The International Building Code, like the CABO code before it, lists presumed bearing strengths for different types of soils. The type and density of the native soil is also important. Source: Table 401.4.1 CABO One- and Two- Family Dwelling Code 1995. Sand, silty sand, clayey sand, silty gravel, and clayey gravelĬlay, sandy clay, silty clay, and clayey silt Soil Bearing Capacities Chart Class of Materials
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Learn how to span soft spots in the soil. Under the weight of a wood house, it won't settle to any significant degree. One-inch-and-a-half or larger gravel is virtually self-compacting as you place it. So when you replace material in the trench, compact it thoroughly, or else use large gravel. Under load, it will reconsolidate and cause settling. 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%. If you don't compact that soil, you could get 1/2 inch of settlement in just the first 6 inches of soil. Use a vibrating plate compactor for sand or gravel soils, and a jumping jack compactor for silt or clay (learn more about compaction equipment in this guide to subgrades and subbases). That's why it is so important to compact the trench bottom.
Soil that's loose has much less bearing capacity than the original soil. Also, soil from the embankment may fall into the trench. When we excavate for the footings, the teeth on the bucket stir up the soil and mix air into it, decreasing its density. So it's the soil right under the footing that is the most critical and also, typically, the most abused. Go down the same distance again, and the pressure has dropped by two-thirds. 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. Soil directly under the footing takes the greatest load, and therefore should be thoroughly compacted.įind nearby slab and foundation contractors to help with your footings.īecause the load spreads out, the pressure on the soil is greatest right beneath the footing. As the load under a footing spreads out, pressure on the soil diminishes.