Best choice between residential foundation designs? 1

[ttech83]

We are building a home in Kaufman county, Texas and retained a geotech firm to perform a soil boring analysis of the foundation area. Upon reciept of this report we forwarded it to a foundation company for design. The company uses an out side design firm to arrange the plans for the home. I spoke to the "designer" about my concerns for cracking inlight of the clay tile roof, portland stucco exterior and the homes long and irregular (saw tooth) shape with a large front porch and attached playroom off the front porch all under one roof.
The geotech gave several options for foundations however, the "designer" seems to promote a waffle slab with no piers. To do so the geotech states 8 feet below the house must be removed, conditioned and reinstalled.This seems to me to be the least desireable way to go, youstill have slab on the ground.
Some of the soil values are as follows,
Clays with a PI of 32 to 41 to depth of 29 ft.
BRAB deign values are: climatic rating- 21
Effective PI -35
support index,c 0.81
If an option for all types are given and a designer perfers the last choice of the geotech is this my "best" choice based on performance or just price?

 

[JAE]

A couple of thoughts on this:

1. If you don't use a "waffle slab" - what I call a stiffened slab-on-grade, your only other options are:
a. Use a suspended structure - either suspending the perimeter and other load-bearing walls with spanning grade beams on piers. This would still leave you with a slab-on-grade and a need to overexcavate - but maybe less than 8'.
b. Or you could suspend the entire floor structure with piers and have a crawlspace below. For a house, suspending an entire footprint of the structure can get pretty expensive (slab-on-grade with overexcavation is usually way cheaper than a structural system on piers.

2. A stiffened slab-on-grade is used extensively across Texas - granted with a variety of results. For your zig-zag house, I would highly recommend using expansion joints through your walls and roof to allow some differential movement - this will greatly minimize cracking.

Eight feet of overexcavation sounds a bit high based on my experience in Texas (9 years in the San Antonion area).

 

[dirtdoctor]

What about a post-tensioned slab? Perform a lesser undercut, install a stiffening mat w/ grid, fabric, stone, etc. (geotech's choice), and use shallow foundations.

 

[JAE]

By the way...isn't Kaufman Co. the county that the old man in the song "Mr. Bojangles" referred to as his birthplace? (per the Nitty Gritty Dirt Band version)

 

[gandersen]

dirtdoctor:

What has been your experience with post-tensioned slabs? I have seen some designs where the tensioning rods were so far apart and the interior grade beams into which they were installed were so shallow that the effective stiffness and corresponding bending strength of the foundation was far less than a waffle slab such as described in this thread.

 

[Focht3]

I'm in San Antonio, but I've done work in Terrell, Dallas, etc. I have a few comments.

Eight feet of "remove, moisture adjust and recompact" is too much. You will have settlement problems if you go that route, unless you replace the soil with properly placed crushed rock base. I'd go with a structurally suspended slab before I chose that option.

A stiffened slab can also work - if it is properly designed and constructed. Ditto for a post-tensioned slab. But don't cut corners to save money! You will regret it in the end!!

You can also look at chemical treatment, although this will require careful pre-screening of sub-contractors to weed out the charlatans and snake oil salesmen. And forget about water injection: it's a waste of time and money that could be better spent elsewhere -

One final point: make the house footprint as close to a square as possible. Floor plans shaped like "L", "Z", or square "U" are very susceptible to damaging soil movements.



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[dirtdoctor]

Right on Focht3!! Living on the east coast, I don't see to many expansive clays like there are in Tejas. But, we do have some areas that are succeptible to differential settlement - I guess this would be similar to expansive clays, particularly if the strata varied across the footprint. If you were to install a stiffened mat using grid, stone, and more grid, would this be sufficient to take care of the high differential settlement found in Tejas?? Just curious on the matter - like I said, we don't have those nasty clays here.

 

[Focht3]

Hmmm,

It hasn't been tried to my knowledge; but theoretically it could be used to reduce differential settlements - under some circumstances. I've had large commercial projects with 8 feet of crushed limestone road base underneath, but those are the exception. (Those are usually on sloping sites - we have to address the cut/fill transition, groundwater, expansive clays, etc. It is more complicated than the typical "flat" site the receives 3 to 6 feet of crushed rock fill.) However, the crushed rock is pretty good, and we're blessed with an abundance of it in town. The rock is very easy to compact to 100+ percent of modified Proctor; settlement isn't an issue most of the time.

San Antonio straddles the Balcones Escarpment - limestone to the north and west of the escarpment, marls, chalks and clays to the east and south. The "chalk" is the toughest problem - discontinuous bentonite seams throughout - this sample has a PI of 2, that one - 100+. Yuck!

