Digging the footing
The foundation of a house is a structure (usually concrete) what holds the walls up. The walls in turn hold the ceiling(s) and roof up. The idea is that the foundation does not move, ever. If it moves so does everything that it’s holding, with disturbing, expensive and possibly disastrous results.
Engineered foundation design
A foundation is only as good as the soil it sits on, or in most cases, in. The soil has to support the foundation and it’s load without moving. So in most cases where there are regulations governing building, a soil test is required. The point of the test is to determine the bearing capacity of the soil and give a reference point for the foundation design. In addition to the soil test, an engineer certified foundation design is usually required as well. In the USA, this will usually add $2000 or more to the cost of building before any building is done. If you have purchased plans, then it’s important that the foundation is sufficent for your soil’s load capacity.
In a temperate climate (the mid latitudes) and towards the poles, the ground has a tendency to freeze when the weather turns cold. Depending on the water content of the soil, this can cause the ground to swell and heave. Anything built over this heaving will also heave. As mentioned before, a foundation that moves is not a good thing, so the normal solution to the heaving problem is to bury the foundation to a level below the freezing zone of the soil. Depending on the climate of your location, the depth may be nothing to over 8 feet (almost 3 meters). In my location the official depth is 30 inches or just under a meter.
Historically foundations were made with rubble or concrete. The material was laid or poured in a ditch that was wider than the walls, to give the walls a “foot” on which to stand. Thus the bottom of the foundation is called the footing. Where the foundation depth is very shallow, the footing may be all that is required before the walls go up. Where it is deeper, there is normally a subsurface foundation wall of block, brick or concrete. Some places have approved subsurface treated wood walls. I personally would not put the effort into a house with subsurface wood parts, treated though they may be.
Rubble is a mix of different sized rocks dumped into a trench. The rubble was compacted and leveled. On this foundation the structure is built. Many very large and heavy structures have been built on this type of foundation, including european castles. While building a foundation of rubble may avoid the use of some cement, it’s likely that the use of heavy equipment like dump trucks for transporting large quantities of rock offsets any ecological benefits of doing so. It is also a very labor intensive activity, to say the least.
Concrete is a mix of sand, gravel and cement (hydrated lime) which forms a very hard rock like material when cured. The formula has been known since early times and famous structures like the Colliseum in Rome are build with it. It is available from concrete companies in large volume or in ready to mix sacks at building supply companies. Equipment is available which allows concrete to be pumped in in it’s liquid state to otherwise awkward places. It is normally necessary to use thick plywood forms to hold it in place while it cures. Normally these forms are removed within a day or two, although to cure strongly the concrete should be kept moist for several days.
There has been some noise made in environmental circles concerning the amount of energy, and thus carbon emitted into the atmosphere, that is used in the production of cement. Some estimates place the total amount of carbon emitted by cement production at 5% of all the carbon humans emit. I don’t know where these figures come from or what their accuracy might be. I do know that so far, there is no other substance that will do what concrete will do. I have read about substitutes for cement made of fly ash (a byproduct of burning coal) but so far have not been able to find a source for them for the residential builder. See: http://www.toolbase.org/Technology-Inventory/Foundations/fly-ash-concrete
The diagram below is a basic foundation design showing the footing, a foundation (or stem) wall and rebar placement. In this case the vertical rebar is left long so as to be used to anchor Insulated Concrete Forms (ICFs) to the foundation wall. If a stud wall were to be constructed, J-bolt would be used. Please note that this plan will not work in all situations and/or soil types. The dimensions are based on a 30″ maximum frost depth.
This is a photo of the inital excavation for the round house. The outside diameter of the house is 42 feet.
Digging the footing trench by hand. I used a rototiller to break up the soil and used water to soften the most rock like.
The foundation digging was done on weekends. I took the day off work for the rebar inspection and another day off for the pour. Below is a photo of the rebar placement for the outside of the footing. There is matching inside rebar that was placed later and smaller pieces wired between them to maintain spacing. Plastic stand-off bridges were placed later to support the rebar 3-4 inches above the trench floor. As the house is built into the hillside the footing steps down so as to maintain at least 30″ below grade. This shows some of those steps.
Once the rebar is placed it’s time for inspection, and a concrete pour.