In response to Mr Murray's letter, we would like to point out that the foundation solution adopted on this site was the most economic given the circumstances; this is something Pennine refers to as 'valued engineering'.
Although Pennine was not involved in the thought process concerning the options available, I would point out the following:
The structure is designed to differential tolerances of 10mm total and 1 in 1000 distortion. The existence of buried structures would generate 'hard spots' and compromise the performance. Therefore they had to be removed.
Why mass excavation? There were numerous foundations, culverts and slabs left over from the demolition of the power station. In some areas the frequency of these buried structures meant it would have been logistically difficult to remove them individually. A mass excavation was therefore going to save time and money, resulting in a happy client.
Why loose tip with vibro columns? The excavated material was crushed and used as backfill to the excavations, thus avoiding disposal of the excavated material off site and importing infill.
The quickest and most cost-effective solution was to loosely end tip the fill and then compact it by installing stone columns rather than rolling the fill in engineered layers. Additional benefits were that the fill did not have to be stockpiled on a restricted site; the grading of the crushed material was less critical; and less stringent quality control was needed when placing it.
The principle of 'sustainable construction' requires minimising waste production and the efficient use and recycling of materials. The potential for greatest savings and environmental benefit occur when recycling on or near site can take place.
I would note that UK engineers are, in the main, mindful of such practical and economic constraints when providing clients with design solutions. Engineers who do not take issues of practicalities, environmental sustainability, safety and economics into account are unlikely to remain in practice for long.
Dr Marc Evans, technical director, Pennine Vibropiling