Foundation over organic soil
Foundation over organic soil
(OP)
The Geotech found organic soil which was apparently left in place by a lazy developer and is recommending a "stiffened" foundation to compensate for approximately 1/2-1 inches of non-uniform settlement over a 5-10 year period.
Question is, what design method do you guys recommend for this scenario? I've designed foundations in the past over clay using WRI's procedure. However, this method is based on the PI of the soil, which is absent here.
The slabs will be mono pours. Preferably not PT due to it's unpopularity/cost in the area. I would think the Geotech would recommend piles????
I've tried calling the Geo, but he won't return my calls....
Question is, what design method do you guys recommend for this scenario? I've designed foundations in the past over clay using WRI's procedure. However, this method is based on the PI of the soil, which is absent here.
The slabs will be mono pours. Preferably not PT due to it's unpopularity/cost in the area. I would think the Geotech would recommend piles????
I've tried calling the Geo, but he won't return my calls....





RE: Foundation over organic soil
You have the expected settlement which is a good starting point but you need to know (or estimate) over what distance this may occur. Then once you have that, you should be able to estimate the forces.
You might also discuss with the geotech what he recommends for the slab-on-grade.
We have dealt with similar situations over the years and I remember getting the Owner to buy into the risk of going with a structure that might experience some settlement due to buried topsoil/organics. The Owner needs to know and buy into the fact that you are creating a design to save him $ upfront but it could possibly settle in the future. If he doesn't want to buy in, then you can go with a more expensive design. It is really his choice, not yours.
RE: Foundation over organic soil
I have dealt with"competitive engineer's" recommendations about "organic soil and many times they are way too conservative. They even talk about future decay of soil that has been organic for centuries, all decay is done.
In some localities the topsoil is on the order of 4 feet thick and no problems occur building on it. Dark color is not always a red flag.
I'd do some checking on what they mean by "organic" before getting too worried about it.
It immediately raised questions in my mind when some long term settlement was mentioned, rather than immediate settlement. I suspect you are getting advice that may be based upon the habit of "stripping topsoil" when that always is not needed, depending on what the topsoil is.
Many a foundation has been built on organic soil with no problems, but doing so only when you know about the soil's properties.
RE: Foundation over organic soil
Basically, I am just questioning what exactly a "stiffened" foundation is?
RE: Foundation over organic soil
RE: Foundation over organic soil
The 6 inches may not be anything to worry about, and 1/2 to 1 inch of settlement from 6 inches of organic soil with roots? There must have been plenty of roots in the test pit. Try to get some more info from the geotech on why this is. This could be an area that was a former slough (etc.) and he may have experience to warrant the added caution.
Were you thinking of putting the footings on top of the 3 1/2 feet of the topsoil fill?
RE: Foundation over organic soil
Digging out the organic zone is not an option because they are wanting to build soon.
Yes, the footings will be at grade level.
RE: Foundation over organic soil
Is this a regular client or a one time only? If the latter then get out while you still can.
RE: Foundation over organic soil
Have you considered putting bored concrete piers under both the footing and the slab, founded down to the higher bearing underlying insitu material. They can be a fairly cheap option in this sort of situation.
HTH
Michael
RE: Foundation over organic soil
I suspect it means doing more than "nothing special".
If some improvement is needed, I'd recommend that the walls have reinforcing top and bottom for sure, possibly two #4's each place, but adding more within the walls cannot hurt.
Rods in the footing may help a little, but not nearly as useful as in the walls (your Structures course 101, remember).
Don't forget to run rods around corners and make the laps long. For some reason ordinary folks get all shook up seeing basement walls with cracks.
On roots in soil, gosh, unless it is sod, I'd not worry. If scattered roots would be something to worry about, my garage on fill, over stumps would have me very sleepless nights. Soil arching is a great thing.
RE: Foundation over organic soil
As far as oldestguy building over fill, he is a professional and therefore understands the risks. Most homeowners and builders do not (or don't care), if something goes wrong it may well be your neck.
If the owner decides that it is too much money and/or would take too long, then that is their problem and they were warned. As far as time to remove and replace, 3 1/2 - 4 feet for most homes is not really that long, as total time to construct a home goes.