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Basement box out

Bachy Soletanche is using both traditional and innovative techniques to lay the foundations for an extension to the British Museum. NCE reports.

Geotechnical specialist Bachy Soletanche, has begun a five-month, multi-million pound piling contract to lay the foundations for the World Conservation and Exhibitions Centre at the British Museum in the centre of London.

Working alongside project manager Mace, the work involves piling for a new building that will house state-of-the-art laboratories and science facilities, a dedicated gallery for special exhibitions, a collections management hub and world-class stores for the library’s study collection.

Irregular shape

The new basement is an irregular cruciform shape created by the adjoining museum buildings, and the secanted wall piles are all installed close to the adjacent, existing Grade I listed British Museum buildings.

This piling package involves inserting bearing piles and secanted piles to produce the new basement. A secant piled basement was chosen as this can deliver the irregular and flexible shape required, and will also give better verticality tolerances. An innovative combination of cased CFA piles and large diameter rotary piles has been employed by Bachy Soletanche as it is considered faster than the more traditional “kelly / casing” rotary bored pile method.

Secant piles

The basement is constructed with 330 linear metres of 880mm diameter piles to create a hard/firm secant piled wall. The 13m to 16m long female/primary piles and 23m to 27m long male/secondary piles will be used for the construction of the basement walls. In addition, 197, 600mm, 750mm, and 900mm diameter load bearing piles up to 30m long are also being installed.

Noise and vibration monitoring in being carried out by sister company Sol Data, using 2 two sets of Noise and vibration monitoring meters plus cameras (NCE 25 Aug-1 Sept 2011). The results are then reported to all interested parties in real-time, via a web-based in-house developed, reporting system. Noise and vibration monitoring is required by the client due to the close proximity of national and international treasures, museum staff and visitors. The new centre is very close to existing museum buildings and this monitoring will help avoid any damage to the building’s structure.

“The new centre is very close to the existing museum buildings”

These results could prove to be invaluable to Bachy Soletanche in future piling works in similar urban environments. Noise testing has been carried out on sites for many years but the demand for vibration monitoring is becoming increasingly popular. The cameras will give an indication of what may have triggered alarms, should this arise.

Box out

An additional innovation has been the slab connection “box out” units that are being installed on the pile reinforcement cages, as an alternative to drilling and grouting dowel bars. These mean that when the wall and basement are excavated at a later stage, the “box-out units” will be exposed for connection to the floor slab reinforcements. This rarely used technique requires a high level of accuracy to get the “box-out” units in the right position and to make sure the cages don’t move when the pile casings are removed. This innovation is faster than traditional drilling and grouting and also delivers noise and vibration, and health and safety, improvements.

Piling started on 14 March 2011 and was due to finish this summer. The British Museum is a world leader in the conservation and display of objects from across the globe.

Key players

Project British Museum World Conservation & Exhibitions Centre
Client British Museum
Project manager Mace
Piling subcontractor Bachy Soletanche

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