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Why do it like that?


I am becoming increasingly alarmed by the apparent lack of real thinking by geotechnical engineering practitioners in the UK. This was highlighted for me recently when I read the short article on page 14 of the November 03 issue of GE.

The article refers to the ground improvement contract executed by Pennine at the old Barton power station site in Manchester. If I read it correctly, foundations were excavated and removed down to bedrock at depths of 3-4m. Granular fill was imported to bring the site up to construction levels and then 2,000 vibro stone columns were introduced to depths of 4m across the whole site.

The objective of the vibro stone columns?

Apparently to facilitate the support of foundations with bearing pressures of 50150kPa!

Since when did 4m of granular fill over bedrock require improvement to support such loads? The structure being supported is a B&Q warehouse - hardly the sort of facility that would be particularly sensitive to settlements.

I find it hard to believe there was no control over the fill and compaction when it was originally placed or that it could not have been retrospectively tested to prove consistency and competency to support such trivial loads.

However, if true, it seems to me that for such a shallow depth of made ground even the most rudimentary form of proof rolling at formation levels prior to pouring any footings would have identified 'soft spots'.

These could be readily removed, replaced and recompacted to eliminate any risk of local differential settlement problems. I am sure this would have cost less than 2,000 vibro stone columns.

Is the consulting geotechnical engineer a lost breed? Has it become too easy for engineers in the UK to go to a geotechnical engineering contracting company and pay them to take away the ground problems or uncertainty without any thought as to whether it is value for money or even necessary?

J Grant Murray, geotechnical engineering manager, Sinclair Knight Merz

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