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KNOCK ON WOOD

GROUND IMPROVEMENT - A ground improvement contract for the foundations of a new 44MW biomass power plant has just been completed in Scotland.

Ten per cent of the UK's energy should come from renewable resources by 2010, the government has decided. This is a tough enough target, but the long-term aim is even harder - to reduce carbon emissions by 60% by 2050.

In an effort to meet these targets, energy ms are increasingly looking to wind, solar, wave and tidal power, and biofuels, to reduce dependence on traditional energy sources such as coal, oil and gas.

At Stevens Croft, near Lockerbie in Scotland, construction is well under way for what power generator E.ON UK (which runs Powergen) claims will be the country's biggest biomass power plant, capable of generating 44MW of power annually, enough for 70,000 homes.

When it begins operation at the end of 2007, the £90M station will initially burn fresh wood and offcuts from local sawmills. The hope is that local farmers will switch to producing fast-growing willow which will become part of the fuel mix for the plant.

Ultimately the station will consume about 475,000t of sustainable wood a year. Use of wood fuel from sustainable forests has been shown to be effectively carbon neutral.

The plant will eventually displace 140,000t of greenhouse gases each year.

The main contractor is German company Siemens Power Generation, which is building the plant with Norwegian rm Kvaerner Construction. Mott MacDonald is project manager.

Construction demanded robust foundations with a high degree of settlement control, so careful attention was paid to ground conditions.

Keller Ground Engineering's Scottish office has just nished installing more than 2000 vibro stone columns on a £185,000 contract.

The design called for a 250kN/ m 2 ground bearing resistance, with settlement limited to a demanding 20mm because the plant's equipment, while heavy, is very sensitive to movement, particularly under dynamic loading occurring during operation.

'Vibro stone column treatment design is all about settlement control, ' says Keller regional manager David Whyte. 'With the highly variable conditions on this site, the ground treatment approach had to be carefully considered to evaluate all the loading and soil parameters involved.' Design responsibility for the treatment was put in the hands of Keller Ground Engineering's chief engineer Derek Egan.

Conditions at the site were challenging. The ground investigation carried out by Fugro revealed Permian New Red Sandstone at depth, overlain by variable deposits of intermixed soils ranging from sands to clays. Near surface recent alluvial deposits were present over some parts of the site but not others.

'Interbedded lenses of saturated silt and sand are notoriously difficult to sample and test, ' explains Egan. 'This made estimation of their engineering parameters - so important given the tight design tolerances - quite tricky.' He was also aware that the liquefaction potential of the silty soils could make installation of stone columns problematic.

However, after detailed analysis of all the soil parameters, and drawing on the experience of treating similar soil conditions for power projects overseas, a treatment strategy was established.

'We were con ent with the right site controls, stone columns constructed by the vibro replacement system could be successfully used, ' Egan says.

The vibrators are powered from on-board generators. Internal eccentric weights within the pokers spin around at high velocity causing the nose cone to vibrate continuously at a set frequency of about 3000 cycles a minute, exerting considerable lateral forces to the ground.

By maintaining the same frequency and amplitude, and noting electric current consumption, Keller says it can monitor stone column construction and the density of the surrounding soils. This is because the harder the poker works, the more power it consumes - there is a direct correlation of current with ground resistance.

Keller's vibro rigs are fitted with on-board instrumentation which records stone column length, time taken and energy used to achieve the depth, and the power used to compact the stone. The stone is compacted in short lifts and these are illustrated on the as-built graph produced for each column.

'Using the available soils information we initially zoned the site with respect to the estimated energy required to drive the vibrating poker to depth and the anticipated stone consumption for the columns, ' Egan says.

'Then using the Vibrocat's monitoring systems we were able to measure the actual energy and stone consumption as work progressed and correlate the variation in ground conditions encountered across the site back to the original design assumptions. This gave us a high degree of control over the whole process.' Keller equates the method to a highly intensive site investigation which, given the number of stone columns, effectively maps out ground variations across the site.

The Vibrocat, one of two rigs used on the contract, is Keller's latest semi-automated machine. It is equipped with a computer system that allows soil parameters for a particular part of the site to be entered and the entire compaction process to be carried out automatically, with minimum operator involvement.

At Lockerbie, reviewing the measured response of the ground to poker energy and stone consumption led to modifications to the column installation.

Initially it was assumed from the site investigation that a top feed system could be used. This method uses a poker vibrator to penetrate and displace the soil before it is lifted out of the ground to enable stone layers to be placed and compacted in turn.

The top feed approach relies on the bore standing open long enough for stone placement.

But higher than expected groundwater levels on parts of the site saw a switch to bottom feed pokers to better cope with the unstable soils, with stone fed down through a housing surrounding the poker directly to the base of the column.

The design was backed up by some supplementary piezocone tests of both the natural ground and areas that had been treated with the stone columns.

The regime required to monitor the performance of the completed vibro solution included carrying out a large scale zone test.

This involved constructing a 2.5m square reinforced concrete pad which was then loaded to 234t using kentledge blocks. This full scale test equated to imposing one and a half times the required foundation bearing pressure of 250kN/m 2.Settlements of 8mm and 16mm at working load and 1.5 times working load respectively were recorded, well within the 20mm limit.

Keller installed the 2000 columns in about eight weeks. Substructure construction was able to start as soon as each area was completed.

'Because vibro is a fully displacement system that generates no or very little spoil, it has a very small environmental footprint, ' Whyte says. 'This is often a key factor, particularly for developers who are concerned about the high cost of offsite disposal.' This makes the method ideal for a project where sustainability is a key element, he adds.

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