Jim Carter said:
I started on a tunnel today. Poured the footings for the walls and installed the walls with rebar in the cavaties. The decision on how to construct the roof hasn't been made yet [Any Suggestions?]. Arched and flat roof are the leaders. Whatever choice is made, it will be dirt and vine covered eventually.
If you're not putting a really heavy load on it, you could probably use 2x8x16 concrete capstones.
My tunnels are under mountains of rock, and some also must support pedestrians on the upper level of the layout. So I make simple, reinforced concrete slabs to cover the tunnels. Here’s how I make them:
On the floor of my garage (a nice level surface), I create a form made of concrete capstones. These are just laid on the floor, not cemented or anything. Over this, I place an unopened plastic trash bag, pressing it down into the form. The concrete won’t stick to the plastic bag.
Next, I toss in a few random pebbles of gravel. On top of them, I place a piece of 1/4" hardware cloth cut to the desired shape. The bits of gravel keep it raised up off the bottom of the form.
Then pour in the concrete, and carefully work it into the hardware cloth so that there won’t be any voids. On large slabs, or where extra strength is required, I’ll add a couple pieces of 3/8" rebar.
After the concrete hardens, simply move the capstones and lift out the slab.