The ever increasing excavation

Expecting the unexpected is always the rule when decommissioning a service station, as graphically illustrated by a recent David Plumb & Co contract at Greenwich, south London.

The fuel site cleaning and decommissioning specialist had been called in to carry out the delicate task of removing an 8,000ltr double-compartment tank from the edge of a redundant site, just a few metres from an operating pub car park.

Says David Plumb director John Moore: “The excavation was so close to the car park that there was a real danger of subsidence if the trench collapsed when we removed the tank.

“We could have guarded against this by hammering in sheet pilings to hold back the earth, but this would have been a very expensive option for the client. Instead, we thoroughly assessed the risks and opted to use immediate backfill, a much more cost effective option, but we have to be very quick and precise, ensuring we have all the inert infill material on-site before we begin, and back-filling the hole as soon as the tank is removed.

“We also had to take extra safety precautions, including temporarily fencing off a 2m-deep section of pub car park. We also had several ‘banksmen’ checking for any cracking throughout the operation and a vibration monitor ensuring there was no possibility of damage to the pub.“

While this operation was a complete success, resulting in a satisfied client and a happy publican, David Plumb’s operatives found a further eight tanks that also had to be excavated.

“We always do a desk-top survey,” says John Moore, “where we check plans held by the owner, utilities companies and the local council, and then an on-site survey with CATs (cable avoiding tools) and test bores, so we suspected there were five more tanks, but the other three came as a complete surprise.”

Eventually, on top of the original 8,000ltr tank and two others that had been previously removed by another contractor, David Plumb & Co eventually removed three 1,000ltr tanks, one 2,000ltr, one 3,000ltr, two 5,000ltr and a 10,000ltr tank. This last was right under the gable wall of an adjacent house, which needed great care to excavate and the need to employ the immediate back-fill technique once more.

“Many forecourt sites have now been used for more than 50 years,” says John Moore, “during which period they will have been altered and redeveloped many times to cater for changing requirements. Over the decades plans get mislaid and tanks get forgotten, so, no matter how careful you are, you can never be certain what’s down there until you actually start digging.”

29 January 2008

 

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