Tunnelling

[Peaky 556]

Supposing I were to construct a long tunnel through my front garden, to negotiate the natural rising landscape.  Why is another matter.  Of course the 'cut and shut' method would be used.  I thought of casting in situ in a dug trench a 'U' section concrete channel to take a twin track, with enough height for the rail, ballast and stock.  This 'U' section channel would have a bit of steel reinforcement cast in, and have a wall thickness of around 2".  Internal dims would be as necessary for a twin track, eg roughly 350 wide x 150 high.  Curves would be included.  Closure would be by cut pieces of concrete paving slab, and then backfill with earth.  For access on a day-to-day basis, every six feet there would need to be a small inspection chamber/access cover, such as the pavement rectangular covers for cables about 350 x 180.  A constant slope is required for the base of the tunnel to drain away. 
The gritty bit is how to restrain the trackwork.  The key requirements are 1. to maintain the track spacing, and 2. to maintain a set distance away from the tunnel walls.  That way no collisions should occur.  Another key need is to keep the rail joints smooth to avoid stock jumping and derailing.  My suggestion for the first two is to provide a single very long sleeper every 8th or 10th sleeper to control the horizontal positioning.  Rail joints can probably be fairly close using plastic fishplates, as temperature changes will be small.  What about vertical control though?  Ideally I could use fine ballast (like fish tank grit) and just let the track bed into this with self weight and that of the trains.  I have a horrible foreboding though that the track would lift where not wanted and risk roof collisions, and this would be largely unseen and inaccessible.  Another way would be to cast in a piece of hardwood into the base of the trough every couple of feet, and screw the track down.
Thoughts welcome.
Cheers, Tim

[hornbeam]

Well I had a tunnel on my old 45mm gauge narrow gauge layout. It was single track, on a curve and about 3 meters long. The floor was cast concrete, sides were blocks and roof slabs which supported the path (also slabs) above it. I ran 1'' by ½'' timber on each side of the track and cast this in and screwed the track down. The timber was raised above the floor so there was a gap under the track. Reason for this was it gave things like small leaves and twigs some ware to go other than causing a derailment- its surprising how much gets blown into a tunnel! I also had a broom that I cut the head down on. This then fitted into the tunnel, and using a long pole meant I could sweep it out from both ends to clear it. Have a feeling that upset the spiders!

It was in use for three years and never had a derailment- was a near miss with a frog once!

[454]

Tim

Tunnel tips

1) Make a section of track on a sub base (not literally but i.e. a plank) & slide it into the tunnel, connect at either end with rest of track. Can then be slid out for maintenance.

2) Never have a tunnel that is greater than 2 arm lengths. To reach centre of tunnel from each end.

3) Have the tunnel much much more commodious in dimensions than the loading gauge prototypically would dictate.

4) Have the pretty tunnel mouths easily detachable at each portal.

5) Position an inspection hatch in a position where one can access in a dignified manner.

6) Provide a close fitting door at each end as closure for when the layout is not being used.

7) Consider drainage.

Survey well, put it in the correct place. Once it's dug it's dug. Re-siting is not an option.

9) Try not to have too much of a gradient in case a loco stalls & comes to a grinding halt in the murky depths.

10) Don't use ballast.

I am sure there are more. These work for me  under my garden shed.

Dave
454

[Peaky 556]

Thanks both, all good experience speaking.  I take it that your track was also screwed down Dave, to the insert.  Despite ballast seeming to be the 'right' starting point, I'm with you both on avoiding it and having a more positive hold-down.  Avoiding it would also allow hosing out the tunnel after severe flooding or landslip conditions!
I like the comment about dignified access; I'll remember that if I'm ever tempted to wear a mini-skirt at a GTG! 
I also like the idea of concrete block sides, as a cheap and robust system, allowing good access for first trowelling the concrete track base level , and making it wide enough to then mortar the blocks onto.  It also gives a commodious height of around 250mm.
Regards, Tim

[454]

Track pinned to sub base on sleeper ends only. Not on sleepers between rails.
Have not screwed track anywhere now. I do not recommend it.

Dave
454

[Moonraker]

There is another tunnelling tip if you live in Australia....before putting your arm in, at the start of a running session, run a train through to shift any poisonous snakes.

And I am not kidding. Only two weeks ago I was working at my bench in my shed and needed to get some sandpaper from the odds and ends box on the work bench. There was a black snake in it. Fortunately I put my hand in at its tail end and noticed it in time.

