Twenty years of the M4 Motorway: construction challenges and economic benefits
19 April 2016
Celbridge Interchange (copyright: Roughan O'Donovan Consulting Engineers)
Author: Paul MacDonald MIEI, CEng, executive engineer/training officer, Kildare National Roads Office
The M4 Leixlip-Maynooth-Kilcock Motorway Scheme is the primary road linking Dublin to the west and northwest of Ireland. It is located in the north of Co Kildare, running in parallel with the Royal Canal. The M4 Leixlip-Maynooth-Kilcock Motorway Scheme was proposed in the Road Development Plan for the 1980s (1979) and the Policy and Planning Framework for Roads (1985).
The need for the scheme related to the occurrence of 19 fatalities and 223 serious injuries on the N4 route from Kilcock to Leixlip between 1970 and 1984. The towns of Leixlip, Maynooth and Kilcock were also experiencing noise levels ranging from 69 to 75 decibels, while air-pollution levels were equivalent to Dublin city-centre levels due to traffic. In 1984, the vehicle volumes in these towns were beyond the traffic capacity of a standard two-lane road and these were set to treble up to the year 2010.
Kildare County Council decided to develop the scheme as a motorway road, based on the benefits of controlled access at interchanges and vehicle restrictions. This ensured less accidents, preservation of traffic capacity given the lack of side accesses and lower vehicle-operating costs due to smooth flow conditions. The route option chosen for the M4 Motorway was the most direct route, located south of the existing N4 route. The route had a superior economic rate of return of 9.3% per year. The scheme also integrated with the Lucan bypass proposal in Co Dublin and had a minimal land severance impact on the numerous horse stud farms in the area.
Design of the route alignment
In terms of the horizontal road alignment, the issues which influenced the design included:
• The need to integrate the motorway into the flat land profile;
• The need to avoid the Castletown House Demesne and preserve the vista views to the Connolly’s Folly obelisk monument and the Wonderful Barn; and
• The need to cross the River Liffey reservoir away from the ESB Power Station Dam at Leixlip.
The road alignment was integrated in a successful manner. However, the route did cross the long view to Connolly’s Folly obelisk and was close to urban housing in Lucan and Kilcock. These constraints were addressed by the vertical alignment chosen for the route.
The vertical geometric alignment of the new road was integrated into the flat profile of the area, therefore avoiding the need for high embankments and undesired visual impacts. The vertical level of the motorway was purposely reduced in particular locations, to protect the two-mile-long view to Connolly’s Folly monument and to tie into the Leixlip and Maynooth interchanges at vertical cuttings. The vertical cuttings provided the additional benefit of reduced noise levels for residents adjacent to the motorway. A ground cutting was also required at Ballygoran Hill, east of Maynooth, to ensure that the sightlines were sufficient for the 120 kilometre per hour design speed on the crest curve through the hill.
The combination of the horizontal and vertical profiles, and motorway cross section, ensured a balance of safe driving conditions, adequate capacity, minimal environmental impact and economic feasibility.
Castletown House and Demesne – a key environmental constraint in the design of the M4 Leixlip to Kilcock Motorway Scheme
During construction, the lack of any significant embankments led to an excess of 400,000 cubic metres of excavated material. The side slopes were constructed at 1 in 4 gradients to balance land take costs for the slopes and ensure safe driving conditions, so cars would not overturn if they accidently left the motorway carriageway. Ground cuttings and embankments greater than 2 metres in height were built at 1 in 2 slopes to reduce land take. However, safety barriers were required on these sections, given the steeper gradients.
A key environmental feature on the scheme was the Castletown House and Demesne. It is an important architectural example of the Palladian building style, which required special mitigation to preserve its visual amenity. Special landscaped earth berms were constructed to screen-off traffic from the protected view to the Famine-era public works monument of Connolly’s Folly, thus preserving its heritage value.
M4 Celbridge Interchange- copyright 2013- Roughan O’ Donovan Consulting Engineers
M4 Motorway interchanges and bridges
The interchanges for the scheme were constructed as diamond-shaped layouts for access to the motorway at Leixlip and Maynooth. Access bridges for bisected roads were built to avoid community severance. The existing Maynooth to Celbridge road was positioned at a skew angle to the motorway on an embankment section, with houses fronting onto this road.
