Pool on Soft/Weak Soils and High Water Table

[RFreund]

We have a situation where our client would like to construct a pool however the soils are weak and the water table appears to be only a couple feet below grade. It would be an all concrete pool

I beleive there are 2 issues - soil strength and buoyancy force:

Soil strength. This does not seem to very 'important' to me as we are taking soil out (say 100pcf) and replacing it with water which is lighter (62.5pcf). If the pool is concrete and made rigid there maybe some settlement but it should not be differential as the surcharge is uniform (water).

Buoyancy force. If the water table is above the bottom of the pool the buoyancy pressure will tend to lift the pool out of the ground if it is not filled with water. Also constructability may be an issue as water must be pumped during construction.

In talking to the contractor they were not very concerned with the water or buoyancy force. They were more concerned with the pore soils supporting the pool. In talking with a geotech, he was more concerned with the buoyancy pressure than soil strength. In researching the subject it seems most pool installers are only concerned with keeping the hole dry for construction as they do not mention buoyancy.

Is buoyancy a big issue or I'm I making too much out of it?

Solutions:
Soil strength - over excavate backfill possibly a concrete slab to work on. Or provide piers with slab.
Water - Determine water table elevation - Place pool above it. Pump water out. Hold pool down.
Am I on the right track?

EIT

http://www.HowToEngineer.com

 

[Brad805]

I would go with a suspended slab option on a deep foundation unless the geotech has provided a reasonable option for soil improvement. I have reviewed leaky pools, and the owner does not want to go down that path. Before you get deep into a design I would however discuss the reality of what you are proposing. A suspended solution will easily double the cost of the structure, and your client may end up re-sizing if budget is a concern.

Brad

 

[ron9876]

Buoyancy can be a problem if water table is high and pool level is low--for some reason. I have seen pools float. Amazing. If soils are bad I would also go with pile support.

 

[hokie66]

If the soil is poor, you may need to use piles. Get some geotechnical assistance. Even though you think the soil condition is uniform, you can still get differential settlement, and a pool which is not level is a sorry sight. Is the pool of constant depth? It is common practice to use hydrostatic relief valves in pools to allow ground water in if the pool is empty, but these valves can be a maintenance headache. If a pool is losing water, the relief valve is the first thing to check, and an open valve will let in ground water, which is not good for the chemical balance of a pool. Raising the elevation of the pool is another option which might help.

 

[RFreund]

Judging by responses it seems that the soil strength is a concern. A did speak to a geotech who will be doing the testing however he was not as concerned with the soil strength. He explained that the weight of the pool is less than the soil you are taking out. He did seem concerned with the water.

The pool will not be constant depth. 6' deep in one end 3.5' or so in the other.

EIT

http://www.HowToEngineer.com

 

[FixedEarth]

The water depth may be for a perched water table and not true ground water table. Your geotechnical firm will clarify this difference in water table terminology.

Consider the use of drain pipes and thick gravel below the pool mat such that any rising water is intercepeted and carried away from the pool. You can route it to a dry circular well backfilled with crushed rock. Keep us posted.

 

[SkiisAndBikes]

If there is sufficient soil bearing capacity, I have used a thickened foundation base slab to provide the additional dead load required to resist buoyancy. You may also consider increasing the size of the base slab beyond that of the pools footprint to engage a bit of soil around the perimeter to further resist the buoyancy.

Agree with the others that piles will be required if soil conditions are poor (to support gravity loads and resist uplift). Your geotech should be giving you clear guidance, if they have been retained to perform an soils investigation (often not the case for residential pools).

 

[hokie66]

Another issue to ask your geotech about, other than the bearing capacity of the soil, is whether or not it shrinks and swells with moisture change. If it is a moisture sensitive clay, then seasonal variations could lead to big problems.