Sunday, January 12, 2014

The Dirt People....

The start of a promising career as a geotechnical engineer.

I mentioned something last week in a post about having to go see my geotechnical engineer, and immediately my friend Betina at Pixel Peeper perked up wanting more info as she works in the geotechnical industry in Florida.  So this post is for Betina, and anyone else whose socks are already sorted, whose kitty litter has already been changed, and who can't find anything but a test pattern to watch on TV.  Zzzzzzzz.....

Here in north Texas we have very unstable soil.  It is a very heavy, black clay, and is hard to build on.  The problem is clay soil acts like a sponge....when it gets wet it swells noticeably, and when it dries out during the hot, dry summer months it shrinks.  The technical terms are "heave" (swell) and "settle" (shrink).  This repetitious pattern of heave and settle breaks apart a home's foundation if it's not engineered properly.

(Betina, I suspect in Florida that isn't your challenge at all, is it?  Aren't you more concerned about the subterranean rock dissolving, leading to sinkholes?)

Before we start construction of a new home we have a geotechnical engineering firm come out and do several borings into the ground.  A big truck raises a boom vertically, then drives a pipe deep into the ground where it penetrates the different strata of dirt and rock; black clay, brown clay, various types of fairly soft limestone, and eventually into rock hard enough to support the weight of a house.  In my area of town these borings usually go down about 20-25 feet.  In the most unstable parts of town (western Dallas county, for example, in the Eagle Ford Shale formation) they might go down 50' or more.

Engineers at a lab then extract the plug of earth from the pipe and analyze it for its "plasticity", moisture content, etc., and give me a detailed report of their findings.  For my boring depth and at my relatively close-in location, this total procedure usually costs $2-$3,000.

The most important thing for me is to know the PVR, the "potential vertical rise"....exactly how much might it heave/settle?  This determines what type of foundation my structural engineer designs.  Our preferred type of foundation (post tensioned) can handle several inches of heave/settle without piers.  Up in the 4-5"+ range we install piers that go down to bearing rock.

FYI, a post tensioned foundation is one where a huge chunk of concrete is poured on top of the ground.  The concrete is only 4" thick, but about every 10' or so there are ditches (beams) that run both front-to-back and side-to-side.  These are generally 10" wide and 30" deep, depending on the stability of the soil.  It's these beams that give a foundation its strength.

Steel cables also run both directions through the foundation, and after the concrete has hardened (cured) for at least a week, these cables are tightened (by pulling on the cables sticking out the sides of the form boards) applying many thousands of pounds pressure per "tendon" which draws the concrete tightly together.  It is "tensioned" (the steel is stretched) "post" (after) pouring of the concrete.  Get it?

As mentioned, if the heave/settle range is high, we install piers.  A pier truck comes out, raises a vertical boom, and drills holes into the ground approx 12" in diameter and 2' into hard rock (20' deep +/-), with 1 pier about every 70 sq ft of first floor space.  These holes are then reinforced with rebar steel and filled with concrete, making a reinforced concrete column.  Then a post tensioned slab is poured on top of the ground with the beams resting on the piers.  This transfers (much of) the weight directly to the solid rock underground.

If the heave/settle rate is off the chart, the deep beams are still resting on top of the piers, but "void boxes" are positioned between the top of the ground and the bottom of the concrete.  These void boxes are essentially just heavy cardboard boxes the concrete is poured on top of, then a short time later the cardboard void boxes deteriorate and ultimately disappear.  This leaves a hollow space (void) allowing for the dirt to heave (and settle) without pushing up on the slab.  I've never had to use void boxes.

Note that in north Texas we rarely put in basements.  That's because: 1.  our soils will move enough to tear basement walls and floors apart; 2.  we don't have to worry about installing a foundation below the permafrost (we have no permafrost); and 3.  if we dig a hole deep enough for a basement we'll usually hit oil.  ;)

Sadly, this is what often happens:  A geotech will do a legitimate subsoil analysis, but an unscrupulous builder will get an unscrupulous structural engineer to disregard the geotech and design a woefully light weight, weak foundation.  This saves money ($10,000+), making the builder look like he's giving a better cost-per-foot value.  