Regards
Peter Lucas
MyLocoSound

[Peaky 556]

Well I'm rather horrified to see that my first post on the subject of this tunnel was seven years ago!
I am, at very last, building the approach trackbed for this long curved tunnel, and the matter of levels is foremost in my mind.  In the interests of keeping a long tunnel dry, in other words stopping the inwash of dirt, leaves and sludge, I'm suggesting a slight gradient at each end with the high point about midway along.  As my approach track work has an incline of about 1:100, would there be anything wrong with trying to replicate this gradient through to the mid point of the tunnel?
As a linked question, rather than have a literal peak between rising and falling gradients, will it be kinder to stock (and drivers) to have a short level stretch mid-tunnel?
All views on the matter welcomed.
Many thanks,
Tim

[cabbage]

Tim,
I would give it a flat floor and drainage runnels either side of the track or a slight slope with an even cross section.

1:70 is normally considered to be the drainage gradient for water - but would be rather steep for a steam powered loco...

Regards

Ralph

[Chris_P]

Your speeds will be less but I remember a talk about TGV routes where the presenter stressed the need for careful transition curves in the vertical as well as horizontal planes as their theoretical design work showed serious dangers from wheel unloading if an uphill became downhill too quickly!!    There seems to have been much previous good advice but I didn't notice anybody mentioning the weight of the emergency access hatch. Having seen narrow gauge garden modellers struggle to lift big heavy slabs, and as none of us get any younger, careful design might be expedient.

[Peaky 556]

Ralph,
There shouldn't be any ingress of water part way through the tunnel, so my suggested gradient of 1:100 is mainly to discourage water entering at either end.  I think I need something like that as my civils skills and equipment are not up to guaranteeing a flat and level tunnel floor.  I predict I'll be more successful in engineering a real value of slope (plus or minus quite a lot), rather than the likely penalty of trying for 'level' and getting it wrong!  Slight slope throughout is not practical as the two ends need to be at the same level.  So I think it boils down to what the gradients should be, and for how far in from the tunnel mouths.

Chris,
Inspection hatches will be of the commercial pressed tin variety, no need for vehicle drive-over capability, so they will be fairly light.

Thanks, Tim

[AshleyW]

 the only foreseeable risk is, live steam loco working uphill with axlepump on, gets boiler level high going into tunnel, on falling the boiler water gets picked up by regulator and loco hydraulics for a short distance, which usually results in a quick burst of speed. if going onto straight, it'll probably work itself out , but if hitting a curve may risk tipping? although given your large radius curves doubt even that'd cause a problem

[Peaky 556]

Ash, I was thinking of putting a bit of camber on the curve, nothing too precisely calculated, just to 'look about right'.  Ought to help a bit...

[Peaky 556]

I have at last plucked up the courage to start digging the 10-yard tunnel.  Here is the start of removing topsoil:


The sharp-eyed might just see the end of the laid track, through the tunnel portal at the far side.  Although it looks like a sharp transition, there is ostensibly (my translation being 'very approximately') a 5m radius lead-in to 4.5m minimum radius around the inside track of the curve.  This overly severe 'tightness' results from the lie of the land, and certain prized shrubs.  I may have to sacrifice one or two Lleylandii from the hedge, but then again who would ever think of those as 'prized'!
Thinking of those poor, but hard drinking and cheerful 'Navvies' that built our railway network, I can employ our scale factor to work out how many of "me" would be working on the job.  Taking the cube of 22.6 suggests that the full size me is the equivalent of over 11000 little Navvies.  How long would that have taken to build this as a cut'n'shut job of only 237 yards?  I don't know but I only have 2 weeks.  Oooo-errr...

[Peaky 556]

A bit more digging out.  I am now in tree root territory, giving much more of a challenge!



The trench is deeper than expected, being nearly two feet now and growing as the piled up earth in the hen run is slowly traversed.  After a mere six feet of dug trench I have accumulated 26 bags of subsoil.  Little or none of this will go back, so I have commenced the spoil runs to the local recycling tip.  Fortunately they welcome "green" waste, as otherwise I would have no idea where to dispose of this displaced spoil.
Chainsaw tomorrow, to despatch those tree roots.  Anyone have advice to prevent it growing again with possibly disastrous results?

[Peaky 556]

Day 5 has seen me continuing the trench around the curve, and have now dug about a 16' length and corrected width and level of the base.  It's quite a wide trench, as I'm adopting a track spacing of 190 centres and an internal width of 430mm, trying to give comfortable clearances between passing trains and to tunnel walls.  The side walls are to be standard concrete building blocks; I would have liked to use 75mm thick ones, but could not find a stockist.  The resulting 100mm thick walls means the trench has to be at least 630mm wide, plus a bit for ease of laying the blocks.
This must explain why the volume of subsoil has been colossal; so far amounting to 87 bags!
We are blessed with dry weather in the midlands, so I will continue and make the most of it.