Sunday, September 1, 2013

Swimming pool expansive soils structural design

Designing swimming pools to withstand expansive soil conditions

Expansive Forces

Expansive soils are found all across the United States and in many parts of the world.  The energy that they can place upon a structure is immense.

The concern with swimming pools is that the soils around the pool will swell and place an inward force upon the shell (called lateral pressure).  The expansive soils literally squeeze the pool to death, cracking it.  Expansive soils under a pool can push it right out of the ground.  And usually this is not an even force, so the pool ends up being out of level, creeps or moves, or the floor cracks.

Once a pool begins to leak, that water saturates the expansive soils making the situation even worse.

May the force be with you

Usually there is little concern about expansive uplift, as expansive soil deposits are usually only a few feet thick.  The act of excavating the pool, removes the expansive soils from beneath the pool shell.  This eliminates the vertical uplift concerns.  In these instances, the pool can be designed to overcome the inward pressures from swelling soils that remain around the walls.

Bench Pressing

Where the expansive soil deposits are deeper than a few feet there are a few options.  Left unchecked, the expansive soils will bench press the pool out of level, potentially cracking the floor.

Over excavation is the least expensive.  If the expansive soils are a reasonable depth, they may be simply removed.  The hole is then brought back up to the required depth with an engineered fill.  The engineered fill can be a cement slurry or imported and compacted road base.  From then on, it is construction as usual with fortified walls (to address the expansive soils around the walls).

In areas with unreasonably deep deposits of expansive soils, the options begin to get expensive.  The most common method is to isolate the floor of the pool from the effects of the swelling soils.  The pool is usually placed upon a series of drilled caissons and supported on grade beams.  The soils between the grade beams are removed and filled with a "void form."  

A void form is comprised of layers of cardboard or EFS foam blocks.  They are used to support the concrete until it is cured.  Eventually the cardboard void form decays and shrinks, leaving a compressible voids or pockets under the pool.  The EFS foam blocks are designed to compress and absorb the energy of the swelling soils.  Not to worry, as the pool is actually supported on the drilled pier foundation.  If the expansive soils swell, they move into these voids, leaving the pool unscathed.

Isolating the Walls

Some soils engineers are over concerned about isolating the walls of the structure from these expansive forces.  This can also be achieved through the use of EFS blocks.  Since they do not decay like void forms, they will continue to support the soils and sidewalks above them.  In the event of compressive forces, they absorb the energy, yet still support the surfaces above.


Keeping the soils around the pool dry, is of course the easiest course of prevention.  Maintaining the caulking in expansion joints and not allowing the decks to drain off into the surrounding planters, will assist in keeping the decks level and the pool structure sound.  

It is best to collect the water on the pool decks and route it a safe distance away from the pool, where they cannot flow back under the pool or decks. Deepened turn down footings around the edge of a patio slab will keep any irrigation water from flowing back under the slab through the base rock material.

This is why you need a soils report before embarking on the structural engineering design for a swimming pool.

Contact the author, Paolo Benedetti of Aquatic Technology Pool and Spa at: or 408-776-8220. Visit his website at: All Contents © Aquatic Technology Pool & Spa, 2013. All rights reserved.

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To contact the author, Paolo Benedetti of Aquatic Technology pool and Spa, email him at: or call 408-776-8220.
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