Digging the basement

[dad-of-4-in-tn]

Hi. I am in the process of building a 16 by 20 two story cabin. I have the shell done am currently working on wiring but I have a question about digging around my foundation.  For the foundation the house has a U shaped wall halfway down each side and acorss the back, the front rests on 3 piers. My question is when digging out the basement how close can I get to the piers in front without causing them to shift? the side walls are 10 ft long which leaves 10 ft to the front of the house.

[glenn-k]

Depends on soil conditions.  Friends in Oregon did it years ago - a foot or two- no major problems - Here I could go to the edge of the block with no problem - at my other place the ground is sand - I just started and had problems with caving when I tried it a long time ago.

[dad-of-4-in-tn]

Building in the hills of TN. Lots of rocks no sand that I've seen as of yet. More like clay.

[Amanda_931]

Parts of Tennessee have a lot of
 [size=18][glb]silt[/glb][/size]
blown over from the Dust Bowl in the thirties, says the septic tank guy.

It kind of looks like buff-colored clay either when it is in a well-stirred up puddle or when it's dry and hard as a rock.  (And there are lots of colors of clay around)

But it is one of the least stable materials to build on.  Less load carrying capacity than anything you are likely to encounter except perhaps sand dunes.

Because of the small particle size and  it can go from hard and rock-like to muddy puddle very very quickly.

So make sure--can you form your dirt into a coiled basket, for instance--before you start digging.

You are quite a bit farther east and north of me.  

And the 18" or more of silt is down in a hollow on my property.  At the top of the hill there are pin-head sized balls of white clay mixed with the silt.

I played with a design for me once that had a small root cellar (something like 7x9) as a basement.  Never built it, though.
 

[desdawg]

Amanda is right about testing the soil. If you dig down below the topsoil layer you can take a golf ball size sample in the palm of your hand and moisten it. When moistened but not sloppy wet you will know if it is sticky like clay and you can roll out a little rope of it (high clay content) , smooth and slick but won't bind together (silt or loam) or if it feels gritty like sand. The top soil isn't indicative of what lies below necessarily so you would need to get down 2' or so. This is just a little informal test that you can do yourself without hiring someone. I gave you three distinct categories but you could have a mixture that falls in between these somewhere as well. This is the short form of the soils testing we do to determine the soil absorption rate for septic design.