Using a rubble trench footer in heavy clay soil with seasonal high water table.

Can it be done?

I have heavy clay soil with almost 0 rocks in it.   It doesn't drain well, and in winter months the water table can be within the top foot of soil.

Frost line here is 30-36 inches.   I'd like to use a french drain and rubble trench footing with a cement bond beam on top, but I'm not sure about the shifting with the expansive clay.

Any suggestions?

Hi Adam.

As you know, drainage is an issue. But unless you breach the clay layer with the bottom of the french drain, you're unlikely to really improve the drainage unless you improve the quality of your soil.

Clays are amazing for hanging on to nutrients, but oftentimes, they can result in soil that's glutinous when wet and cementitous when dry to the point of cracking. Sometimes this is due to a mineral deficiency issue. In calcium-depleted clays, often adding gypsum grit in, as a calcium amendment that won't affect the soil pH, will cause the clay to unlock.

People also throw tonnes of biomass at clay soil to improve it, to great effect. I like working with woodchips, when available, but any chopped crop residue will do.

I feel a rubble trench footer is a great idea, and making it as deep as you can afford to go is a good way to keep your structure's feet dry. But in terms of earth drainage, I would suggest biomass trenches rather than rubble downgrade from the structure itself. It would be good to have biomass components that will retain structure under load and decomposition, so I also feel biochar would be excellent in this capacity.

Basically, instead of the rubble trench leading away from your rubble trench footer draining into something like a 6' x 6' x 6' pit of rubble, which stands a good chance of simply filling up with water and then backing up into your rubble trench footer and into your walls, the fill in the downgrade trench and drain system becomes perhaps entirely biomass and biochar, to be topped up with more biomass as necessary, a soil life bioreactor in soil fed by the water you don't want in your walls.

-CK

The first critical question is, do you have any slope away from the building site? If you can dig a trench leading from the bottom of the rubble trench to drain away water to the surface somewhere downhill, you will be fine. Then it is just a matter of deciding on the best method of keeping the water flowing, whether rubble, biochar as Chris suggests, or something else.

If your site is so flat that there is nowhere you can drain water to, digging down well below frostline and also wide enough that the rubble at the bottom of the trench spreads the load out widely, you may be able to work even with a waterlogged footing. How dry does the clay get a few feet down in summer? If it dries out and cracks deeply, you may have an untenable situation for anything short of a reinforced concrete mat under the whole structure. If it stays moist deep down, you may not run into major swelling/shrinkage issues.

Another thing I would do in your situation is to raise the floor level of the structure, along with the ground outside for several feet around, to more positively redirect water away from the foundation and keep the floor and wall base farther above water level.

Thanks for your reply Chris.

I'm not worried about the draining as much as I am the swelling and contraction of the clay shifting the trench over time and making an unstable foundation.

So I guess I'm asking, is this a real concern, or is it unnecessary to worry about?

Also details on the trench... It would be 1 foot wide. 3 foot deep. And the drain tile would slope to daylight.

Hi Glenn.

Yes it has a slope to it that leads towards a stream.   So I should be able to have the rubble trench's drains go to daylight.  

But I still will have significant expansion and contraction depending on drought in summer.     So maybe some years won't crack as much as others.



What would you suggest for building the site up before the build?

How big is the structure?
You may need to drill piers down to better soil.
Put some reo bars in them and fill with concrete to the bottom of your proposed footing, then pour the footing with mesh inside it.

I would have to rubble trench along side, not under the footing

Amending clay in the manner I suggested will result in a transformation of the greater soil structure in appropriate conditions. This would translate to the soil not behaving in the manner that concerns you. Increasing the ability of water to pass through your soil will literally keep it from swelling, and increasing organic matter in the soil increases its capacity to retain moisture over dry periods.

Results may vary. The methods I describe might be completely inappropriate for your specific soil type. But it has worked for some.

-CK

What did you decide to do?  I spoke to a structural engineer for ours, which is heavy black prairyland clay, and for us he said to use a min of 18" crushed limestone.  We will be building an earthbag, and hyperadobe home.  

As Glenn suggested, drain to daylight and build the site up.

I live in a very flat high clay area. In my last property I built up a couple acres by a total of 3’ and sloped the entire area away from the house/garage in all directions. That kept the amount of water at the base of the foundation to a minimum. Cracking of the ground is always going to be an issue. As long as the footings are below the frost line and sloped out as Glenn suggested you shouldn’t have an issue with shifting.

My other suggestion would be to do a sloped insulation/water skirt around the house. At least 2” thick rigid insulation sloped 1/4” per foot and extended out 4’ or more. That will not only push the water away, but also move the frost line. If you don’t want to use the rigid insulation then go with a layer of straw sandwiched between heavy plastic sheeting. Anything to insulate and waterproof.

I have lived in a +100 year old house with a rubble stone foundation. As long as I maintained the proper slope for drainage I did not have a problem in our heavy (almost pure) clay, even with the frost line down at +60”s.