heavy duty Bauer BG-22 rigs were used on the project, along with service cranes and all other essential plant and machinery, the site required careful co-ordination to enable the smooth running of the work which was surrounded at all times by pedestrians and traffic.”
BSL effectively took on the role of main contractor for installation of the piles. In this capacity BSL were responsible for the provision of welfare and offices, setting out, crane and excavator hire, spoil disposal, security and breaking down the piles upon completion.
“As the sole contractor on the site, we have worked very closely with the client to ensure that the project has run on course. And now that the piling work is finished, the contiguous wall will be excavated; then we will revisit the project to install temporary ground anchors.” Concluded Steve Mallinson.
The full project is due for completion at the end of 2008, with the Snow Hill development set to become one of Birmingham’s most prestigious landmarks.