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FOR PEAT'S SAKE...

GROUND IMPROVEMENT - Surcharging of peat and lime stabilisation of site-won material is improving ground at a housing site in south Wales. Alan Jones reports.

Ground improvement has been ongoing at the Ynysallen land reclamation site, a 17ha residential development in Birchgrove, Swansea, since March 2002.

A peat bog covers nearly half the site, which has a varied history. In the 19th century there was an ironworks in the south-east corner and from the 1930s to 1960s there was a 14ha coal storage area with windrows leading from adjacent mineral railway sidings.

Since then the peat bog has returned to nature with . y-tipping taking place in some areas, and the south east of the site has been used as a coal storage.

As well as the peat bog, which is up to 3m deep and covers about 8ha, ground conditions consist of up to 2m of soft alluvial clay, silt and sands and glacial till beneath. Bedrock is the Carboniferous Grovesend Beds of the Upper Coal Measures, although there are no shallow mine workings below the site.

The client for the £5M ground improvement and site infrastructure work is Cardiff-based Macob Projects. Arup is engineer for the scheme, brought in at the conceptual design stage in 2001. Walters UK was the main contractor for three previous phases of earthworks, and is back on site for the fourth phase.

It was clear that a variety of ground improvement measures were needed before development could go ahead, so the following strategy was developed (see plan):

The site was divided into nine development areas.

Elevated areas of glacial deposits (areas 1, 2 and 3) were cut to form construction platforms.

In areas around the edge of the bog (areas 4, 5 and 7), thin layers of peat were removed and replaced with site-won structural fill.

Other areas of peat (areas 6, 8 and 9) were treated by the application of a phased surcharge load, comprising unsuitable site-won material (predominantly soft alluvium, coal dust, slag and clinker waste), to induce primary consolidation.

Following the completion of the phase 1 surcharge, 50,000m 3 of the fill was treated by lime stabilisation to raise ground levels in area 7.

Peat is a highly compressible material and its consolidation is largely governed by the way the pore water is held and released.

It is present in macro-pores (intercellular water), micro pores (interparticle water) and as adsorbed water, the amount of each depending on the structure of the peat and the degree of humi. cation. As the water is expelled, the peat consolidates and structural rearrangement of the peat occurs.

The aim of surcharging at Ynysallen was to induce primary consolidation, eliminating longterm differential settlement between piled houses and the surrounding ground, and to provide a suitable formation for constructing roads, services and utilities.

Following initial site clearance, the . st phase of surcharge was placed during summer 2003.

Carrier drains were excavated into the surface of the peat and a geotextile drainage blanket placed over the surface to transport expelled groundwater into perimeter drainage ditches. A 1.2m thick layer of compacted structural fill was placed above the drainage blanket as the permanent construction platform.

Settlement monitoring posts were placed on a 25m grid and about 90,000m 3 of temporary fill was used to form a 2.5m thick surcharge.

It was estimated total primary confiolidation beneath the surcharge would be 200-700mm. The calculated primary consolidation for the permanent site loads was 100-300mm.

The actual recorded settlements for the first phase surcharge ranged between 108mm and 574mm, and averaged 300mm (see box).

The phase 1 surcharge was complete after 15 months, with the temporary fill removed from October 2004. The phase 2 surcharge used 40,000m 3 of this fill, while the remainder (50,000m 3) was treated with lime by subcontractor Bennett Soil Technology.

Treated fill was placed and compacted in layers to raise ground levels in areas 7 and 7a by up to 2.5m and to build a structural platform in area 9.

The moisture content of the surcharge fill was too high to allow compaction and about 2% of lime by volume was added to lower this to an MCV between 8 and 12.

The lime could not be pulverised without first being raked by excavators to remove cobble and boulder size fragments of slag and clinker.

Construction of the houses is complete in fi ve of the development areas and ongoing in two others.

The fourth earthworks contract is now on site, which involves removing part of the phase 2 surcharge fill and a 550m diversion of a 600mm water main. Ground improvement is required along the water main corridor. This will partly comprise peat removal and replacement, and an increased 5m high surcharge in the phase 2 area.

Earthworks should be complete in spring 2007 following a fifth phase of the contract.

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