Soil analysis helps avoid pool damage, failures
By Julie Knudson
Surprises during construction are never good. No pool contractor wants to bid out and start a job, only to start digging and realize there are soil issues that will increase costs or impact the project’s schedule.
The risky business of soil
“You want to make sure you’re putting the pool structure into competent soil,” says Scott Cohen, president and garden artisan at The Green Scene Landscaping and Pools in Chatsworth, California. Cohen, whose work has been featured on HGTV, is also a construction-defect expert witness for swimming pools, concrete and landscape, and the author of The Candid Contractor, a book that covers a decade of the errors affecting pool and hardscape contractors Cohen has seen. He has found soil issues are the most common culprit behind problems with pool construction and hardscape.
Some soil-related issues appear more often than others. One is the installation of pools in expansive clay soil, which is prone to absorbing water, swelling as it does. “Building on it is like building on top of a sponge,” Cohen says. “Those soils can shrink and expand substantially depending on how much moisture is introduced into it.” Even if the pool contractor takes this issue into account, adjacent concrete work or other hardscape installation can still trigger big problems. “The pool coping has to be completely separated from the pool deck with an isolation joint down the full depth of the slab,” Cohen says. Because clay soil shrinks and swells with moisture — and because concrete used in the pool and hardscape may also expand and contract due to changes in temperature — Cohen says, “It will create pressure on the bond beam if not isolated properly.”
What can a soil analysis do?
Soil conditions can be vastly different even within a small area. “We built 40 pools in a Calabasas neighborhood, and in some lots we found native soil; some had fill soil,” Cohen says. “In some we were breaking rocks.” His team found that even locations just across the street from each other had the potential to present pool contractors with very different soil types.
Visual inspections conducted by a pool contractor may not always be enough to reveal the soil details necessary to ensure their installations are a success. More information may be needed about both the active and inactive zones of soil, and consultation with a geotechnical expert is sometimes recommended. “We think of the top four feet of earth as the active zone,” Cohen says. Moisture levels in the active zone fluctuate considerably during rainy periods, droughts and other weather events. Below that four-foot layer — in the inactive zone — the moisture level is usually more stable. “If your pool shell is six feet deep, the bottom of the shell is anchored in relatively consistent soil,” Cohen says. Patios and other hardscape features, however, are built on top of the active layer. “Those are moving up and down or side to side, and we don’t want them to damage the pool.”
In those environments where soil composition may be questionable, an analysis provides crucial insight. Once conveyed to the engineer designing the pool shell, the results of the analysis will drive a number of installation details. “A lot of standard plans will have three or four different methodologies for construction depending on how expansive the soil may be,” Cohen says, “and you have to design to accommodate those site conditions or you may experience failures.”
Does your project need a soil analysis?
Full soil tests, which can run as much as about $4,000, aren’t needed on every project. Cohen says his team has “every pool excavation bottom inspected to confirm the soils are competent before we install our pool shell.” A complete analysis may be called for if site conditions or other issues raise concerns. “We might see trees on a slope that are leaning downhill, which would be an indicator there could be slope creep,” Cohen says. It’s the type of situation where an evaluation by a geotechnical expert would provide important information to the pool engineer. The local municipality may also have their own rules.
Finding the right expertise to identify potential soil issues is important, but it isn’t needed every day. Because some projects are time sensitive or partner contractors may be on a tight schedule for adjacent hardscape work, it’s useful to develop a relationship with an experienced soil expert before they’re needed.