In central Texas we primarily deal with one of two types of soil conditions. Almost everything to the east of I-35 will be black dirt/clay, and everything to the west of I-35 will be rock. How you build in these soils is slightly different, but extremely important. With the rock soil, the process is fairly simple because you are not dealing with much movement, but the excavation will be more difficult. However, when building in clay, or “expansive soil”, there are a few things I recommend doing to help ensure the structural integrity of your pool. First – you need to over-excavate the pool by six inches in depth to allow for gravel to be placed in the bottom of the hole. The gunite is then shot on top of the gravel. The gravel serves several functions. It works as a french drain under the pool to help get rid of excess water, it allows for the soil to expand upward under the pool without shifting the structure, and it eliminates the possibility of differential sediment under the pool.
Most of this may sound like I am speaking a different language, but I will explain. By placing the gravel in the bottom you achieve what is called point to point compaction. This means that there is a structurally sound platform to build on, and if the ground below the pool swells, it can expand into the void between the gravel rather than move the structure. The term differential sediment means that there won’t be two types of soil under the pool, which can settle differently and potentially cause structural damage to the pool.
The good news is that if you are using a reputable contractor, they will already be aware of this, and will have taken the necessary steps to handle either condition you have, and there is little cost difference between them.
Thank you for reading, and I hope this helps.