The geology under the streets of Dublin would present formidable obstacles to engineers attempting to build a Luas tunnel. Yet a safe working tunnel could be built, "if we are prepared to pay", stated Dr Adrian Philips, associate professor at the department of geology at Trinity College, Dublin.
There is no way to determine these costs yet, Dr Philips said, because "to get the sort of information you need, you need to drill holes", and this would be "very difficult and expensive".
The planned £200 million Dublin Port tunnel will be about 2.5 km long. It follows a route mostly well away from historic or very large buildings. There was plenty of space for most of the 100 bore holes drilled during the survey for this tunnel and little difficulty keeping away from buried gas, water, electricity and sewer lines. The same would not be true in central Dublin, he pointed out.
Bore holes could be drilled in the streets, in turn causing serious traffic disruption. The tunnel would be unlikely to follow the line of the street on its proposed route from St Stephen's Green to Broadstone, however, and this would mean drilling on private property. He doubted that property owners would be keen on this, given "the idea of having a drilling rig brought into people's back yards and the anxieties of subsidence".
The Geological Survey holds a database of bore hole information giving depth to bedrock and the materials deposited on it, stated Dr Ronnie Creighton, a geologist with the survey. "For the greater Dublin area we have about 5,000 bore holes," he said, providing a "bedrock map" of the city.
The data were accumulated as holes were drilled for various projects. It shows that the Liffey once flowed north-east from Islandbridge towards Drumcondra, looping across the north city and back towards the East Wall. The bedrock is very deep along this route, he said, up to 100 ft down.
If the data could be gathered and a city centre route agreed, engineering challenges would then come into play. The single greatest difficulty would be posed by ancient river bed channels cut into the limestone bedrock at the end of the last ice age, Dr Philips said.
Sea levels were 45 m lower at that time. Dublin's rivers, the Liffey and then later the Poddle, Bradoge, Stein, Camac and others left channels through the limestone. There was a "mini Grand Canyon", with bedrock cut by the River Stein, plummeting 100 feet deep below the front of Trinity College, Dr Philips said.
As the ice melted and sea levels rose, the retreating glaciers dumped "boulder clay" into the channels, Dr Creighton said. Sand and gravel were also flushed into these channels, leaving Dublin with its current geological picture.
Geologists are unsure exactly where these channels are, however, because of changes in the routes of all of these rivers over the centuries. The St Stephen's Green to Broadstone route would cut into an unknown number of these channels.
This leaves tunnel engineers with at least two options, to go shallow, running through bedrock where available and then across the deposits of sand, gravel and boulder clay, or to go deep, getting into bedrock below the original 45-metre deep Liffey river bed. Both options present their own problems, Dr Philips said. "It is not a simple tunnel."
Both would put the Luas tunnel under sea level so excavation would be liable to water seepage or possibly flooding, particularly when cutting through gravel.
"These channels are deep down, well below sea level. In effect, what you have are very large water mains," he said. Loss of water into tunnels would be expected to reduce water pressures in the ground, possibly causing subsidence under buildings.
The deep option would be below these soft deposits but the limestone could be liable to inherent weaknesses, fractures that allowed the rivers to cut channels in the first place, Dr Philips explained. Water could penetrate along fracture lines or demand more reinforcement during building to ensure strength in unstable conditions.
Mr Donal Mangan, Luas project director, said the next step was to begin tests to find what lies underground on the route. This could be done with sampling, followed by intensive drilling as a final route began to emerge.