When I started thinking seriously about starting this I knew I had to do a few simple tests first that would cost me a bundle if I had to hire an Engineer and do them even before I applied for a permit.
They'd want to know the different types of soils, how thick each seam was and most importantly I wanted to know how far down the water table is.
I live in Florida and generally if you dig a hole a few feet down you'll hit water but I'm in the Panhandle of Florida so we actually have some hills here. The lot slopes to the lake enough I was able to dig a bottom level into the hill giving me a pretty safe place to hide if we get a bad hurricane. The house is extremely reinforced and all the walls are shear walls meaning at least one side has plywood glued and nailed every four inches, including a second layer on the inside of the outer walls which also went a long way to soundproofing the place. I have more anchors than required and threaded rod going from the footers to the trusses including the interior walls.
The molding work on the ceiling of the great room is actually tied into the roof trusses and the floors are all 3/4 plywood tongue and groove and glued and nailed plywood every four inches for a subfloor. It's covered by 3/4 by 2 1/4 Oak tongue and groove running perpendicular to the floor joists. The Kitchen, Halls and Bathrooms I tiled but the bulk of the house has the Oak making a very stout roof for the bottom level.
My workshop also has plywood ceilings so that part is especially strengthened to the point of a tornadoes shredded the top two floors you'd be safe huddling under the work benches.
I tend to ramble because I crushed my head three years ago in an accident so back to the intended topic of how to dig a hole under a house and not have it fall on me.
My primary method was that I felt I could safely dig along side of a footer and pour a 2 foot high concrete wall so long as I did it in stages the sides would not cave in for a few day, especially if I misted it with water from the hose. I drove rebar about a foot into the sand or clay and left the rest sticking out the top so the final bottom to top pour would bind everything together. I also after the concrete had dried and I had the forms moved down another 2 feet I would use a small spade to find the bottom of the previous rebar, and using a piece of pipe for leverage I'd bend it into the area of the new pour at a 45 and then bind it to the rebar for the next pour. The end result is a very solidly secured concrete wall 10 or 12 feet high shaped like an inverted wedge to discourage it from sinking into soft ground. Also I decided to only do 10 or 15 feet at a time but never the length of an entire wall in one shot so I didn't have the sides blowout like in a mine accident.
I spent 7 years building the house I don't want to cause it to collapse and the wife would never let me hear the end of it.
I then figured I could keep repeating the process essentially forming a very tall staircase and when I had enough head room I'd pour a final layer 8 or 10 feet high to tie it all in. Billy down at the building department would have a fit if I told him I wanted to do this and I'd need a real engineer to put their stamp of approval on it and it's not the type of job you can start and walk away from halfway done if something comes up, so this is too much for me to do as a solo project and I;m not actually Licensed Florida Contractor.
I did figure I should do a test shaft to see if it actually worked and get a record of what was down there and I would definitely need to know the water table height and the thought of having to assemble a mini drill rig in a crawlspace seemed pretty expensive but by staring long and hard at it for a time and a day, I decided I could dig a shaft by hand right next to the access port and cover the stairs with a snug piece of plywood then shovel it out the port onto the stairs then into wheel barrows and dump it out back.
How hard can that be?
I had a limited budget but I do own a mortar mixer and had a great stockpile of rebar and three inch pvc pipe left over from something I cannot recall and some lumber I'd used already for concrete forms and yes there is a Plus side to OCD especially if you have the room to store it until you need it.
I chose not to mention to the wife what I was planning mostly because she would have said no but I had just about used up all my savings finishing the house but I still had my emergency $1000 so I did some math and bargained for a lower price on 80 pound sacks of Redi-Mix if I bought 6 pallets and so I snuck a look at the wife's calendar to see her next doctors appointment and her caregiver would drive her so I set up the deliver for when she was gone and had a space cleared in the bottom garage that I could store all of this, and had some pieces of pipe on hand to walk like an Egyptian and move the 3000 pound pallets to the far back of the garage by rolling then and then using the next pallet to roll the first to the back until all six would be safely hidden inside.
The wife was running late though and they showed up just as she was leaving...
Fate Strikes Again but my wife knows I was still not quite right in the head in fact she'd say I was like that even before hitting my head and is used to me just jumping up in the morning to start working on something and I did after all build her a house and miraculously she didn't ever really ask me what the cement was for.
Once I had the cement I thought about forms and I decided if I am to do this and make a 2500 square foot bunker I need the forms to be modular so I could keep reusing them over and over.
I decided on a few basic sizes, 2' by 8',2' by 6', 2' by 4', 3' by 4', and I made them with the braces in each size identical so when I needed to make 50 of these I could drill around the perimeters in a standard pattern so I could just bolt these together, pour the concrete, then unbolt them the next day and move on. Trial and error taught me that staple gunning a smooth layer of 6 mill plastic makes for a much smoother wall than plain plywood and you just need to replace it every few pours.
With the forms made and the 6 yards of concrete safely hidden and covered with a tarp just to be safe I started digging next to the access port so I could get some standing room. The first five feet of soil is sand I believe referred to as Orange Ruffie but I'll need to check my soil survey of Walton county to be sure.
Regardless sand is pretty easy to shovel although the very top layer was had dried out pretty solidly and had some clay mixed in I had to spray it with the hose to soften it up. You actually need to keep the sand moist or the sides will cave in quickly and I made a point of misting the ground before I quit working on it for the day.
I made this approximately 7 by 7 since as I went deep it would get narrower and I was about 18 inches between two columns supporting the beams so this was a good spot to test. On the first 2 foot pour I also poured and formed a beam between the columns with a lot of heavy angle iron cut to fit tight between the columns.
The I started getting the hits from The Office of Homeland Security which I figured was just because so my of my traffic comes from Walter Reed Hospital and the rest from our troops abroad.
Then the cable company knocked on my door and said some magic piece of equipment let them locate the source as somehow coming the crawlspace under the house and they needed access under there to check for problems...http://senseless.livejournal.com/229844.html