Certainly one of the least conspicuous but most critically important design considerations for an on grade or in-ground commercial swimming pool structure are the geotechnical conditions (soil conditions). Variations in geotechnical conditions can drastically influence the design approach for the pool structure, pool shell construction methods, and in some cases the pool maintenance protocol as well. Soil properties can dictate the difference between a simple slab-on-grade design and a more elaborate structural solution requiring deep foundations, over-excavation, soil conditioning, subsurface drainage, piping encasement, or some combination of the like. This blog post will provide a brief introduction into the basic geotechnical considerations influencing pool shell design.
An early step in the design process for any earth supported structural system - be it a building foundation, swimming pool, or bridge design - is to study the geotechnical conditions of the project site. This must be done by a qualified geotechnical engineer who will provide data on the soil properties and recommendations to be used in structural design of the foundation system. This study is done by analyzing soil samples taken from borings drilled on the project site. For swimming pool design, it is typically recommended that one boring be taken for each 5,000 SF of pool area, and if multiple pools are included in the project, the borings be located accordingly with at least one taken for each pool. Required boring depth is determined by the geotechnical engineer based on soil conditions, but in any case should not be less than 5 feet below the greatest excavation depth of the swimming pool.
Data collected from boring samples will be used to determine subgrade soil properties, including but not limited to soil gradation, density, bearing capacity, shrink and swell potential, plasticity, lateral earth pressures, and groundwater conditions. This information is then utilized to determine the most appropriate method of pool shell structural design. One of the most critical considerations influencing the structural design approach is the swell potential of the soil. Soil that exhibits high swell potential is known as “expansive soil” and can pose a great threat to the performance of an in-ground pool structure if not adequately addressed. Expansive soils are generally made up of clayey materials which have the ability to absorb water. These soil types swell or shrink dependent upon the moisture content of the soil. For certain highly expansive soil types, a volume change of up to 30% may be possible. As you might imagine, even a minimal volume change of soil supporting an in-ground structure can be catastrophic for a swimming pool shell. Soil movement can cause cracking of the pool shell, uneven heaving or settlement, and damage to below grade piping systems.
Some common design solutions in dealing with expansive soils are over-excavation of the site to replace expansive material with more suitable soil, and/or structurally supporting the pool shell with deep foundations. When over-excavation is necessary, the geotechnical engineer will recommend the depth to which high plasticity or expansive soils must be removed from the site to limit swell potential. The soil material that is used to replace the expansive material is known as “select fill”. The select fill must exhibit the necessary soil properties to limit swell potential and to support whatever structural design approach will be utilized for the pool shells. It is also very important that the fill material be properly placed and compacted to achieve the density necessary to support the foundation system.
When the structural solution requires deep foundations, such as drilled piers or friction piles, the pool structure is often constructed over a void space designed to allow for soil swell to take place without influencing the pool slab. The deep foundations serve the purpose of supporting the pool shell by distributing the load vertically or by bearing on subgrade material of greater capacity, such as bedrock. When a void space is utilized, the expansive soil layer is then free to exhibit volume change without exerting pressure on the structure above. Of course, with this structural solution, the design of the below grade pool piping system must be carefully considered such that soil movement does not affect buried piping beneath the shell. Structural encasement of PVC piping is a common solution in this design scenario.
Another perhaps slightly less common approach to dealing with swell potential of expansive soils is to moisture condition the soils in order to minimize the possibility of volume change occurring during or following construction. The goal with this method is to saturate the expansive soils prior to construction and maintain the moisture content throughout not only the project, but theoretically the life of the facility as well. This is only appropriate for certain soil types and generally only those that do not exhibit extreme shrink/swell potential. Moisture conditioning can take on several forms and may involve injecting water into the native soils; excavating, conditioning, and compacting native soils in a series of lifts; or a combination of these approaches to achieve optimum moisture content. Once the conditioning has taken place and moisture content of the soil established, it is critical that this be maintained. Maintenance of moisture content may involve limitations on duration that excavations remain open, capping conditioned soil with select fill or paving to retain moisture, and even irrigation of landscaped areas.
