Thursday, July 10, 2014

Swimming Pool Design in Expansive Soil Conditions

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

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: 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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