The Wikipedia article states that the new tunnel (is really two tunnels) will have two single track side by side bores, with passageways connecting the two tunnels allowing trains to move from one tunnel to the other every thousand feet or so. Why two tunnels instead of one large double track bore?
Just a suggestion: a derailment or other calamity is less likely to shut the entire system down if the tracks are kept separate. It may also be less expensive to drill this way.
Isn’t the double bore configuration for extremely long modern rail tunnels pretty standard now? I know the Channel tunnel was built that way.
I can think of several advantages…if the tunnels are being built with Tunnel Boring Machines than smaller diameter (and thus cheaper to buy and operate) machines can be utilized. there are also the safety and redundancy advantages mentioned by another poster. And correct me if I’m mistaken but wouldn’t a larger diameter double track tunnel actually require the removal of more rock to contruct compared to two single track tunnels (given a circular tunnel bore)?
Bingo!
It’s a cost thing, just heard it from one of the engineers on the project. Apparently its a number of factors that add up to it being cheaper to bore two smaller tunnels verses one big one…
The 2 bores vs. 1 bore would cut the amount of rock removed by exactly half. But a smaller bore is also lot easier to line and support than a bigger one. Viewing the diameter of the tunnel as a span (the same as a bridge), the twice-as-larger diameter would need 4 times as much bending strength, if the unit load per foot of diameter is constant. Actually, that load isn’t constant, because the ‘prism’ of rock above the tunnel that needs to be supported is much, much larger for a wider tunnel - again, roughly 4 times as much. So you might be looking at as much as 16 times more support being needed !
With the crossovers between the two bores, more efficient/ productive methods like 1-way traffic in most of each bore can be implemented - i.e., loads of tunnel lining material (cement, steel, etc.) in on one track, loads out (broken rock) on the other.
- Paul North.
So far, in my humble opinion, all the answers so far are wrong. It has absolutely nothing to do with excavation, the amount of rock removal, or anything like that. It has to do with AIR and air circulation. It is impossible to force air into very long tunnels and have it reach the work face. The Discovery Channel has had several programs dealing with long tunnels such as the Chunnel and the Gottard Base Tunnel, and all have the same problem – air flow to the work face.
What is done is two tunnels drilled side-by-side with spaced connecting tubes between them. Air is pumped in one tunnel, it passes thru the farthest-in cross tube, and exists thru the parallel tunnel. The closer cross-tubes are sealed off to prevent an air short circuit. It’s is the only practical way to keep people from suffocating at the working face, and it is good in controlling the heat.
I think the soil conditions also played a part.
Here is an interesting article and a photo to illustrate Paul’s point.
A little geometry might help. A single bore of radius 2 (for two tracks) would remove 4 units of rubble. Two bores of radius 1 (1 track each) would remove 2 units of rubble (area proportional to the square of the radius). So two bores is half the amount of earth removed. That alone would be a big advantage.
I thiought there was also something related to the fact that one of the two tunnels can be shut off if needed, and passenger and staff evacuated to the other one.
Petitnj, that’s true… good thinking. So why then were tunnels with two tracks ever built in the past?.. CP’s Connaught Tunnel was one of the longest tunnels in its day yet was built to accommodate two tracks. It was single tracked in 1959 due to clearance issues brought on with modern taller freight cars. Connaught Tunnel was built when mechanized excavation was in its infancy… surely minimizing the need for removal of rubble would have been top of mind for those engineers.
As we look at the cross-section area of a tunnel and consider how much as to be excavated for a one-track tunnel and how much must be excavated for a two-track tunnel, it is easy to forget that comparatively little, if any additional vertical clearance is needed for two tracks than is needed for one track. When I read the first post on this thread, I thought, “of course, the total amount of rock to be removed is the reason for two bores.”
As Ulrich says, one hundred years ago the easiest way to bore the Connaught was the one that was used. And, it is well that they made the tunnel higher in the center than on the sides. Just think of what NS and CSX have had to do in making it possible to carry today’s loads that are higher than those of the times when various lines were constructed.
As an aside, how many of you have had the experience of passing a coal train going upgrade in N&W’s Elkhorn Tunnel? It is rather ear-splitting.
good point although in the case of the Gotthard project they did drill a number of vertical shafts down from the surface which connected with the horizontally advancing tunnel bores at regular intervals and were used to provide supplemental ventilation as the work advanced.
