Foundation over organic soil

This soil must not be too highly organic otherwise the geotech would have you take it out.

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.

How organic is it? How stiff is it? What in the way of local problems are there in same situation.

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.

The topsoil was silt/silty sand over fine sand. They then put 3-1/2' of new soil over top. I guess the geo is worried about the roots he encountered in the 6" layer of original dark top soil that were left by the developer. Truthfully, this is probably what we build 95% of our houses on, just this contractor decided he wanted a test pit dug.

Basically, I am just questioning what exactly a "stiffened" foundation is?

As the oldestguy said, many buildings have been built over low organic material with no detrimental effects, but you better be sure what you are dealing with. The geotech needs to give you a litttle guidance here.

How much more would it be to dig out the extra six inches verse designing a stiffened foundation? How sensitive is the structure to differential settlement?

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?

This is for single story residences. Differential settlement would be a problem. I agree that 6" of top soil with what I would describe as some or few roots wouldn't result in what I believe is 1/2-1" of settlement.

Digging out the organic zone is not an option because they are wanting to build soon.

Yes, the footings will be at grade level.

Lack of information on the geotech, poor quality contractor...it may be a good time to get out while your PI is still intact.

Is this a regular client or a one time only? If the latter then get out while you still can.

Don't know for sure but it appears from your postings that the problem is not specifically the fact that you have organics. It sounds more like your actual problem is that you have contaminated fill. Basically just a layer of cr*p.

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

Stiffened foundation? Probably a local term.

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.

Can you determine how well the fill was compacted, and what exactly it contains? If not, I would recommend digging it all out. For all you know it could have just been dumped and then smoothed over. You may dig it out and find that it had been used as a garbage dump (it is rare, but it does happen). I would personnaly recommend digging it all out to competent natural soils, especially since differential settlement will be an issue.

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.