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A bomb in the bucket

EARTHMOVING

ONE OF the most serious hazards Alfred McAlpine faced when it arrived at The Bridge was the fact that, like much of the Thames Gateway, the site had not avoided the attentions of the Luftwaffe during World War II. The only way to be sure workers were going to be safe was to bring in specialists Bactec to carry out a full geotechnical survey.

'Bactec used two technologies; obtrusive and non-obtrusive ground survey, ' says McAlpine site manager Kim Barrows. 'The unobtrusive is a ground radar using a magnetometer that finds metals in the ground to an effective depth of 3 m.

But it is extremely difficult in the made ground because they get a lot of signals. We had to try to identify whether it was a bomb or not. We screen out any thing under 50 kg and then have to go back in with a backhoe and one of their bomb technicians.'

Where piles were going deeper than 3 m a probe with a magnetometer head was put down the hole first. Any anomalies can then be checked out by inserting further probes nearby. The team found no bombs during this survey, yet still ended up with two UXB incidents.

The first was a small incendiary device in an area next to a high-pressure pipeline crossing the site.

The pipeline had to be closed while it was dealt with, a situation that also occurred when a 250 kg bomb was found underneath powerlines from the nearby Littlebrook Power Station.

Mr Barrows says interference from the pylons meant the bomb survey methods could not be used effectively in the area, so the bomb was not discovered until it showed up in an excavator's bucket.