When Swimming Pools Pop
When summertime brings warm sun and hot weather, many homeowners try to beat the heat by taking a dip in a cool swimming pool. Some are lucky enough to have their own backyard pool to use at their leisure. But being a pool owner is not always like floating on an inflatable lounge chair, coconut drink in hand. Routine maintenance, cleaning, and proper equipment are crucial to keeping a pool in good working order, and when those things don’t happen or are ill-timed, serious and potentially permanent damage can occur to the large financial investment that is a swimming pool.
One type of property damage claim that Degenkolb’s Forensic Engineers see regarding residential pools is when the in-ground portion of the pool lifts–or pops up–out of the ground. Not only has the shell of the pool experienced irreparable damage, but the surrounding pool deck, the underground plumbing, and adjacent structures are also affected. But why does this happen?
In-ground pool shells are typically constructed of gunite, a material similar to concrete but applied via a spray gun directly onto the undisturbed soils of a pool excavation. Six inches of gunite creates the pool shell, and advantages to using it are its structural strength and variations in surface shape. Homeowners and designers can create whatever shape of pool they want, whether that’s a rectangle or more free-form curving shapes, like tunnels that lead into a home and swim up bars! Once the gunite dries and a plaster finish coat is applied, a shell is formed that holds the pool water.
The weight of the pool varies dramatically when empty or full, and this is where the real structural problems start to arise. In areas like California, the moisture content of the soils and the water table elevation vary from season to season, with more water in the ground during the winter months. If the local water table elevation rises to the depth of the pool due to a wet winter season, the buoyant force from displaced water tries to lift the shell up out of the ground. Fortunately, the weight of the gunite shell, the surrounding deck, and the water in the pool counteract these forces and keep the structure in the ground. But what happens when the water level of the pool rapidly drops or the water is removed from the pool for maintenance reasons?
Without the weight of the water in the pool, an elevated water table could produce large enough buoyant forces to lift the shell up out of the ground. Everyone has seen the extremely large cargo container ships and wondered how they stay afloat. Well, it’s all about Archimedes’ Principle, which states the buoyant force on a submerged object is equal to the weight of the volume of displaced water. Unfortunately, this is the moment when irreparable damage to the pool can occur. Not only are the plumbing pipes, electrical conduit, and surrounding deck damaged, but the soils under the gunite shell consolidate and fill the void—the shell cannot drop back down to its original elevation when the ground water table drops again.
But what if my empty pool pops out of the ground in the summer? Or what if my completely full pool pops out of the ground? Those types of questions require digging deeper and investigating the true cause of the elevated water table. Sub-grade plumbing leaks, crack formation in the plaster finish coat and gunite shell, defective equipment, improper installation methods, bond beam and deck coping separation, and surrounding topography are some examples of the true cause behind a popped swimming pool and can be very difficult to identify without proper experience and expertise.
So where would a pool owner or any interested party go to find out information about the ground water table in their neighborhood? Well, a couple of options would be to consult with a Contractor or Forensic Engineer specializing in swimming pool construction and installation. Another option would be to visit the California Department of Water Resources website and research historical data for wells.
If you are a pool owner and want to protect that fun and wonderful investment in your backyard try to keep an image in the back of your mind of ancient Archimedes lounging in his bathtub exclaiming that single word of discovery and enlightenment, “Eureka!”.
Read more of Steve Moore’s blog articles here.