Geotechnical reports will sometimes have recommendations for deep piers or drilled shafts that reach to non-expansive soils way below the surface or at least to the bottom of the weather-affected zone wherein moisture contents vary seasonally. These piers can range between 0.4 to 0.7 m (1.5 to 2.5 ft) in diameter. If built through deep clay layers, a casing is necessary to keep uplift from occurring due to skin friction on the pier, or bell-footed piers are required. One complaint with this system is cost as piers can be more than 7.6 m (25 ft) deep. Therefore, this option leads to several variations:
- Frame support. For this method, a frame comprised of horizontal beams is engineered and built on top of the piers to support the pool’s weight. Box forms (void forms) are installed between, and under, the frames to keep the expansive soil from lifting the frame itself or the pool it supports. The frame and pier arrangement is based on the pool’s shape.
- Pool structural floor. For this method, the pool is engineered to support its own weight (plus water) by installing monolithic pool floors with integral beams instead of a separate frame to span the piers. Carton or void forms are still necessary to keep the expansive soil from contacting the pool shell.
- Precast concrete or steel piles can be driven to support the pool or pool sub-frame.
- Auger piles can be ‘screwed’ into the soil to support the pool. Augers must be designed to sufficient depth to resist uplift from clay soils.
Moisture level maintenance
Another theory is to maintain high moisture levels below the pool to prevent shrinkage caused by the soil drying out. To do this properly, the contractor must install two things:
- A water source; and
- A path for the water to saturate the pool subgrade.
Simply installing a crushed stone blanket beneath the pool does not work as it will cause the moisture content to rise and fall with the seasons. If a crushed stone blanket is specified, it should be drained to a sump (i.e. reservoir) or to ‘daylight’ to prevent water from accumulating beneath the pool. While this may be the least expensive alternative, this method is probably the least effective as well.
Resisting the pressure
Engineers should design their pools to be stiff to resist pressures on pool floors and walls when built in expansive soils. A properly reinforced pool should resist soil uplift and loss of soil support due to shrinkage. As a result, it is often necessary to install ‘double matted’ reinforcing steel. If something has to move, it is always better if the entire pool can move and/or flex without fracturing the shell. After all, levelling a coping or even a gutter or vanishing edge is not impossible and costs far less than the preventative methods necessary to keep the pool absolutely level.
The pool designer and engineer must also consider the pool type when recommending a stabilization method. Obviously, the tolerance for movement of a skimmer-designed swimming pool is much greater than it is for a vanishing-edge or perimeter-overflow pool. When choosing a method for building a swimming pool in expansive soils, the owner must be included in the negotiations. After all, neither the engineer or pool contractor selected the site; therefore, risk should not be transferred to them due to the owner’s decisions to save money.
If the owner’s tolerance for movement is small then he or she has to be informed that more costly building methods (e.g. piers and concrete frames) need to be employed. In some cases, foundation costs can exceed the value of the swimming pool. On the other hand, if the owner accepts some risk of movement, a less expensive method can be used.
The owner must be shown their options with regards to repair costs versus the expense of the original installation. For example, the cost of levelling a pool beam and replacing the coping could be a pittance when compared to a $100,000 foundation. In either extreme, the owner should be the one to accept the risk or pay the price for certainty. The pool contractor who fails to bring the owner into the decision-making process assumes liability for any failures, no matter how well-meaning his/her intentions.