Void Boxes under Tiltwall Panels- Useful??

[PatSablatura]

I'm in the commercial tiltwall construction business and have heard more and more people talking about needing to put 12" void boxes underneath the tiltwall panels spanning between the drilled piers.

How is it that a panel which weighs about 40,000 lbs. and has a thickness of only 6-7 inches can be lifted upwards by an expansive soil ?

It seems to me the shear weight of the wall would counter any potential for vertical rise or expansion of the soil.

Thanks, Pat.

 

[dik]

Assuming that the panel spans 15', is 6" thick and weighs 40K, then a 40 psi swelling pressure will lift it... Expansion of freezing water can exert a pressure of 2000 psi, in case you're curious... I don't know what pressures are exerted by swelling clays, but 40 psi doesn't seem like much.

 

Oh, I'm in Texas, I don't think it EVER freezes here.

Right. I agree, but how do i figure the 'expansive uplift pressure' exerted by the existing subsurface clay?

Is this force a function of P.I. or liquid limit? or is there some equations explaining it?

and when it expands, wouldn't some of the uplift pressure be dissipated into the sourrounding soil?

Thanks,

Pat.

 

[dik]

Maybe a geotekkie can step in and fill in the blanks.

Clays are generally produced from a feldspar type of rock and may only be a few molecules thick, but are much longer and wider.

The swelling characteristics, I understand (and may be subject to being beaten for error), come from molecular scale electrical charges on these small clay 'platlets' having an affinity to water (water is a polar type molecule and would attach to the platlet surface). I would suspect that the magnitude of the molecular charge would determine the affinity to water and, hence, the degree of swelling and swelling pressures. Different types of feldspar would produce different types of clay and the variation in composition would be the determining factor. It is also likely modified by any salts that become 'attached' to the platlets.

I can see some of it being dissipated to surrounding soils as easily as I can picture the swelling surrounding soils having an effect on it.

 

[JAE]

Pat - we always (in San Antonio) used voids under tilt panels. Expansive soils can be exceedingly powerful and it was our "policy" not to tempt fate with it. We found in one case where an entire 3 story concrete framed building was lifted off its piers (actually, the piers failed in tension) approximately 9".

A narrow (6&quot panel doesn't have much surface area but I never knew any geotechs down there who would recommend otherwise.

 

[PatSablatura]

Thanks for the info.

Here in Austin... it'll be solid rock for 10 feet then whammo, Super sponge clay for 10 feet...

We specialize in tiltwall construction and I'm the only engineer here... it's a pretty country-fied organization and thus there's a lot of ''That's just the way we do things''. So, I'm constantly faced with the dilemma of explaining the basics of engineering to my Superintendents.

I suppose on highly expansive soils that lay over the entire lot as shown in the soils boring report, I'll go ahead and use void boxes.

Gracias,

Hey 'JAE'... I'll be in San Antonio for the Cinco De Mayo soccer tournament this weekend... Where do I party? and particular place on the river walk? Besides the super-commercialized touristy ones like Dicks last resort? maybe a dive or two... with a Dartboard???

 

[JAE]

I haven't lived there since '91 but we just went down there a month ago. Dicks was the first thing I thought of before you mentioned it. There's a place called Little Hipps that is just north of downtown that serves great burgers. Best Mexican food: La Fogata on Vance Jackson Blvd. Best BBQ: County Line (but fairly commercial). Best eclectic food: 410 Diner at Broadway and I410 (great chicken fried steak). Most out-of-the-way: The Grey Moss Inn (way northwest of town somewhere....in the yellow pages.)

You really can't go too wrong anywhere in SA. Have fun!