Foundation Footing Fundamentals

Footing Fundamentals

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Under every house is a foundation, and under most foundations are footings. Most of the time we take footings for granted, and usually we can: For typical soils, a common 16- or 20-inch-wide footing can more than handle the relatively light weight of an ordinary house.

On the other hand, if you build on soft clay soil or if there's a soft zone under part of your foundation, there can be trouble. A footing that performs well in good soil may not do so well in weak bearing conditions. We don't often see outright failure, but it's not uncommon to see excessive settlement when soil bearing capacity is low.

If the whole house settles slowly and evenly, some additional settlement is no big deal; but if settlement is uneven (differential settlement), there could be damage. A frame house with wood siding and drywall interiors can probably handle up to 1/2 inch of differential foundation movement, but even 1/4 inch of uneven settling is enough to cause cracks in masonry, tile, or plaster.

It's the unusual situations that cause the most trouble. When the footing is laid out off-center so the wall misses its bearing, when you encounter a soft zone on site, or when the footing is undersized, the builder faces a judgment call. If you think there's a problem ahead, you know you should stop and call an engineer. But if the risk is low, you'd like to keep the job moving.

In these tough cases, it's helpful to understand the bearing strength of soil and the reasons behind footing design rules. In very strong soils, minor mistakes probably aren't a big deal. In weak or marginal soils, however, it's best to be very cautious -- some of the solutions contractors think up may not really work.

I'm a consulting engineer as well as a contractor, and I get called in to a lot of problem situations. I find that people understand the problems better if they have some background knowledge. For the benefit of builders in the field and at the risk of oversimplifying, I'm going to use non-technical language in this article to briefly explain a little about how footings work and to present some ideas for dealing with special situations. As you look at the solutions I recommend, however, keep in mind that high-bearing-capacity soil is assumed. Any time you're in doubt about the soil under your foundation, you'd be wise to get professional help.

Why Soils Matter

In addition to providing a level platform for forms or masonry, footings spread out the weight of the house so the soil can carry the load. The load spreads out within the footing itself at about a 45-degree angle, and then spreads out in the soil at a steeper angle, more like 60 degrees from the horizontal.

Because the load spreads out, the pressure on the soil is greatest right beneath the footing. By the time we get down below the footing a distance equal to the footing's width, the unit soil pressure has dropped by about half. Go down the same distance again, and the pressure has dropped by two-thirds. So it's the soil right under the footing that is the most critical -- and also, typically, the most abused.

When we excavate for the footings, the teeth on the bucket stir up the soil and mix air into it, decreasing its density. Also, soil from the embankment may fall into the trench. That loose soil has much less bearing capacity than the original soil. That's why it is so important to compact the trench bottom (use a vibrating plate compactor for sand or gravel soils, and a jumping jack compactor for silt or clay). If you don't compact that soil, you could get 1/2 inch of settlement in just the first 6 inches of soil.

If you dig too deep and replace the soil to recover the grade, you are adding back soil that has expanded by as much as 50%. Under load, it will reconsolidate and cause settling. So when you replace material in the trench, compact it thoroughly, or else use large gravel. One-inch-and-a-half or larger gravel is virtually self-compacting as you place it. Under the weight of a wood house, it won't settle to any significant degree.

Soil types and bearing. The type and density of the native soil is also important. The International Building Code, like the CABO code before it, lists presumed bearing strengths for different types of soils (see "Soil Bearing Capacities," below). Very fine soils (clays and silts) typically have lower capacities than coarse granular soils (sands and gravels).

However, some clays or silts have higher bearing capacity than the values in the code tables. If you have a soil test done, you could discover that you have a denser clay with a much higher bearing strength. Mechanically compacting the soil can also raise its bearing capacity.

You can get a pretty good idea of the soil bearing capacity in the trench bottom using a hand penetrometer. This pocket-sized device is a spring-loaded probe that estimates you the pressure the soil can resist and is calibrated to give readings in tons per square foot. In my opinion, every contractor and building inspector should have one of these -- it can help you avoid a lot of trouble.

Footing Dimensions

So, how does soil bearing capacity relate to the size of footings? The footing transmits the load into the soil. The lower the bearing capacity of the soil, the wider the footing needs to be. If the soil is very strong, the footing isn't even strictly necessary -- just the soil under the wall would be enough to hold the building up.

You can look up the recommended footing size, based on the size and type of house and the bearing capacity of the soil. As you can see, heavy houses on weak soil need footings 2 feet wide or more. But the lightest buildings on the strongest soil require footings as narrow as 7 or 8 inches. Under an 8-inch-thick wall, that's the same as saying you have no footing.

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