Plug technique reduces noise and prevents migration

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.