ground bearing problems

[sweep888]

I produce pools and have been told about doubts from the local council engineer that the pool structure is in his opinion not man enough for the ground bearing pressures.This has never been a problem in over 200 installs.The pool is only 4m x 2m x 1.2m deep and constructed in 30mm x 60mm box section in a gate formation.with central braces ,fixed to a 150mm concrete base and then a 225mm concrete collar at floor level.This is not my field and would like to know what is needed to satisfy this inspector.Thanks

 

[BigH]

If the pool is wholly below ground, tell him that water weighs less than the soil . . . Soil - typically 120 psf; water about 60 psf. Hence if you take out 4 ft of soi - you remove a load of 480 psf; when the pool if full, you add a load of 250 psf - a net unloading. This is the principal by which the first skyscrapers were built in Chicago (the Auditorium) - bouyant raft foundations.

 

[MiketheEngineer]

BigH -

Don't you mean - pcf - per cubic foot??

 

[Riggly]

No Mike- psf is correct. 4ft (soil) x 120pcf = 480 psf. The only thing is I did not see where he applied the weight for the 150mm (6in) concrete base.

I agree with BigH. If you pool is above ground, however, there will be added load to the foundation.

 

[sweep888]

Thanks BigH at last someone talking sense I can understand.
This is what the inspector emailed back ,any thoughts.
"Many thanks for your email with further details on the pool construction.

I have discussed this with the Council's Engineers and their opinion is the structure does not appear adequate to resist ground bearing pressures. It will be necessary to provide engineers calculations to justify the wall construction and specifically the fixing detail to the concrete base to prove the capability to resist ground bearing pressures. Alternatively an engineers design could be provided for a retaining wall construction. "
Still cant fathom what he wants or if he knows what he wants!!
Thanks

 

[kelowna]

It seems clear to me what he wants.

From what you wrote, the soil capacity is not in doubt. The capacity of the pool's walls is in doubt and he wants you to provide engineering drawings to prove that capacity.

Check your local building code. Most likely he is right to ask for the drawings. Contact a local engineer and talk to him. It should not be very expensive.

 

[sweep888]

Thanks kelowna
Are you saying he wants the pool structure capacity and you feel I should consult a structural engineer, is that right?.
Why do they keep talking about ground bearing loads?
The pool is 40mm x 60mm galvi steel in a five bar gate formation welded ends, panels are 1.2m high and 2m long and bolted together with 10mm rod, washers and nuts.On the centre join is a full height 600mm wide brace bolted to the two panels.The panels are rawl bolted to the 150mm concrete base.At floor level is a 225mm concrete collar around the pool.
The frame is then covered with 20mm ply and then welded polyprop.
Our pools can stand alone out of the ground with no problems.
Any help with this would be much appreciated.
Thanks

 

[jhoulette]

They are misleading you by using "ground bearing" When I read what the inspector said, I think he means lateral soil pressure (horizontal movement). As in the soil pushing on the walls into the pool. Not the soil below the pool not strong enough to resist the weight of the pool (vertical movement).


Jim Houlette PE
Web:

http://www.evstudio.us

Online Magazine:

http://www.evstudio.info

 

[Ron]

It would appear your inspector and the lay council have no understanding of engineering. The pool EXERTS a bearing pressure on the ground. It only RESISTS a pressure from the ground if there is a net positive hydrostatic pressure.

The only "pressure" exerted by the ground is the lateral pressure on the side walls of the pool (assuming no hydrostatic uplift). If the walls of the pool are sufficiently robust to resist the lateral pressure (as with a retaining wall), and the floor of the pool is sufficient to resist any hydrostatic uplift from a high groundwater condition, then you have satisfied the design constraints. The safety factor (at the least) is the pressure exerted in the opposite direction by the water that fills the pool.

 

[apsix]

It's not us saying you should engage an engineer, it's the council;
"It will be necessary to provide engineers calculations to justify the wall construction...."

 

[BigH]

I'm not sure a 1.2 m high wall needs to be engineered? Rockery walls in Washington of up to 6 ft (if I remember right) don't need to be. 1.2 m - that is the depth limit at which OSHA still permits vertical excavation slopes. Ron is right on uplift - only a problem if gwl near ground surface and if that's the case, excavation would be problematic, perhaps. I'd suggest he find out if there is a code requiring the engineering calculations for walls less than 2 m high. Of course, this doesn't let him out of the hook for not providing some kind of wall that is robust enough for the lateral earth pressures.

 

[msquared48]

BigH:

We have standard details for rockery walls up to 8 feet here. Engineering above that...

That would retain anything Ron could muster in Florida.

Mike McCann
MMC Engineering

 

[kelowna]

Regardless of what we think, the Council requires engineering calcs / drawings.

So check local building code, if engineering is required for that kind of structure on those conditions, you will have to provide it. The soil pushes over the side of the pool and they seem concern about it.

By the way, they are not saying the design does not work, it might be a legal requirement were you are to engineer such a structure.

As I said, talk to a local engineer and get the design validated and signed. It should not be expensive.

 

[Ron]

Mike...we have mountains here in Florida! I live on one such that is about 20 feet above the creek across the street. My driveway slopes about 2-1/2 percent and my pool is 9 feet deep. Flatlander engineering challenges!

We get our dizziness from other sources...not the heights!

On another note, I did a failure investigation on a retaining wall a couple of years ago in Central Florida. It was about 20 feet high, no engineering and constructed out of segmental block. It was sliding nicely!!

Agree BigH...not sure why they'd want to bother engineering a 1.2m wall.

 

[msquared48]

That's four feet - a typical concrete stem wall for a partially buried basement. This is kids stuff!

If you asked me, they just want more $$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$
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$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$
$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$

Mike McCann
MMC Engineering

 

[sweep888]

Thanks everyone for your info.The planning inspector now says the info from BigH has made things worse as it is inward ground pressure they have a problem with.
I am going to engage a structural engineer to assess the frame which is rawl bolted to the concrete base and the cantilever brace at the centre of the long side.Does anyone
think the frame is a problem?