URGENT STRUCTURAL assessment of Edinburgh's Union Canal was under way as NCE went to press, following a breach early on Sunday that sent more than 12M litres of water sluicing into city centre streets.
According to one waterways expert, investigators will look at a sewer laid in a trench across the canal between October 2001 and January this year which may have leaked and contributed to the collapse.
A brewery belonging to Scottish Courage and five flats were inundated when water started gushing from the Lochrin Basin, where the Union Canal terminates, at around 4.30am on Sunday morning.
Water escaped through the canal's puddled clay bed, under the 6m wide towpath.
Around 150mm depth of water was lost over an 8km stretch of the Union Canal before British Waterways and city council workers finally plugged the leak in the clay bed with sandbags and rock.
Edinburgh-based consultant Carl Bro was appointed on Monday morning to carry out a detailed inspection of the basin, which has now been sealed off and drained down. It is expected to report late this week.
One theory is that a waterproof geotextile lining laid on the canal bed to seal it after the sewer was laid may not have been adequately keyed into the surrounding clay, creating an escape path for water. The water could then have been able to migrate through granular material used to backfill around the sewer pipe, eroding it to the point of rapid failure.
But British Waterways chief engineer George Ballinger said other causes were also being scrutinised. These include the presence of weak materials and cavities in the ground, leaking water mains, and the possibility that historic construction work may have led to overcompaction of soils, contributing to weakness.
The sudden rush of water left a 1.25m diameter hole some 7m deep in the canal bed. Escaping water eroded ground beneath the towpath causing it to collapse but it appears not to have damaged the concrete canal wall or a brick retaining wall supporting the towpath, said Ballinger.
Though it is at the tail end of the recently refurbished Union Canal, no major repair or maintenance work has been carried out on the Lochrin Basin, said a British Waterways spokeswoman.
INFOPLUS www. british-waterways. org