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Stabilising the sleech

Soft ground City centre sites are rarely straightforward to the foundation engineer. Dave Parker reports from Northern Ireland.

Many of the world's major cities sit on estuarial floodplains, for good historic and socio-economic reasons, and Belfast in Northern Ireland is no exception.

But a strategically excellent location is no guarantee of its suitability for large scale construction, as examples such as the recent flooding of New Orleans demonstrate.

There may be little risk of hurricanes or storm surges battering Belfast, but its geography has its own unique drawbacks, as the team building the vast £150M ($264M) Victoria Square development found out.

Main contractor for the project is FGA, a joint venture of Farrans (Construction) and Gilbert-Ash (NI), with Farrans taking on responsibility for the £19.5M ($34M) construction of three storeys of underground car park and retail. Local knowledge is vital.

'Eight metres of sleech is something you only find in Belfast, ' says Farrans design co-ordinator Stephen McCaffrey.

Sleech is the local term for soft estuarine quasi-thixotropic deposits that underlie both banks of the River Lagan in the heart of the city.

Up to the end of the 19th century the entire centre of the city was supported on a forest of timber piles, most of which are still performing their original function. But 19th century engineers never attempted to build deep basements, certainly not on the Victoria Square scale.

The 150m square excavation would be challenge enough, sleech being one of the most unco-operative soils in the UK, but at Victoria Square sleech is just one of the underground nasties.

McCaffrey says this could in fact be 'the worst place in Belfast to build a deep basement.' He goes on: 'Our research shows that 300 years ago half the site was under the river.

'Maps from 1819 show there was a dock on the site - our preliminary excavations uncovered a timber slipway. But our real problem is that there are two distinct aquifers below the sleech, both artesian. There used to be a water bottling plant on the site, which should have given us a clue.' At ground level the site acquired by developer AM Developments housed a number of ageing government offices, multistorey car parks and a much smaller shopping centre dating from the 1980s. These were demolished, but the site remains tightly hemmed in by buildings, some of them listed, and access is poor.

Basement footprint is 21,500m 2. Designer Benaim's solution was 4,000t of 'high modulus sheet piles' - basically two standard piles welded together and stiffened by universal beam sections welded on to the outer side - around the entire perimeter, driven down 22m to the sand and gravel. This perimeter wall cut off most of the water flow across the site.

Some 3m of made ground above the sleech was then excavated and steeply inclined ground anchors installed to stabilise the wall.

'At an angle of 55° these anchors put significant compressive forces into the piles, ' McCaffrey points out.

'But there was little alternative if we were to avoid the existing foundations around us.' Up to three rows of ground anchors had to be inserted in some places around the perimeter as excavation proceeded. More than 500 were used in total, with strutting frames stabilising the corners.

Dewatering wells 40m deep helped to keep the sleech under control, although McCaffrey says it can never be totally trusted.

In all, the contractor had to excavate 240,000m 3 of material to get down to formation level, 10m below ground level and 8m below the effective water table.

Such a high water table means that once the dewatering pumps are turned off, the basement will try to float. So the main purpose of the more than 1,300 piles installed by an FK Lowry/Stent joint venture is to hold the building down, via friction between the piles and the subsoil. Where strutting was planned and piling from formation level impractical, 180, 759mm diameter bored piles were installed from ground level 30m down, with the upper 10m cut away once formation level was reached.

Over most of the site, however, 600mm diameter CFA piles 20m deep were the order of the day, and 1,200 were installed in total.

'We couldn't use CFA piles from ground level because the reinforcement cages would have been too long to handle, ' McCaffrey reports. 'In the end we didn't need as much strutting as expected as the developer got permission to use more ground anchors instead.' An extensive monitoring system was needed to check on the stability of the perimeter wall and the settlement, if any, of the surrounding buildings. The readings, says McCaffrey 'were in line with expectations'.

Polish concrete works subcontractor Format is well advanced with the basement slab construction. C32/40 concrete is being used, waterproofed with Pudlo admixtures. Five tower cranes service the site, with two more due. Dotted at key locations are 10 concrete placing pump booms.

In the longer term, stability of the basement walls will come from the propping action of the normally reinforced basement floor slab and the upper of the two post-tensioned prestressed concrete floor slabs above it, McCaffrey says.

He adds: 'With slabs on this scale, characteristics such as elastic shortening, creep and drying shrinkage have to be allowed for if you want to achieve watertight construction.' Just nine months are programmed for the completion of the basement structure, after which Gilbert Ash will play the leading role in the construction of the complex superstructure with landmark dome.

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