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.  :)

S