Talking with a builder right now about building a new house (first time building). I am trying to determine the best type of slab to build by weighing cost vs. structural integrity, etc. Specifically, I'm wondering what the advantages/disadvantages are between a post tension slab and a traditional rebar slab with under ream piles. This is along the gulf coast so I'm worried about soft soil. Both types seem designed to deal with the soft soil. Any folks out there who are well versed in the concrete world who can offer guidance? Builder is offering to finance a project but will only build a traditional slab with under ream piles.

Post-tensioning the rebar places the concrete under compression to help prevent cracking. The is useful for things like long spans of beams on a highway overpasses, but will not prevent the concrete from settling in the soil. The piles are what prevent the vertical shift/settling of the foundation. A post tension slab can still settle, tensioning just helps with uniform settlement.

In my opinion, post tensioning a house foundation is overkill unless there are extreme conditions. I'm not a structural engineer, but if you are worried, I would just have one look over the foundation design before construction starts.

Also, post-tensioning is more important in clay soil. Clay soil will expand with moisture, and put forces on the concrete. The post tensioning helps prevent cracking in this scenario.

Clay soil is more of a threat than 'soft' soil.

FYI... It isn't "clay" that is the problem... Certain clays, based on their mineralogy are more susceptible to shrink/swell than others. The beaumont formation is one of those clays. Clay in the underlying Lyssie formation are not as prone to the problem.

Agree with the above. Call a geotech, if possible.

Post Tension will keep cracks tight, which is good, but if the slab has insufficient sectional stiffness, you'll still see large deformations in the House itself. This can be just as bad as a cracked slab.

Go here: https://websoilsurvey.sc.egov.usda.gov/App/WebSoilSurvey.aspx

Find the address of the house and create an "area of interest" of the property
Go to soils data explorer tab
Find soil physical properties. Look for "Plasticity Index" (PI)
Find PI for depths of 0-36", 36-60", and 60"-120". Post those numbers here and you will probably get some better suggestions (although this does not replace the foundation design of a PE, which you will have to have in some incorporated areas and if FHA is involved in the lending).

I am not a PE, but I do have an engineering degree and more importantly, I just know stuff. And the answer will likely just get you what you need to give to the PE to design your foundation.

Not a fan of post-tension. Any swage or clip/keeper will eventually corrode/concrete spalls out around it and you no longer have compression. Then what? Designing a foundation with the proper amount of steel and beam depth/piers to do the job works every time. You may also be looking at void forms under the beams, but let a PE tell you that.

The best type of slate is to do whatever type a reputable engineer designs after doing a geotechnical soil investigation..........every lot is different.

Most folks don't worry that much about their slabs when building because they figure they'll be out by the time it becomes an issue.

SWCBonfire said:

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Go here: https://websoilsurvey.sc.egov.usda.gov/App/WebSoilSurvey.aspx

Find the address of the house and create an "area of interest" of the property
Go to soils data explorer tab
Find soil physical properties. Look for "Plasticity Index" (PI)
Find PI for depths of 0-36", 36-60", and 60"-120". Post those numbers here and you will probably get some better suggestions (although this does not replace the foundation design of a PE, which you will have to have in some incorporated areas and if FHA is involved in the lending).

I am not a PE, but I do have an engineering degree and more importantly, I just know stuff. And the answer will likely just get you what you need to give to the PE to design your foundation.

Not a fan of post-tension. Any swage or clip/keeper will eventually corrode/concrete spalls out around it and you no longer have compression. Then what? Designing a foundation with the proper amount of steel and beam depth/piers to do the job works every time. You may also be looking at void forms under the beams, but let a PE tell you that.

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0-36" - 47.9
36" to 60" - 47.7
60" to 120" - 51.9"

If I'm reading the definition correctly, higher this number the wider the moisture content range is to maintain plastic clay characteristics, which appears to be a bad thing... indicates a more expansive soil.

I always get soils report and then send that along with the plans and let my structural engineer handle the design. Majority of the time it is post tension, which i like as it keeps the home moving on a single plane as your soil shrinks and expense.

Old timers and traditionalists in the building business will stay conventional (rebar slab) on piers is the best, when in fact in certain conditions it isn't. I have repaired way more conventional slabs than I have post tension slabs in my 18 plus years in this business.

I only had one instance where a structural slab was recommended (carton form void boxes) as this is more common in commercial applications but also by far the most expensive.

All that being said it also comes down to economics and peace of mind, which is why i leave it to the expert to design the foundation, then i have him on site to do a pre-pour inspection, then i have him as a pour moniter while we place concrete, and then i have him do a post pour inspection and/or performance letter if post tension. Never had an slab issues working it this way (knock on wood) and hope to keep it that way.

