Concrete Column, Pier, Footing, Tank

[Archie264]

Colleges,

I have a situation where there is an existing concrete column connected to a cast-in-place pier (below a slab) that in turn bears on a footing. There is a proposal to put a sump tank adjacent to the pier, below the slab and above the footing.

All of the loads associated with the tank would be minor compared to the existing loads on the column and footing.

Does anyone foresee a problem with this? I’ve calculated the system as though the column extends directly to the footing (i.e. using the column depth and pier depth as one continuous column) and the numbers bear it out. But I’d appreciate your thoughts on the matter. Thanks.

 

[JAE]

From your description I can't see a problem but a sketch might help.

It sounds like the pier is simply an extension of the column.

If the tank has any significant weight/load and it is adjacent to the footing, then there might be some degree of overburden on the footing and thus possibly cause some additional settlement - i.e. the underlying soils would be compressed a bit more and respond to that compression by consolidating a bit.

If the tank is not over the footing but adjacent to it, their respective "bulbs" of pressure below each would overlap and also result in some consolidation/settlement.

 

[Archie264]

JAE,

A belated thanks for your response. Minor exploritory excavation revealed the system to be a large cassion, not the pier and spread footing system shown on the drawings. The column load is quite large (~1,000 kips), so there is no concern of additional load from a sump pit. But, the sump pit could be on the order of 6' x 6' x 6', so I don't want to get it too close to the caisson.

Per the original drawings the caisson likely bears on strong soil, or even rock, and should only extend 12' or so below grade. So I don't think skin friction plays any significant factor in the foundation system, but I'd still rather not install a "void" adjacent to the caisson, even if the void is a vault of reinforced concrete.

What say ye learned folks? Thanks.

 

[JAE]

Is there lateral shear coming through the caisson? A "void" on one side could eliminate some passive soil resistance. That is all I can see as a problem.

I think the pit would still provide confinement to the caisson.. Remember that drilled piers/caissons don't have the same requirements for ties/confining reinforcement that columns do so the surrounding earth being removed might be an issue.

 

[Archie264]

That's a good point. Time to break out the CRSI Handbook, ACI 318 Section 22, Terzaghi & Peck, and whatever other resource I can think of...