The Dublin Port Access Tunnel which is a design and build contract is currently the largest Transport Civil Engineering project in Ireland with a current value of almost 680 million euro, which includes the contractors tender sum, insurances pre tender design and land acquisition costs.

The Tunnel will provide a link between the existing M1/M50 motorways to the Port of Dublin by means of a twin tube dual carriageway. Dublin City is virtually unique in Europe in having the country’s premier port situated in the heart of the city, with all of the trucks travelling to the port having to battle their way through narrow, congested streets that were never designed to carry them.

The project is being built by the NMI consortium, which was formed between two International construction companies, Nishimatsu Construction Co ltd and Mowlem PLC together with a local Irish company, Irishenco. The overall length of the scheme is 5.6km of which 4.5km is in tunnel. Nishimatsu are forming the central bored tunnel section (2.6km long) using two 11.77m diameter Tunnel Boring machines with Mowlem PLC completing the rest with cut and cover methods at either end including road, bridge and control building construction plus M&E fit out.

Coastway Ltd (CWL) are sub-contracted to Nishimatsu to provide all the survey / setting out requirements on the bored tunnel section of the Port Tunnel. The existing Primary control network was established by GPS methods for topographical survey purposes with an updated report published in July 2001 including additions for controlling the North and South cut and cover sections.

The project grid is based upon the Irish national grid , using the IRENET95 GPS point at Howth as a base. A single central scale factor was then established to convert the system to a local (plane) grid.

A network of nine primary points were observed by GPS, with the origin being the IRENET control point at Howth pier. Three of these points were existing points from the original survey, one point was from Mowlem’s and the remaining five installed by Coastway to strengthen the network or improve the position/quality of the original marks.

An additional set of observations were made from the IRENET 95 OSI survey control point at Dunlaoghaire to Howth and the primary network as a check on the overall results to ensure there was no ‘swing’ in the network.

As well as the primary control a secondary control network was established by conventional total station traversing methods linking the North and South sections and some of the primary control points. This was referenced from 2 baselines between points at the Northern and Southern ends.

All coordinates of the stations forming the network were derived from GPS observations or by conventional total station methods. These results were then computed using Leica’s Ski software and ‘Starnet’ least squares adjustment package.

The two tunnel boring machines were controlled by the ‘ENZAN’ alignment system which utilises a robotic total station fixed on a bracket to the tunnel wall which ‘locks’ onto and coordinates targets fixed to the TBM.

Once the relationship of the relative position of these three prisms to the alignment of the TBM is established, the precise position, direction and grade of the machine can be calculated. This information is processed by the Enzan software and fed up to a terminal in the main office where the TBM’s progress can be monitored.

CWL are also responsible for maintaining the alignment system, periodically moving it up and performing independent traverse checks from the main tunnel control to keep the machine on course.

As well as keeping the machines on line, the CWL team are responsible for all the setting out, as built surveys, wriggle surveys, surface and tunnel monitoring.

The setting out itself has involved complicated calculations which have been processed using Liscad and NRG software, which are also utilised to process as built data.

Alongside the Survey and setting out runs the Surface and Tunnel monitoring, consisting of a network of almost 1000 surface points installed in fields, streets and houses levelled from a network of 60 benchmarks.

Monitoring is also carried out inside the tunnel to detect 3-D movement at retro targets around niche, lay-by and NATM excavation areas.

The monitoring is processed through Liscad and Excel producing daily a mountain of graphs tracking the data.

Alongside the Survey and setting out runs the Surface and Tunnel monitoring, consisting of a network of almost 1000 surface points installed in fields, streets and houses levelled from a network of 60 benchmarks.

Monitoring is also carried out inside the tunnel to detect 3-D movement at retro targets around niche, lay-by and NATM excavation areas.

The monitoring is processed through Liscad and Excel producing daily a mountain of graphs tracking the data.