sonotube foundation for a slab on grade 2

[lubos1984]

Visited a site today where the contractor installed 6 sonotubes about 4' deep and then decided to pour the 8" concrete slab and sonotubes in one pour. The sonotubes have rebar tying into the slab and the slab has a rebar as well. This slab will have a shed and a gazebo.
The way this was constructed was abit puzzling to me. Should the contractor have chosen either a slab on grade without any footings so that the slab bears directly on the soil or a pier /footing and beam system where the footings would have taken the load ?
Would the current system present issues where as a result of frost heave the slab will function more as a beam / footing system since the sonotubes will provide resistance to any frost heave experience by the slab ? Right now the slab was poured on undisturbed soil over a drainage layer. In the future would the tie in to the footings present this issue ?
Thanks for all your input!

 

[MIStructE_IRE]

“Should the contractor have chosen either ....”

The contractor should not be choosing anything, surely the engineer should be specifying the slab/base type?

If the ground is good, and assuming he’s stripped the topsoil, this will likely act as a ground bearing slab/raft in any event.

If the ground is poor, or shrinks away from slab soffit, you could have punching issues?

From what you describe however I don’t think I’d be concerned.

 

[HouseBoy]

I agree with MIS.

Some other thoughts;

No dimensions given or pier placement but I assume the piers are at the corners and mid length of the two longer sides. How much actual load the piers will take before they "settle" (relative to the slab bearing on the sub-base) is hard to say but it doesn't seem worrisome.

For frost heave to be an issue, there just be water trapped under the slab. If you have good drainage there (and it looks like you would since the slab area appears to be "high" relative to the surroundings) then I would not expect too much of a problem from that.

About the slab itself - 8" thickness seems like a lot of thickness and so punching shear should not be a problem unless the pier diameter is too small.

I'd be curious about how much reinforcement is in the slab. 8" seems pretty thick (nothing wrong with that) but it's a number game, pier size and location and resulting slab span combined with the reinforcement is going to signal whether or not a "structural slab" really exists. Stiffness of the bearing soil (at 4 ft depth) will be a factor too in how the foundation behaves.

Again, it does not sound like a structural problem is very likely and I don't THINK the slab is going to perform much differently whether the contractor placed concrete monolithically or in two separate placements.

 

[lubos1984]

Thank you for the quick responses MIS and HB!

The contractor was doing this work without a permit and the inspector upon finding out requested that an Engineer look at what was done.
In this case the piers are 12" evenly spaced out along the perimeter. Reinforcement consisted of a wire mesh in the slab and 15M rebar connecting pier to slab.
I was thinking of analyzing this by assuming how the slab would behave as if the piers were not there. Considering the slab thickness and the loading of just a gazebo and shed, I would think that the resulting loading would not present an issue.

Total slab area is 25'x15'.

 

[HouseBoy]

Agreed. Piers won't have much effect.
Curious proportions
8" slab with wire mesh! Wonder where the mesh is located ... NOT

As far as "analysis it"... I don't know what quantitative evaluation can be made but I would not expect the behavior to be a problem (as you state).
I think one can provide an opinion but I don't know what more beyond that one can really do with it.

 

[oldestguy]

If this sits on "frost susceptible" ground,it easily can heave because water in the ground obeys few gravity rules and can migrate to where ever there is a demand and frost lenses have a high demand effect,

 

[BAretired]

The contractor was doing this work without a permit and the inspector upon finding out requested that an Engineer look at what was done.

I would be very careful about providing a written opinion on that slab. Clearly, the contractor did not know what he was doing. There is a very good chance that differential movement will take place during the life of the structure. If you provide an opinion suggesting that there is nothing much to worry about, you could be setting yourself up for a lengthy and expensive battle in court. Personally, I would not touch it with a barge pole.

BA

 

[MIStructE_IRE]

Agreed, walk away. You don't know ground conditions, if concrete was compacted, reinforcement laps/locations etc etc. and for what, a few hundred dollars?

 

[Ron]

The piers create a significant lateral restraint to the slab, and with little reinforcement, I would expect wide cracks to open from shrinkage. If he wants to salvage the slab, have a cutting company come in and cut the slab from the piers and just let the slab float, though that will probably cost more than just replacing the slab.

 

[lubos1984]

Thanks for the comments. Assuming:
-the ground conditions are suitable
- an adequate drainage layer is provided below the slab
- the piers extend below the frost line
does the monolithic pour of the pier and slab create a structural issue ? If the soil provides little bearing resistance (since the piers don't have footings) and the slab begins to act as a "floating slab" i would think cracks could form at the underside of the pier / slab ? The Owner indicated he's not too concerned about this.

 

[dhengr]

Lubos1984:
It’s becoming crazier by the day. Nobody wants to pay for our honest advice, certainly not if it is the least bit conservative or comes to an unfavorable conclusion, which could likely prove to be true on down the line. They tell us they don’t care about the cracks in the slab, but will forget that statement was ever made when the cracks actually form, and then want us to make it right. They tend to tell the engineer what the design should be, in some detail, and then the engineer should meet this cheap (or crazy) design, and the code, if he/she wants the job, and at their low fee to boot. And finally, they didn’t need our help (the cost of real engineering advice and help) to screw it up, in the first place, with their own vast knowledge and experience; now they want us to fix it for them, at min. cost, and assure them that all will be o.k. Mostly what they want is for us to be the insurer of last resort. We get a $500 fee, and they get our insurance protection, neither the inspector or the contractor give a damn about real engineering.

 

[Ron]

@dhengr....well said!

 

[bhiggins]

An 8" slab is pretty beefy, but the slab is likely under-reinforced and does not have any crack control joints (and it's too late to install them now). If the subgrade is unstable then the slab will likely crack uncontrollably. The piers may cause lateral restraint which will also cause cracking. If any of the piers move differentially, then the slab will crack uncontrollably. The piers do not have sufficient depth to rely on for any significant frost heave resistance so the slab will likely move.

 

[oldrunner]

Take is out and start over with an engineer. Need someone to determine what the soil is going to do. It was an illegal project. Not sorry for the contractor.

 

[Rabbit12]

oldrunner, while I agree with you if the OP doesn't sign off on this I'm guessing the contractor will find an engineer that will. Too many people in this world that are just looking to make a buck.