Soil Bearing Capacity

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Soil bearing capacity refers to the ability of soil to support load applied to the ground. Bearing capacities have a safety factor built in to prevent failure. So, the ultimate bearing capacity would be its perceived point of failure with no safety factor built in.

Soils beneath the footings of a concrete slab are under the most pressure so soil compaction is necessary. Use a vibrating plate compactor for sandy and gravel soil consistencies, and a jumping jack compactor for clay or silt based soils.

Pole embedment depth is calculated based upon the type of soil at the construction site, frost heave depth, whether there are "open walls" or not, and the size and design of the building it must support. A hand penetrometer can be utilized to measure soil bearing capacity and provides readings in tons per square foot. Your local building code office can help with soil bearing requirements in "some areas" of the country, others not so much.

 

Soil Bearing Capacity Table
Type of Soil Load Bearing
(Pounds Per Square Foot)
Rock w/ Gravel 6,000 psf +
Gravel 5000 psf
Sandy Gravel 5000 psf
Sand 3000 psf
Silt Sand 3000 psf
Silt Gravel 3000 psf
Gravel w/ Clay 3000 psf
Clay 2000 psf
Sandy Clay 2000 psf
Silt Clay 2000 psf


Remember that after digging holes for your posts, level out with gravel, rather than throwing soil back in the hole. The soil added back will have expanded by as much as 50%. Under load it will settle and potentially cause problems for your project.

Also, the size and thickness of the footer (concrete the post sits on) is based upon the same factors discussed above.


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