Slab vs Helical Pier vs Concrete Pier

[ocaj11]

I would like to get some opinions on this topic. We are getting ready to build a house on expansive clay soil and I'm trying to figure out which foundation to use. I've read lots of opinions on the Internet, but still can't figure out the most cost effective way to proceed.

I would like to build approximately 2000 sf in a single story, but, I would consider a 1.5 or 2 story if that was less expensive. My plans are to pay someone to do the dry in and I'll do most of the rest as time allows. I'm not in a big hurry once the house is in the dry. I'm located in Southwest Arkansas around Texarkana and I believe the neighbors (1/3 mile) soil test came back with a plasticity around 45 (not 100% on that). A few of the questions I have:

1. We talked to a helical pier installer and he said they usually go around 6 foot deep here to get the required torque. They seem to rely on the torque value to define satisfactory depth. I'm sure the seasonal moisture variation is deeper than 6 feet. What keeps the pier from moving when the soil expands/contracts with more/less moisture? The installer quoted (ballpark) $165/pier at a 6 foot depth. That's $8,250 for 50 helical piers.

2. Drilled concrete piers are an option, but I think you need to go around 15-18 foot deep to keep the expanding/contracting soil from lifting the pier. There is a guy that drills the holes for $100/hr and I have no idea how many 15 foot holes he can drill in an hour.

3. For a concrete slab foundation, I'm not comfortable hauling in a couple of feet of foundation sand and building on top of that. I'm pretty sure we would need to use some sort of pier to support the foundation during the expand/contract cycles. If I'm going to need piers anyway, wouldn't it be cheaper to go pier and beam? We have a 12 yard dump truck and dozer-trackhoe-backhoe to dig out a few feet and backfill with the proper material, but that seems like a lot of work to me and I'm not sure it would be effective.

4. We built a 40 x 60 concrete pad for our metal shop 3 years ago. We weren't too concerned with the slab for the shop, but we did hire the so-called best concrete guy around to do the work. He poured a 4" slab using 12" footings and rebar on 18" (I think) centers. He installed the vapor barrier as well. Eventhough we have a water drainage issue (haven't had a chance to correct this yet) on the back side of the shop, we've only experienced a crack approximately 1/16" wide and 20 foot long. There isn't any seperation to speak of.

5. If we get the slab on proper material and utilize a proper slope to get the water away from the house, is it possible to avoid a busted slab by digging deeper footings, reinforcing the footings with extra rebar and running the interior rebar on 12" centers? 5" slabs significantly better than 4" slabs? Could we pour on 4" of compacted gravel with a drain under the footings to wick any water away?

Basically, there are too many options to choose from and I'm trying to build a proper foundation without killing my budget.

Opinions???

 

[mjncad]

The Front Range of Colorado is loaded with expansive Bentonite soils. Get a good registered P.E. (Professional Engineer) structural engineer to design your foundation.

A skilled engineer will pick the best combination of pier and foundation type based on your soils report.

This is one are you do not want to skimp on.

 

[ocaj11]

Thanks for the reply mjncad.

I know the P.E. route might be the best route, but, from my past experience with them, they tend to over engineer things to make sure they cover any liability they may have. Over engineering is certainly not a bad choice for a house foundation, but it comes at a hefty cost.

My grandmother's house was built on Bois d' Arc blocks sitting on top of the ground. It was built around 1915 and is still standing today. Of course, the floors are crooked and the blocks had to be replaced in the 70's, but the house is still there.

I'm trying to find that point that's slightly better than good enough. There's probably too much guess work involved to do that without professional assistance.

 

[Jsheds]

If you want to skimp on the project, the foundation isn't the place to do it. Most other parts of a structure can be easily upgraded at a later date, but that's difficult to do with a foundation.

Any soil with a plasticity index over 25 is considered expansive. Soil with a PI of 40 has the potential for significant movement. Many of the larger home builders use ribbed post-tensioned slabs on ground - I have been told they are the most economical foundation for homes on expansive soils. The design of the foundation will be dependent on many factors, from potential vertical rise of the soil, vegetation around the foundation, the size and shape of the structure' etc. A professional engineer will be able to evaluate all of these factors and design a foundation that will fit your needs and soil conditions.

Do a web search for the 'foundation performance association'. The have a lot of information available regarding foundations on expansive soils.

 

[Kendall69]

Here's some reading for you.

Bonded post-tensioned concrete.
Prestressed concrete - Wikipedia, the free encyclopedia

Post-tensioned concrete comes of age: while traditional reinforced concrete construction hasn't changed much in the past four decades, post-tensioned design and construction methods have become simpler - Cover Story Concrete Construction - Find Articles

Post-Tensioning Institute

 

[ocaj11]

Jsheds,

Thanks for that information. I'll certainly look for the 'foundation performance association' and read what they have to say. I've done a lot of research on this subject and I don't think I've been to that site yet.

Have a great day!!

 

[mjncad]

Lots of Colorado home builders have gone bust while making the lawyers rich because they didn't spend the bucks to engineer a proper foundation for expansive soils.

I'd rather have Formica counter tops instead of Granite or Corian; or mid grade cabinets instead of custom cabinets, etc in exchange for a properly engineered foundation.

Best of luck on your project regardless of how you decide to engineer the foundation.

 

[ocaj11]

I'm all too familiar with poorly built foundations. It's going to be my main concern and I plan to make sure it's done properly.

What I would like to do is figure out what over-engineered is and take the money I would spend on soil tests and P.E. foundation designs and put that money into the foundation. Right now, I'm leaning toward heilical piers approximately 10 foot deep into stable soil and deeper than the seasonal moisture variation. I'll bolt the subfloor to those helical piers.

Before I make the final decision, I'm going to read the information from the foundation performance association.

This site is a great resource and I appreciate all the replies....

 

[pat32rf]

Not at all familiar with Texas soils( up here we like basements) but I have always been told by concrete people that a 4" thick slab is suitable for a sidewalk only. Anything structural such as a basement, garage floor or floating slab should be 6" minimum up here if you don't want it to crack.

 

[mjncad]

Not at all familiar with Texas soils( up here we like basements) but I have always been told by concrete people that a 4" thick slab is suitable for a sidewalk only. Anything structural such as a basement, garage floor or floating slab should be 6" minimum up here if you don't want it to crack.

If only the slabs were a true 4" and 6" thick. Out here the guys forming 4" slabs love to use 2x4's and 2x6's for the 6" slabs. Which means you are getting a 3-1/2" and 5-1/2" slab respectively.