Top down

I visited two highly impressive sites where Careys have gone top-down...ish

Seeing some major temporary works and engineering on site really took me back to my contractor days. It’s easy to overlook the skill and expertise that the construction industry has, that is until you see a complex job and have some of the challenges explained first-hand.

Old Embassy: Stood on Level 5 looking East over Grosvenor Square towards the city. L-R, Jamie, Jason, myself, Tommy and Will

I visited two Careys projects in Mayfair recently, both extremely impressive in terms of the engineering ambition for permanent and temporary works. Thank you to Jason Carey, Tommy Carey and the project teams for hosting.

The first was the old American Embassy on Grosvenor Square. This is a development by Qatari Diar, with Multiplex now Principal Contractor and Careys retained on the concrete package after completing the demolition and basement box as Principal Contractor.

The second, just a few streets away, was the equally impressive site of Audley Square. This development is by Caudwell Properties, with Careys currently working on the basement box and Mace about to start building upwards from the new ground floor slab.

The scale and complexity of both these projects is seriously impressive.

The ground beneath the embassy has been hollowed out to create four levels of basement, including what is thought will be the largest ballroom in London (and possibly further afield). In a sort of hybrid top-down, the basement was excavated whilst the new above ground construction was taking place. This helped balance the unloading of the ground and limit heave - a critical point considering the listed facade and first floor dia-grid floor was being precariously held in place to a very tight tolerance. The hybrid approach reduced the need for lots of basement propping but meant there were large enough slab openings and blue sky to be able to slip-form the core from B4.

At Audley Square, the term hybrid top-down takes on another meaning. Careys split the site into two zones, with z1 following a true top-down approach alongside an open-cut in z2. Even z2 has some elements of top-down though, as a perimeter strip of the ground floor slab was poured prior to excavation to firstly prop the capping beam and secondly provide logistics space for muck-away from both zones.

And just in case you don’t have a contracting or civils background, top-down is when you form a basement by digging down whilst at the same time constructing upwards above ground. Not only does this save time but it also means that less propping is needed because the new structure is in place before the soil is excavated away and acts as the propping system.

Old Embassy: Looking up at the first floor dia-grid soffit. Spot the old and new.

Old Embassy: The retained Grade II-listed facade. This will be the frontage of the new Chancery Rosewood hotel.

Audley Square: Excavation on hold as second level of props are being installed. Blue sky of z2 can be seen, with muck-away through the side of z1 in the distance.

Audley Square: Looking down into z2 and the perimeter strip of slab, with z1 superstructure columns punching off the slab to the right.

Thanks to the Careys team: Jamie Smith, Will Benson, Michael O'Shea, Paul Chira, Jon Croxford, Michael Bassett