Over the past year I have been called to inspect several pool failures on hillside lots. If you are a home builder or homeowner contemplating the installation of a swimming pool on a lot that is on a hillside or adjacent to a downward slope, there are some precautions that you should consider.
Although most municipalities require compaction and testing of soil on lots where homes will be built, I have found that the soil in areas outside of the actual house footprint are often poorly prepared or tested. It also appears that soil from the house construction process is occasionally left on the property and, during the final construction grading process, is pushed out over a hillside without proper compaction.
It is common for new homeowners to wait several years before engaging a swimming pool contractor to design and install their “backyard resort”. By the time the backyard construction begins there is little evidence of any inappropriate soil deposits or lack of compaction. Most people believe that soil becomes compacted over time and provide proper support for their new pool. It is also a belief that, because the pool bottom is deep in the ground, any un-compacted “top soil” is not a problem because the pool depth acts as a footing into bedrock.
Be aware that a concrete swimming pool is a rigid structure and has little or no flexibility. Any supporting soil settlement or movement can cause cracking of the pool. Even a small crack in a swimming pool structure can result in a substantial amount of water loss with the result of saturation of the supporting soil. On a hillside lot, saturation can result in even further unstable soil conditions with possible disastrous results.
The majority of swimming pools are properly designed and engineered to withstand the site soil conditions and never suffer damage. Proper design and engineering may include extra structural components such as extra steel, concrete, deep footings or even caissons. I do not discourage the construction of hillside swimming pools; however, certain precautions are advisable. Although most housing developments require grading plans and soil testing, the recommendations may not include the soil where the pool will be built. On hillside lots I recommend, as a minimum, the following:
- Hire a reputable soils engineer and testing lab to determine the suitability of the soil where the pool will be constructed.
- Insist that a structural engineer be engaged to review the recommendations of the soils engineer and provide a specific design for the proposed pool in conformance with the soils engineers findings.
- Insist that the soils engineer inspect and perform tests of the soil at the bottom of the swimming pool excavation and/or any required footings or support structures to insure compliance with the engineering requirements.