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The Hole Story

When a large hole appeared in one of Belfast’s busiest city centre roads, fingers pointed to a tunnelling operation completed a year earlier. Gemma Goldfingle looks into the cause of the Cromac Street collapse.

When tunnellers on the Belfast Sewer project hit hard underground dolerite shafts, contractors kept working without realising they had set off the chain of events which led to a road collapse 12 months later.

The collapse opened a 15m2 hole in Cromac Street above, disrupting one of the city’s busiest streets.

It also occurred despite extensive work to de-risk the project, which passes through unpredictable and hard to tunnel ground.

Tunnel designer Atkins began working on the route a full year before it went out to tender in order to “take out the uncertainties and risk”.

Despite finding the most favourable tunnelling strata, the route couldn’t avoid the difficult dolerite shards.

“We expected to hit 10 sections of dolerite. In fact, we hit 14”, said Paul Ronicle, project manager for the Morgan Est/Farans JV that excavated the tunnel.

When the tunnel boring machine fractured one of these unexpected dolerite shards, this led to the Cromac Street collapse.

“The earth pressure balance machine hit a dyke underneath Cromac Street. The hard dolerite material fractured and created a void,” admitted Ronicle.

Pressure grouting was carried out immediately after the incident to fill the void.

However, some of the fractured dolerite and spoil had arched beneath the void preventing the area being grouted.

Tunnelling is rare in Northern Ireland. Experts say this is largely because of its incredibly complex ground conditions.

“Atkins monitored the tunnel for settlement up to six months after completion but nothing unusual was found.”

So when Northern Ireland Water (NI Water) announced plans to build a 9.5km long, 30m deep sewer tunnel through the centre of Belfast the stage was set for a very challenging project.

Belfast’s geology is a tunnelling engineer’s worst nightmare.

Just 2m below the surface lie the notorious Belfast sleech strata, a difficult mixture of sand, gravel and boulder clay.

Sleech is the local term for the soft, estuarine, quasi-thixotropic deposits that underlie both banks of the River Lagan in the heart of the city.

The material is a two-layer deposit, with the upper layer being a higher permeability sandy sleech and the lower a clayey sleech.

The upper, sandy sleech layer, located between 1m and 3m below ground level, has proportions of sand and clay of 20% and 10% respectively, whilst below this level the proportions reverse, with a maximum clay fraction of 38%.

University College Dublin lecturer and former chairman of the Geotechnical Society of Ireland Kenneth Gavin said: “The strata is very challenging, both compressible and low strength. There is no uniformity, making it incredibly difficult to tunnel through.”

Site investigations for the project identified that the ground was also riddled with troublesome dolorite dykes, which are particular to the region.

These vertical shafts of hard igneous rock rise shard-like through intervening bedrock layers as close as 20m from the surface.

The tunnel’s depth ranges between 15m to 30m. As dolerite is harder than its surrounding rock it is extremely difficult to tunnel through, especially when unexpected.

Tunnel_Boring_Machine_August_07

Ronicle admitted that, regardless of how big, no project could prepare him for the “tremendous unpredictability of Belfast’s geology”.

The 15m² hole in Cromac Street, appeared on 7 November 2009, over a year after the void was created.

“It certainly is unusual to take 12 months for the void to emerge,” said NI Water head of wastewater Bill Gowdy.

“Atkins monitored the tunnel for settlement up to six months after completion but nothing unusual was found.”

Industry sources said it would be difficult to pick up the presence of such a void.

Ronicle also admitted that the hole at Cromac Street was not the only damage caused during the sewer tunnel excavation.

“Cracks appeared in Chancery House, an old Victorian building in the centre of Belfast, while tunnelling was taking place only metres away. We have been served a writ by the owners of the property for hundreds of thousands of pounds,” he said.

Remedial works in Cromac Street were completed in late November and the road reopened a week after the collapse.

But the future of deep tunnelling in the Northern Irish capital looks doubtful after this troublesome debut.

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