SWIMMING POOL PROBLEMS
Every year we investigate a number of failures at residential swimming pools. Typical problems include ripped vinyl liners, shifted metal sidewalls and heaved concrete plunge areas. This article concentrates on problems with the walls and plunge areas of in-ground vinyl lined pools which generate most of the failures that we see.
The basic construction of a vinyl lined in-ground pool begins with the excavation of the plunge area. A thin layer of concrete is placed over the soil surface in the plunge area and shallow end. A footing consisting of a narrow concrete pad is poured around the perimeter of the pool to support steel panels that form the upper 3.5" of the pool walls. Additional support for the steel walls is provided by steel angle braces. Usually the base of these braces is anchored in concrete.
A plastic coping is normally located at the top of the steel sidewalls. The coping holds the top of the vinyl liner in place. The weight of the water in the pool holds the liner against the walls and bottom of the pool. A concrete deck is usually constructed around the pool. The deck may be supported in several ways; on the soil surrounding the pool (often backfill), on steel braces similar to those supporting the sidewalls or on small concrete piers.
The most serious damage experienced by in-ground pools occurs when the concrete base and/or metal sidewalls shift or collapse. The structure of the pool must resist lateral earth pressures and ground water pressures. The lateral earth pressures act on the pool walls in the same manner as on the basement wall. Both act as a retaining walls, holding the surrounding earth in place. Earth pressures are highest where the soils are weak and wet. Water adds weight to the retained soil and reduces strength.
Groundwater at any level above the bottom of the pool acts in two ways. It provides an upward buoyant force that can pop a pool out of the ground in extreme cases. The groundwater also adds significantly to the lateral pressures that must be resisted by the walls.
The primary force resisting the earth and the ground water forces is generated by the water in the pool. It holds the walls in place and provides a counterweight to the buoyant effect of the ground water. For this reason a pool is at much greater risk of failure of the walls and base when the water is removed. The National Spa and Pool Institute recommends that only contractors empty pools. The metal angle braces installed around the pool also resist the lateral earth pressures.
Vinyl pools are not designed for use in areas with high ground water levels. Where this is the case it is best to not build a pool or to construct it on an earth fill pad above the ground water level.
From this discussion the following questions should be answered when investigating heaved or collapsed pool walls:
≈ Was the pool empty at when it collapsed, and if so why? This often leads to answers related to ongoing repairs or liner problems.
≈ How old is the pool and what is the condition of the braces supporting the wall?*
≈ Is there any rusting of the metal panels?*
≈ Where is the water table? Occasionally homeowners can relate this information based on the presence of nearby sump pumps or old wells.
≈ Was the concrete base for the walls and braces installed?*
≈ Is the pool area built up compared to the surrounding grade? If so the pool walls may be backfilled with unsuitable material (just about anything from tree stumps to boulders) and the pool itself may be resting on topsoil that is settling with time.*
The questions marked by an asterisk may require removal of the liner and/or excavation beside the pool. Sometimes this type of investigation is not justified given the relatively small dollar value of the pool claim. Often investigation is warranted as the following discussion of some of our recent experiences shows.
Often the liner is partially removed when a pool problem occurs. This has allowed us to observe ground water seeping through rusted steel panels and cracks in the plunge area leading to erosion of the soil supporting the concrete. In one case we determined that the extensive cracking and heaving of the concrete in the plunge area was caused by a loss of water over the winter resulting in frost heaving of the soil in the plunge area which was exposed to cold temperatures (the soil was formerly insulated by the water and ice above).
Using a hand shovel we have discovered wall braces that were rusted through and obviously made out of sections of scrap and determined that the concrete pad used to support the metal walls was missing.
Using a small backhoe, typically operated by gardening contractors we have observed: pool walls constructed on topsoil; heavy clay backfill behind walls where sand was specified; deck supports placed incorrectly and supported on topsoil. Be sure your summer swimming hole has none of these problems!
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Walters Forensic Engineering | 277 Wellington
Street West, Suite 800 | Toronto, ON M5V 3H2