Engineering features of the M7 Newbridge Bypass and M9 Kilcullen Link motorway
28 March 2017
The M7 Newbridge Bypass scheme consisted of the construction of 12 kilometres of motorway between the M7 Naas Bypass and the dual carriageway across the Curragh Plains. The scheme provided congestion relief for Newbridge town, which was experiencing traffic flows of over 30,000 vehicles per day due to its location on the Dublin to Cork and Limerick routes. The scheme was also supported by the European Social Development Fund to enhance access to Ireland within Europe.
The selection of a motorway cross-section with a geometric design speed of 120 kilometres per hour for the scheme was an important design decision to ensure safe driving conditions on the bypass at a time of increasing car ownership levels in Ireland.
The route selection for the M7 Newbridge Bypass was determined based on engineering, environmental and socio-economic factors such as accident and travel-time savings. The horizontal road alignment of the bypass was influenced by:
- The need to avoid the Great Connell Abbey monastic site and the parallel Corbally branch of the Grand Canal;
- The selection of an optimum crossing point on the meandering section of the River Liffey between Newbridge and Kilcullen; and
- The need to tie in with the existing M7 Naas Bypass and the N7 dual carriageway road south of Newbridge. The Ballymany interchange at the southern end of the scheme was located to form an appropriate transition marker between the suburban area of Newbridge and the sensitive Curragh Plains.
The factors which influenced the vertical geometric alignment of the bypass include the integration of the motorway into the Kildare Central Lowlands landscape in a sensitive manner and the crossing of the River Liffey. The landscaping measures included for the planting of 200,000 trees and 300,000 plants by Kildare County Council staff under the supervision of Dublin County Council Parks Department in order to further integrate the scheme in a visually appropriate manner.
Over two million cubic metres of material was excavated during construction with 1.4 million cubic metres of material being reused based on its optimum moisture content characteristics which determined the maximum strength. This was a significant economic benefit during construction. This illustrated the importance of geotechnical investigations at pre-tender stages which determined the potential for re-use of excavated materials for embankment construction and the related estimation of project costs.
Drainage and bridges
The drainage for the scheme included for the installation of 48km of surface water drains. The drainage outfall from the Ballymany Interchange at the Curragh Plains to the River Liffey was designed to be of an exceptional length. The pipe length was 4km to the outfall where it traversed through the abutment of the bridge into the river.
The scheme included 12 major bridge structures including the River Liffey crossing. A range of different bridge deck construction methods were used including cast insitu concrete decks, pre-stressed beams and post tensioned box decks. A number of the bridges required precast concrete piles to be driven for foundation support due to less stable ground conditions at particular locations. The construction of the River Liffey bridge was completed in a careful manner so as to minimise environmental impact on this important aquatic habitat.
The importance of the Curragh Plains as a world-renowned gallops for thoroughbred horses was considered as part of the access provisions of the scheme. Two special horse access bridges were constructed to facilitate safe access for horse trainers between the Curragh Race Course and the Curragh Plains.
The bridges were aesthetically designed to incorporate soft visual features appropriate to the open views on the Curragh including the distinctive ‘horseshoe’ shaped concrete column supports for the access bridges. The range of different horse industry stakeholders with an interest in the Curragh access bridges created a challenge for the Kildare County Council engineers regarding communication of the bridge sizing options to the parties in a non-technical and understandable manner.
The bridges were also designed to sustain military tank loading at the request of the Department of Defence to facilitate army manoeuvres from the nearby Curragh Military Camp. The Defence Force officers inspected the bridges after construction to ensure that openings were present to place explosive ordnance for demolition of the bridges in the unlikely event of a war to prevent access to the Curragh Camp.
The Kildare County Council resident engineer pointed out the 50 millimetre drainage outlets under the precast panels on the bridges would serve as ideal locations for the demolition explosives of the Army Corps of Engineers.
Ireland’s first motorway-to-motorway interchange
The M7 Newbridge Bypass incorporated the first motorway-to-motorway interchange in Ireland to link the parallel M9 Kilcullen Link Bypass to the M7 Motorway. A skew concrete box culvert ‘tunnel’ with special dual power supply for internal lighting was constructed at Greatconnell to facilitate merge of the traffic from the M9 Kilcullen Link onto the M7 Motorway. The M7 Newbridge Bypass was opened to traffic on 16 June 1993.
The M9 Kilcullen Link scheme was a 6km length of motorway that was constructed in parallel with the M7 Newbridge Bypass. The M9 Kilcullen Link was opened on 24 October 1994, which facilitated the removal of traffic travelling between Dublin and Waterford from the towns of Naas and Kilcullen once this scheme was integrated with the parallel M7 Newbridge Bypass scheme.
The M9 Motorway linked to the M7 Motorway at the Greatconnell Interchange and a second interchange was constructed at Kilcullen to facilitate access to Kilcullen and Athy from the M9 Motorway.
The location of the M9 Link was designed to account for significant environmental features including: Castlemartin House and Horse Stud Demesne near Kilcullen, the pre-Norman ring fort at Dun Aillinne which is the largest of its kind in Ireland; and the early Christian monastic site at Old Kilcullen, which was founded in the fifth century by Saint Patrick according to historical record.
A cast insitu concrete bridge was constructed at the River Liffey crossing near Castlemartin. The bridge included sheet piling at the river bank edge to prevent scouring and damage of the bridge abutment concrete and earthworks during floods.
During construction, the overhead power lines near the bridge unexpectedly induced static electric charges in the steel components of the bridge deck which necessitated special earthing of the deck by the Electricity Supply Board to enable safe construction. Access bridges were incorporated into the scheme to facilitate access to adjacent quarry operations, which are a common feature at Kilcullen.
The M7 Newbridge Bypass and M9 Kilcullen Link Motorway were important road schemes which were developed by Kildare County Council to facilitate traffic relief for the towns of Newbridge, Naas and Kilcullen. Both schemes were designed and constructed to account for the unique historical and environmental features in the area while also accounting for the needs of the important horse breeding industry located in this area of County Kildare.
The M7 Newbridge Bypass and M9 Kilcullen Link Schemes were important first steps in the process of developing the major inter-urban road network from 2000 onwards to ensure full transport integration for Ireland within the European Union.http://www.engineersjournal.ie/2017/03/28/m7-newbridge-bypass-m9-kilcullen-link-motorway/http://www.engineersjournal.ie/wp-content/uploads/2017/03/m9.jpghttp://www.engineersjournal.ie/wp-content/uploads/2017/03/m9-300x300.jpgCivilKildare Co Co National Roads Office,roads,Roads and Transportation Division