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SOFT OPTION

INNOVATIVE GEOTECHNICS

A new UK vibro technique has proved its worth in soft marshy ground on a regeneration project in the east of England.

UK foundation contractor Pennine has developed new solution for ground improvement in estuarine and coastal regions, particularly suitable for work in alluvial clay and peat.

Pennine's system combines traditional vibro techniques with a water flushed method that reduces disturbance to peat strata as the poker pushes through. It also creates bulbous bases to vibro concrete columns, which aid toeing in to the clay and so create a stronger base to the column.

'We have tried and tested the new solution with great success, ' says Pennine founding partner Andy Russell. 'We are confident we have found an answer to the long-standing geotechnical problems experienced in many areas of the UK, particularly in the south and east of England, which have marshy ground.'

The system was recently used in East Anglia as part of the £140M Nar Ouse Regeneration Area scheme at King's Lynn in Norfolk. The first phase, covering 28ha, includes a business park, about 700 homes, a new road, a park and community facilities.

The public and private partners behind the project are King's Lynn and West Norfolk Borough Council, developer Morston Assets, Norfolk County Council, East of England Development Agency and English Partnerships.

At the heart of the redevelopment is the Nar Ouse Regeneration Route, a new 1.8km single carriageway section of the A47. Norfolk County Council secured £7.7M of government funding for the road.

Expected to carry more than 11,000 vehicles a day, the road will relieve King's Lynn of through traffic and include footways, a cycle route and a bus priority route into the town centre. It will mainly run on a 2.5m high embankment and includes a new bridge over the River Nar and refurbishment of two bridges and two large culverts.

Working for contractor Birse, Pennine stabilised ground along the route. Russell says: 'The project is being carried out bang in the middle of the Cambridgeshire Fens, infamous in construction for its difficult soft land. It presented us with one of the most difficult geotechnical situations we have experienced in more than 15 years.'

The water table is high and soil conditions were 'challenging' consisting of very soft compressible alluvium deposits up to 6m thick with stiff clays beneath.

Russell says: 'To use traditional methods of slowly loading the ground, up-filling and then constructing the road surface would have been far too costly in time and money, because of the size of area involved and the need to provide access to the regeneration project.'

Trials showed it would take 20 weeks for the completed embankment to settle to the point where the road could be built using traditional techniques.

Pennine Vibropiling's solution on its £500,000 contract was to use vibro stone columns (VSCs) and vibro concrete columns (VCCs) with a load transfer platform to address settlement control beneath the new embankment, building over the weak ground and creating transition zones between the rigid piled bridge structures, halving the surcharge period.

A method was also devised of inserting the columns using vibration techniques that would keep disturbance of the peat to a minimum.

Pennine adapted its in house rigs to enable water to be flushed down the borehole at the same time as the poker was being vibrated into the ground.

The water then saturated the peat, keeping it in place as the poker pushed through. Exactly the right amount of water had to be used or an even bigger swamp problem would have been created, Russell says.

Because the team was having to toe the VCCs into overconsolidated clay rather than the usual granular strata, Pennine developed a method of reinserting the poker to create an enhanced 'bulb' at the base of each concrete column.

Russell says: 'We thoroughly tested the solution by rebuilding an embankment on the site (10m square and 2m high) and undertook a series of full-scale trials, which we were able to compare with a second trial section without [vibro]piling, to monitor the consolidation of the ground.'

This approached proved highly effective.

'Not only did it significantly improve drainage of the areas, speeding up primary consolidation of the ground from 20 weeks to 12 weeks, but some secondary consolidation was also recorded, ' Russell says.

Following the trial, Pennine installed 500 VCCs between 10m and 11m long into the clay using its Stratacaster piling rig, plus a further 3,000 VSCs between 7m and 8m long into marine sand using its Terrafirmer piling rig.

Pennine's work finished at the end of September. The road and remediation of the area are due to be completed early next year.

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