While expansive soils are one of the more challenging soil properties and often one of the first to be considered when determining a design approach, there are many other geotechnical conditions that influence the pool shell. The pool wall design - whether it be a cast-in-place concrete wall, pneumatically applied concrete wall, or a stainless steel panel wall and buttress system – must take into account not only the pressure of water acting on the interior face of the pool walls when the pool is full, but also the pressure of the soil or backfill materials acting on the exterior side of the pool walls as well. This force that the soil exerts in the horizontal direction is known as lateral earth pressure. The measured or calculated lateral earth pressure conditions based on soil properties will dictate the pool wall structural design and the excavation and backfill requirements behind the pool walls. This criteria, along with the type of soil, may influence preferred construction methods for the pool shell. For example, shotcrete or pneumatically applied concrete construction methods for pool walls are most efficient when a vertical cut excavation can be supported and the concrete materials shot against earth. When very sandy or soft soil is present, supporting a vertical cut to the full depth of the pool excavation may not be feasible. When this is true the pool excavation must be “laid back” at an angle to prevent the excavation from caving in and pool wall construction will involve formwork for placement of concrete, or a stainless steel panel and buttress system, or a combination of the two depending on the depth of the pool. The anticipated construction methods based on geotechnical conditions will have an impact on the cost of the pool construction, and also sequencing of the construction among trades, especially for indoor pools where adjacent building foundations are involved.
Another very important geotechnical consideration is the presence of groundwater. We have already discussed in this article the effects of moisture content on certain types of soil, but regardless of expansive soil, subsurface water must be considered in the design of a pool structure as well. The design must consider naturally occurring groundwater relative to the water table in the area, and also the possibility of surface water infiltration on the site. If groundwater is present at excavation depth, dewatering methods will be necessary to facilitate construction. Once the pool is in place, groundwater levels that are above the lowest elevation of an in-ground pool structure will exhibit buoyant forces on the pool shell. If the pool structure has not been designed to counteract buoyant forces due to hydrostatic pressure, there is risk that an empty pool shell may literally “float” out of the ground causing significant damage or catastrophic failure of the pool system. If groundwater levels are naturally high on the site relative to the foundation elevation, a subsurface drainage system to remove groundwater from the soil surrounding the pool shell may be necessary. The subgrade preparation for the pool slab and in some cases the backfill material for the pool walls must be designed to allow for drainage of subsurface water when build-up of hydrostatic pressure is a possibility. Groundwater levels can fluctuate significantly in certain areas of the country or on certain project sites. A pool owner or operator must have an understanding of how the pool structure has been designed to perform and whether or not it is safe to drain the pool for cleaning or maintenance. A method of observing or monitoring groundwater level relative to the pool slab should be provided, such as a sight well or piezometer. If a permanent subsurface drainage system is installed, it must be ensured that this system is functioning properly before making a decision to drain the pool, especially if the system involves the use of pumps to remove water. Hydrostatic relief valves in the pool floor are another fail-safe against the effects of hydrostatic pressure on an empty pool shell.
While this post just barely “scarifies” the surface on the subject of how geotechnical conditions can influence the design and performance of a swimming pool, I hope that it has introduced some of the broad topics that must be considered when planning a new pool or even when evaluating issues with an old pool. As discussed, it is critical that a qualified geotechnical engineer be consulted to determine the soil properties on any project site. Soil properties can vary quite dramatically in some areas, so just because the existing structure on the adjacent site employed a certain design approach does not mean that same solution will be appropriate for your pool project. Each site must be evaluated according to the project type, and the design solution developed with project goals in mind. This process of establishing design criteria for the pool structure will ensure expectations are met with regard to cost of the pool shell construction, long term performance of the pool shell, and maintenance requirements. Simply put, several small holes in the dirt are required to determine how best to fill a big hole in the dirt with water.