Obviously, however, that method would be much,much more difficult and expensive with a subsea project like the Channel tunnel…
PDN and others: Lets take a look at the various engineering constraints. First look at bore diameters ( radius for figuring spoil ).
1. What are going to be the clearance requirements? If double stack trains are going to be run then essentially US plate “H” will be needed?
2. What are the CAT clearance requirements?
3. In a single bore how fast would trains pass each other? Probably the faster passing speeds the further apart trains would have to be.
4. track bed depth requirements. Rail heights.
5. Higher probable single bore loads might require a thicker tunnel liner increasing bore diameter.
6. Using these numbers then calculate diameters needed. So would single bore be 1/2 diameter of twin bore ?.
tunnel boring
7. smaller diameter bores may proceed faster for spoil removal. Believe that 4 boring machines were used. Reduction of construction time very important as money spent is just running up interest charges until bores producing revenue.
Operational points – Experiences of other tunnels especially the Chunnel.
A
8. The very slight chance of loose lading striking opposite train eliminated in twin bores.
9. Fire damage repain can be completed without shutting down whole tunnel.
10. Maintenance on single track will not have to stop when another train passes.
11. Emergency ( will happe
Today, most (but not all) Tunnel Boring Machines (TBMs) are designed, built, and used to work as a large rotary cutter - similar to a rotary snow plow - with the diameter equal to the largest required clearance, either horizontal (2 x 1 side distance) or vertical. Happily, those distances are not much different, and provide the most depth in the middle, where the track is - and the most overhead clearance also in the middle, where double-stacks and the catenary would need it. To do the same with a 2-track bore would involve a lot of wasted excavation at the middle, both top and bottom.
Because the CP Connaught Tunnel was essentially hand-drilled, it was a lot easier for those guys to custom-cut the excavation to a non-circular shape, to minimize excess rock removal. Also, since steam locomotives were still used back then, providing enough cross-sectional area to allow for the smoke to escape would have been an important consideration, too.
On the ventilation aspect - that’s another good reason (but not the only one).
- Paul North.
Good incites, makes alot of sense.
I think in was in 2010 or 2011 that they ‘broke through’ for the first segment of the railway tunnel; There was a Forum Thread here, on that event.
Here is a link to a Company[AlpTransit Gotthard] Video ( You Tube) linked @ https://www.youtube.com/watch?v=3qI1xQX7Cg8
Also to the Company site of AlpTransitGottard which has lots of pictures, it does contain quite a bit of History, and the process that enabled the start of the Gotthard/Cenari Tunneling Project.
Linked @ http://www.alptransit.ch/en/home.html
And if interested search the You Tube site for 'Gotthard Base Tunnel"- Plenty of related videos, more if you are able to understand or read German.
I’m planning to go there next year for the grand opening. I’ve never witnessed anything earth shattering great in person… this will be right up there with the opening of the Suez Canal, and the Empire State Building.
Referring to the Connaught Tunnel, while it was indeed double track the track centers were only 13’-0. Modern practice is for greater separation for a number of reasons.
The most effective mean to support a span is some form of arch (ranging from circular to an A-frame). You may think a mountain is solidly immoveable but there is in fact a lot of pressure from the weight above when you create a void.
A TBM naturally gives a circular outline for the roof. When a tunnel is excavated by drill & blast, a shallow irregular arch is the usual cross-section. Any lining also smooths out this general shape. The wider the tunnel, the taller the arch becomes and that extra height means extra excavation. There may be some tunnels wide enough for three tracks but they will be the exception.
I haven’t done any calculation, but given the relatively close track spacing in the Connaught tunnel, quite probably putting both tracks in one bore minimized the amount of waste to be removed. Incidentally, there were small pilot bores dug beside the main tunnel to aid construction. They did not go the full length and saw no further use.
And you indeed have lots of valid comments from earlier posters.
John
Thanks for posting this photo ! It illustrates the effect of the different diameters for 1 bore vs. 2 bores far better than my description, especially at the wall at the far end.
Very interesting caption for this photo in the linked article:
“Wye Junction - Wye Junction is in the western tube of the Gotthard Base Tunnel. The right tunnel is the main western bore, while the left tunnel is a connecting spur into the main eastern bore.”
- Paul North.