DART+ (DART Expansion)
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Cut-price tunnel options compared:
- 2019 proposal but cut the western end: Heuston-Stephen’s Green-Pearse-Docklands: 5.25 km
2. Drop SSG, Heuston-Docklands via Pearse: 4.89 km (360 m shorter)
3. Drop SSG, Heuston-Docklands via Tara St: 4.78 km (480 m shorter than #1, 120 m shorter than option #2)
Because you’re mostly travelling east, but have to be facing roughly north at the eastern end of the tunnel, the savings in tunnelling turn out to be negligible for these “shorter” options. Also, the Tara option involves more curves and theoretically could be slower from station to station than the Pearse one as a result.
And it’s only the station build cost that’s being saved: the only significant saving on tunnelling cost is at the Western end of the project. Any change of routing through the city has a negligible effect. Personally, I think dropping SSG would not save enough of the project cost to compensate for the loss of passenger benefit from removing it.
As an aside, the direct Heuston-Northern Line link with no intermediate stations was ruled out in both studies as being of very little passenger benefit: people don’t want to go to either of those places, they only get out there now because they have to: they actually want to go to Dublin centre. As far as I see it, the whole point of a DART tunnel from a passenger’s point of view is the provision of two new DART stations at SSG and Christchurch.
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That's exactly the point I was making. Seeking a “short” route through the city is mostly a waste of time. Once you’ve dropped one of the station from the plan, you have saved so much that you can basically bring the tunnel wherever you want. I don’t see any easy wins for building the city-centre stations either. Of the proposed stations, SSG could be cut-and-cover, but it would be the mother of all court battles to get permission to dig a huge pit in the Green (and/or closing SSG South or North to all traffic for three years); Christchurch could be dug down more easily, but it's closing a major traffic artery in the city (maybe not a bad thing in the long run, but a major headache during). Pearse will have to be mined, because there’s just no room nearby, plus the best routing of the tunnel is directly under Pearse station itself.
The difficulty is compounded by Metrolink, which has placed its tunnel at 20 m below ground for the most part, which in turn makes it easy to dig out a station, as shown in the Metrolink plans. Twenty metres is the closest to the surface you can reasonably go, as it guarantees you won’t affect any modern basements or buried infrastructure that you didn’t know about (although ML did rediscover a Victorian-era sewer during planning!). So, because Metrolink is as high as it can be, wherever the two lines cross, the DART tunnel must go under Metro.Generally, heavy rail lines like DART are built at a gradient of 3.33% or less, with many operators preferring to keep things below 2.5%, but the shallower you make these gradients, the faster and cheaper the service will be to operate. ( IÉ doesn’t publish their specs on this, but maybe someone here knows?) That limit is the biggest problem with putting a DART station near to the planned Metrolink line if you want to use cut/cover construction for the DART station.
The Tara Street option would be the most difficult interchange to construct, as it would need to be a very deep station (at least 35 m below surface), because it would be right next to the Metro structures which already extend down to around 30 metres depth here (parts of the Tara metro station themselves reach to 35 m below surface, although most of the main box sits approximately 26 m below surface):
At that depth, cut/cover in a tight streetscape really isn’t going to be any cheaper than mining.A direct-to-Pearse option is a little better in terms of depth, because the crossing point with the Metro tunnel would be at the end of Leinster St, south-east corner of Trinity, 350 m from Pearse, where the Metro tunnel itself drops to about 29 m below surface, so the DART tunnel would need to be at around 38 or even 40 m below surface. Climbing 20 metres in 350 is a 5% gradient - far in excess of the desired.
Running the tunnel via St Stephen’s Green puts that Metrolink/DART crossing point 750 m from Pearse. The metro tunnel isn’t as deep here, and there’s enough of a run to come back closer to the surface, but in practice, the streetscape above would almost guarantee that this needs to mined. The only way it wouldn’t is if the box itself was dug down under Merrion Square, and connecting pedestrian ramps were mined out to Pearse station (250 m) - and to SSG Metro (300 m). I suggested this in passing as a cheapo option for dropping St Stephen’s Green before, but it’s really a “least bad” solution rather than a better one: it’s inferior to having a shorter direct link at Pearse.
All that said, I would question the wisdom of dropping a station in the first place.
[Depth information is from the MetroLink alignment plans, here: Metrolink General Arrangement - Dublin City Council ; Tara Street box diagram from Metrolink Structures - Dublin City Council I have assumed DART Underground would be a single-bore tunnel; if it's twin-bore, take about 4 m from the required DART depths, but it doesn’t change much]Post edited by KrisW1001 on3 -
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One minor annoyance about the Merrion idea is that the ML station has its entrance turned away from Merrion Square, and the corner of the station building that faces that way has the services in it, but it the will was there it's nothing that couldn't be worked around.:
My figure for gradient comes from trawling the internet, and I would assume it’s for electrified heavy rail (passenger) in all conditions, but each railway appears to have their own limit. Freight railways, especially outside of Europe, prefer extremely shallow gradients (0.25%). My limited studies of mechanical engineering would tell me that EMUs can take higher gradients than Locomotive-hauled trains as they have more driven wheels, and as you say, rain or leaves don't fall on rails that are in tunnels. There are some places, however, where you just can’t get around the problem of too high a climb in too short a distance and a cut/cover station at Tara is one.It’s not so much the energy used as the effect on service speed - you can’t accelerate as fast up a steep slope as a shallow one. Where stations are fairly close together, that difference may be significant.
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