That Sinking Feeling

Project Details

Project Description

Engineering
That Sinking Feeling
Without A Soils Report, Your Backyard Pool May Be At Risk.

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.

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.