My neighbors pool down the street has all these wrinkles in the bottom of their liner.
A person I work with has a pool and their patio decking around the pool has separated from the other part of the patio.
These are typical comments I hear from potential customers when I meet with them. Most of these issues are related to the soil conditions that exist in their yard space. I have written an article that might place some of this in proper perspective.
A swimming pool is ultimately supported by the ground around it. The ground can be comprised or bedrock, soil or a combination of the two. The only concern with bedrock is how to excavate the space, but with ground that is part soil or completely made up of soil, things get a little more complicated. Your pool builder will have to take into account the type of soil and how it will affect your swimming pool and the surrounding decking. Each type of soil comes with a unique set of challenges that we want you to be aware of so you can ask your pool builder how they will handle problems that might arise with your soil type. Additionally, determining what type of pool shell to place in your yard could depend on your “soil type”.
The soil issues that are most likely to affect Arkansas pool builders are:
- Expansive clay
- Ground water during excavation
- Lateral (sideways) soil support
- Pool shell settlement
While this isn’t as common in Arkansas because of our warmer climate, “frost heave” is also something you should be aware of.
Expansive Clay is one of the biggest concerns for pool builders in Arkansas because it is a very common soil type here that can create expensive problems if the pool isn’t built to accommodate it. Clay soils undergo volume changes with moisture variations. When there is more moisture available, they can swell, and when they dry out, they shrink. The forces behind this volume change can be substantial, easily enough to lift concrete decking and even a pool shell. This soil can be found through out the United States, but it is more of a problem in areas with semi-arid climates (Arkansas) where there is a distinct wet and dry season.
The maximum depth to which clay soils seasonally change moisture content (and therefore volume) is known as the active zone. If the active zone is greater than the pool depth, expanding clay soil can readily lift a pool (expansion pressure can be as much as 15,000 pounds per square foot). If this is the case with your project, your pool builder has a couple different options. The structure can be strengthened to help it move as a complete unit or it can be separated from the soil using piers and void boxes.
However, it is more common for the active zone to be less than the pool depth. In this case the only complication could be decking that rises and potentially cracks. This is known as deck heave. There are two common methods to deal with this. One is to remove 6 – 24 inches of the expansive soil and replace it with a non-swelling soil. The other option is to cause the clay to swell before the construction by pre-moistening it.
(pool excavation site with expansive clay)
Ground Water is another problem your pool builder may encounter. It can be problematic during swimming pool construction and long term. Ground water can seep in through the excavated walls and cause instabilities and cave-ins. Pooled water at the bottom of the excavation site can also hinder placement of reinforcement materials and gunite.
If the groundwater seepage is not excessive, then the most common approach is to place a layer of clean gravel (6 to 12 inches) in the bottom of the excavation with a sump pump installed near the main drain area.
The buoyant force from elevated ground water can cause the pool to float and pop out of the ground if it is empty. Any time groundwater is present, a pressure relief valve should be installed in the bottom of the pool along with a gravel layer, which allows water to easily flow to the valve. Always consult a pool professional before draining your swimming pool.
Lateral (sideways) Support is necessary to support the weight of the pool walls and water. The soil around the pool should “push back” against this weight and support the pool shell. The problem occurs when the soil around the pool hasn’t been properly compacted. If the soil is loose it is softer and can’t always support the weight of the pool. This is usually seen in spaces that have had fill dirt brought in like new construction sites. If the new soil is not properly compacted, it won’t offer as much support as the swimming pool needs. Pool builders can work around this by structurally strengthening the pool (gunite / concrete pool ) so it acts as a free standing pool instead of relying on the support of the soil.
Pool Shell Settlement much like lateral support is a problem only when new soil has been brought in and is not properly compacted. If the fill has been placed under controlled conditions with proper moisture conditioning, compaction and testing, it is known as “engineered fill” and may be relied on for structural support, (parrot bay pools in this case, always gets the soils engineer to “sign off” on the compaction testing, before proceeding). If not, it is known as “uncontrolled fill,” and it cannot be relied on for structural support of a pool shell. Building a pool on uncontrolled fill can result in cracking once the pool settles.
Frost Heave is something we are unlikely to see in Arkansas because it usually only occurs where there is sustained freezing. However, it doesn’t hurt to be aware of it. It happens when rising moisture within the soil is trapped below a frozen surface layer. The moisture condenses and freezes along the bottom of the layer, forming “lenses” of ice. Over time, these lenses can build up and cause the ground (or decking) to heave and crack. If a pool is drained over the winter, frost heave can crack the floor of the pool. The best defense against this is well drained soil and not allowing water to pool around the shell and decking. The pool should also not be drained over the winter unless your contractor assures you of good drainage in the underlying subsoil.
l you have any questions or concerns about your soil type, chances are we have dealt with soil types previously in your area. If you would like to discuss this you can call our office at 501-664-6861 or email larry@parrotbaypools.com.
Click here to see a video of the excavation or read about the concrete pool construction process here.
