swimming pool open tank slab frost depth

huangyhg

swimming pool / open tank slab frost depth
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)
i have been thinking about unheated building (or exposed exterior slabs) a lot lately. where i am with a frost depth of about 1'-0", any special provisions under the slab of an unheated building or exposed exterior slab are pretty much ignored. we just drop the perimeter footings of a heated or unheated building below frost level and that's it.
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.
mkberry,
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
thanks haynewp.  we too seem to ignore any special treatment of slabs unless there is a special design consideration.  the local building officials don't seem too concerned about swimming pools as they are considered acessory structures.  i was just at a site visit the other day and saw that the wading pool was completely frost protected with a structural slab, but the bottom of the main pool was right there to be placed on grade.  they have been installe this way for some time now with success.  i am just concerned that there is some hypocracy by protecting the wading pool from frost, but placing the pool on the ground.
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.