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 footings 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. Soil that's loose 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.
Learn how to span soft spots in the soil.
Soil Bearing Capacities
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. 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 calibratedto 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.
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