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Columns of soil stabilised with cement and lime have firmed up ground on the site of a new flight simulation centre in Hampshire.

Mike Walter reports.

Helicopter and corporate aircraft pilots will be trained in a new flight simulation centre planned at Farnborough Airport in Hampshire.

The two-storey steel frame building will cover 5,000m 2and house 14 flight simulation machines. But before construction could begin, the site needed to be put on a firmer footing in the poor quality peat and clay which lies up to 3.5m deep.

To strengthen the ground Roger Bullivant has installed vibro-replacement columns made of treated soils rather than the conventional stone or concrete. Each column was formed by compacting soil enhanced with cement and lime.

This ground improvement technique is a development of Bullivant's Hardrock injection system and forms a crucial element of foundations.

Enabling works were followed by the laying of a 600mm thick temporary piling platform of stabilised soil. This allowed plant to get on to the site and install about 1,900 vibro replacement columns to improve the bearing capacity and settlement characteristics of the ground.

The top 300mm of the piling platform was then remixed and reworked with cement to form a stiffened soil raft that will sit beneath ground bearing slabs.

Bullivant's senior design engineer Stephen Coatsworth says: 'Extensive laboratory and field testing were undertaken to gain strength and stiffness data for the Hardrock injection columns. This data and some previous site trials were incorporated into finite element analysis of the strip footing stanchion foundations.

'This work demonstrated the viability of much reduced strip foundation size, resulting in substantial cost savings on the project.'

The flight training facility is being built by Bullivant's sister company BuildXpress on behalf of Flight Safety International. The foundation system specified had to be capable of supporting not only static loadings but the dynamic effects of simulators in motion.

To this end, two types of ground bearing slab are to be placed for the building; a 350mm thick slab for the simulators and 150mm slabs elsewhere. In addition, 28 precast concrete piles have been installed to a depth of 8.5m where loadings will be greatest.

Use of soil instead of stone to create the vertical columns for strengthening the ground had two significant benefits. It allowed use of selected site-won materials that would otherwise have had to be disposed of, and it meant aggregate did not have to be imported to site.

A borrow pit, later backfilled with most of the job's arisings, generated the source materials for forming the columns. Mixing was carried out with one of Bullivant's two Wirtgen machines on site, adding 3% of quicklime and 4% Portland cement by volume.

The process of creating a vibro-replacement column of stabilised soil begins with driving a tapered steel mandrill - at Farnborough to a depth of 5.5m or refusal if this was met at a shallower depth.

The resulting conical void is initially lined with Hardrock materials to stabilise the bore, then repeatedly filled and compacted with stabilised material until the column is complete.

Inspection of trial columns in deep trial pits before construction confirmed the viability of the installation method.

At Farnborough columns have been installed at between 1.75m and 2.5m centres and measure about 250mm in diameter at depth and about 500mm at the surface.

Bullivant's soil stabilisation manager John Bateman says development of a vibro-replacement column using stabilised soil is a huge step forward for ground improvement.

'Development of this system marks a major watershed for improving difficult ground and has given us an even better understanding of soil stabilisation, ' he says.

Once injection was finished, plate loading tests were performed which gave 'excellent results' Stabilised material was also tested for stiffness, strength and moisture content by a UKAS accredited laboratory on site.

Independent testing specialist Clear Structures undertook seismic testing to measure soil stiffness. Tests were carried out before and after column installation and surface stabilisation, demonstrating a doubling of soil stiffness at shallow depths.

Erection of the flight simulation building is due to begin shortly and BuildXpress will use its own precast concrete wall panels designed to reduce acoustic interference and improve insulation.

Commercial development manager Ian Alcorn says: 'The panels are double skinned to give excellent thermal insulation qualities and provide good acoustic insulation. The buildings must prevent the flight simulators from generating noise over and above the ambient noise at midnight of around 35 decibels because of the close proximity of a runway which is only 50m away.

'The Farnborough Air Show takes place at the end of July and we have been asked to complete the main structure of the building before then. We gained access to site on 22 March, so it is going to be a tall order.'

Fitting out will continue until the end of the year and the flight simulators should be ready for takeoff by next spring.

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