Uplift anchors on residential slab on grade?

[YankeeeSJR]

The area is Southeast Texas, expansive clays. A friend is constructing a new residence and wants under reams placed under a conventional slab on grade foundation. The opinions around here are mixed as to whether to tie the slab and the piers together, to resist uplift as well as settlement, or have the piles support the foundation during shrinkage cycles only.
No void forms will be used and the "pad" is select fill placed on native soil that has been grubbed, but not much else. Pad will be compacted by the dozer during the filling operation.
18" diameter with 24" diameter bell.

Any advice would be helpful.

 

[dgillette]

If the piers are tied to the slab, seems you would need lots of steel both top and bottom in the slab, since the bending moment could change sign with changes in moisture. If the piers only support the floor slab against settlement, there is potential for the interior walls and finish to have problems if the floor gets lifted relative to the structure that the walls are attached to. Either way, the slab could be called upon to bridge a long span. The walls of my house in CO are supported by piers socketed into bedrock; the basement floor slab floats. When the house was about 30 years old, we had a big drought, and the slab dropped (unexpectedly) as much as 4 cm! I need to have some new finish carpentry done to hide the gaps below the baseboards.

(I also have the problem of the expansive clay working its evil on the basement wall. We had a wet spring and summer here, and the previously small movements got big enough to cause a major "Oh-s___" moment when I discovered it. About to have three counterfort walls built on the outside of the basement (as well as some improvements I've been considering for years). Big $$$, not covered by insurance. The counterforts will be tied down by helical anchors, and they will have void forms under them.)

I claim no expertise on this subject aside from observations at my own house; in fact, I hired a guy who knows the local practice to do the geotech work for the repairs to my house, contractor to be selected later this week.

Bon chance!

 

[TDAA]

Unless you have one major slab, of you tie the it to the piers, you will crack the slab.

Piers below, just for settlement are a waste of money unless your subgrade cannot handle the net pressure that might be placed on it.

Piers used to resist uplift are designed to anchor the bottom of the pier well below the zone of moisture variation. They are typically designed to resist the uplift due to skin friction on the pier only, and not to a larger area.

If you have a slab-on-grade, with no void, you will have to resist the pressure of the swelling soil on the entire slab. What is the swell pressure of the soil? It cold easily be in the 10,000 to 25,000 psf range. I hope there are a lot of thoose 18 inch piers planned.

 

[JAE]

If he's going to the expense of putting in drilled and belled piers, then he probably should go the whole 9 yards and suspend the floor slab with a crawlspace (vented) below.

As the others have suggested, the forces on the slab if tied down to the piers would require tremendous steel anyway.

For a residence, and enough closely spaced piers, the reinforcing of a suspended slab might not be that great due to limited span lengths and residential loads being light.

 

[hokie66]

Agree with others that if you don't have a void, tieing down the slab will only result in heartache. Depends somewhat on the moisture condition when the slab is placed, but the risk is too great.

 

[YankeeeSJR]

Thanks for the input. I will let him know the pros and cons and he can discuss with builder. I assume that he will get some type of warranty with the slab and he can base the decision on that as well.

 

[TDAA]

If it is warrantied, there are most likely still provisions for slab movement. Those movements may crack the slab and still be considered within the anticipated performance.

The only way to eliminate the differential movement from the foundations, and to reduce the likelihood of movement due to expansive soil is to use a structural floor.

 

[apsix]

From someone not living in the US, what's the definition of a structural floor?

 

[dgillette]

In this case, one that can span between piers, spread footings, or whatever, and does not need to be supported by bearing on the ground surface, which would be called "slab on grade," and would need only light reinforcement.

 

[YankeeeSJR]

Just as a clarification. The under-reams are located under the grade beams, both the perimeter grade beam and the interior grade beams as well.

I haven't designed a residential slab in quite a while, but if I'm not mistaken, I believe a portion of the design of the grade beam was based on soil shrinkage/swell and the length of the unsupported grade beam. Reinforcing and depth of beam designed as a continuous beam and a cantilever. So in that respect, if designed properly, it should already have been designed as a structural slab.

A far as swell potential, I agree that the forces due to expanding soils would be tremendous. In my initial discussions I didn't really consider those forces. I have inspected tunnels under residential foundations during cast iron plumbing re-routes and would usually find a gap between the bottom of the slab and the soil, I suspect due to settlement/consolidation of the fill. I would expect a pile supported grade beam would experience similar consolidation of supporting soils and create a gap or void.

Around here significant amounts of money are spent to install driven piles, steel or more commonly concrete, or to install under-reams, to jack up the foundation after problems are encountered. Heaven forbid the few thousand bucks spent to get a properly designed and constructed foundation be budgeted in from the start.

I'll stop preachin' to the choir.

I think he is taking the right approach, by installing piers. Anyway, I presented the options and opinions gathered here and amongst collegues, the contractor states that he has constructed them both ways, tied and untied, with satisfactory results. Again thanks for your input and opinions.

 

[boo1]

If a wood flooring system was installed over the pier and beam foundation, the surcharge would be reduced and releveling would be easier.

 

[aeoliantexan]

I agree that tying a slab on grade to the piers is asking for a cracked slab. Using piers to limit settlement due to shrinkage is fairly common in Texas, often in conjunction with water injection, lime injection, or excavation and replacement with the clay compacted well on the wet side, all intended to reduce swell potential. The piers then limit settlement that occurs due to drying or dessication by trees and bushes, usually near the perimeter of the building. If the clay at the time of construction is relatively wet, several percentage points above the plastic limit, this approach may work OK.

Why spend money on piers based on the contractor's non-site-specific experience and not spend it on engineering? Consult a reputable geotechnical engineer.