Void Forms for Expansive Clay

[vandede427]

I'm designing a building that is on a site with expansive clay soil. Deep foundations (drilled shafts) and grade beams will support a structural slab at the ground level. I've looked at several different products for this application; the more typical "cardboard" type and the "rainy day" metal type for grade beams and slabs.

What I'm looking for is whether or not void forms completely degrade and absorb all the soil swelling pressure or if they still transmit some of the upward swelling pressure. I can't find this information on any of the products' websites.

The geotechnical report for this site says that swelling pressures could be as high as 2000 psf if no void form is used. It recommends void forms to help "reduce uplift forces" but it doesn't say what that reduced force will be. I understand it probably differs based on each product, but I can't find that info.

 

[rowingengineer]

general rule of thumb for these is that you can assume about 60% of original depth. I spec

http://bildavoid.com.au/main/page_bildavoid_supavoid.php,

they have some test data in the techincal section.

Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that they like it

 

[vandede427]

No, that's not what I'm looking for. The 60% of the original depth is the requirement of the form to keep its shape during the formwork phase, before the casting of the concrete.

I'm looking for data where the compressive strength of the form has been tested after casting of the concrete and water has soaked the form.

 

[hokie66]

vandede,
You need to read again. The link rowingengineer gave states that the void after flooding is 60% of the original height of the void former. It doesn't collapse 60% due to the wet mass of the concrete.

 

[dik]

I usually use one of two products. One by Beaver plastics called Frost Cushion. They have two products, one with a higher compressive strength and one with a lower one. The former is used for beams and walls and the latter used for forming slabs where there is a larger area. The other company is Plastifab and they market similar products. I've spec'd void form to use against a backfilled wall where the adjacent soil (other than the fill) is rock salt which creeps and exerts a lateral pressure. By having the voidform 'crush' the lateral loads on the cantilevered wall were minimised. It was the biggest sump pit I had ever designed... about 18' wide, 70' long and 7' deep...

Dik

 

[BAretired]

GeoSpan, manufactured by Plasti-Fab is an excellent voidformer. The compressive stress at 50% Deformation is 50 kPa (7.3 psi).

Edmonton clay is expansive and I usually specify a 4" thickness of GeoSpan under grade beams. This seems to work reasonably well in most cases, but there are times when a greater thickness is warranted.

http://www.plastifab.com/applications/geotechnical/compressible_fill/geospan.html

BA

 

[cntw1953]

BAretired:

Curios and want to learn more on this.

So, the soil has a potential to exert 2000 psf pressure (after DL) on the structure, then there are two scenios:

1. use 4" form to obsorb 50% of pressure, and design the grade beam/slab to resist the rest (7.3 kpa approx = 1000 psf).
2. use 8" form to relief entire pressure.

I don't know if I have worded my question correctly or not, you can see I am lost in somewhere.

Apreciate your comments and further explanation.

 

[vandede427]

cntw, I think what BAretired means by 50% deformation is similar to the 60% of original depth after flooding mentioned above.

Basically the question is "how much compression can wet cardboard trasmit?"

In my head, I'm thinking of it as the linear-elastic / perfectly plastic stress strain curve we assume for steel. The wet void form will carrying the swelling pressure, transmitting an increasing amount of compression to the structure, until it finally crushes and continues to crush while applying the same load.

BAretired says his product is 7.3 psi. That's the number I was looking for. The geotech eng said the metal product crushed at 500 psi (which shocked the crap out of me) but I think he may have meant 50 psi. I saw an online brochure of the product and it's nothing more than corrugated chicken wire. There's no way that stuffed with-held 500 psi.

 

[vandede427]

7.5 psi (50 kPa) is about 1000 psf. Are you telling me you design foundations for 1000 psf applie uplift?

 

[cntw1953]

vandede427:

Quote: Basically the question is "how much compression can wet cardboard trasmit?"

That's exactly the question I have in mind.

No kidding, 1000 psf uplift? I think you need to follow up on BAretired's lead to select a form with proper thickness to further reduce the pressure to a more manageable level. Keep me posted on your findings. Thanks.

