An air void disturbed by tunnelling work 12 months ago has been blamed for the collapse of one of Belfast’s busiest roads last weekend.
Northern Ireland (NI) Water has accepted responsibility for the 15m² hole that appeared in Cromac Street on Saturday.
Asphalt on the road stayed largely intact but sagged above a large void which had opened up beneath it.
“It’s thought that the driving of the 4m diameter tunnel may have disturbed a void.”
Bill Gowdy, Northern Ireland Water
The collapse occurred 15m above the 4m diameter Belfast Sewer tunnel, which was completed in November last year. It is thought that tunnelling work carried out 12 months ago set up the chain of events which led to the collapse.
“It’s thought that the driving of the 4m diameter tunnel may have disturbed a void and caused [the ground around the void] to disintegrate,” said NI Water head of wastewater Bill Gowdy.
“The disintegration allowed the void to move upwards. When it eventually reached the underside of the asphalt carriageway, the road sagged under its own weight and caused a depression in the road”.
The Atkins-designed £120M tunnel is part of a three year project to reduce the risk of flooding in Belfast.
A Morgan Est/Farrans joint venture completed the 9.5km sewer using an earth pressure balance tunnel boring machine (TBM). The tunnel was driven at depths of between 10m and 30m.
NI Water claimed voids or “air pockets” are commonplace in Belfast’s complex geology. The tunnel was driven through Belfast sleech, a notoriously difficult mixture of sand, gravel and boulder clay.
Geotechnical experts expressed surprise that air voids would exist in the sleech. “The strata is very challenging, both compressible and low strength, but I am not aware of any so called ‘air pockets,’” said University College Dublin lecturer and former chairman of the Geotechnical Society of Ireland Kenneth Gavin.
One geotechnical expert claimed that the most likely scenario was that the tunnelling operation created the void, rather than simply disturbing it. Gowdy insisted that voids do exist in the sub-strata, although they normally cause no problems.
Consultant Atkins monitored the tunnel before construction and up until six months after completion. Nothing unusual was found.