Geogrid has been successfully used for asphaltic warehouse entries, truck lanes, etc. But the expansive soil movements required to mobilize it would be too much for most buildings. Swell pressures in the highly bentonitic soils can exceed 16 tsf - too much for a grid (or a building!) to resist and prevent movement...



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[ben1212]

I work in Austin as a structural engineer and we have some areas in town (especially to the east) with highly expansive soils.

The problem with remove and replacing 8 feet of soil is getting the select fill compacted properly. On a house job I would not do this, there is less quality control on house jobs. On commercial jobs it is much easier to get QC and testing labs on site. If the select fill is not compacted properly it will settle also. Nevertheless, I would recommend a suspended slab and beams on void forms that bear on underreamed piers. This is what I have done in the past. The void forms are less costly than a fully suspended slab with crawlspace. Check out

http://www.surevoid.com

Control joints or expansion joints should be located in the slab where appropriate and it should be stressed to the general contractor the importance of controlling cracks in finishes such as tile, masonry, etc.

It is also not a bad idea to note your concerns about differential settlement in a letter to the owner or contractor to CYA, because afterall this is not an exact science. Foundations on soils such as these are prone to movement even after you fully suspend it on piers. I've prepared such letters in the past and even convinced one owner to sell the lot and build on better soil (his decision was also based on the fact that houses in the same neighborhood had movement problems).

 

[canoeman]

OK I’ll bite
Having grown up in the east where nearly every building had a basement, and then moving to Dallas, Texas 30 years ago, where only very expensive houses have basements, I have to ask why?
I’ve been working in construction here for 30 years and still can’t fathom the idea that basements wouldn’t solve 90 percent of the foundation problems I see here.
Here in the Dallas area we see a great deal of weathered chalk which is at or near the surface like over by Love Field.
When they built the drainage tunnels under central expressway the chalk was praised for its stability and workability. From my experience working with the stuff it’s very stable. It’s the Black clay which is expansive.
So could someone tell please tell me why we still adhere to the notion that you can’t build a house with a basement in Texas. I built one under my house and it solved my clay uplift problems during our annual wet dry cycles.
Comments?

 

[Focht3]

You are referring to the Austin Chalk Formation. This formation is found along most of the Balcones Escarpment, which extends from the Uvalde area, through San Antonio, along the I-35 corridor into Dallas, then on toward Texarkana. Geotechnical experience with this formation is pretty extensive since it passes through quite a few of the larger cities in Texas (Dallas, San Antonio, Austin, Waco, Temple.)

Most of the formation is a pretty good chalk, and it did hold up pretty well in the Central Expressway cut. But it is important to understand two things about this formation:

1. It contains lenses of montmorillonite, and
2. It weathers to expansive clay.

The Eagle Ford Shale also weathers in a similar manner. The Eagle Ford has a well-deserved reputation for its' expansive nature; but the Austin Chalk can be just as problematic.

Many a developer has been fooled by the 'chalky' nature of this formation. One of my competitors here in San Antonio claims that 90 percent of the local foundation problems are in the Austin Chalk. He may be right on this point.

But don't take my word for it; ask one of the larger local foundation repair companies. I'll bet they can name a few of your neighbors that have had foundation work done.

Why no basements? For the longest, land was cheap. You have to deal with expansive soils. And there's no local expertise with basements.



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[JAE]

Focht3 - a little off the subject, but all this talk about Texas has the nostalgic part of my brain going. I used to live in SA (in the 80's) - worked for a grand old structural firm down there that has since been bought out. (WESCO).

The expansive clays certainly amazed me upon my arrival in SA in 1982. It took me a number of years before I started realizing that when I saw one part of a structure that had "settled", it was really rather the other part had been raised.

One of our projects involved using "snake-oil" - can't remember the firm, but they were touting it all over Texas and in the Denver area as well - a kind of red cranberry material that was pumped into the ground in a grid fashion -

The theory was that this chemical "satisfied and ion" or something.... whereby the water wouldn't force the platelettes of soil to turn on end, thus, the expansion. Is it still being used down there? (Left Texas in 1991).

The problem was simply getting it distributed into the ground.

 

[Focht3]

So you worked for "old man Simpson", eh? (Or Willard Jr. - Sr. was probably gone by the time you went there.) He was a real pioneer - few realize that he was the driving force in developing a mechanized drill pier rig, and a big reason that drilled pier construction is so prevalent now.

You were referring to STTI and Jack Karam - and his now "famous" bug juice. Jack sold that part of his business to Hayward Baker about 10 years ago; they are still using it, but not as often. Jack is still around - inventing new equipment and processes for other construction needs. He's a real character -



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[JAE]

Yes, 8 1/2 years under Simpson Jr./Steadman/et.al - and even the very old M. Gerhardt (started engineering in 1927). Perfect firm to begin a career with.