Boring new tunnels vs widening existing tunnels - costs?

[ijmad]

I've been wondering about Crossrail/CR2 and thinking to myself, if it were possible, could it be more cost effective to widen an existing Underground deep tube tunnel to take a larger train?

I know there are massive impracticalities here from the point of view of keeping existing services running, and there may not even be useful routes, but there might be a few places on the tube where it'd be possible (e.g. re-using some of the W&C's alignment for something else, don't shoot me, it's just a thought experiment!).

But would it actually save money? I'd assume all the existing tunnel rings would need to be removed before a tunnel boring machine came through behind, which might require extra machinery and pose additional risks.

On the other hand I'd imagine you'd save a whole lot of money not having to worry as much about whether the alignment is clear, moving other tunnels/utilities, staying on course, and overall there'd be less sediment to remove (although still some). Deep tube tunnels seem to be about 3.5-3.6m in diameter whereas Crossrail 1's were 6.2m, meaning the tunnel cross-section is about 3x the overall size.

Are there any (non-historical) projects where tunnels have been widened in this fashion?

 

[edwin_m]

I'd guess they would go for a new one every time.

Removing the rings will be difficult when they have been bolted together for 100+ years, and leaves the tunnel prone to collapse. Many tunnels are also grouted to fill any voids outside the rings, which will have set solid and could impede both the removal of the rings and the work of the TBM. If it is a tube tunnel there is also another one nearby for the other direction, and even if the enlarged tunnels don't physically overlap the earth in between might no longer be enough to ensure stability. And when you get to a station there are all sorts of other tunnels nearby to contend with. The only recent example of enlarging a bore I can think of was Farnworth, which didn't go exactly to plan and was only necessary because the portals had to stay where they were. There have been collapses during historic tunnel enlargements such as Penmanshiel in the 70s and on the rebuilding of the original City and South London line to take standard size Tube stock.

Also almost all existing Tube tunnels are in use. Crossrail will provide an express service on a similar route rather than replicating all the existing stations, which would lose service if their tunnels were taken over.

I think at the sort of depth we are talking about the positions of all existing tunnels would be pretty much understood - the maze of utilities isn't always well mapped but will be much shallower than a Tube tunnel. There might be a risk of old mine workings at depth in some places, but London has never been a mining area.

 

[Ken H]

LT did that in a few cases. they enlarged the Central Line tunnels by inserting packing between the lining segments. That allowed them to run the same trains as the yerkes tubes. Think something was done to old C&SLR(*) tunnels
Also when doing the new step plate juctions($) for the Victoria line, they excavated round the old tunnels by hand and put in the new larger linings in while the old track and lining was supported on trestles. Then, the old lining was removed, and at the end the track was lowered into its new place. Think they did all that at night when the trains were not running.
Think something similar was done north of Euston for the complicated Northern Line junction
See this image for the Camden Juction - you can see step plate junctions clearly

*City and South london Railway. Became City branch of northern line
A tube tunnel where the segments get bigger and bigger. Used for turnouts

 

[Ken H]

step plate discussion here - https://www.railforums.co.uk/threads/step-plate-junction.176295/

 

[edwin_m]

LT did that in a few cases. they enlarged the Central Line tunnels by inserting packing between the lining segments. That allowed them to run the same trains as the yerkes tubes. Think something was done to old C&SLR(*) tunnels

http://www.railwaysarchive.co.uk/documents/MoT_Borough1923.pdf
The above is the link to the collapse that happened during the enlargement of the C&SLR tunnel, also including a description of how the work was done.

At each place, small gangs of men, working either by shield or by hand, removed the existing cast iron segmental lining ring by ring, excavated the surrounding clay and substituted the new lining to form the enlarged tunnel. … for the convenience of the public a four-minute service of trains was being maintained during the day-time.

I don't recall ever hearing of anything similar being done on the Central.

 

[Ken H]

http://www.railwaysarchive.co.uk/documents/MoT_Borough1923.pdf
The above is the link to the collapse that happened during the enlargement of the C&SLR tunnel, also including a description of how the work was done.


I don't recall ever hearing of anything similar being done on the Central.

the central wasnt built to the same diameter as the yerkes tubes. They had to enlarge the tunnels to take 'standard stock' during the 1935-40 new works programme, which was done to relieve unemployment.
The outside electric rail was set at a higher level than other lines because of the tight tunnels. Dont know if thats still true.

