A concrete tunnel is one way to connect two sections of your property. By going underground, you can avoid any inclement weather and reach the connecting area safely. It is also an effective way to run utility and communication lines without blighting the area with utility poles. The size of the tunnel depends on your needs and can range from a crawl space to something you can drive a truck through.
Mark the path of your tunnel on the surface. Be sure to avoid any major obstructions or buildings. Also, if there are any trees or utility poles in the path, you may want to reconsider. While it is possible to burrow a tunnel without breaking the surface, it is dangerous and complicated. Be sure to consult the local utility companies in this process to avoid intersecting any utility corridors.
Dig the corridor. Using a backhoe or other industrial digger will save time, but it can be done by hand. If the strata through which you are digging is hard rock, you will need to use a reciprocating hammer attachment on the digger. Dig the corridor a minimum of 1 1/2-feet wider than the desired width of the tunnel and a minimum of 3 feet deeper than the maximum depth.
Lay the French drains (a length of wide gauge PVC with holes drilled through) the full length of the corridor. If you are making a tunnel wide enough for human traffic, there should be three drains, one on each edge and one in the middle. Make sure that these are below the uniform surface of the bottom of the corridor and resting on a bed of gravel.
Fill the area with gravel to the level of the bottom of the tunnel. This will provide drainage and will both help to keep your tunnel from flooding and keep the ground from subsiding away from the tunnel itself. Spread and tamp the gravel so that it provides a fairly level surface. Do not try to compress it fully, as that would defeat the purpose of the drain.
Lay a layer of vapor barrier on the gravel a minimum of 6 inches wider than the tunnel floor. Erect the concrete forms for the floor. Spread the reinforcing bar across the width of the floor and the full length of the run. Remember to keep at least 1 1/2 feet of space between the edge of the pad and the wall of the corridor. Spread the concrete as level as possible to void puddles and water accumulation and about 4 inches thick.
Bend and erect a frame for the walls of the tunnel out of reinforcing bar. You can use any shape you like, but an arch is usually best for structural integrity. You can do this part in sections, but be sure that your sections overlap. Erect the concrete forms inside of the frame so that there is no less than 3 inches of space between the reinforcing bar frame and the forms and outside so that there is approximately the same space. Spread the concrete evenly over the area covered by the forms. This will provide a 6-inch thick, steel reinforced wall for your tunnel and should minimize the risk of collapse.
Cover the exterior of your tunnel with vapor barrier after the concrete cures. You may also choose to cover the walls of the corridor. This will minimize the space through which water can seep into the corridor and potentially into your tunnel. Cover the tunnel with at least 6 inches of gravel.
Bury the tunnel. Be sure that there is nobody inside of the tunnel during this process.
Landscape the surface as desired.