Cement Stabilized Post Backfill

[Ernest T. Bass]

Hi all,

I think I asked this question indirectly through a different thread in the past, but the time has come for us to decide on a few things right away, and I hope to get an answer from you smart people... 

We are building a PT wood pier foundation for our smallish cabin. Our soil is rotten.. An expansive clay loam. Since we have absolutely no drainage, I think that backfilling around the posts (4' deep) with the typical gravel type mix is only going to cause trouble, as I think the holes would fill up with ground water and stay that way. I have seen others pour concrete all the way to the top of the hole surrounding the post, but that leads to big problems when moisture seeps down between the two.

Also, these are 24'' diameter holes, so any imported fill is going to cost $$.

Would it be possible to mix a small amount of cement in with the excavated dirt and pound it back in the hole it came from? As far as I can tell, there would be two concerns..

#1 The stabilized clay would still absorb moisture and expand in the winter.

#2 It would behave too much like concrete and trap moisture against the post.

Am I missing any concerns? Would this method work?

If there is any potential for trapped moisture seeping down around the post, would several inches of gravel underneath the stabilized soil help drain it out?

Thanks,
Andrew

[MountainDon]

How deep is the clay/loam? Does it go on forever?

Why a 24" diameter hole when you are planning on using PT piers? Is it for the footing? That I can understand.

Why PT piers; why not an 8" or so Sonotube and poured concrete piers if the ground is so wet and you do seem to be concerned about that?

Or even Bigfoots with Sonotubes?

Sorry all I had were questions.

[Ernest T. Bass]

Yeah, the clay just gets harder and harder the deeper you dig.

Yes, the wide hole is for the footing. We'll probably drill something more along the lines of an 18'' hole and widen by shovel at the bottom, but there's still a lot of fill there..

The wood piers worked out cheaper and easier, especially since we will be hauling in the materials by hand.. But, even concrete piers would need a stable backfill, right?

[glenn kangiser]

You should be fine with soil cement backfill, Andrew.  Once you stabilize it with cement, it is no longer expansive.  That is where part of the word stabilize comes in.  The rest of it is that it is no longer easily water soluble.

Usually around 5 to 7 % cement is enough to stabilize it.  You may want to try a sample first and just use a bit over the minimum necessary. Try a couple different mix amounts and see which one feels best.  It won't be super hard but will be fairly stable in water in a couple days. The clay will flash off like concrete, ie: in about an hour or two it will set up pretty good --- not as hard but a definite set.  It will still transfer the water away from the treated post as the rest of the earth dries out as it allows the migration of water through it without dissolving.  It is about the same as rammed earth.  Rock and sand added to it up to around 50% could also help minimize change effects but is not necessary.

I would put a good concrete pad - with crossed rebars on the bottom to help spread the load to the soil below.

[bobtheengineer]

check out this link http://www.postprotector.com/

Its a sleeve to protect the post from any contact issues.  It might be the ticket for something in constantly wet soil.

[Ernest T. Bass]

I would put a good concrete pad - with crossed rebars on the bottom to help spread the load to the soil below.

Yep, that's what I'm planning.. I was thinking more of a # pattern for such a wide pad...

And, (stupid question), we're talking about Portland cement, not redi-mix, right? I've never mixed my own concrete, so I'm not familiar getting plain old cement.. 

Also, do you think the layer of gravel is unnecessary? You made it sound like the stabilized earth would still be somewhat drain-able, which is what I was hoping for..

Thanks! This really helps set my mind at ease..
Andrew

EDIT: That's an interesting product, Bob... one of those simple type things that you never think about. 

[glenn kangiser]

Yup -- cement is cement, redi-mix is concrete mix ie: has rock and sand mixed in.  Cement is not concrete - it is an ingredient of concrete.

The soil cement will be pretty firm and will wick moisture through it.  Not much need for gravel if there is no place for it to drain to.

I have seen the post protectors, Bobtheengineer and I think they could be good or bad.  If they are 100% sealed and the wood going in is dry maybe I'd trust them, but if the wood is wet or there is any split or other way for water to get in then they become an incubation chamber for bugs and mold -.  The only 3 posts I lost here were protected with multiple plastic layers but the water in the logs drained into them and rotted them completely out along with pine beetle larvae who just - had a great lunch took about 4 years to be noticable - 6 years all were gone.

[MountainDon]

Think of cement (like portland cement) as the glue that holds all the other parts (sand, gravel) together. Concrete is the finished mix/product. Most people don't use the terms correctly. 

[glenn kangiser]

I was taught by my ex-wifes uncle who was a union concrete man, so I had to learn to use the terms correctly.  To bad we had to quit using him -- he was too slow.

The cement holds the dirt together too and reacts with the clay to firm it up and stop change.  Clay that is stabilized with cement for floors does not get as hard as non-stabilized natural clay that just dries (if it doe not get wet).  It's kind of like air entrained.  It would like to get smaller and harder but the cement stops it mid shrink.  Therefore a natural cob floor is more durable with linseed oil on it than a stabilized clay floor.  Dishes don't break when you drop them on earthen floors.