This is a long post, but it didn't take near as long to write this as it did for me to repair a foundation where corners were cut. If I can save someone that backache it's time well spent.
I'm not an expert, but I am experienced. In the last year I've repaired the foundation on my current house, and I've dug the foundation for the new house. Here's what I've learned with a bit of speculation thrown in.
We all know that some material supports a foundation better than others. My existing home sits on clay as does the new one. A fellow I talked with who is in the foundation repair business told me that clay is one of the worst of all materials to build on. Not so much because it is unable to support weight; it does that fairly well, but because of the unstable nature of clay. It's pretty much like building on a sponge. It expands when wet, and contracts when it dries out, therefore the support for the footers is constantly changing. OK, I can't change the material I'm building on, or the one my current home sits on; what did the expert tell me to do?
First off, he told me to keep clay wet in times of drought. That keeps it from shrinking and allowing areas to drop. He also recommended piers sunken deep into the soil in stategic areas for the footers to sit on. The piers should extend well beyond the clay down to bedrock. This would cost thousands of dollars, and was, of course, what he did for a living. I took his advice into consideration, but didn't go the piers route. I believe he is dead on with the moisture though. Most soil has some degree of organic material in it which expands and contracts with moisture. We should pay attention to keeping it damp, but not overdo it. We all know how much support we get from a mudhole.
When I dug the ditches for my new home's footers I went about a foot deeper than is normal for my area. Two reasons. First, to get beyond the disturbed soil resulting from stump removal. I took out over twenty stumps within the perimeter of the foundation before I started digging. We want undisturbed soil for the footers to sit on. Second reason was that the deeper we go, the less likely for the soil to react to a temporary drought. I figured that would be more stable soil. I did notice that the deeper soil was fairly moist even though the surface layer was dusty from a very dry fall. All that dry weather was over though as soon as I got most of the ditches dug. That's when the monsoons kicked in and dumped six inches of rain on my project. I had tons of mucky mess washed into the ditches and cave ins of the walls, but that's another story. I finally got to the point where I could put rebar in the ditches.
My code called for two number four bars at three inches of the bottom and at least three inches from the sides of the concrete. Code also called for the concrete to be at least sixteen inches wide and eight inches thick. That's what I've got on the current house and, remember, I've had to repair it. I wanted to do better, so I went with twenty four inch ditches for the perimeter walls, and twenty two inch for the interior piers. All ditches were continuous and connected to the perimeter wall. I also went with number five rebar and installed two rows stacked. Everyone who saw what I was doing thought I was crazy, until I explained my reasoning. For those interested enough to be still reading, here goes.
I understand that rebar is installed on the bottom side of the concrete to support the weight that is pushing straight down, and I believe it does a good job of this. The tensile strenghth of rebar is impressive, even if it does bend rather easy. If there is a soft spot in the supporting material under a straight stretch of concrete with rebar imbedded in it towards the bottom, and this soft spot allows the concrete to begin to sink, it results in an attempt to stretch the rebar on the bottom, not to bend it. Of course if the concrete settled very much it would bend the rebar, but we're already in trouble when that occurrs to any degree. The problem with the foundation on my house didn't happen in the middle of a long run though, it happened on a corner. Rebar on the bottom of concrete does very little to keep a corner from sagging because there's very little tendency for the rebar to stretch, but if you stack the rebar, now the rod on the top portion of the concrete would have to stretch considerably for the corner to sag. My two rods of rebar run three inches off the bottom and three inches down from the top, with a separation of four inches between the rods. That ought to help keep the concrete from sagging in the straight runs as well as in the vulnerable corners. At least it can't hurt.
For those interested in how I repaired my foundation, I'll make it short as I can. My house is a brick veneer with a crawlspace. The corner that settled caused the bricks to crack in a stair step design about twelve feet from the corner on two sides of the house. I dug out on the outside of the foundation until I was deep enough and wide enough to get a twenty ton jack under the corner. I sat the jack on a half in thick plate of steel for support and jacked until the jack quit lifting. It was working. The crack in the bricks was much smaller, but still there. I needed more lift and removed more dirt until I could get two more jacks under the foundation. Finally, with three jacks lifting to capacity, the foundation and the bricks were back where they belonged. Now to get it to stay there. I had a huge hole now to get access under the foundation and the hole extended out from the original foundation nearly thirty inches and under it more than a foot. I drilled into the old foundation in numerous points and inserted rebar leaving about two feet protruding. Then I filled the hole with concrete hoping the rebar would tie the new concrete to the original. After a year, so far, so good, but I had to go buy another jack. Those jacks are still down there imbedded in the concrete, but a small price to pay for a foundation that doesn't sag.
Trust me, more time, effort, and money dedicated to the foundation is well spent, if for nothing more than peace of mind.
Good luck with it.
Tom
