Looking over a recently revised manufactured home installation manual, (DAPIA approved in Feb. 2018), I ran across something that I thought was worth a closer look.
In the section of the manual that talks about footings, I found a chart titled “Foundation Embedment Factors for Cohesive Soils” (there is also a chart for Non-Cohesive Soils).
In case you are wondering, cohesive soils are described as clay, or soil with a high clay content, which has cohesive strength. Cohesive soil does not crumble, can be excavated with vertical sides, and is like plastic when wet. Cohesive soil is hard to break up when dry and sticks together when submerged. Cohesive soils include clayey silt, sandy clay, silty clay, clay.
On the other hand, non-cohesive soils are loose soils like sand, or sandy soils.
Ultimately, by using these charts and directions, you can increase the maximum load per footing based on the depth that the footing is embedded in the soil.
Let’s try to work through the process. This manufacturer tells me that a 14’ wide home, in the south (20 psf) roof load zone, with piers spaced 8’ apart, has a pier load of 5640 pounds (per pier).
Since I like to auger round footings, the chart for circular shape footings says that I need a 28” round footing at a soil bearing capacity of 1,500 PSF. But my auger is only 24” diameter! Well, this is where the “Foundation Embedment Factors for Cohesive Soils” chart might come in handy!
Since I can only dig 24″ round footings, I need to start there. The chart for circular footings tells me that a 24” round footing can carry 4710 pounds (again 1,500 Soil bearing capacity). I know my 24” round footing will be 36” deep (for example, to get below the frost line) in a clay (cohesive) soil, so I can multiply the 24” footing capacity by the 1.56 as indicated in the cohesive soils chart and my 24” footing works! (4710 pounds x 1.56=7347 pounds, well beyond the 5640 pound load needed to support the home).
It is not as confusing as it seems. You just need to know if your soil is cohesive (clay) or non-cohesive and know the footing size and depth. Use the chart to determine the embedment factor and multiply the footing capacity by the factor from the chart.
A few important things to consider before you start reducing footing sizes. Currently, this is specific to only a few manufacturers. You need to check with the Quality Control Manager at your factories to see if they allow you to utilize embedment factors. One manufacturer told me that they provide these designs only upon request.
Next, you need to have a very thorough understanding of the soil at the job site. You need to have all of your documents in order and make certain that the building code official (and possibly the purchaser) understand how you are calculating these footings sizes/loads. As always, keep good records for your installation files, including copies of these charts.
While I am not an engineer, I do think that the embedment factor is to be applied only to poured in place concrete footings. But you can verify that with the factory.
Finally, on the one manual I reviewed, there is a big typographical error, labeling the chart for non-cohesive soils, so be cautious. Stay in touch with Quality Control folks and watch for further changes to the manuals.