 

[vandede427]

I'm just really having a hard time believing that a 12" x 12" piece of soggy wet cardboard is able to withstand 1000 pounds.

And if it is that strong, why is the product designed to be so strong when all that it is need to carry is the self weight of the concrete; 100 psf for an 8" slab or 400 psf for a grade beam?

Am I missing something here?

 

[cntw1953]

I think it is to be investigated that while the form will collapse 50% of its original volumn under 1000 psf load, will, or how much, the load remains on it. I don't quite get it.

and, as the question you have raised, what is the form of complete collaspe, by what pressure?

Maybe I missed something, but that's why I am sticking to it

 

[dgillette]

Bear in mind that the geotech's estimate of uplift pressure is with essentially zero upward movement of the soil surface allowed. It shouldn't take a whole lot of movement to relieve the uplift pressure to close to zero.

Last month, some were installed under some new grade beams at my house (on swelling clay) with the trade name AVOID - clever, eh? I believe I could have stomped them flat without much trouble. They were just cardboard boxes with internal stiffeners, designed not to crush under the weight of the concrete, about 4 psi in this case. They could not have been very stiff in compression.

 

[cntw1953]

Good point.

So the question should be "what is the anticipated volumn change of the soil", a pure geotechnical question, am I correct?

 

[vandede427]

In doing my calcs, I just found something in the ASCE load combos that's a little disturbing. If H (earth) is a downward or lateral force then everything is fine. But if H (earth) is an upward force then there's a problem with the load combos.

For checking pile reactions worst case downward, there isn't an ASD load case that has D and L but not H. For designing the grade beam's worst case downward, there is a case with D and L but not H, but that case is only 1.0L and not 1.6L.

If I blindly went by the combos listed I would be assuming that the uplift swelling pressure occured 100% of the time (which isn't true) and would never get a worst downward case.

Does anybody agree with this?

 

[vandede427]

I think IBC 2003 saw this issue. The pulled H, F, T, and S out of the combos and just said "apply as necessary."

But IBC 2006 pulled all load combos and reverted back to ASCE 7.

 

[dgillette]

"So the question should be "what is the anticipated volumn change of the soil", a pure geotechnical question, am I correct?"

That's part of it - how fast does the pressure reduce with swelling allowed, versus how fast does the void form pick up the load with compression? Seems it would take a significant amount of upward movement of the underside of the beams to get the piers (normally in compression) into tension. (Don't ask me to quantify "significant.")

 

[cntw1953]

dgillette:

I think a leason is learned today. Thank you very much.

 

[BAretired]

GeoSpan is not soggy cardboard. It is an expanded polystyrene product whose properties are not affected by the presence of water. It is very easy to indent the material with finger pressure.

The attached graph shows Compressive Stress vs Percent Deformation.

The magnitude of heave varies from place to place. It may be caused by swelling clay soil, frost heave or both. If you experience a 2" heave and use 4" GeoSpan, the Percent Deformation will be 50% and the pressure would be about 50 kPa. If you get only 1", the deformation is 25% and the pressure would be about 40 kPa according to the graph.

If you are expecting more than 2" heave, then you should use a thicker voidform.

7.5 psi (50 kPa) is about 1000 psf. Are you telling me you design foundations for 1000 psf applied uplift?

The answer to this is "You bet!" A 12" x 24" grade beam weighs 300 plf so the net uplift is only 700 plf. You likely wouldn't have less than 3 - 15M top and bottom, so the uplift is carried by the grade beam spanning between piles.

BA

 

[vandede427]

I don't mind designing a grade beam for 1000 psf applied uplift, but not an 8" slab spanning between grade beams.

But it appears you don't have to. It appears that properly designed cardboard products will on average degrade down to 30% of the initial strength.

Read this

http://surevoid.com/product/testing.php

it is testing done by a manufacturer to determine strength reduction. they show that strength reduction can reach 80-90% based on the amount of moisture absorbed.