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LET'S BE AVIN YOU

Dry soil mixing has proved a winning substitute during construction of a new road at Norwich City Football Club.

Use of dry soil mixing on a road project at Norwich City Football Club's Carrow Road ground has removed the need to excavate and take away huge volumes of material from the site.

The new road will service an upmarket housing development, Riverside Heights, on what was once a distant car park behind the Canaries' ground.

Just over a year ago the club sold the land to Taylor Woodrow, whose Bryant Homes division is now developing it.

Although the project is phased and on-going, the 150m long access road, built to local authority highways standards, needs to be complete before the fi rst residents arrive in September. The plan is to carry out most of the work in the summer closed season.

In places the road runs just 10m from the back of the home stand - and right past the entrance of Norwich FC chairman Delia Smith's restaurant, Delia's, giving construction workers an upmarket alternative to the local greasy spoon.

For the construction team the major challenge has been installation of drainage infrastructure enabling works, particularly given the poor ground conditions - and accommodating the work around home game fixtures.

The Riverside Heights name refers to the site's location beside the River Wensum and the geotechnical conditions refl ect this local geomorphology. Site investigation identified just over a metre of fill overlying peat to about 3.5m, overlying sands and gravels typically below 5m. Groundwater level is quite high, about 1.5m below the surface.

Jon Spalding, associate director of the football club's design consultant Millard Consulting Engineers, says the fi rst option was to dig out and replace the fill and peat. This was almost immediately discounted because of cost and programme - and because it would have proved hugely disruptive to both the club and Bryant Homes.

The next approach was a scheme making use of vibro concrete columns (VCC). This solution still needed fairly large scale excavation and involved casting a reinforced concrete slab, up to 2.5m below ground level, to support the drainage runs and manholes.

Although there were considerable programme and logistical issues, this was the preferred solution at the beginning of the year. But around this time Millard approached Keller in connection with the VCC work, and Keller had other ideas.

Keller suggested using dry soil mixing instead. This Swedish ground improvement technique is particularly suitable for soft clays and peat.

Keller, working with Swedish sister company LCM, believes the technique has plenty of UK potential.

Spalding was certainly convinced of its potential and as a result 'quickly changed the scheme'.

The great benefi t was speed, not just in the execution of the ground treatment, but because the approach greatly simplifi ed the whole project.

In the fi rst instance it meant ground improvement work could be comfortably completed in a four-week period in March, in which there were no home fixtures.

In fact Keller installed columns on a 1m grid down through the fill and peat with one rig in just two weeks.

More critically, when the main contractor moves in at the end of the season, it will be able to safely excavate through the stabilised ground and lay drainage services directly on the improved ground - making the whole operation much more contained, quicker and cheaper.

After backfi lling around the service installations, a load transfer platform will be laid down, allowing construction of the road pavement to follow shortly afterwards.

Spalding says although the original VCC approach was marginally cheaper than dry soil mixing, use of the latter reduced total project costs signifi antly 'because it removed the need for a substantial reinforced concrete slab to support the road's drainage infrastructure'.

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