If you’ve ever sat through a class on soils or engineering, you know that the teachers—engineers themselves—will first grab your attention with spectacular photos of pools that have cracked in half, popped out of the ground, or worse yet slid down a hillside. Most of these tragedies start with one place—the ground. And most of them could have been prevented with a soils report. These types of problems aren’t restricted to difficult environments such as hillside projects. They can happen on the most innocent-looking lots when home and pool builders don’t make sure there’s proper compaction.
Uneven Ground. When constructing a home, it’s important to have the proper soil conditions. But when building a pool, it’s critical. “What normally would not be too big a problem for the house can be a major problem for the swimming pool,” says Ron Lacher, a civil engineer and president of Pool Engineering Inc. in Anaheim, Calif. “With a house, if there’s a little bit of settlement you might get cracks in the stucco or on your floor slab underneath the carpet. Not to minimize those problems, but a swimming pool is a vessel. If it cracks, water begins leaking [into the soil] and whenever you have water in the soil, the original problem usually gets worse.”
The cut-fill transitions that result from grading can create a particularly unstable environment. In these areas, disturbed soil has been piled on top of native material to flatten the lot. If not properly compacted, settlement will occur, and the pool will move and possibly crack. “By far the biggest damage to pools is caused by loose fill,” says Neil Anderson, president of Neil O. Anderson and Associates, a civil and geotechnical engineering firm in Lodi, Calif.
Cut-fill transitions are the primary culprit of pools gone bad. Home builders obtain the required soils report for the house pad, signifying that the earth can support the abode. Soils engineers generally recommend leaving the home on native soil and relocating fill into the backyard. Most municipalities don’t regulate this soil, classified as “landscape fill,” so the home builder oftentimes doesn’t have it tested for proper compaction. When the pool builder enters the picture, he or she may ask if the soil has been tested. “[The home builder] says yes, but he only tested the house pad and pushed all the fill into the backyard,” Anderson says. But now the pool is primed to rest on “uncontrolled fill,” which is not tested or certified. As rain and gravity take their toll, the ground will settle, possibly subjecting the pool and plumbing to movement and cracking.
If the fill varies in depth, problems increase as the soil compresses at different rates. “We could have a backyard where one side of the yard has 10 feet of fill, and the other side has 40 feet,” Lacher says. “That fill may be compressible, therefore, we could have rotation and cracking of the pool.”
The Solution. This problem is completely preventable. Home builders should alert the soils engineer and grading contractor when a pool will be built so the soils specialist can check the backyard as well as the home pad. The grading contractor needs to properly compact the pool area, especially if the fill is more than a couple feet deep. If the backyard contains compacted fill, the soils engineer should certify the compaction as engineered fill. For the pool builder, it’s important to see a soils report and make sure it addresses the backyard. If not, he should ask the home builder to bring in the soils engineer. They’re generally on retainer for the home anyway, Lacher says.
At the very least, the engineer should inspect the pool excavation. He or she will check the bottom of the hole for firmness and make sure it matches the original soils report, Lacher says. This is generally less expensive than doing a complete soils investigation. “Pool excavations are giant test pits to us, so they tell us a lot of the information we need to know,” Anderson says. “A typical soils report may cost you $3,000 to $5,000, where a simple excavation inspection may only cost $500.” If the bottom of your excavation is still in fill, however, they may have to investigate more deeply.
