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Feat of clay on the Broads

ENVIRONMENTAL GEOTECHNICS

A lightweight aggregate from Scandinavia is helping prevent settlement on a road widening scheme over soft ground in Norfolk. Max Soudain reports.

Improvements to local roads were a condition of planning permission for a retail and light industrial scheme near Great Yarmouth in Norfolk.

Norfolk County Council wanted the upgrading to cope with increased traffic generated by the scheme, which includes a large supermarket.

This involved widening the approach roads to the Gapton Hall roundabout from one lane to two - an engineering challenge because it meant extending an existing embankment out over soft marshy ground, with the risk of differential settlement between the new and original carriageways.

A solution was found in the form of Optiroc Exclay, a lightweight aggregate fill that reduced the load on the soft ground while providing the bearing capacity for the new embankment.

Exclay is made from marine clay expanded at 1150°C in a rotary kiln to form lightweight ceramic granules with a honeycomb core. It conforms to the Environment Agency's drive to increase the use of recycled materials and secondary sources of aggregate.

Exclay has low thermal conductivity, is completely inert and its resistance to chemical attack is similar to that of glazed tile or glass, explains Optiroc Exclay's David Hodgson.

Its porous structure means it can absorb moisture after it has been placed, with long-term moisture content above the water table about 25%. It has excellent insulating properties and is used for lightweight concrete blocks and panels as well as lightweight granular fill.

Hodgson says the material is particularly suitable for geotechnical applications because it has low density and is relatively strong. This means it can be used in combination with more traditional capping materials to provide a high bearing capacity on poor ground.

Typically, a grading of 10mm to 20mm is used, with a bearing capacity of up to 200kN/m 2when the material is placed under a reinforced concrete cap.

For example, using Exclay against a piled bridge abutment means less horizontal earth pressure is exerted than traditional backfill, Hodgson says. This reduces settlement and improves stability. He adds that stability is also improved on large scale cut and fill operations and construction on soft soils, as less load is imposed on the ground.

At Great Yarmouth, main contractor RG Carters (Builders) is building a new Tesco superstore and industrial units.

The low-lying area sits between the River Yare estuary and a Broad, one of the inland waterways for which Norfolk is famous.

It is naturally marshy and the water table rises and falls with the tides in the estuary.

Widening involves extending the road sub-base and pavement on to the verge, covering the whole of the existing embankment and extending it to accommodate the new carriageways.

Road construction is being carried out by Jackson Civil Engineering.

Loading over the marshy ground is obviously critical, with the main concern being differential settlement between the new and existing road lanes. To minimise this, designer Boreham Consulting Engineers specified lightweight aggregate fill and chose Exclay.

Working in sections around the roundabout, Jackson cut away the embankment at a right angle to the line of the new outside edge of the carriageway.

A geotextile membrane was then laid and some 1,500m 3ofExclay placed using a mini hydraulic excavator, which tracked over the material compressing it to minimise surface settlement. A further layer of geotextile was placed on top, creating an Exclay 'sausage' and preventing migration of the material.

The whole area was then covered in topsoil and seeded.

The Exclay will work together with 4m deep wick drains and a drainage blanket that have been installed in the new earth structure. The system is expected to accelerate settlement to completion in about six months, compared with two to three years using conventional fill.

Advantages over natural fill

Exclay has been used across Europe since the late 1950s, particularly in Scandinavia, says the firm's David Hodgson. The Great Yarmouth job is the first large-scale use of the material in the UK.

In Norway, 250,000m 3of Exclay was used in building and upgrading of railways between 1996 and 1998.

It has also been used to replace settled ground beneath roads on thick peat deposits, as backfill to a culvert to reduce settlement and earth pressure on the structure, to prevent settlement of a railway track widening and as approach embankment fill to bridge abutments to prevent differential settlement in soft soils between the approaches and the piled abutments.

Hodgson says the main barrier to use of the material in Britain is that many of the soil tests in the UK Specification for Highway Works are not appropriate for it.

A Transport Research Laboratory report into the use of Exclay concluded the material had a number of advantages over natural fill materials as a structural backfill in bridge abutments and retaining walls, especially in areas of embankment over soft ground.

'The very low density of Exclay compared with natural fill materials will lead to greatly reduced earth pressures and settlements both in short and long term and hence to improved ride quality, ' the report said.

'It will also greatly reduce differential settlement between bridge abutments and embankments.'The report requested 'a site-specific departure' from the specification to allow its use on highways work.

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