Buyers just take the concrete for granted and instead concentrate on the pretties....appliances, granite, light fixtures, faux finish paints, etc.  Then a couple of years later the foundation fails and the builder disappears and it all winds up on TV, giving all builders a bad name.  *shaking head*

So nothing happens without a geotechnical engineer first identifying what type of soil conditions we're dealing with.  If you have a well-built home on a good, solid foundation, it's because you had a thorough geotechnical analysis, a competent structural engineer, and a conscientious builder who didn't cut corners.  

Answer your questions, Betina?  If not, let me know.  :)



  1. If I ever need to build a home in Texas, I will first give you a call. And I think I would like to go with a basement and hope to hit oil.

    Now I have socks to sort.

    1. You're assuming I didn't keep the minerals rights. ;)

  2. Dang! I knew I should have demanded a basement in one of our Texas homes. I'd be like Jed Clampet now.

    I am new to this Florida place, but do know there is about 2 feet of very sandy soil on top of sea shells. The County decided to put up a new fence on the path I walk each day. They drilled through a foot of sand and then 3 feet of tiny sea shells came out of the holes. The shells were mostly intact and about a 1/2 inch long. I may take a camera with me today and get a pic. Thanks for the informative post.

    1. Ha! Drill a hole in the ground and shells come up. That's funny. :)

  3. I learned a lot reading this informative post. Thanks.

  4. I'm flattered that you wrote a post just for me - and such a detailed, informative one at that! Yes, you did answer a lot of my questions. It seems you guys in Texas are doing it right. I knew of the shrink/swell clays you have there and wondered how you dealt with it.

    Here in Florida we have some of those clay soils (Gainesville, Lake City, Tallahassee area), but houses and foundations are built without a geotechnical investigation beforehand. And during especially dry or wet years (when the clay shrinks or swells and does its damage), homeowners get terrible cracks and damage to their walls and foundations. Oftentime in Florida, they think it's a sinkhole causing the damage. Then we do a sinkhole investigation, only to find out it's the clay causing the problem. And while the homeowner's insurance will pay to repair damage and to fix a sinkhole issue, they won't for damage due to shrink/swell clays. They are just plain out of luck...

    Foundations on residential buildings are typically a concrete slab-on-grade, and code requires a 12-inch embedment. Sadly, the loose sands that we have here as the top layer are frequently not compacted enough, which leads to differential settlement...

    You know, if you ever make it to Florida or I make it to Texas, we need to sit and have a beer and talk dirt and foundations. I'm fascinated by how much better Texas does it - we always assumed that Texas was such a pro-business state they just let businesses do their own thing and cut corners wherever they wanted.

    I know New York is considered to be somewhat of a "nanny state" by others, but they don't require a geotechnical investigation ahead of time, either. In a new suburb of Buffalo a few years ago the houses suddenly started having major problems - foundations that broke in half, walls that fell apart, etc. It turned out that this one suburb had the shrink/swell clays. The homeowners turned to their insurance companies for help and their claims were all denied. Now dozens of homeowners are stuck with houses that are more expensive to repair than what the houses are worth.

    Thanks again for writing this post!

    1. You're welcome Betina. I can't imagine how a builder can put down a foundation without a structural engineer's input, nor how a structural engineer can do a design without knowing what soil conditions he's up against. Don't the builders there have any liability? I wouldn't dare even think of doing business that way.

      I can't speak for the entire state, but I can say that here in the DFW Metroplex no city will issue a building permit without submitting an engineered foundation design, which includes a geotech investigation. But as it has and always will be the case, shysters will make end runs around the rules.

      Have a beer and talk dirty to each talk DIRT to each other? Great! I'm sure that would be fun for us, but I'm guessing our significant others would be bored to tears. :)


    2. It appears that rules and regulations vary widely from state to state. I mentioned your post and the process in Texas to my co-worker today (a geologist) and he suspected that the shrink/swell clays in your area are so well researched and documented that something HAD to be done. He cynically opined that once enough people here in Florida got sued, some rules might get tightened, too.

      I agree with you on the talking dirt...K and Dave would probably roll eyes and/or start snoring! LOL.

  5. I found this fascinating! I remember in 8th grade being really interested in science because we studied geology. Wonder why I never pursued that interest?