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EEJ: Inside an Engineering Enterprise

Clarges, London

Clarges in Mayfair is a £228 million ‘super prime’ residential and commercial development in the heart of London overlooking Green Park and Buckingham Palace. With construction works commencing in August 2014, Laing O’Rourke will complete the office element in mid-2016 and the main residential element in early-2017.

Developed by British Land, the scheme consists of a detached high-end residential building fronting Green Park, a semi-detached commercial building, a new terraced headquarters for the Kennel Club of Great Britain (which was based in a building on the site, now in the process of being demolished) and an element of affordable housing.

The 8,700m2 reinforced-concrete residential building at 82-84 Piccadilly (Block A) provides 34 apartments, ranging in size from one to five bedrooms, arranged over ten storeys and three basement levels. The apartments benefit from a gated mews with secure drop-off, basement parking, a private wellness spa including a 25m swimming pool and fully equipped gymnasium, sauna, steam and treatment rooms, private cinema and other lifestyle services.

The seven-storey, steel-framed commercial building behind the new mews (Block B) provides 4,400m2 of high-quality prime office space and 1,400m2 of retail and leisure space fronting onto Clarges Street, plus a triple basement. Adjoining this is a new five-storey, steel-framed headquarters for the Kennel Club (Block C), spanning Clarges Street and Bolton Street. Next door, at 6-12 Clarges Street, is a six-storey, reinforced concrete block of intermediate and affordable rented apartments (Block D). Blocks C and D have a single basement level each.

In September 2014 British Land announced it had achieved an average sale price of £11.6 million on the first 18 apartments in Block A, working out at a record £4,750 per square foot. According to British Land: ‘The success achieved at Clarges Mayfair is testament to its unrivalled position, and the quality of its design which sets a new international benchmark for residential development in London’. Laing O’Rourke’s design partners on the project are Squire and Partners (for overall architecture) Martin Kemp Design (for interiors) and Waterman (for structure and services).

Digital engineering and DfMA

In addition to being bounded by busy London streets on three sides, the Jubilee Underground line runs in close proximity to the site. In order to address the associated logistical challenges, Laing O’Rourke used digital engineering to design and plan the project, maximising the application of offsite manufacturing. This reduced the amount of activity on and around the congested site, with deliveries managed through a bespoke booking system to ensure components arrive ‘just in time’ for installation.

Innovative construction products and systems being used at the site include E6 megaplank precast solid concrete floors, lattice plank precast composite floors, twinwall and solid precast structural walls, castform insulated perimeter walls, smartwall, fully serviced partition walls, precast columns, precast stairs and landings, vertical precast single-sided lattice wall, precast façades, clad in brick and Portland stone, plant modules and bathroom pods.

Digital engineering has also enabled the project team to highlight risks and identify solutions. For example, visualisations have been produced for use in safety briefings to address potential hazards, while clash-detection software has ensured the building services have been suitably designed around the structure.

Digital engineering is also used to design temporary works, ensuring that when components arrive on site they can be accommodated until they are fixed in their permanent state. By anticipating and planning effectively through technology, Laing O’Rourke has been able to reduce the overall duration of the project, as well as manage environmental impacts, making construction a more sustainable process.

Structural systems 

Twinwall is one of Laing O’Rourke’s primary solutions for precast structural walls. Manufactured by in-house business, Explore Manufacturing, in Nottinghamshire, it consists of two 70-75mm-thick precast concrete leaves tied together with lattice girder reinforcement. The units are quickly craned into position over starter bars and then held upright with two or more push-pull props. Small splice bars are inserted between each unit and, when all units are in position, concrete is poured into the 60-100mm spaces between leaves in 1.5m lifts.

For a typical floor in Blocks C and B, it took one day to install the precast concrete building core, three days to complete the structural steel connections and install, and a further day to complete the megaplank and concreting works, making a total of five days per floor.

Laing O’Rourke uses its own multi-skilled site team to install twinwall and all other structural systems, including steelwork, in line with its policy of ‘direct delivery’. As well as minimising the number of people required on site and ensuring optimum use of crane time, it also provides end-to-end control of the delivery process − from design to manufacturing, delivery and installation.

