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.
Carl
Railroader Emeritus (practiced railroading for 46 years--and in 2010 I finally got it right!)
CAACSCOCOM--I don't want to behave improperly, so I just won't behave at all. (SM)
Ulrich 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?
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)?
"I Often Dream of Trains"-From the Album of the Same Name by Robyn Hitchcock
carnej1And 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.
http://www.popsci.com/technology/article/2010-10/after-14-years-worlds-longest-tunnel-breaks-through-swiss-alps
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.
Johnny
GN_Fan 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.
Obviously, however, that method would be much,much more difficult and expensive with a subsea project like the Channel tunnel...
Paul_D_North_Jr 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. Paul North.
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. Paul North.
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 happen ) evacuations, protection of crew and passengers much quicker and safer.
12. Fire protection ventilation can be more reliable. Note problems of WMATA.
All the other points in this thread are also important.
Ulrich 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.
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).
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
rdamon I think the soil conditions also played a part. Here is an interesting article and a photo to illustrate Paul’s point. http://www.popsci.com/technology/article/2010-10/after-14-years-worlds-longest-tunnel-breaks-through-swiss-alps
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."
The solution has been used ever since the Hoosac Tunnel was drilled in the late 1800's. It's simply . . . compressed air. It can be pumped a long distance in a pipe much smaller than the bore, which is easy to arrange during the construction phase. It can be used to both power equipment (such as rock drills), and to ventilate the working face with fresh air, right at the point of need. That can come either from the exhaust discharge from the tools, or a dedicated vent for the workers, both of which create 'positive pressure' at the working face. That in turn causes the stale air to naturally flow out of the partially completed bore to the outside (distance and resistance to flow affects which opening, etc.).
Further, when compressed air is discharged to a normal pressure environment (uncompressed), it cools considerably from the expansion (remember Boyle's Law in chemistry or physics: PV = nRT, so T = PV/nR), the same as when you bleed air from a bicyle or car tire. So that solves that issue, too.
As Paul North says, during construction ventilation at the working face is fairly straightforward. Just standard mining technology. The high pressure air that is brought to the face has only one way to go and that is back to the entrance.
I think what you are talking about are the ventilation issues once the tunnel is complete. Now you are trying to force a long column of air in a very large bore to start moving. Instead of an air compressor and high pressure, now all that is available are large fans. I understand that it is feasible up to about 5 miles; after that the weight and inertia of the air makes it increasingly problematic. That is why ventilation in CPR's 9 mile Mount MacDonald Tunnel is split, with a ventilation shaft near the mid-point. Fans are located at the east portal and at the bottom of the 1/4 mile deep ventilation shaft, with nearby doors (that open for trains) in the tunnel to force the air to flow in the desired direction.
It is possible the producers for the Discovery Channel got confused, or perhaps it was not clearly described in the program. TV documentaries can be fascinating to watch but do not necessarily tell a complete story or interpret technical details 100% correctly.
Ventilation during normal train operations in tunnels is 'critically necessary' with steam locomotives, and just 'necessary' with diesels - note the constraints on spacing/ timing trains to be about 25 mins apart through the Cascade and Moffatt Tunnels, etc.
But the need for ventilation is probably 'minor' with electrics - which is what the Swiss and Italians will likely have running through this tunnel. Hence another reason that two bores are not absolutely needed. (The Mount MacDonald tunnel is a single bore, correct ? And yet it can run with diesels.) Note that ventilation shafts are not practical for long underwater tunnels . . .
The added resistance from the 'piston-in-a-tube' effect of the displaced air pressure wave at high speeds is more challenging, and a second bore would surely help solve that, as noted above. But the same relief can be obtained by making a single bore just a foot or two larger all the way around to provide an escape path for the trapped air.
instead, the real reason for the 2nd bore is the anticipated traffic levels. First, with a single bore, no train could proceed until all opposing trains have arrived at the near end (unless there are passing sidings in the middle). That would make the 'cycle time' between opposing trains very long.
Second, if the tunnel is important enough to construct, that means a considerable amount of traffic is anticipated. As with electrification, advanced signals such as CTC (and some other upgrades), by the time it's actually needed and can be paid for out of the traffic revenues, the line would have a traffic volume that would necessitate 2 tracks anyway.
Two single bores were mandated by the Swiss BAV (Federal Office for Transport), like the USDOT and FRA combined. They did not want the whole railway line shut down by fires like those experienced by the Channel Tunnel or the Road Tunnel at Mount Frejus. The photos show the location where one of the double crossovers will be located. The tunnel has three outside access points besides the portals, two were drilled from the side into the location, one just south of Erstfeld, and one just north of Bodio. In addition a vertical shaft was sunk from Sedrun to a point near the center of the planned tunnel. Four TBMs were used. In additon there was a section of each bore where the TBMs could not be used due to the type of rock in that area. This covered a distance of several kilometers north from the point where the Sedrun shaft intersected the tunnel. This portion had to be excavated by drilling and blasting followed by mucking out the spoil. The bores were excavated by TBMs starting from each portal with the spoil moving back out behind them, some of the spoil rock was recycled to be used in the concrete of the tunnel reinforcing sections and some for the poured concrete floor (slab track). The rest was transported by regular rail to planned disposal sites. Once the TBMs reached the location where the side galleries intersected the tunnel the concrete plants were moved and the rock and material flows were diverted to the side galleries rather than via the portals. In the mean time the drilling and blasting was progressing north from the vertical shaft intersection with the rock being hauled to the surface via a skip like in a mine. That rock not reused was hauled away via the Matterhorn Gotthard Bahn, a Cog railway which is part of the route of the famous Glacier Express, to another disposal site. At its deepest point the tunnel sits under 800 meters of mountain(1.3 miles deep).
