Swimming pools subgrade 9

[Rippst20]

Hi, planning to construct a pool on soil, that has low bearing capacity,mostly sandy soil and relatively with a high water table,like above 15 iinches from pool floor,walls and floor is determined to be 8 inches with two layer rebar at bottom and top, wall height like 5 feet,what can be done to improve subgrade soil to prevent settlement of pool, thinking of compacting grade soil after excavation, Is there anything I can do without fill in gravel, what to watch for to be sure the compaction will do the job,

 

[EngineeringEric]

Install helical piles and new construction pile caps for the pool

 

[MotorCity]

Compacting poor soil does not improve the quality of the soil, it simply leaves you with compacted poor soil. You have a few options: deeper foundations to bypass the poor soil, strip and replace the poor soil, or soil improvement (lime or cement treatment). How was the floor designed to accommodate the soil conditions?

 

[RPMG]

The soil removed is twice as heavy as water, so it has the bearing capacity required and doesn't settle. Also, poor natural sands can presumably support 2,000 psf, which is a lot greater than a 5'-0" pool.

 

[r13]

I like the dummy soil theory But I think geotechnical shall still be consulted.

 

[phamENG]

I agree with RPMG. The biggest issue with pools and high ground water is usually flotation before they're filled (or over-draining during the winter). Yes, even concrete pools. If you don't believe me, look up "SS Selma." Though I don't know if I'd call 45" below grade "high water table." But then everything's relative - this time of year parts of my yard are perpetually 4" below the ground water level.

 

[oldestguy]

RMPG has it right (the only one so far) . The use of piling, etc might only be considered if you secure to the piles and use them for anchorage in case someone dewaters the pool and the drain valve is stuck (a common problem) when there is a high water table, and you don't want it to float.

 

[Rabbit12]

You have an 8" slab with two rebar mats? Yikes.

Are you the contractor?

 

[STrctPono]

8" slab with 2 layers of reinforcing seems a bit overkill. I've never designed a pool before, but I would think that the buoyant uplift load while the pool was drained would be the largest load on the pool floor. 15" water table above pool floor would be 78 psf of uplift pressure. However, your slab weighs 100 psf.

oldestguy, what about soils susceptible to settlement. You still think that deep foundations are not necessary?

 

[oldestguy]

StrctPono
The two stars on RPMG are there for a reason from experienced engineers.

 

[Mad Mike]

Agree that the excavated soil ensures sufficient bearing capacity and rules out settlement...

PROVIDED, the site is relatively level and the excavation therefore uniform.

This principle gets applied incorrectly on sloping sites all the time- where one side of the pool is in relative cut and the other side in new fill or even significantly different depths of relative cut- those pools do settle and crack; I've seen them! Then again, none of them had a fully reinforced concrete shell of the thickness mentioned here.

All the best,
Mike

 

[Rabbit12]

STrctPono, by the time you provide 3" clr on the bottom (cast against earth) and 2" clr on the top (exposed to water) you have 3" to place two mats.

 

[Rippst20]

I'm like in client position.I dont have steel lay out specifications right now, but just know rebar diameter is given 10 mm and spacing is like 8 inches.I think floatation will be the main problem, when pool water is drained, and I guess floatation can cause tension on both sides of the floor, how can two layer be a over-kill.Draining water after excavation and soil improvement can do the job.I think RPMG has a point.Agree with RPMG and EnginneeringEric.Thanks for all your view, comments.

 

[Rippst20]

I think I need to give more deatail about the subgrade soil's condition, bearing capacity etc. THANKS all of you.

 

[STrctPono]

Rabbit12, hence why I would think a single layer of reinforcing would work. 3" clr. + #3 or #4 rebar + 2" clr. = 5.5" thick shotcrete. I'm just not seeing the force demand on the pool floor (or even the pool walls) to require 8" thick with 2 layers of reinforcing given the water table that the OP stated. Perhaps his Engineer knows something that the OP is not conveying.

 

[hokie66]

I agree with RPMG, but also with Mad Mike. If the site is level and uniform, go for it.

As to uplift, make sure you have a hydrostatic relief valve, so if the pool is emptied, the water level can equalize. But also remember that these valves require maintenance, and can be a main source of leakage.

 

[r13]

Rabbit12,

You can have two layer mats of #4 with inner bars offset to have 1.5" clearance.

 

[cvg]

its not unusual for pools to leak water or for the piping around and under the pool to leak, perhaps increasing the water table much higher than 15 inches and thus resulting in the need to rapidly drain the pool. so upload could be a lot more critical.

15 inches above pool floor (23" above bottom of concrete) is more like 122 psf and assuming a leaky pool you could have 2 - 3 feet above the floor resulting in over 200 psf of uplift