SLAB ON GRADE DESIGN FOR EXPANSIVE SOILS 2

[alumpkin]

Generally looking for some guidance on how to design slab on grade for expansive soils. Geotech is recommending a monolithic grade beam stiffened non-structural slab supported by straight shaft piers. The recommendation is to remove the top 5 feet or so of the expansive clays. Below these clays are soft limestone with clay seams. These top five feet are replaced with compacted structural fill installed in lifts. The grade beam system is about 4 feet deep and 2 feet wide. So, here are my questions:

1. What expansive loads are used to design the slab?
2. Do you design the slab as typically would for the uniform and point loads and disregard the expansive loads since you have removed the most problematic clays and installed a grade beam system?
3. Design as a two-way plate?
3. From what I can determine online this is a complicated problem and not much data is out there for practical use.

Any thoughts would be appreciated!

alumpkin
CU 1982


Alan L. Lumpkin, MS, PE
Greenville, SC

 

[dik]

Can the structure tolerate movement? How expansive? May need perimeter grade beam, piles and a structured slab.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[JoelTXCive]

I do not think you design specifically for the behavior (expansion/contraction) of clays.

I would state it as you apply your expected loading; and then design for the soils uniform support conditions. The subgrade modulus is how the support conditions are quantified.

There are several methods out there..

WRI Method - I have used this extensively for small slabs. It is a code approved method; however the more you use it; the more you realize that you always get the same results no matter what the inputs are. I would classify this as an empirical method dressed up with some math. The WRI method has its roots in the residential industry; so basic design assumptions are that you have a reasonably rectangular slab with a perimeter brick load. You need the bearing pressure; along with soil PI & Unconfined Compressive Strength.

Finite Element Modeling - All you need is a subgrade modulus and allowable bearing pressure. This is my current go-to method. I use IES Web's Visual Foundation Software. It's very easy to use and you can do monolithic slabs along with ribbed slabs. You can do highly complex loading if you need to; or you can keep it real simple. They have a free 30 day trial and a bunch of videos on youtube.

PTI Method-even if you are not doing a Post Tensioned slab, some people still use the design parameters to calculate moment demand. You need the geotech to give you the PTI design parameters to use this method though.

How big or small is your slab?

 

[dik]

Still need to know what movement, and if this can be tolerated.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[hokie66]

You would have to ask the geotech again, but it sounds to like the removal and replacement of the top 5 feet of moisture sensitive clay is to overcome the problem, thus allowing your design to proceed as normal.

 

[JoelTXCive]

Valid point Dik.

 

[dik]

and what is the construction if no movement can be tolerated? Slab on compacted backfill?

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[WARose]

1. What expansive loads are used to design the slab?

Assuming the slab never is in physical contact with the ground: none.

2. Do you design the slab as typically would for the uniform and point loads and disregard the expansive loads since you have removed the most problematic clays and installed a grade beam system?

Yep. However for the piles/piers.....I seem to recall some sort of drag/down drag load on them if they are in such soils. (This is assuming you still have some below after the removal of that top 5' you spoke of.) You need to get with the goetech on that.

3. Design as a two-way plate?

Yep. Me personally, I typically use a FEA package for it (like STAAD).

 

[dik]

With soils, your moisture change is generally in the first few feet... piles extend beyond that and the added uplift/download is easily accommodated. Voidform is used to isolate the slab from the soil... 50 years back I remember using beer boxes (empty of course) with 1/8" masonite for bearing and accessory support. You may find excavating 5' and re-compacting new fill still permits movement... if no movement, it has to be structured. I've led a sheltered life... I've never used FEM for slabs. I've used it for gas bearings and teeth... and that's it.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[EireChch]

Do you guys in the US not use expanded polystyrene directly beneath the slab to absorb the heave forces and movement. In the UK its common for expansive soils, the amount of movement is assessed based on the plasticity index of the CLAY. That determines whether you have like <50, 50-100 or 100+mm of movement, something like that anyways. Seems like a very good solution.

 

[hokie66]

Expanded polystyrene is very stiff stuff. It is commonly used in place of compacted fill to prevent lateral pressure on walls. It is not a suitably compressible material over expansive soil. For that, you need a material that will deteriorate with moisture. Degradable cardboard void formers under structural slabs and ground beams are commonly used, with the beams spanning between piles.

 

[HTURKAK]

Dear alumpkin ,

The expansive clays are defined as fine grained soils with a PI of 20 or higher ...

Apparently , the top 5 feet or so of the expansive clays will be removed and replaced with compacted structural fill installed in lifts..

If you provide a thick compacted Subbase and base , i think you are not required to use grillage of grade beams .

I will suggest you to look (ACI 360R-92 Design of Slabs on Grade ). There are worked examples and provisions for expansive soils..

If you provide the size of SOG, proposed loadings etc, you may get better responds..

 

[dik]

We have products that use expanded foam that are designed to minimise compressive strength... we also have a cardboard type honeycomb product that do the same. Have used it often and also underneath exterior gradebeams. Have to be careful... if it's compressible but capable of loading a few psi... it adds up over a sq.ft.

hturkak... it still depends on the requirment for the amount of movement. If no movement, I'd structure it.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[EireChch]

Void former was actually the word I couldn't think of earlier when I wrote my post! But my point was a proprietary system that is cheap and easy to install by that really works!

 

[Enable]

I try to avoid the use of void forms whenever I will need to walk on the surface after placement, which is usually always. They are, by design, susceptible to moisture and walking on them after a rain is an absolute nightmare. It's near impossible to lay them in large areas, get the steel done, and pour without a major incident.

I much prefer compressible insulation for this purpose (as a contractor). Though it is more expensive and does confer greater possibility of load transference.

 

[dik]

I'm not so sure... the entire main floor of the Cornwall Centre in Regina was placed on void form... about 100,000 sq.ft. I have had them collapse in rain and they often place them under poly and use 1/8" masonite on top to provide a surface and prevent accessories from poking through.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Enable]

I've done the poly and it turned out worse due to imposed drying constraints. Note I said near impossible, not impossible. Perhaps I should have qualified that in my experience I have found it near impossible, and I have attempted on a good number of occasions. Quite surely this product still exists so people must be getting it done. I just prefer compressible insulation.

 

[dik]

Other than a couple of void collapses, caused by the contractor... I've never had an issue with it... just different experiences, I guess.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[alumpkin]

Hi Guys,

Thanks for all of the input and responses you provided. Some of you have asked about perimeter grade beams and such. Please note that I describe the grade beams sizes in the original post. Now, please see the attachments for you information. There is a slab plan showing grade beams and layout. Also, please see the pdf from the Texas group called "The Foundation Performance Association". See page 13 where it specifically calls out the system the Geotech engineer recommends which is a grade beam system on piers that has a stiffened non-structural slab that is soil supported vs a structural slab (shown on page 12).

My thinking is the structural slab is a designed slab (WRI or other methods) and the non-structural slab is designed for point loads, temp and shrinkage, as a typical SOG would be designed.

Alan L. Lumpkin, MS, PE
Greenville, SC

 

[alumpkin]

Guys,

Here is other document.

Alan L. Lumpkin, MS, PE
Greenville, SC