Foundation options on steep slope

[old_guy]

Our Tennessee property is mostly very steep.  The rise from the water is about 500 feet, and it completes this rise in about 500 - 1000 feet. The hilltop is relatively flat but, of course, we wish to build where more difficult.

There is an area near the water where we would like to build 3 cabins (16x24? plus porch) for our 3 children.  We would hope to rent these cabins when not used by family.  The slope in this area is about 45 degrees (12/12, 100%, ...).  Forgive me for not knowing the proper terms to represent this.  In any case, I am fishing for ideas regarding foundations for these cabins.

Rather than have the long axis of each cabin parallel to the hill, we would prefer to rotate each approximately 30 degrees, so one back corner will be deep into hill.  This gives a much better view, and probably simplifies water control/diversion from above cabins.  I think that we could put proper footings in the hillside, with poured concrete foundation walls under that portion of cabin, but would need to support the porch and downhill portion of cabins on poles.

I have apparently clouded my mind with this approach to the exclusion of others, and would be delighted to hear different ways, or even just that I have made some major errors in my thinking here.

[NathanS]

On steep slopes like that, you've got to be careful with erosion. On some steep forested land, you might notice that the bottom of tree trunks are J shaped, from soil sliding down the hill as the tree grows.

Investing in 3 cabins, I think it would be worth having an engineer design the foundation.

The USGS soil survey map should give you an idea of what you'll be dealing with when you start digging.

[old_guy]

Nathan - thanks for your response.  The state forester did a soil survey several years ago.  My big concern is not so much the makeup of the soil itself, but rather what we will find a few feet under the surface.  There is a 30 foot tall rock face sticking out of the hillside a few hundred yards downstream, and hitting solid rock would make things more difficult.

There is an old road cut into the hillside between this build site and the water, running parallel to both for a couple hundred yards.  It has been there at least since it was last logged (70-90 years).  The downhill (filled) side of the road has melted some, but there is no sign of any erosion or slumping on the uphill side.  I am comfortable that the soil is strong and stable, but am searching for a good approach for the foundations.  I am expecting that having 1/3 - 1/2 of the building supported on concrete footings set on virgin soil in the hillside should address the concerns that the building will move when build on piers.

Per your suggestion, we will probably get input from an engineer before we finalize any plans.

[DaveOrr]

If you were able to pin your foundation to bed rock it would be much stronger and no shifting would ever happen.
My place is pinned to bedrock although it is much flatter than your lot. I have 8X8 & 6X6 PT piers set on galvanized saddles pinned into granite and secured with non-shrink grout.

[old_guy]

Excellent point about pinning to the rock, Dave.  The rock I found elsewhere on the property is not granite hard.  When they had to widen an old road into a rock hillside for timber removal they used a hydraulic hammer to split the rock loose and/or just pound it down.  If there is solid stone/rock I will be delighted to pin to it and reap the stability.  Your pictures showed clearly how you did that.  My biggest concern in that regard is that the rock may be a vertical or sloping wall, just a few feet under the sloping surface of the hillside, which may prevent me from digging into the hill.

[DaveOrr]

You can use a rock saw to cut benches into the rock to support your piers on.
Should be able to rent one for the job.

Even if the rock is softer like sand stone it is still stronger than dirt. 


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