Build a Catacombs Beneath Your Basement

Building catacombs may not be a typical weekend project. If a person is intent on trying it, a lot of planning should be done before starting.

Steps

  1. The first consideration should be legal questions, such as local and state building codes and permitting. A project like this can fall under various planning and zoning board jurisdictions, not to mention insurance liabilities.
  2. If a person either finds the local building codes allow this project, it will require a substantial engineering effort. Because this is an excavation project, you must determine the strata you will have to dig through, their stability and drainage conditions. The first "strata" will be the basement floor, of course, which is often cast in place reinforced concrete.
  3. When you determine to go ahead with building the tunnels which comprise your catacomb labyrinths, and have investigated the type of material you will be digging in, you also have to consider safe access and egress, support structures for the walls and ceiling, and disposal of excavated materials. If the soils are stable, cohesive material, this whole process is not too complicated, but if it is sandy, loose soil, more thorough engineering than this writer is capable of is needed. I will continue to offer advice considering the material to be consistently cohesive and stable.
  4. Most likely, digging will begin several feet outside the walls of the basement, going down diagonally toward the basement floor so that the ceiling will be below the sub-grade of the floor and the wall foundation at the location you intersect it. The reasoning here is that you will be removing at least several cubic yards of earth and it would not be desirable to transport it through the living area of your home. The distance from the wall to the beginning of the entrance must allow the floor of the tunnel to slope gradually enough to allow you to either carry or wheelbarrow out the excavated material.
  5. As the tunnel progresses, overhead shoring must be installed to support the weight of earth overhead, or overburden. This will require rigid vertical supports, and galvanized steel pipe with mud sills of either treated 2X12 lumber or steel plate with beams or corrugated galvanized steel sheet metal placed horizontally across the ceiling. The spacing of this "post and beam" shoring is normally dependent on the stability of the earth, but a minimum spacing would be one span for each distance of one half the width of the tunnel way. In other words, a three foot wide tunnel in stable soil would require a post on either side every {{safesubst:#invoke:convert|convert}}.
  6. When you have reached a floor elevation sufficient to allow a head space to the ceiling and are deep enough below the basement wall footing that digging horizontally underneath it will not weaken the structure above, level out the tunnel floor and proceed at grade the distance you would like to go, continuing to shore as you dig.
  7. After making the initial tunnel shaft, leveling out the floor, and shoring, the catacomb can basically be completed by digging the niches in the side walls, installing lights or torch sconces, and even laying up masonry walls if you wish.
  8. The catacomb builder may want to make the permanent access to his creation through the cellar floor, and this would be the time to proceed to this step. Using careful observation and measurement, locate the tunnel underneath the basement floor, and assuming the floor is concrete, use a rotary hammer-drill with a long, small diameter bit to drill a "probe hole" to the tunnel ceiling. The builder may find renting this tool and its accessories more practical than purchasing it, as it is an expensive tool item. Next, using a diamond aggregate saw blade on a demolition saw, cut a hole in the floor the size you will want to use for your access way. Take care removing this section of sawed concrete, excessive force breaking it up into removable pieces can cause the tunnel to weaken or cave in from the vibration. After the floor is removed, dig out the hole with post hole diggers, standing on the concrete, as at some point, the tunnel ceiling will give way and some of the material will collapse into the tunnel underneath.
  9. After the vertical shaft is excavated, the final step in the process would be to install a ladder and "hatch" or access hole cover. Oh, and getting rid of a large pile of earth excavated in the previous steps.

Tips

  • Be prepared to "seal off" the entrance if rain is likely, the tunnel is below grade, and a little water there will create a large mess.
  • Be careful of underground utilities, especially at the beginning of the operation. Gas lines, water lines, sewers, electrical services, and telephone cables which coincide with the tunnel will set you back if they are encountered during the project.
  • Use strong, heavy duty tubing or other material to shore and support the tunnel sides and roof. Normal soil weighs about 120 pounds per cubic foot.
  • Allow enough distance from the basement wall to slope the entrance tunnel gradually enough to facilitate either carrying or wheel barrowing the excavated material easily. There will be many trips up this ramp.

Warnings

  • Methane. Some (if not most) methane pockets are surprisingly close to the surface. This is because methane is a product of organic decay. If your house lies in the valley between two mountains, then you can be positive you will release methane in your tunneling. While not a major problem, or a major risk, it would be silly to blow your house up with a silly home improvement project. After a NJ home was tightened up, with modern insulation and house wrap, it actually blew up after two years. The cause was methane coming in thru the stone basement walls.
  • As before mentioned, typical soil weighs much more than you may realize, so multiply the safety factors, and if in doubt, consult a geo technical engineer.
  • Gas. Just because your home does not have gas appliances does not mean that you don't have a gas pipe running to your home. Remember gas predates electricity! Sometimes these were cut and capped in the foundation. Some may be abandoned, (gas predated electricity), but it's difficult to tell when you're looking at a few inches of capped pipe.
  • Radon. This might be an additional path for radon to travel into your home.
  • Hidden foundation problems. If your foundation has eroded from the outside of the wall, then you will not know this until you dig it out. This is the often the case with mortared stone, the mortar may have fallen out leaving the outside dirt as a major support structure of the foundation (and the house is slightly sinking one way or another an inch at a time). You will need to repair this if it's uncovered. If all doors in the house open and close properly and you don't see major cracks in the walls, then this is not likely a problem. If you notice a crack in the plaster or sheetrock going diagonally, then this might be a problem and the foundation wall has already fallen an inch or more.
  • This project increases the walls, and the odds of this currently "freak" accident... (when houses are sealed tight, and methane is not allowed to escape outside). Clearly, you might need a vent pipe for your little mole project to allow the methane to escape before entering your home. Also use a fan to get plenty of fresh air while you dig, and vent well.
  • Mold. If you are not sufficiently above the water table, then this will bring more airborne mold into your home.

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