Paul_D_North_Jr Ventilation during normal train operations in tunnels is 'critically necessary' with steam locomotives, and just 'necessary' with diesels - note the constraints on spacing/ timing trains to be about 25 mins apart through the Cascade and Moffatt Tunnels, etc. But the need for ventilation is probably 'minor' with electrics - which is what the Swiss and Italians will likely have running through this tunnel. Hence another reason that two bores are not absolutely needed. (The Mount MacDonald tunnel is a single bore, correct ? And yet it can run with diesels.) Note that ventilation shafts are not practical for long underwater tunnels . . .
Actually it is vital for passengers even with electric motive power, ambient air temperature in the tunnel is 42C.
The added resistance from the 'piston-in-a-tube' effect of the displaced air pressure wave at high speeds is more challenging, and a second bore would surely help solve that, as noted above. But the same relief can be obtained by making a single bore just a foot or two larger all the way around to provide an escape path for the trapped air. instead, the real reason for the 2nd bore is the anticipated traffic levels. First, with a single bore, no train could proceed until all opposing trains have arrived at the near end (unless there are passing sidings in the middle). That would make the 'cycle time' between opposing trains very long. Second, if the tunnel is important enough to construct, that means a considerable amount of traffic is anticipated. As with electrification, advanced signals such as CTC (and some other upgrades), by the time it's actually needed and can be paid for out of the traffic revenues, the line would have a traffic volume that would necessitate 2 tracks anyway. - Paul North.
Yes, traffic levels will require two tracks right from the start. The Swiss built the Lötschberg Base Tunnel with 1/3 2-track and 2/3 single track, and it couldn't handle all the traffic after less than one year from completion. But the Swiss were smart about the financing of both Base tunnels(not surprising really). They placed a tax on all trucks and buses crossing the Alps in Switzerland weighing more than 3.5 metric tonnes. The trains using the tunnel will only pay for maintainance and services like dispatching and electric power. The two tunnels are already substantially paid for, and very little bonded debt was required as the tax was imposed several years before construction commenced.
beaulieu: In your post 2 above, did you mean 1,800 meters instead of 800 ? (to get to 1.3 miles)
Let's see: 42 deg. Centigrade = 42 x 1.8 = 75.6 + 32 = 108 deg. Fahrenheit.
Why is it so hot ? Is that 'natural', or the result of train friction, traction motor heat disspation, etc., or a combination of the those ?
I've heard of deep African diamond and gold mines getting that hot, but they're like 5,000 ft. deep, and mostly closed-end shafts, etc. Is there really that much geothermal heat being radiated out at that depth ? [Inquiring minds want to know, not least because my house is heated by 2 geothermal wells that are only 250 ft. deep with a 3-ton capacity (36,000 BTU/ hr.) ground source ("geothermal") heat pump. We're doing just fine at 69 deg. F inside while it's been as cold as -3 deg. F in the past few days. (House is 3,200 sq. ft. of main living space, plus a lot of inside basement and attic area, and is also superinsulated by most standards: R-24 in the walls, R-48 in the roof. But I keep having to explain to people that the heat doesn't come from the sun warming the Earth's earth; instead it's the result of radioactive decay in the molten magma core deep inside, radiating out to the surface of the Earth. But I didn't think it was that strong . . . )
Paul. If memory serves some heat comes from Aluminum 27 ( half life ~725,000 years so should be depleted except for nature's newly made ) and Potassium 40 ( half life ~ 1.2 billion years providing ~ 25% of earth's interior heat now )
beaulieu Yes, traffic levels will require two tracks right from the start. The Swiss built the Lötschberg Base Tunnel with 1/3 2-track and 2/3 single track, and it couldn't handle all the traffic after less than one year from completion. But the Swiss were smart about the financing of both Base tunnels(not surprising really). They placed a tax on all trucks and buses crossing the Alps in Switzerland weighing more than 3.5 metric tonnes. The trains using the tunnel will only pay for maintainance and services like dispatching and electric power. The two tunnels are already substantially paid for, and very little bonded debt was required as the tax was imposed several years before construction commenced.
(*Very hard to quickly find any kind of overall length dimension, even an approximate one.)
Quite true, Paul--but shouldn't you be in bed; it's after ten o'clock, Eastern Time (I just finished my evening meal)?
Paul_D_North_Jr beaulieu: In your post 2 above, did you mean 1,800 meters instead of 800 ? (to get to 1.3 miles) - Paul North.
Sorry, my mistake, the deepest point is 2,500 meters which equals 8202 ft or 1.55 miles. The vertical shaft at Sedrun is located in a valley so is much shorter at only 800 meters.
BTW bore diameter of the finished tunnels are 29 ft. Horizontally and 31.4 ft. Vertically. The greater vertical distance is to allow for Cantenary. The bores must have been cylindrical after the TBMs were done. Also there were two zones were drilling and blasting had to be done, the second was north from the Faido side access point
I assume the heat is due to the depth. Like the other long tunnels in Switzerland they are cooled by cold alpine water which is then used for irrigation and heating in Greenhouse complexes. Waste not, want not. The sale of the warm water helps cover costs.
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