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Dunsany Manholes and Railways Don't Mix

While doing a preliminary survey of the Clonsilla Navan Rail Users Ireland members spotted a sewer main being laid on a section of the former alignment from approximately 200 m north of Dunsany Bridge southwards to Warrenstown College just north of Drumree. This sewer is part of a new system linking to Dunshaughlin.

Inquiries were made with Meath County Council whose engineer seems to believe that placing a sewer main underneath a railway right of way is an acceptable practice, international best practice would indicate otherwise. The pipe is placed off centre on the alignment on what is the left side if facing northwards towards Navan. Manhole covers are to be found approximately every 100 m and would directly obstruct the track formation. Plans from Meath County Council indicate the presence of a total of 9 manholes on the alignment.

Despite many requests neither Meath CC nor the consulting engineers responsible for the sewer have been willing so far to give a response on the record. Meath CC are clearly uncomfortable. A Rail Users Ireland presentation give 21-11-2005 in Navan was brought to an abrupt end when the matter was raised, likewise at a public meeting held on 6-2-2006 the Meath County Manager was unconvincing. The sewer main is an engineering problem, given some time and some money it can be overcome but the question to ask is why it was ever laid in the first place?

The manhole access is not compatible with the provision of a railway. Even if there were no manholes the presence of the sewer main could lead to difficulties with respect supporting the load of track directly above, for foundations required for signalling masts and overhead electrification masts. Iarnród Éireann policy is that all bridges over railway lines shall allow for 2 tracks and shall make provision for electrification. For the bridge at Cannistown which Meath CC failed to make provision for in the M3 planning application the An Bord Pleanála ruling stipulated that the bridge allow for a double track electrified railway.

In the context of evidence that a two track railway is envisaged placement of the sewer main anywhere underneath where track may run is irresponsible. If the line is electrified the clearance under the bridge at Dunsany (and others) would need to be increased, the most practical method being to drop the level of the railway alignment to achieve the nominal 4.8m clearance, this could expose the sewer main or reduce its cover to such an extent that it was a risk of damage from the railway above. If Meath CC wished to access the sewer for any reason this would require closure of the railway line, if the pipe sprung a leak the railway line would require total closure. This is not an acceptable state of affairs.

You Can't Have Manhole On A Trackbed

Iarnród Éireann are quite clear that manholes have no place near the trackbed and require relocation,

Where sewers are buried at a substantial distance below the trackbed they will not be relocated, however manhole access will need to be relocated away from the trackbed.

However with a longitudinally laid pipe such as the one in Dunsany repositioning of the manholes is not possible since the manhole access has to be directly above the sewer as such it is clear the sewer main would demand relocation away from the trackbed.

The Evidence

Below is a picture of the manhole positioned adjacent to Dunsany Bridge. The photo is taken from the south side of the bridge and faces towards Navan.

Dunsany Bridge

Note the manhole, it is approximately 4.5 m from the left of the bridge and 8 m from the right side. The manhole is 9.1m from the arch. These measurements relate to the retaining wall in advance of the arch, the width narrows under the arch. More photos.

The width of the bridge at Dunsany is approximately 12 metres which is less than the minimum required by current safety guidance, but is still adequate if additional safety mitigation steps are taken. Given the positioning of the manhole it would make a two track railway impossible and would make a single track extremely difficult.

Verdict

Meath County Council continues to play its dangerous game. To allow for reopening of the line the manhole access to the sewer will require removal from the alignment. This is a stated requirement from Iarnród Éireann. In line with international best practice no pipeline of any type should be situated directly below the track and should be placed as far as is practical away from the track alignment. As it stands the manhole covers are situated in the majority case where the Navan bound track of the 2 track railway would be placed.

Removal of the manhole access by definition will require the diversion of the sewer pipe away from the railway alignment.

The Clearance Requirements

The table below shows the clearances required for any new railway works. Reopening of an abandoned line falls into this category. Currently open lines are exempt except where new structures are put in place which are required to comply. First things first a warning this is about to get very technical with dimensions, numbers and design standards. All dimensions are in millimetres.

Measurement DescriptionLength (mm)
Width of lineside cess walkway700
Lineside cess walkway to swept envelope of train up to 165 kph431
Lineside cess walkway to swept envelope of train 165 kph to 200 kph1,286
Width of widest train on network3,200
Maximum swept envelope of train3,420
Rail to swept envelope of train909
Approx. width of sleeper (varies by type)2,740
Recommended clearance between passing trains up to 165 kph380
Recommended clearance between passing trains 165 kph to 200 kph450
Clearance between outer rail and bridge abutment4,500
Irish Standard Gauge1,602

Under certain conditions reduced clearances are possible but only after seeking a derogation from the safety body in charge and also having put in place extra procedures such as derailment containment to minimise the risk.

The Context

All bridges over the Clonsilla Navan alignment pre date modern safety requirements and are sub-standard in width and will all require a derogation from the safety body in charge at time of construction of the reopened railway. The exact width of each bridge varies but all are of a common design with a width of approximately 12 m, current regulations demand 14.4 m. Provision of derailment containment features would allow the usage of the existing bridges. However there is no possibility of slewing track either side to avoid the sewer main as this would compromise the already substandard clearances.

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Last Updated: October 16 2007 22:37:27
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