effect of water saturating clay soil on footings

[Superchief]

I am builder for a project, this will go to engineering before permitting but I am trying to solve a conceptual problem so as to put together a preliminary design and budget. Time is of the essence and my structural engineer is hospitalized for the next two weeks so I am turning to the forum for preliminary advice.

20,000 gallon 15'x40' swimming pool is proposed to sit on a concrete vault with footings bearing on clay soil.

1200 ft3 storm water system is proposed to be under the pool in that vault (hard to locate storm water anywhere else on the site) and will therefore perc into ground around walls and footings. The pool will have precise detailing so any movement can cause significant issues.

Because of the water + clay I am concerned about water saturation weakening the bearing capacity of the footings.

So, my question is whether cost effective solutions such as larger or deeper footings are likely to resolve all issues. If so, this is likely the optimum design and I will proceed accordingly with schematic design and then send it to the engineer when he returns.

 

[jayrod12]

Larger footings would accommodate a lower bearing pressure, however depending on the lower allowable bearing pressure it may make a end-bearing footing foundation unfeasible. May need to look at a deep foundation system of driven or bored piles. I practice in an area of highly plastic clays and for anything where someone says precise detailing and movement causing significant issues we immediately go to a deep foundation. All types of shallow end bearing foudnations (footings/pads) are subject to some movement. Whether that is a tolerable amount is up to the designer.

I hope there's a geotechnical engineer on the project to get accurate recommendations from a soils perspective. If there's not, I highly recommend engaging someone.

 

[r13]

Larger footing has more surface area subjects to the uplift, it might not solve your problem. Depending on the depth of the clay layer, replace with well graded, and well compact backfill can be an effective alternative to deep foundations (mainly piles). Also, increase the thickness of the base mat is quite effective way to offset most of the problems. It can works alone, or in conjunction with other method. You shall discuss/call your structural engineer, and get a geotechnical engineer involved, if necessary. As the structural engineer may have already seen the soil report, and thought about it.

 

[cvg]

i am a bit confused, most swimming pools do not have footings, they are a shell bearing directly on subgrade. except yours is sitting on a slab. vaults typically dont have footings either and this is a rather small vault. so what are the footings supporting? percolation into clay will be difficult if not impossible. wetting a clay subgrade adjacent to footings is a recipe for disaster. suggest engineering get involved before any construction starts

 

[rowingengineer]

I would look to create a suspended pool with sacrificial/compressible form under on piles such that clay expansion cannot affect the pool.

Other than that I agree with CVG, get an engineer that isn't in hospital to help.

 

[Superchief]

Thank you all,

When I say precise detail JAYROD it is because there is an infinity edge and you do not want that out of level so I need to protect against heaving and settling.

I agree pools don't normally have footings CVG, but this could be going over a building (the vault) so I am assuming either footings or an alternative such as piles. The vault isn't necessarily small, it is possibly the entire area under the pool (the actual minimum area would depend upon the perc test that we have not yet done, as I have to show that I can recharge 100% of the volume in 24 hours).

Being a residential builder I am familiar with piles but don't do them (unless you count sonotubes :}) ... I am sure I can identify the right sub, that doesn't worry me, but I anticipate that if I have to use piles then that will significantly increase the foundation costs.

If I were to simply set my footing deeper then the primary cost would be taller stem walls which isn't so expensive; is it conceivable that going some reasonable distance deeper (for example, 36" which would be 72" below the frost line) might solve the problem without bringing in a specialty sub? A quick calculation says that only adds $4600 to the budget, can't imagine I can do piles for anything close to that.

Regarding the geotechnical engineer, I know the soils are clay to at least 8' everywhere we dug, including in this area, is there really value in getting a more detailed soil analysis? I put in a rain garden nearby on the site (within 50') and there we hit sandy soil at 11' but can't imagine it is worth going that deep to get an extra 1000psf.

 

[r13]

Can you provide a conceptual cross section view to show the relationship between the vault and the pool? If it is direct under the pool how the piling can be an option?

is there really value in getting a more detailed soil analysis?

Because the "deep nature" of structures in this project, the clayey soil, and the potential troublesome seepage, or high ground water situation, the geotechnical consultation fee is rather a small token to everything considered, and for the sake of make certain that nothing would go wrong.

 

[oldestguy]

This is a darn complicated project. The geotech engineer you contact should be very experienced. This is no easy thing to resolve. Ask if he -she has worked on any situation like this before. They must be very experienced.

 

[PEinc]

I have to show that I can recharge 100% of the volume in 24 hours

Perk's not gonna happen with clay under the open bottom retention vault.

http://www.PeirceEngineering.com

 

[Superchief]

we normally dig deep enough to perc which on this site is probably 11' based upon earlier digging we did for a rain garden, but the surrounding soil will still be clay unless I excavate a much larger area and fill and compact it, which presents its own issues.

I did get a reference for a geotechnical engineer I could follow up with, but it may all be moot because I was now told I need full size manhole access to the vault and I can't do that under a pool, though we are looking at some options if we extended the vault past the pool at a lower elevation.

Thanks for all the help guys!

 

[PEinc]

Just extend the MH access beyond the footprint of the pool?

http://www.PeirceEngineering.com

 

[Superchief]

it is an aesthetic issue we need to see if we can solve - no one wants to look at a manhole next to their expensive pool and landscape

 

[PEinc]

Then you need to extend it farther away from the pool. You don't have another choice. Tell the owner to live with it.

http://www.PeirceEngineering.com