Below the surface: added force on pool walls may require a specially engineered shell

Custom Home, March, 2004 by Rebecca Robledo

Pool builders working with custom homes often face challenging conditions. For example, the design may call for a pool to be built very close to the house. In other instances, a hillside lot or loose fill create unstable environments. Sometimes the best solution is to call in a structural engineer and "beef up" the pool shell.

A thicker shell is needed when the equilibrium between the outside force (the earth) and the inside force (the water) has been thrown off. Extra concrete will help the vessel hold up under special conditions. However, where the extra concrete is placed depends on the site conditions.

As more designers are tying the aesthetics of a pool to the home's architecture, they often site the pool right next to the house--an effect that can look beautiful, but that places extra pressure on the vessel. The foundation of the house exerts an added lateral, or horizontal, load on the pool from the outside, and the water isn't sufficient to counteract it. This imbalance occurs when building a pool near any structure or mass, including retaining walls, covered patios, pool houses, and even an ascending slope.

There are two simple rules for finding out if a nearby structure or mass can affect the pool. One states that if the pool is as far away from the house as it is deep, then you're OK, says geotechnical civil engineer Neil O. Anderson, president of Neil O. Anderson & Associates Inc. in Lodi, Calif. You can also see if the pool is free and clear of the mass by drawing a 45-degree line from the structure's foundation toward the pool site. This illustrates how the soil would splay under the force of the house, says structural engineer Ed Gutierrez, a principal of J-E & Associates in Arlington, Texas. If the pool intersects that line, then it will need extra reinforcement. It may also mean that the pool crews should take precautions to prevent the excavation from caving in during construction.

One way to ensure against a cave-in is to flash the surface before beginning work in the hole. Flashing means to cover the sides of the excavation with a 1- or 2-inch layer of gunite or shotcrete to keep the soil from drying out. This is especially critical in granular soils, such as sand. Conversely, more cohesive soils, such as clay, may allow you to work closer to the structure without extra reinforcement. It's a good idea to consult a geotechnical engineer to find out if this is the case.

On the engineering side, experts use computer analysis to determine just how much force the structure will impose on the pool walls. The closer the structure, the more load it exerts and the more reinforcement the pool will need. To help the pool stave off this added outside pressure, the engineer will probably design the shell with added gunite of shotcrete on the water side of the pool wall reinforcement, says Ron Lacher, structural engineer and president of Pool Engineering Inc. in Anaheim, Calif. This goes back to the very principle behind reinforced concrete: The concrete stands up to compression, or pushing; and the steel withstands tension, of pulling. With added thickness on the water side, the gunite of shotcrete can best resist the compressive forces of the home's foundation pushing in.

Other times, the pool wall must withstand force from the water pushing outward. If an outside wall is exposed or sitting in loose fill, there won't be enough pressure to counter act the force of the water. In those instances, a structural engineer will specify that the pool builder put the extra concrete on the backside of the wall. The concrete can resist the compressive force of the water, while the steel sits closer to the other side, enduring the stretching that occurs as the water pushes outward.

Whenever you're contending with extra pressure, a structural engineer should be brought on board to provide the specifics of how the shell should be reinforced and where the new thickness should be applied.