How to build a Large Tunnel by Duncan Thomson

After some careful consideration, I decided to make a fairly permanent roadbed inside of what will be a tunnel. I cut a trench about 2.5 inches deep, and set some 1/8" X 2" Masonite as a form. Held in place with some mini-stakes, and set to the 1.5% grade that needs to run up/down the hill. The roadbed is "6" wide.

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What you need Levels (carpenter's and torpedo or line level) Cement  &  Mortar (number of bags determined by length of tunnel) Extra aggregate (small or fine gravel) Mixer, or wheelbarrow, or mixing tub Flexible forming (1/8" masonite works well) Small stakes (1/2" X  3/16" X 6") 3" squares of cedar (one for every 1.5 feet of roadbed) 1/4" square welded wire screen, or plain old chicken wire Finishing trowel or wood float The materials for the rest of the tunnel will be on following pages...

Then I cut up some 1/4" welded wire screen to use as remesh

Next, I poured a 2 bag concrete/1 bag mortar mix into the mixer (plus about 4 gallons of 3/8" decomposed granite for extra aggregate), pouring the first half depth, then placing the screen in, and finishing the pour to the top of the forms. One thing to note was that the bagged Redi-Mix that I used had very small aggregate, as opposed to that which comes in trucks. Nothing much bigger than 5/8".I also added about four gallons of 3/8" crushed granite to the three bag (2 concrete/1 mortar) mix, to reduce the amount of "fines", and provide a bit more "structure" to the mud. After the pour was in, and the initial floating was done, I sank some 3" square pieces of cedar fence planking into the concrete, to give me someplace to secure the track inside the tunnel. I don't want any shift of trackwork inside the tunnel. The following pictures show the blocks in place, after the final troweling, curing, and wire brushing.

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Heat Expansion The Rail Line is located in the arid Southwest near Phoenix, Arizona. Track expansion and contraction shouldn't be a big issue where this tunnel is located. While the track leading in and out of the tunnel itself will be free floating, and exposed to significant temperature changes, the thermal delta disipation should be fairly rapid once the rail is "subterrainian". There is plenty of area available for the exposed rail to move around, so I think it will be okay with the securing inside. Also each section of rail that is in the tunnel will be secured with Split Jaw clamps, to further eliminate any misalignment issues.