Swimming Pool / Open Tank Slab Frost Depth
Swimming Pool / Open Tank Slab Frost Depth
(OP)
We have a debate about frost heave at the interior slab of a shot-crete pool or open tank. Example: A 20'x 40' pool that is 5' deep is well below our 30" frost depth, but if it were totally drained in the winter, what are the effects of frost on the slab in the center of the pool? Points being made are:
1. The 20'x40' area is small enough that the 5' depth is protected from frost.
2. The gravel and under-drain system will reduce potential frost heave to a small enough amount that the slab will tolerate the deflection.
3. It should not be emptied anyway.
What do you think? Along those same lines, would a wading pool that will always be drained and only bears 18" below grade (not to frost depth) then require tall perimeter walls and void under the interior slab, or would the entire thing move up and down as a unit with frost? (The Plumbing connections are designed to flex)
1. The 20'x40' area is small enough that the 5' depth is protected from frost.
2. The gravel and under-drain system will reduce potential frost heave to a small enough amount that the slab will tolerate the deflection.
3. It should not be emptied anyway.
What do you think? Along those same lines, would a wading pool that will always be drained and only bears 18" below grade (not to frost depth) then require tall perimeter walls and void under the interior slab, or would the entire thing move up and down as a unit with frost? (The Plumbing connections are designed to flex)






RE: Swimming Pool / Open Tank Slab Frost Depth
I understand that colder regions can have ice lenses form underneath a slab from capillary action of the water table.
This can cause the slab to crack and bulge in places. There are frost protected slab foundation articles you can find that show insulation under the slab to help prevent this.
For your case, the pool slab is 5 feet from the top of the high grade but it's not really below frost level when empty. I count the frost level as being measured from the exposed grade (or top of drained pool slab) down.
For exposed exterior slabs, I recommend you call your local building official and ask what is required or normally done in your area for frost protecting under the slab.
RE: Swimming Pool / Open Tank Slab Frost Depth
In addition to frost, check the uplift forces from the local water table and soil conditions, if the pool is drained.
Your slab bottom is probably designed for the 5'-0" water above, how will it respond if the load is reversed and becomes 2'-0" from below?
I've owned a couple of inground pools over the years and saw the effects of draining (high water table, liner floated up, substrata cracked)
jjf
RE: Swimming Pool / Open Tank Slab Frost Depth
JJF - Good point about the ground water pressure. These pools have under-drain systems connected to sump pumps as well as a pop valve in the drains. Should the ground water begin to build pressure, it will flow into the pool from the floor drain. I don't think this is possible with a liner pool like you had. Also, there should always be a geotechnical report referenced. Sometimes we do have expansive or collapsable soils and the pool is completely structural. Any ground heave is not an issue there.