In short; After all of the cash & effort that has gone into completing it, I am terrified that when emptied it will pop out of the ground and be rendered useless. (~$25k to repair a popped pool)

Everyone says - this is a risk with in ground pools. I am trying to better understand and quantify that risk.

I have searched for a few days for some decent info on Hydrostatic pressure and the chances of a pool popping out of the ground.

I've not found anything as detailed as I would like so I have drawn some pictures and am going to post my assumptions here for comments.

Again, I stress, what is written below is my assumption, if it happens to be factually accurate that is a chance combination of a basic physics education and luck!

When an in ground pool is full of water it exerts a force downwards due to gravity (weight) that is larger than that of the hydrostatic pressure, if there is any, that is exerted upwards on it by the water table (Figure 1).

When that pool is emptied, the force exerted downwards by the pool is less. I envisage three potential scenarios depending on the height of the water table:

Figure 2: The water table is lower than the bottom of the pool. No hydrostatic pressure exists so the pool does not move!

Figure 3: The water table is so high that the force exerted by the hydrostatic pressure exceeds that exerted by the weight of the pool and the pool pops!

Figure 4: There exists a point of equilibrium where the upwards force exerted by the hydrostatic pressure equals that of the weight of the pool. As long as the water table is lower than this point then the pool will not move

Making the bold assumption that the above theory is correct I tried to calculate where this point might be for my pool. I went to school in Europe so my calculations are metric (apologies)

The pool is an irregular shape and the equations for volume and surface area get pretty complex pretty quickly so I have used the following assumptions in this calc:

The pool is now rectangular with straight sides, an average depth and a flat bottom. (30' x 19' x 4' = 9.1m x 5.8m x 1.2m)

Air has no density/mass

Shotcrete/gunite made with normal weight aggregates will have a density of approximately 2323 kg/m3 (145 lb/ft3)

Shotcrete/gunite average thickness 20cm (8")

So for the pool to "pop" (or float) it needs to be displaced by a volume of water that weighs as much as the empty pool.Archimedes said:If an object is less dense than water (if it floats on water), it displaces a weight of water equal to the weight of the object

To calculate the weight of the empty pool. We find the surface area of the gunite: (2 x L x D) + (2 x W x D) + (L x W) = That gives about 90 square meters

We then acknowledge the fact that the pool's not square and we want to be cautious and so knock it down to 80m

^{2}

We multiply the surface area by the thickness of the gunite (80m

^{2}x 0.2m) and get a volume of gunite equal to 16m

^{3}

We multiply this volume by the density of gunite (16m

^{3}x 2323 kg/m

^{3}) and get a mass of just over 37,000kg

What we really want to know is how high does the water have to be in the water table around the pool to pop it.

Mass of the empty pool = Mass of water = Density of water x volume of water

Splitting that down:

Mass of the empty pool = Density of water x length of pool x width of pool x height of water table above bottom of pool

Rearranging :

Mass of the empty pool / (Density of water x length of pool x width of pool ) = height of water table above bottom of pool

Putting the numbers in:

37000 / ( 1000 x 9.1 x 5.8 ) =~ 0.7m (2' 4")

So if the water table around my pool is higher than 2' 4" from the bottom of my pool it may pop!

Am I wildly off the mark here and how do I find out where the water table is?

Does it really matter? Am I making a mountain out of a mole hill?