GC with SE Concerns

[Kaspen]

All, currently constructing a building that the construction team does not feel comfortable with the SE foundation design. The building is designed with 2' diameter 66' deep drilled caisson utilizing skin friction no bedrock was encountered. The soils are soft to medium stiff clay. Concrete grade beams placed from caissons to caisson with the slab being monolithic placed with the grade beams. The building is an energy plant supporting multiple cooling towers on the roof and chillers with up to 24" diameter piping supported from the cast in place roof deck. The average span between the 20 drilled caisson is 15' up to 27' apart. During a meeting the SE informed the team the soil below the grade beams need to support 1500 psf. Is this common to use grade beams as spread footings and skin friction from drilled caisson?

 

[jayrod12]

I would say that mixing end bearing shallow foundations and piles is a no-no where I am. Either the caissons are supporting the structure, or the slab is. Wouldn't be both. It would take too much movement of the deep foundations before the shallow foundations began supporting load.

But I don't have the full picture. So maybe there's something I'm missing.

 

[TigerGuy]

This is a pile supported mat. What's the problem?

 

[MTNClimber]

I agree with jayrod12. You don't support the same structure on both shallow foundations and piles. It's one or the other. Get some clarification from the structural engineer if that is his intent. If so, ask the geotech to weigh in if this is acceptable. If that is their intent, get it in writing. If the owner understands your concerns but wants to proceed with their engineer's design, then the choice is up to you if you want to be involved in the project, which may be fine or have more headaches than it's worth.

 

[TigerGuy]

Again, it's a pile supported mat foundation, or a pile supported raft. They are commonly used in different areas of the world. If the soil lacks the capacity to support the mat foundation, piles are installed beneath to transfer some of the load to deeper strata.

Even if the mat is not continuous between grade beams, the concept is the same.

 

[Kaspen]

Thank you for responding all of you. TigerGuy, I was not thinking of a Pile-Supported Mat Foundation, but it is a very good question. This would go against the recommendation of the Geotech report some SE do that. It's only an 8" slab with #4 each way. I would have anticipated a lot thicker concrete and more bar for this type of foundation system.

 

[MTNClimber]

TigerGuy - That's new to me! Doing a quick look, a tiny section in Coduto talks about "pile-enhanced mats" being potentially used more in the future but no information on how to approach its design. Nothing is mentioned in my version of Das or Bowles. Can you provide any references that give details?

 

[Kaspen]

TigerGuy, I took a couple of the structural sheets out and attached them for reference. Soil report called for the building to be supported on Deep Foundation Drilled Shafts with the floor slab to be structurally independent of any building footing or wall to reduce the possibility of floor slab cracking caused by differential movement between slab and foundation.

 

[JAE]

Ya, on this one I would NOT do this personally - rely on supporting the structure on deep and also shallow levels of earth.

The problem is that the skin friction behavior under load - i.e. the "stiffness" of the piling/piers is probably much different than the behavior/stiffness of the earth under the slab.

Also the loading is probably a LOT different between the pier areas and the slab areas.

The slab is reinforced with a single mat of rebar at mid-height - suggesting that this is a slab-on-grade and not intended to span between beams but be supported by the earth below.
The beams have stirrups at 24" o.c. while the depth, d, of these beams can be as low as 45" (48-3) which, if loaded and spanning would require in some places a max. stirrup spacing of d/2 or 22.5".

With different loading and stiffness, this would mean that under load, the heavier loaded areas of the beam/slab structure will be supported differently, and deflect differently, under the different loads and stiffness areas - causing all sorts of internal stress on the structure that probably weren't considered by the SE in design.

The only possible way I'd do this would be to ensure that the slabs actually span (i.e. reinforce as structural slabs and not mid-depth SOG's.
Also I'd make sure my pier design and grade beam design can fully span these distances and not rely on the soil for any support at all other than as an initial form for the concrete.

 

[TigerGuy]

I would start here

https://www.structuralguide.com/pile-raft-foundations/

I've never had the misfortune of designing one, I just know they exist and aren't as uncommon as they once were.

 

[JAE]

The drawings the OP posted aren't a raft foundation. It is a pier supported beam and slab floor system with the slab only 8" thick (not a raft by any means) and the slab not reinforced to span between beams. Not sure what the system would be called but not a raft.

 

[MTNClimber]

TigerGuy, I saw that too. Decided to download the references it listed for some light reading on a rainy day.