Plan for the Worst Case Scenario
Whenever designing or building a swimming pool in expansive soils conditions, you must plan for the worst case scenario, says swimming pool designer and expert witness Paolo Benedetti of Aquatic Technology Pool and Spa at www.aquatictechnology.com.
This means that you must adhere to to the following procedures:
- Obtain a Geotechnical report (soils report)
- Have the structure engineered to withstand saturated soil conditions when the pool is empty.
- Ensure that water does not collect around the outside of the structure.
- Isolate the structure from the soils.
Soils Report
The structural engineer cannot possibly know how strong to design the structure if he does not know that characteristics of the soils.
The soils report will define the load bearing capacity of the soils. It will also quantify the expansiveness of the soils. From these criteria, the structural engineer can determine the strength of the structure.
Structural Engineering
Whether one uses generic or custom structural engineering, it must exceed the forces the soils will exert upon it (these are known as soils surcharges).
The structural engineering will define the thickness of the walls and floor; the size, placement and spacing of the reinforcement steel, PSI strength of the concrete and any special mix design requirements.
Custom engineering is always recommended, as the structural engineer will also review other potential surcharges (from buildings, slopes, snow, surf, wind, etc.).
Empty and Saturated Condition
The structural engineer will design the structure to resist surcharges when the pool is empty. Why empty? Because some day the pool will need to be drained to be remodeled or refinished.
And because the engineer cannot predict the condition of the soils when the pool is drained, he must assume that the soils will be fully saturated and swollen.
Keeping the soils dry
Installing concrete decks around the pool will help keep the soils adjacent to the structure dry.
Drainage should be installed to prevent water from sheeting off of the decks and into the surrounding landscaping. This will only force the water to seep back under the decks into the sand/rock base, defeating the intent of keeping the soils around the pool dry.
Placing drainage rock or drainage panels under or around the pool is fruitless, unless there is a means to discharge any water that collects under the pool. This could be a drain to daylight (if there were a slope nearby) or a dewatering well with a sump pump in it.
Isolate the pool from the expansive soils
Another method is to reduce the surcharges around the pool from expansive soils. This can be achieved by over excavating the soils and later back filling with non-expansive materials. Of course this requires that the pool be built differently, since the concrete cannot be shot against the soils.
Sand bags, void forms or geo-voids can also be installed behind the reinforcement steel. Properly braced, the concrete can be shot against these surfaces.
Contact the author, Paolo Benedetti of Aquatic Technology Pool and Spa at:
info@aquatictechnology.com or 408-776-8220.
Visit his website at: www.aquatictechnology.com.
All Contents © Aquatic Technology Pool & Spa, 2013.
All rights reserved.
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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.
Drainage
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:
info@aquatictechnology.com or 408-776-8220.
Visit his website at: www.aquatictechnology.com.
All Contents © Aquatic Technology Pool & Spa, 2013.
All rights reserved.
