Harrow and Hillingdon Geological Society

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Representing Public Concerns in Urban Areas: an example from Dublin

Dr Michael de Freitas - Imperial College London

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Pictures from the original presentation courtesy of Dr Michael De Freitas.

In Dublin many of the properties situated along the banks of the River Liffy have basements. The roads in this area are the centre of major traffic congestion problems. Heavy lorries shake the buildings causing severe damage and occasionally falling into the basements or the river. Everything arriving in the port is taken either along the river or by lorry across country to the motorways for delivery elsewhere. It was decided, therefore, to build a tunnel under the area to link to directly with the M1. This is urban Dublin and most houses were built in the 1930s and were expensive to buy.

Geologically, the area is covered in glacial deposits to a depth of about 10- metres lying on what is basically limestone. A total lack of information to local residents from the construction company, council etc, caused a lot of scaremongering about house damage, collapse and even disappearance. There was much concern leading to protests. The tunnel, which takes about 5 min to traverse, was started on a council estate in the Marino area of the city.

	The limestone here is massive, grey and contains clay. It is jointed containing large blocks and is also steeply inclined. Overlaying this is a dense, hard glacial deposition containing granules and clay. It is difficult to define where the glacial deposits end and the limestone bedrock begins = glaciotectonised limestone layer. It, therefore, made it very difficult to define the position of the tunnel roof and it was decided to dig deeper. The project required two tunnels of about 4-5 km in length, one for each direction, one north to south and the other south to north.
	The tunnel digger comprises a huge machine which pulverises and grinds the rock ahead of it. The lumps of rock passed into the machine, along a belt and the debris was taken away in trucks. Large chunks of rock could not pass on to the belt and stuck to the front of the machine and ground more rock. A second type of equipment shaves the rock and this is followed by a machine to erect a prefabricated tunnel lining. The only unsupported ground is at the front of the tunnel. The direction is controlled by lasers although the machine itself is driven by a man to control the thrust.
	Houses along about 100 yards of the tunnel were old and no precondition survey was done before work commenced. It also went under a recreational area causing more protests. One concern was how you know exactly what is under the house particularly when selling.
	Sample bores were drilled and appeared ok. This part of the talk was well-illustrated with cartoons. The trouble with the residents increased essentially because nobody was told what was happening.
	Vibration was a major worry as it radiated in all directions along seams, by-passing some parts and shaking others. Radiators and lights were shaking, there was a lot of noise and a general shambles. There were no experts available to explain and the only person was a PR man with no expertise to answer questions, hence another disaster.
	A further problem was settlement which is of two types, 1. Total - causing damage to services etc or 2. Structural - this is differential, due to sagging buoyancy or hogging buoyancy. Uneven settlement causes collapse as foundations do not adjust to ground movement. Drainage of water into the tunnel can cause consolidation and settlement.
	All the public concerns caused much wasted time and hence money. The distance affected increases directly with the depth of tunnel and the character of the geology.

Lessons Learned

	All the public concerns caused much wasted time and hence money. The distance affected increases directly with the depth of tunnel and the character of the geology.
	Employ an independent panel of experts Use: extensomometers, inclinomometers, piezometers (level of water in the ground) and seismometers or field instrumentation and have a technical response available 24/7.

The quality of life in Dublin is considerably better with the lorries underground. This was an exercise in political geology which took about 5 years to prepare and was opened last year.

This was a very interesting and well-illustrated talk. It introduced us to the difficulties experienced by geologists but alerted us to pitfalls of not keeping the public informed at every planning stage.

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