Behind many of London鈥檚 historic facades lurk brand new buildings offering modern high-quality accommodation in prime locations 鈥� at prices to match. And delivering these premium projects often demands a great deal of engineering ingenuity.
Contractor O鈥橩eefe has recently completed the very delicate enabling works for the 拢9.5m redevelopment of an elegant Edwardian building in Mayfair.
The firm has demolished nearly the entire structure at 13-14 Hanover Street, leaving only the original fa莽ade supported temporarily on its bespoke structural steel frame.
Behind this fa莽ade, O鈥橩eefe has excavated a cavernous two-storey basement and has just finished the process of building the in-situ concrete retaining walls and basement slabs.
The project, for developer Frank Capital and designed by architect Blair Associates, will ultimately deliver new Class A1 commercial premises comprising five floors of office space with retail space on the ground floor. The site also includes 6-7 Pollen Street, a narrow thoroughfare running perpendicular to Hanover Street.
Besides the technical demands of retaining the building鈥檚 original fa莽ade, O鈥橩eefe has had the additional challenge of demolishing a building with party walls on three sides.
鈥淚t鈥檚 a very delicate job,鈥� comments O鈥橩eefe鈥檚 project manager, Kevin Hartigan. 鈥淎ll the neighbouring buildings are fully occupied and one of them is a Michelin-starred restaurant.鈥�
The works have had to be very carefully planned and all noisy operations scheduled so as not to disturb the neighbours. 鈥淲e鈥檙e talking to them all the time,鈥� explains Hartigan. 鈥淲e let them know when we need to plan noisy work, and they let us know when they need quiet. We work around them,鈥� he adds.
Noise, vibration and dust all need to be minimised. 鈥淲e have site monitoring equipment set up to take readings all the time. We have a surveyor come in every week and we get a weekly report,鈥� says Hartigan.
鈥淚 also send a weekly report off to our client. Plus we have a monthly meeting with the local authority environmental people,鈥� he adds.
Traffic is another top priority for the site team, says Hartigan: 鈥淕ood traffic management is essential because this is a very busy location. We鈥檙e on several bus routes 鈥� all the buses come down here!鈥�
Due to the extra basement level, O鈥橩eefe has had to underpin to a depth of up to 7m along all of the party walls. The retaining wall along Pollen Street was originally specified as a steel sheet-piled structure, but O鈥橩eefe modified the design to use contiguous bored concrete piles.
Hartigan explains: 鈥淭he original design would have interfered with a mains electric supply running along Pollen Street. The cables would have had to be lifted and moved temporarily to the other side of the street.
鈥淥ur solution was to install contiguous piles in front of the existing retaining wall, which got us round the problem.鈥�
Logistically, one of the biggest challenges has been to coordinate the works so that O鈥橩eefe could simultaneously carry out underpinning and piling works while demolishing the upper storeys of the old building.
鈥淲e also designed and installed all the temporary supporting steelwork for the fa莽ade, which is very complex,鈥� says Hartigan. The steel was designed in-house and fabricated by Mareth Fabrications, based in Slough.
By mid-February, O鈥橩eefe had poured the lower basement slab and was beginning to install the reinforced concrete basement walls. 鈥淣ext, we鈥檒l be fixing the formwork for the B1 basement slab and then continue up to ground floor level,鈥� says Hartigan.
After that, construction of the new building, on top of the basement and behind the original fa莽ade, can begin. That will be carried out by another contractor, yet to be appointed. However, O鈥橩eefe continues to maintain an interest in the project and has already priced the concreting for this next phase.
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