One warning: Don’t try to jump ahead and tie the steel beforehand. Lacher has seen this happen several times, with a bad result. “The soils engineer will see something that requires remedial action and they’ll have to take the steel out to do that.”
In cases where a pool is being built on unfit fill, the soils engineer will recommend one of a few options. If the bad soil only goes down a couple feet, you’re generally OK. Once it goes 5 to 8 feet below the surface, you will probably have to overexcavate to native ground, then backfill the pool with gravel.
If the native material sits more than 8 feet below ground, you may need a pier-and-grade-beam system. “The solution is not to take out the fill and recompact it,” Anderson says. “You’re going to remove a lot of the soil anyway when you dig the pool, and it’s very expensive to compact fill. Usually if the house already exists, there’s not enough room to do that because you have to move all the fill away and bring it back in lifts.” Where a pier system will add about $20,000 to the project, Anderson says, recompacting may run between $40,000 and $50,000.
Extraordinary Measures. In addition to loose-fill scenarios, there are other when a soils engineer should be called to a pool site:
- Hillside installation. The soil on hills can act in unpredictable ways. For example, it could be subject to slope creep, where the top layer slowly moves downward. A soils engineer will provide the proper data to a structural engineer who can design piers, footings, or other supports that may be needed for these installations.
- On-ground pools. If the vessel will be built up from the ground more than 2 feet, it will increase the load placed on the soil, and should be checked for potential settling. “When it’s constructed in the ground, the weight of the pool and water is less than the weight of the soil you excavate to put it in, so you have not increased the load,” Anderson says. Soil weighs 120 pounds per cubic foot, while water only weighs 62.4 pounds per cubic foot. If you build the pool on the ground, however, you’re adding an extra load.
- Problem soils. There are problematic soils in a variety of regions around the U.S. Expansive soils, most commonly found in Texas and California, can cause serious deck heaving. The corrosive material of the desert regions, and organic soils or peat deposits in the Eastern regions can settle over time. A soils engineer should check to see what kind of support is required to withstand such conditions.
- Strange excavation findings. If you find different materials from one side of the pool to the other, call in the experts to make sure the vessel won’t settle unevenly. The same holds true when you unearth large tree stumps, old septic tanks, or other evidence that the yard was used as a dumping ground.
- Signs of distress from other structures. If you see a home, driveway, or other existing structure with significant cracking, you could have a problem site on your hands and will need a soils report.
