Belfast is built on land riddled with the kind of air pockets that led to the dramatic weekend collapse of Cromac Street — yet nothing like this has ever happened before.

The city was built on a material known as Belfast sleech — a complex mixture of sand, gravel and boulder clay that has caused serious headaches for the team building the Belfast Sewer Project.

The nature of the soil was the main reason why the team had to add three months to the £130 million scheme — every time the huge tunnelling machines building the stormwater sewer from Cromac Street to the docks moved from one type of substrate to another they had to be recalibrated.

To build the huge four-metre diameter tunnel the boring machines — affectionately known as ‘Lucille’ and ‘Lady Jane’ — pushed forward through the ground which caused pressure on the surrounding substrate.

Cement grout was injected from the inside to the outside of the tunnel forming a shell, but also filling any air pockets in the ground surrounding the pipe.

However, Northern Ireland Water’s head of wastewater procurement Bill Gowdy believes the air pocket responsible for the weekend damage may have lain some distance above the route of the tunnel, out of the range of this grout.

“What may have happened is that one of these voids that might have been some metres above — the force of the tunnel going through the earth forced it to move,” he said.

“It would have been like a small cave within the ground structure — so the roof fell down into the soil and it gradually ate its way up to the surface.

“Eventually it would have come in below the asphalt carriageway where there was nothing to support it and the carriageway simply dropped.”

Northern Ireland Water was quickly notified when the first signs of the collapse were spotted and has now excavated the road to reveal the cave. A hole about two metres deep was dug out and has been filled back in again.

“BT, NIE and the gas providers have been involved but none of their utilities were damaged,” Mr Gowdy said.

“We have been able to make excellent progress. Compaction tests are now being carried out and the plan will be to put a concrete raft forming a lid across the excavation. It will take a few days to harden before we finish the road reinstatement.”

The massive storm overflow tunnel beneath was designed to relieve the serious flooding that has affected parts of Belfast, including the lower Ormeau, at times of torrential rain. The tunnel was scheduled to go into use next month and that schedule has not been affected by the weekend disaster.

“This was a very rare and unusual event to happen. The geology of Belfast is complicated but it has never been a problem before and this is unlikely to happen again,” Mr Gowdy said.

“The tunnel is all built under public roadway and does not pose a threat to buildings or houses. I couldn’t anticipate anything like this happening in the future — it’s a very rare event.”

Factfile: how it happened

Belfast is built on land that is perforated with air pockets because of its unusual consistency.

To build a huge storm tunnel massive boring machines pushed forward through the ground 20 metres underneath Belfast.

Cement grout was injected from the inside to the outside of the tunnel to fill any air pockets around the pipe.

One air pocket, lying some distance away from the tunnel below Cromac Street, was missed.

Disturbed by the work, the air pocket’s roof fell down and it slowly ate its way upwards.

Eventually it caused subsidence directly below the road surface, causing the Tarmac to buckle.