The Dublin Port Tunnel project was a €752 million project that was officially opened to traffic on 20 December 2006. The project saw the construction of twin tunnels to form a two lane dual-carriageway connecting Dublin Port with the M1 motorway close to Dublin Airport.
On the northern section of the Dublin Port tunnel contract the main contractor (NMI Consortium) was to construct an 850m long section of cut and cover tunnel from the northern portal to the piled/diaphragm wall section. The busy N1, the main link road from the North and airport into Dublin, was situated just a couple of metres from each side of the excavations.
To monitor pore pressure and lateral ground movement in the excavations, extensive instrumentation was installed by itmsoil, including 80 inclinometers and 240 de-airable vibrating wire piezometers. Geotech Specialists Ltd of Cork formed the holes, and monitoring was carried out by the main contractor.
The work started after completion of a trial excavation alongside the main site to determine the construction characteristics of the boulder clay strata, with lenses of sands and gravel overlying limestone bedrock. The trial involved the same instruments as per the main cut and cover section, inclinometers and VW piezometers and installation services, our expertise ensuring the instruments were installed to specification.
Before excavation started on the main works, instrumentation was installed every 20 metres on both sides. Inclinometers were installed for measuring lateral ground movement, along with de-airable vibrating wire piezometers at 12m, 8m and 4m depths to monitor negative pore water pressure in the boulder clays during the excavation. The piezometers were all terminated to data loggers for the automated recovery of data; these were downloaded daily by the main contractor.