Cob Concerns

In the exploration phase of our home building journey. We have a few questions that I hope someone may be able to answer.

We seem to have good clay. My clay, sand, and hay test bricks feel as hard as real bricks.

Our concern is standing water. We occasionally have heavy rains and have a flat plain as a site. There are no hills or otherwise to build higher.

Once a decade we can flood with standing water for a week or so. A few months in any given year we can get a deluge with a few standing puddles and sloppy soil. In the summer we can reach 100f and our ground cracks everywhere.

My question is will a high stem wall permit us to build a cobb home in our area considering the above?

I am thinking we can also build up the interior of the home a foot or a foot and a half higher than grade so if we do get standing water that penetrates the stem wall it won't touch the cobb, and the interior of the home being elevated won't be underwater itself.

Thoughts?

I suppose we can always get a pad of stabilized soil placed to elevate it over before we built but that of course would incur additional costs.

Thanks for your knowledge.

An image would help a lot.
Building above the flood level is always important, particularly with earthen walls.
There are a few ways to get above the flood level;
RAISE GROUND LEVEL
- dump and compact fill from somewhere either;
- soil
- rock
- clay
- or even rubble with some crushed rock to finish it off.
BUILD A STEM WALL made from;
- rocks
- formed concrete
USE STUMPS TO ELEVATE THE FLOOR
And have a stem wall for the walls.

When you mention the ground cracks, you may have an expansive clay that will always crack when it dries, by mixing with sand or getting soil from another part of your property
you may be able to use that material for building.
Please carry out some basic soil tests to check though.

Hi Tamar,

Welcome to Permies.

I'd think you'd definitely want to build your impervious stem wall/finished floor level well above any possible flood! Unstabilized cob subjected to saturation will not bear load. If the base of the wall gets wet, the wall is definitely going to fall down.

You might also look into various stabilizers. Road engineers think about this a lot because of course roads are out in all weathers but have to support the weight of heavy vehicles. Your local road crews might have a lot of good ideas. Usual stabilizers are lime, maybe lime + waterglass, some portion of cement, or "enzymes" (about which I would like to know a whole lot more, so tell me what you find!).

But if I were building in a flood plain, much as I love cob, I don't think I would do it. I think I would sink piers and build a lighter-weight house raised well up off the ground. This is what I did with my own timber-frame cabin here in Vermont -- the slope here prevents it being an actual floodplain, but the water table is permanently at 18" (0.5 m) and I didn't want to try to deal with building a heavy-duty load-bearing foundation in a swamp. If you have access to wood, I'd sink piers and frame the house, then wattle-and daub or straw-clay between to keep the weight reasonable. You can't raise cob on piers of course; it's way too heavy for that sort of point-loading. The advantages of piers over a high stem wall are 1) price, and 2) if that flood water needs to move, it can do so without pushing on your foundation as much.

But either way would work. Just make sure you don't underestimate how deep that water may someday get!