Plug technique reduces noise and prevents migration

Pennine Vibropiling's innovative use of its vibro concrete plug technique has provided an alternative design solution for ground improvement at the new B & Q Superstore being built in Kidderminster.

The site was formerly occupied by carpet manufacturers. As a result of the weaving and dyeing factories, boiler houses and several large fuel tanks, significant ground contamination is present.

Pennine provided the main contractor, Watkins Jones, with an alternative method of ground improvement based on stone columns installed using its vibrocompaction technique.

Initially, the Environment Agency objected to this proposal due to concern relating to the possible migration of contamination down the stone columns into the underlying sandstone acquifer which is utilised by Severn Trent Water for the public supply. Whilst this is rarely a problem in the long term, because of the amount of hard cover over the site when the development is complete, during construction the situation could arise where a sudden deluge might cause migration.

However, based on its previous experience of similar situations, Pennine was able to satisfy the Environment Agency that the introduction of lean mix concrete plugs at the base of the columns would prevent the possibility of any transfer of contaminants.

Computer monitoring
In the right geology this type of plug prevents the aquiclude - the impermeable clay layer covering the aquifer - being pierced while still enabling Pennine's ground improvement technique to be used even though aquifers are present at greater depths.

Quality control is extremely important when using this technique and computer monitoring is used to ensure that the plug is in the correct position. Over 2,000 stone columns and concrete plugs have been installed to support both the floor and main foundations.

The use of a concrete plug to overcome the possibility of migration was pioneered by Pennine who developed the technique. It is achieved by using a specially adapted poker which enables a small charge of concrete to be pumped at a depth controlled by use of on-board instrumentation. Stone can then be fed down the flot using bottom feed equipment or conventionally from the top.

There is increasing concern by the Environment Agency about potential contamination of the aquifer as more and more brownfield sites are developed. Depending on the specific situation, this innovative technique can often be used to provide a solution and safeguard the environment.



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