All four building cores are generally made from a mixture of twinwall and precast solid wall units, the latter being propped and then grouted into position. Solid precast columns with column shoes are generally used within the floor areas of Blocks A and D and the substructure of Block B. These are lifted straight onto holding-down bolts, with no requirement for propping, and then grouted. A threaded bar connection at the top provides continuity with the next level.

In the commercial Blocks B and C, the main superstructure is provided by structural steel frames. The steel columns and beams are manufactured and delivered by an external supplier but these too are erected by the Laing O’Rourke site team.

Flooring systems

The flooring system in the steel-frame buildings is made from E6 megaplanks. These precast reinforced concrete units are up to 9m long, 2.8m wide and 8t in weight. They do not require propping and provide an instant structural floor, allowing follow-on work to start immediately both below and above. The megaplanks feature a unique studded joint detail which, when concreted with the studs on top of the steel beams, forms a continuous solid floor.

Each megaplank takes just 12-15 minutes to install, meaning a whole Block C floor could be laid in a day. Clarges is the first project to use the E6 megaplank and, after only a few floors, the enhanced speed and safety – compared with conventional in-situ slabs on metal decking – had already become apparent.

The floors in residential Blocks A and D are made with lattice planks, which are a precast floor system that contains all bottom reinforcement. The 75mm-thick planks with embedded lattice girder reinforcement are propped in position with falsework and then an in-situ reinforced concrete floor is poured on top. While these require more effort to install they allow the creation of thicker slabs − in this case up to 425mm − with a higher acoustic rating.

Non-structural walls

Internal party walls are made from castform panels. These have a solid 60mm reinforced external concrete leaf, a 60mm acoustic and fire insulation in a light-gauge steel frame, and a gypsum fibreboard inner lining. These are grouted, mastic-sealed and screwed into position, all from within the building.

Internal partition walls in the commercial Blocks C and D are made from smartwall units, which are serviced, and insulated gypsum fibreboard panels complete with all mechanical and electrical services. They are propped in position on timber base tracks and then fixed to the slab above as soon as it is completed, allowing the props to be removed and electricians and plumbers to start ‘second fix’ work.

One of the final construction elements on each building is the prefabricated façade panels. On Block D these were complete with brick cladding, windows frames, glazing and Juliet balconies. Glass reinforced concrete feature bandings were installed separately. On the other blocks the cladding consists of Portland stone columns and beams, into which the glazing will be fitted subsequently. All façade units were manufactured by Laing O’Rourke businesses Explore Manufacturing, Vetter, and GRCUK.

Modules and pods

Some of the most complex offsite units are the plant modules, which are manufactured at Laing O’Rourke’s Crown House Technologies factory in Oldbury, West Midlands. These come complete with all mechanical, electrical and pipework systems, and are fully tested and commissioned in the factory prior to delivery. 

Similarly complex are the bathroom and toilet pods for residential Block D, which are manufactured at Laing O’Rourke’s Modulor factory in Dubai. These fully fitted and commissioned units are delivered to site as large boxes. They are then lifted in a purpose-built cage to the designated floor, rolled into position on pallet trucks, with just enough clearance below the slab above, then connected to the building services.

The use of modules, pods and all other offsite manufactured units results in better quality control, faster installation, fewer trades on site, minimum space required for material storage and virtually no waste.

Jack slab

A further innovation is taking place in the basement of Block A, where the first and second basement slabs were cast on top of each other, on the ground, using lattice planks. The first basement slab is in its final position, but the second-level slab will be held immediately beneath it, using tie-bars while excavation and concreting takes place for third and final basement level. The second-level slab will then be lowered using hydraulic jacks to its final position.

The main advantage of the jack slab method is that it allowed the second basement slab to be constructed and in clear, open conditions using the tower crane. This top-down approach is made possible by the use of steel and precast concrete plunge piles, which are used throughout Blocks A and B.

Conclusion

Throughout construction, the Clarges project team has embraced the opportunities afforded by a variety of DfMA solutions, and has used them to bring certainty and clarity to the delivery process.

The innovative E6 megaplank developed by the Engineering Excellence Group has been pioneered on the project and found to give immediate programme, safety and quality benefits.

The site team believes it has managed to achieve a 50 per cent saving in workforce and 30 per cent reduction in programme compared to conventional construction techniques.

David ScottStructural Engineering Director for the Engineering Excellence Group
Brett GeorgeProject Engineer, Clarges

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