This layout would have required an extremely high overbridge with high embankments beside the houses, thus creating a negative visual impact. The engineers decided to forego the skew-bridge option, given these difficulties. They tied the road into another minor road, which was bridged directly over the motorway at a cutting, thus providing the optimum solution.
Interestingly, the planning for the M4 Motorway Scheme in 1986 did not envisage the need for an interchange at Celbridge. However, the arrival in Leixlip of two key computer technology companies, Intel Ltd and Hewlett Packard Ltd, during the late 1980s created a need for direct access to the M4. This illustrates the concept of ‘generated traffic’ due to industrial development being located close to a new motorway to derive access benefit. This creates new traffic-access requirements which can be almost impossible to predict by design engineers when schemes such as the M4 Motorway are being designed and therefore must be addressed at a later stage.
The bridge over the ESB reservoir at Leixlip was originally designed to be a portal bridge sitting on rock-bed either side of the River Liffey. However, the ground investigations indicated the presence of fissured rock, which fractured easily. This was judged to be unsuitable ground for a portal bridge foundation.
The solution was a re-design of the bridge to a simple span structure with innovative piled foundations. This new piling technology involved placing 750mm diameter reinforced steel cages in the bored holes and then pumping concrete from the bottom upwards with a concrete feedpipe. Beams were cast on the piles taking care to avoid contamination of the reservoir which supplied drinking water for Dublin city. The simple span deck was then cast over the River Liffey reservoir, thus ensuring a stable structure.
The opening of the M4 Leixlip –Maynooth – Kilcock Motorway Scheme in 1994 by the Taoiseach, Mr. John Bruton, Mr. Michael Nolan, Chairman of Kildare County Council and Ms. Joan Burton T.D.
The drainage system for the motorway was a piped French drain system, with drainage outfalls to the River Liffey and Lyreen stream. The road drains at the edge of the carriageway were designed for the worst storm in a five-year period, so that flooding at the edge of the motorway would only occur once or twice in a decade, which is considered to be an acceptable level. The pipes crossing under the motorway are designed for the more severe 50-year storm, given that culvert flooding crosses the entire motorway and hence must be restricted to being a once-in-a-lifetime event to prevent skidding accidents.
A drainage problem arose near Maynooth five years after scheme construction, when a littering problem downstream at Bond Bridge created flooding upstream at a motorway culvert. Post- construction drainage remedial works were undertaken at Bond Bridge to prevent further flooding on the M4 Motorway route.
Development of Celbridge Interchange
A number of years after the M4 Motorway Scheme was completed, the access requirements for the international computer firms Intel and Hewlett Packard led to the design of the new Celbridge Interchange proposal. The new interchange ensured direct access to the M4 from Celbridge and the adjacent industrial parks via a dual carriageway link road, which bridged over the parallel railway and Royal Canal.
Again, the key environmental consideration was the impact on Castletown House and the need to preserve this important amenity. The Celbridge Interchange Scheme was completed in 2003 and continues to serve the access needs of the locality and firms such as Intel and Hewlett Packard, which are key companies in the information communications technology sector of the Irish economy.
The Intel Leixlip plant, which makes semi-conductor processor chips for computers and phones, is one of the top three Intel plants in the world and is currently investing €3.6 billion in its plant upgrade at Collinstown. This investment illustrates the important role that schemes such as the M4 Motorway and Celbridge Interchange play in integrating the Irish economy into international business markets, through enhanced access to ports and airports. It also highlights the key role played by the Kildare County Council engineers, consultants and contractors who contributed to both these projects.
On this, the 20th anniversary of the M4 Leixlip-Maynooth-Kilcock Motorway Scheme, and the Celbridge Interchange which followed later, it is important to note the role of Kildare County Council in developing road infrastructure which preserves and integrates the heritage of the county with the transport requirements of Ireland’s modern industrial economy.http://www.engineersjournal.ie/2016/04/19/m4-motorway-construction-2/http://www.engineersjournal.ie/wp-content/uploads/2016/04/Celbridge-interchange.jpghttp://www.engineersjournal.ie/wp-content/uploads/2016/04/Celbridge-interchange-300x300.jpgCivilconstruction,Kildare County Council,roads