 

[AM9]

the central wasnt built to the same diameter as the yerkes tubes. They had to enlarge the tunnels to take 'standard stock' during the 1935-40 new works programme, which was done to relieve unemployment.

I think that several of the stations on the original section of the Central line have had their platforms extended. This has involved building a larger tunnel outside the running tunnel and then inserting a step plate. The extensions can be seen where the platform dips down at the entry end. That is because the rise to the platforms to assist with braking is not as steep as the falling gradient at the departing ends where the gradient is rather more to give departing trains as boost when starting.

The outside electric rail was set at a higher level than other lines because of the tight tunnels. Dont know if thats still true.

Yes I believe that it is still true, but the adjustment is a simple operation at a depot.

 

[Tio Terry]

Bearing in mind that any new tunnel, or major works to an existing tunnel, requires the provision of a side walkway to allow emergency evacuation of all passengers, including PRM, then I think it would be new every time.

 

[ijmad]

Thanks for all your thoughts folks. I think you are all probably on the money that it would be impractical or cost prohibitive (possibly more costly than brand new!) in practice.

 

[Ken H]

I think that several of the stations on the original section of the Central line have had their platforms extended. This has involved building a larger tunnel outside the running tunnel and then inserting a step plate. The extensions can be seen where the platform dips down at the entry end. That is because the rise to the platforms to assist with braking is not as steep as the falling gradient at the departing ends where the gradient is rather more to give departing trains as boost when starting.

Yes I believe that it is still true, but the adjustment is a simple operation at a depot.

There are 2 ways of using gradients for helping with braking and accelerating. Saw tooth where you have a shallow gradient down and a steep one up, and hump, where you have a steep down, a level bit and a steep up. As Cl309 says, this complicated platform extensions.

 

[yoyothehobo]

Also consider that London Clay is probably one of the easiest ground materials to tunnel through in the world

 

[higthomas]

Note that they recently did something along these lines at the Farnworth tunnel near Preston.

Murphy had to fill the up-tunnel with 7,500m3 of foam concrete before re-boring it using the UK’s biggest tunnel boring machine (TBM), Fillie, to create a bigger tunnel housing two tracks with room for overhead electric cables.

(https://www.murphygroup.com/projects/farnworth-tunnel)

Sounds like it was easier to bore a new tunnel (well an old tunnel filled with concrete) than widen an existing one.

 

[Ken H]

Also consider that London Clay is probably one of the easiest ground materials to tunnel through in the world

True dat.

 

[edwin_m]

Note that they recently did something along these lines at the Farnworth tunnel near Preston.


(https://www.murphygroup.com/projects/farnworth-tunnel)

Sounds like it was easier to bore a new tunnel (well an old tunnel filled with concrete) than widen an existing one.

As I posted further back, the portals of the new tunnel had to be pretty much in the same place as the old ones, otherwise the cutting at both ends would have had to be widened. As the tunnel isn't very long most or all of its length would have to overlap the existing bore, so a solution had to be found to make that work. Without the constraint of having to keep the track roughly in the same place, I expect they would have built a brand new tunnel some distance off the line of the old one.

 

[Bald Rick]

Other things to consider.

The line would have to be closed, and track signalling etc removed before work takes place. Note that the C&SLR did this when they enlarged north of Moorgate, nearly a century ago. South of Moorgate they tried to keep trains running, but gave that up part way through (as mentioned above). The tunnel enlargement was by about 2’4”.

The only cost saving would be spoil removal. Everything else would be the at least same: TBMs, segments, staffing, grouting, etc. Then there would be extra costs - design would be more complicated; interfaces with the existing tunnel (keeping the pressure up in water bearing ground, for example, removing the existing lining and grout, removing the rail systems kit, and not least the cost of not running a service for a couple of years (at least).

Stations would be quite a challenge. Bigger trains means more passengers, that means bigger platforms, more and wider passageways, more escalators, bigger ticket halls, more entrance / exits at street level, more emergency acess. All to be built around an existing station, many of which would still be open for traffic on other lines.

Finally, many existing tunnels are VERY close to other ‘things’ underground. It might sound easy to dig an extra metre or two around an existing tunnel but there are going to be other things in the way, particularly building foundations and other tunnels.

Overall, it would be more difficult, more expensive and most importantly much more disruptive to passengers.