Building on expansive clay soils?

Started by DutchMo, January 03, 2015, 08:44:02 PM

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DutchMo

Hi everyone,

I've been reading posts here for about a year, but I figured it was finally time to join the fun.  After searching for our wilderness retreat and hunting land for about 18 months, we've finally made an offer on 80ish acres about 60 miles from our primary residence.  I'm excited to begin the process of developing our get-away-from-it-all spot!  Once we close and drop a huge chunk of change on the table, it will be back to saving up for a couple years so we can start to build, and we will be planning in detail while we save.

As part of the closing process, we had a soil morphology test done where we will build to determine suitability for septic.  We were approved for an engineered shallow field system, as we have a huge layer of clay at about 24 inches.  This is pretty typical for my area (mid-Missouri), so I know others build on this stuff, but I want to make sure I do it correctly.

Now on to the question: what special considerations do I need to take building a foundation on expansive clay soils?  I'm considering a full perimeter foundation, either a slab on grade or a crawlspace.  I'm also looking at possibly a frost-protected shallow foundation, or maybe even an on-grade mat foundation if that is warranted.  I'll probably hire the foundation out once we've got money for it, and do the rest, as much as possible, myself.

Some sources say you have to engineer a special foundation.  I've heard from others that everything will be okay if I just put 6-12 inches of gravel beneath my foundation, add adequate drainage, and grade away from the build.

Does anyone have experience building on this type of soil?  Any stories on what works or what doesn't when building in clay?

Thanks in advance,
Dutch

Ps.  I always enjoy seeing everybody's pictures, so here's a couple shots of my boys who came out with me for the soil evaluation.  The excavator operator was very cool and insisted the boys climb in the cab for a pic!




hpinson

I'm not able to help answer your question, but the picture of your boys in the excavator made my day.

w*


John Raabe

I think your best advice for expansive clay soils is to get opinions from local foundation installers and/or a soils engineer. It sounds like you are on the right track, but there are some locations where freezing and expanding clay soils can tear concrete walls apart with things like "ice lensing". Local solutions are likely to have evolved to solve such issues.
None of us are as smart as all of us.

MountainDon

Quotewill be okay if I just put 6-12 inches of gravel

That might help IF there is slope away from the gravel trench to drain the water away. Otherwise the gravel may simply become a place water can flow into easily. The gravel would also need to be properly compacted so it does not settle after the concrete is done. John's advice to check locally is good advice.

I believe there are different requirements for shallow frost protected foundations depending on if the building will be heated continuously or not (as in a part time recreational cabin left unheated over the winter.)

Just because something has been done and has not failed, doesn't mean it is good design.

UK4X4

My soil report came with suitable foundation designs for the plot,

But through investigation into local issues and previously attempted and repaired foundations we were able to exclude some of them and eventually designed a foundation that the builder, soil engineer and enginever who signed off the design, all agreed with.

We have sand silt, expansive clays and winter runoff to contend with.

The house foundation was eventually poured on top of 4 ft of crushed  rock and drainage pipes, which basicly removed any water and hence any issues with erosion of sand silt and expansion of the clays.

Take some time to talk to others in the area, and see what has worked and what has failed.......my  closest  neighbor for example had 14" of movement from one side of his house too the other, others had cracked foundations before they'd even finished construction and others who,s deck left the house during its first winter

All those designs were thrown out, my conditions are relatively difficult though but the process should be repeated.....check to see what works in your area !


DutchMo

Thanks all!  I knew I'd find some good advice here.

I'll talk to my soon-to-be neighbor and ask what he has done, as well as definitely talk to building professionals in the area.

I plan to have the foundation poured professionally anyway, so I'll have the advantage of a local builder's input, but I also like to learn for myself as much as possible also, if only to be more conversant with the professionals working with me.

UK4x4, my soil report was done for septic only.  I did ask the soil scientist if he would recommend an engineered foundation and if he did that type of work.  He said that he could provide specs for an engineered foundation, but that the engineering would be overkill (expense of the engineering work would exceed the cost of just putting in drainage).  He's the one that suggested building on a gravel pad instead.

UK4X4

Gravel does allow drainage but the stones are mostly round and dont form a stable platform, even when compacted they just bounce arround

crushed rock drains the same but naturally forms a stable platform as the shards mesh with each other, compacting makes it even stronger.

we added drainage in a grid under our rock, diverted to the front and rear of the land, the topsoil etc is all graded away from the foundation,i still need to add some 'ground drains" to stop erosion and remove the water faster as in places the runoff is making holes.

I added stengthening beams to the design to make the base rigid enough to move in one piece, basicly just like the perimeter edge drop on a flat foundation making it more of a raft.

Mines probably over kill, for where I am but its working so far and I'm not concerned, whatever happens my foundation will be still there for my grand kids !

DutchMo

Quote from: MountainDon on January 05, 2015, 01:39:09 PM
I believe there are different requirements for shallow frost protected foundations depending on if the building will be heated continuously or not (as in a part time recreational cabin left unheated over the winter.)

MountainDon, you are correct.  Per a PDF guideline from HUD, seasonal dwellings are considered unheated spaces.  Basically the only difference for unheated spaces is that the insulation extends under the entire foundation, and there has to be six inches of gravel or crushed rock beneath the insulation.

Here's a pic from the PDF:

garyc

I live in lincoln. co. missouri we have the same problem . What we do is dig down 2 feet deeper and fill with 1'' clean rock and drain tile on both sides of the footings.
If it wasn't for bad luck . I would 't have any luck at all.