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Is foundation safe underneath a cabin

kyrral's picture

I am hollowing out a space underneath my cabin to create another room that will end up being a box within a box on its own foundation. the cabins orginal foundation is 6" wide, a couple of feet tall with a 22' length. Below ground level the foundation is roughly a foot wide (uneven) and extends below ground level about 2 feet. I am removing most of the dirt along the inside face but not undercuttting the foundation.  Starting about 6" below the deepest part of the foundation I will run a 6x6 trench parallel with the original wall for my footing. My question is: have I compromised the original foundation? It seems to me the weight is downward on the foundation and not lateral so I don't think the original could fail (by buckling maybe)?

In locations with "expansive" (post #207520, reply #1 of 3)

In locations with "expansive" soils or where the frost depth is more than a foot or so the soil can exert substantial lateral force on the foundation.  What you want to do can be done, but it should include some sort of reenforcement of the existing foundation against lateral force, probably by carrying your new wall up a foot or so and backfilling (or pouring concrete) between.


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Two issues to resolve, (post #207520, reply #2 of 3)

Two issues to resolve, removing the soil above the bottom of the footing, and removing the soil below the bottom of the footing.

The soil above the bottom of the footing increases the bearing capacity of the soil under the footing. That's one of the reasons we bury footings instead of putting them on top of the ground. This could be critical in soft soils with low bearing capacity.

The soil below the bottom of the footing confines the soil, keeping it from bulging out from the pressure the house puts on the soil. All soils have this tendency, but some soils bulge more than others. The heavier the load on the foundation, or the softer the soil, the more lateral spread will occur. Usually, a safe way to excavate next to a foundation as you're doing is to not remove the soil in a 45 degree wedge below the footing. But this will make your basement narrower.

One technique to excavate vertically is to not excavate the entire wall length at one time. Divide the length into reasonable parts (4', 6', maybe 8') then construct your basement footing & wall in sections.

A related issue is the depth of your excavation. If you dig a trench deeper than 4', safety requirements usually mean you need to shore up the sides or slope the sides of the excavation. 4' is a typical depth, your local soil conditions may mean a shallower depth requires special precautions.

You could (probably should) consult a geotechnical engineer in your area. If you try this without a geotechnical report, and damage your house structurally, it's not likely your insurance will cover it.

cm (post #207520, reply #3 of 3)

The soil above the bottom of the footing increases the bearing capacity of the soil under the footing. That's one of the reasons we bury footings instead of putting them on top of the ground.

 

Frost depth I know of-never heard this one b/4.   Are you saying that the load of the structure besides bearing straight down, also pushes out? 

 

What does that say about drainage systems and stone backfill?  or for that matter, the basement/crawl to the inside?

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