Best of luck.

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I have repaired way more conventional slabs than I have post tension slabs in my 18 plus years in this business.

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There are probably a lot more conventional slabs to be repaired than post tension, especially if you figure in years when they became popular. And there are plenty of them that were made shoddily.

OP, that is some relatively expansive stuff. I'm sure the PE will want to know PI even deeper. Rough estimate based on what I've seen on a conventional style slab in similar PI would be a 4" minimum slab w/ #3's 12" on center; 10" wide x 30" or 33" deep perimeter beams w/ (2) #7's top and bottom & stirrups on 18" centers; interior beams 10" wide x 24" or 27" deep of similar construction.

This is probably a good starting point for estimation purposes and will still require the input of a PE. It is worth exactly what you paid for it, maybe less.

I ended up talking to some engineers, and the consensus was that the under ream piles with a traditional slab is considered the better slab (was talking in general terms, of course, not in my particular case). I obtained a soil analysis done for the subdivision, bore site very close to the lot, and there is a more sandy layer (PI of 18) about 8-10 ft down to anchor to with the bell footings. The builder told me that they just keep digging until they hit that harder layer. I figured that if I witness the change in color of the soil (which I know from the analysis) while they're digging I can ensure they've gone deep enough.

FlowCtlr said:

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I ended up talking to some engineers, and the consensus was that the under ream piles with a traditional slab is considered the better slab (was talking in general terms, of course, not in my particular case). I obtained a soil analysis done for the subdivision, bore site very close to the lot, and there is a more sandy layer (PI of 18) about 8-10 ft down to anchor to with the bell footings. The builder told me that they just keep digging until they hit that harder layer. I figured that if I witness the change in color of the soil (which I know from the analysis) while they're digging I can ensure they've gone deep enough.

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TLDR WARNING- many words ahead...

If you have a geotech report look and see if it gives you BRAB numbers for a rebar slab, Em/Ym numbers for post tension designs, or pier depths recommendations. If you have any of those numbers an engineer can get you a foundation designed for around $2,000 unless you are building a truly monstrous house. If the geotech report doesn't give you that info for your lot you'd be looking a another $1-2000 for a site specific soil report. This cost may in fact save you money in the long run. We have customers that plan on doing deep beams/piers because that is what they always do. If they can get a good soil report we can scale back the foundation design and save more money on concrete/reinforcing than they spent on engineering fees.

Look for a page that has info that looks like this:



The builder is giving you questionable information as far as how deep piers need to be. You need to stop thinking about how soft/hard the soil is and think about the potential expansion of the soil. Your house will end up weighing between 150-450 pounds per square foot. The soil can handle it. In fact, your house is so light that it will move up and down if you choose the wrong type of foundation. There is a common misconception that we use piers in this part of the world to get down to "bedrock". You would have to drill hundreds/thousands of feet to hit bedrock in much of the gulf coast. We use piers in clay soils to fight uplift and soil movement. Think of the pier as an anchor that prevents the house from lifting as the soil moves. You likely have an active zone of 4-5 feet. This is the area in the soil that has shifts in moisture content and consequently moves up and down. You want to anchor your piers a minimum of 2 times deeper than the active zone. You are using the friction of the soil on the sides of the pier to support the foundation and to hold the pier down against uplift. In your case going only 8 feet with the piers would barely get you to 2x the active zone. This is further complicated with the fact that you don't count the area above the bell/ream into your uplift calcs. If you do an 8' deep pier with a 24" bell you only end up with 2' of "anchor" below the 4' active zone. Check pages 60-61 of the link below to give you an idea of the active zone around the Houston area and common pier depths. They sampled 23 different geotech companies' reports and found average pier depth to be 11.7'. Page 13 also shows a diagram of what I mean by the active zone. Lots of math too if that's your thing.

http://www.foundationperformance.org/projects/FPA-SC-16-0.pdf

Your builder shouldn't have any reluctance to use and engineer. Right now, if the house breaks in half due to a bad foundation the liability is on him 100%. If he uses an engineer then the liability moves to them. This should help you sleep better because the engineer probably has better insurance. I grew up in a house in the panhandle that had major foundation problems. The builder used the same foundation design that he used everywhere else in town. It just so happened that our neighborhood had way more clay than the rest of town. My parents sued(along with many other folks in the neighborhood) and won. Builder went out of business, declared bankruptcy, and my parents never saw a dime. Having an engineer on the hook will hopefully mean that you don't have an issue with the foundation but will definitely mean you have someone else to foot the bill if you do have a problem.