In response to J Grant Murray's letter (GE February 2004), the process of ground treatment as described by Marc Evans has been used, to my knowledge, for over 15 years and has proved a cost-effective method, particularly for dealing with old power station and gas works sites where there is a high incidence of buried obstructions.
Where the replacement fill is granular the process is not 'stone columns' but a rapid deep compaction process. The benefits of using this approach, compared to the more old fashioned method of rolling in layers was well described by Dr Evans.
In the same issue, Ken Watts mentions the ICE Specification for Ground Treatment, NHBC Standards, BR 391 and BR458 Specifying stone columns/ dynamic compaction.
When these were first produced most civil engineers and developers were unaware of these processes or of the details needed for procuring or supervising them, and over the years the documents have been updated to cover advances in the processes.
However it was always understood the design must be carried out by an experienced geotechnical engineer.
The design of ground treatment, particularly dynamic compaction, is by no means a precise science and a conversant independent geotechnical consultant should always check the design for larger or more sensitive projects.
Graham Thomson, retired ground treatment consultant