Tunnel-building at Winter Creek
Many people have asked how I built the tunnel. This feature more than any other
seems to attract non-enthusiasts like no other. So here's an overview:
The Design
The tunnel is built on a 13m radius curve on a 1in40 downgrade. This limited the
choice of construction methods. I had really wanted to build a concrete tunnel
to the proper curve profile, but after lengthy investigations with local
concrete experts I deemed it too hard/expensive. Ideally I would have build a
short former and moved it progressively, but this would have meant many trips
with a concrete truck and pump - not cheap. The alternative was to build the
former for the whole length of the tunnel and pour in one go. Thats when I
decided that if I had to build a wooden tunnel former anyway - it might as well
be a timber lined tunnel.
Digging
The first step was to dig out the trench: Up until this point I had done all
the digging by hand - great exercise, but a bit much in this case as the trench
is over 3m (10ft) deep in places - (check out photo three: thats a 6ft step
ladder on top of the tunnel!) This was before the 2.5 tonne Cat digger arrived - so a local
contractor was contacted.
The Local Digger operator was intrigued by the project - turns out he is an HO
modeller and relished the opportunity to do some scenery making on a large
scale. Ended up moving a couple of hundred m2 of soil
(covered by a resource consent). This was placed on the other half of the
horseshoe curve to build the ground up by about 1m. Becuase the slope is quite
steep just below this point the new slope merges
with the top of the existing slope and results in the feel of an impressively high
embankment. The trench was dug out on an extra 300-500mm or so to allow for
drainage works.
Some Novaflow and heaps of scoria rock brought the level back up, and have meant
no drainage problems whatsoever. I hand-trimmed the walls of the trench to be
exactly the width of the frames + the cladding + 10mm or so.
Framing
I studied engineering practice for retaining walls and upped the specs a bit
for the actual tunnel. The frames are 125mm (5") square house piles put together
in a series of rectangles. The idea is to transfer the force from top to bottom
and one side to another. The frames have an inside measurement of 1500mm x
1000mm (60x39 inches).
I asked Dave Giles what measurements would suit his big
shays. The Manukau club's tunnel is 900mm wide and Dave said it just wasn't
enough clearance for him to get out/off the locomotive and suggested the 1m.
If you look closely at photo 4 you will see the vertical members of the frames
are let into the bottom and top members. I felt this would add strength in what
is essentially a compressive force situation and this has proved correct (more
on this later). The frames are held together with gang-nails and large steel
pins, placed so they
are not visible from within the finished tunnel.
.
Frames are spaced at 900mm centres. The photo at the right was taken when I was
busy getting them aligned both with the curve and the gradient.
I used an awful lot of 200 x 50 (8"x2"), H4 retaining wall timber. Each length
was cut to match a specific frame to frame measurement and rammed home. Most
needed me standing on them and hitting with a heavy post rammer to force them between
the dirt wall and the frames. Because they all have random overlapping joints
and are under tension with the curve and rammed between compacted clay & frames
- the structure was very solid at this point!
Once the basic box was complete, the portals
were constructed, the top and exposes sides covered with heavy-duty polythene
sheet, and novaflow/scoria placed along the tops of the sides where the banks
had been sloped back (for safety during construction - I didn't fancy being in a
straight sided trench 3m deep)
I left the project at this point for the
winter to check all the drainage etc.
The fill was a few loads of free fill obtained
from a local contractor. It was dumped into the trench from one side and stayed
that way for 3-4months. THIS WAS A MISTAKE! The fill put slow but steady
pressure on the tunnel box and it moved out of square on 4-5
frames. The scoria placed each side happily resettled and there was no way
short of a bomb that the frame could be coaxed back to plumb. So I am stuck with
it. The fill has been properly distributed for about a 3 years now and the
tunnel has not moved in any direction.
The final photo shows the 2.5 tonne
digger used to fill in over the tunnel. I admit to some trepidation while
driving it over the tunnel for the first time!
All and all it was a really interesting
project, and I am glad I attempted it and not leave it as a cutting. I have a
deepish cutting further down the line that was dug last summer and I must admit
to measuring it up for a second tunnel....oh-well some people never learn!
