Beach entries, sun shelves and other shallow features are especially vulnerable to the potential havoc wrought by expansive soils. Learn why and what to do.
In addition to design and construction, I do quite a bit of work as an expert witness. This involves inspection of damaged or failed pools, investigating the cause, and forming opinions as to what went wrong and how it should be corrected. Over the past decade I’ve been seeing one particular problem over and over, resulting in some lengthy and very pricey lawsuits.
Currently, I am involved in several cases where Baja shelves cracked where they intersect the rest of the shell. In all three instances, the projects were built in clay soils with a potential for expansion. When soils below the shallow beach entry or sun shelf became oversaturated, they swelled and caused the shelf to heave up until they were 1- to 2 inches out of level.
This also can happen with spas, decks and other shallow features built in expansive soil. While the problem seems common enough, at least in the California market where I work, I don’t see the industry addressing the issue. This worries me as a businessman and pool contractor, because I think it negatively impacts our reputation as an industry.
Here, I’ll explain the problem, steps to avoid it, and methods I use for these projects.
Common missteps
Building in clay or expansive soils requires special measures. I believe that, when pool builders fail to take the proper precautions, they do it for one of three reasons:
1. They don’t realize they’re in clay soil.
2. They don’t realize just how expansive it can be.
3. They don’t want to spend extra money on the job.
I’ve seen these mistakes from reputable pool builders. Sometimes even knowledgeable contractors think, ‘Oh, it’ll be okay. It hasn’t been a problem before.’ Others simply don’t know about expansive soil, so they just build to the simplest specifications of a standard plan.
To be clear, it’s much more expensive to build pools to withstand expansive soil. But failing to do so can exact a steep cost, not only financially, but in time and headaches as well. Remember the old adage; “An ounce of prevention, equals a pound of cure.”
Here’s a typical scenario: The contractor builds the pool in dry soil. At some point after construction, moisture is introduced through rain, runoff or irrigation. This causes the soil to swell, put pressure on the pool walls and eventually crack and/or heave the shell. And it doesn’t need to move much to develop a structural crack.
Once a crack forms, the problem exacerbates: More water slowly weeps from the pool into the clay, causing the soil to further swell. These leaks are difficult to detect at first, because the water moves so slowly. But eventually the clay can swell enough to shift, twist or heave the pool out of level.
Shallow features such as beach entries, sunshelves, portable spas and decks are especially vulnerable to this. I see sunshelves dug down about 18 inches — the minimum depth listed in some standard plans. This puts them squarely in the soil’s active zone, typically the top 3 to 4 feet. Here, the earth moves up and down as the moisture level changes as a result of rain or irrigation. (The depth of the active zone can vary in different areas, so consult a soils geotechnical expert for site-specific recommendations.)
Below the active zone, moisture generally remains pretty consistent, so the ground doesn’t move as much, resulting in a steadier foundation for the deeper pool shell.
If set in the active zone, a sunshelf can move, heave and shrink at a different rate than the pool shell. A “hinge crack” results at the bottom or corners of the shelf, where it joins the pool shell. Water slowly leaks from the pool to the ground, causing the clay to swell and, over time, heave the entire pool.
It can eventually affect the foundation of the home and adjacent structures. Water can even migrate downhill and harm properties below. I’ve seen damage to house foundations, retaining walls, decking, slides and tennis courts.
Needless to say, these failures become very expensive for contractors, because they can involve a lawsuit in the hundreds of thousands of dollars. I recently went to inspect a project — along with about a dozen other experts and attorneys — all on the clock at $350 per hour. It adds up fast. These cases typically drag on for years. Depending on the type and amount of liability insurance you carry, the company may or may not pay to defend you. If a case goes all the way to trial, there are often clauses that exclude legal fees.
Special measures
If you’re working in expansive soil, first know this: Standard engineering plans often will not suffice.
For instance, many standard plans only require that a sunshelf or beach entry be dug a minimum of 18 inches deep. That’s not enough if the soil is too expansive: Water can too easily migrate under the shelf and wet the soil. Consult the engineer to learn what soil conditions the plans apply to. In my experience, most are insufficient when the soil has a high potential to expand. Then we need a special detail.
If you aren’t familiar with the soil in an area, obtain a soils report. It only costs about $5,000 in my market, and it takes about three to five weeks to get the results.
In many areas where I work, it’s not a question of whether the soil is expansive, but how expansive. If I know an area to have clay, we’ll assume the soil on the site is also highly expansive, and we’ll build for that.
With Baja shelves, we dig 3-4 feet deep. This gets below the soil’s active zone and reduces shifting of the shelf. Then we form a steel cage at the bottom and another at the top of the shelf, according to our engineer’s specs. We form a six-bar bond beam for highly expansive soils. Then we fill the void with concrete. In extremely expansive clay, we build a double steel cage for the entire pool.
This involves a lot of extra concrete — we might put 8 to 10 yards into a sunshelf. So there’s definitely an added expense.
As an alternative, some engineers might recommend digging a deepened footing around the bond beam, going down 3 to 4 feet to reduce the amount of moisture that gets under the shelf.
We take similar steps when constructing attached spas. If moisture gets below a spa that is attached to the side of the pool shell, it can start to lift and create that hinge effect and crack.
With spas, we will excavate at least 4 feet below grade to get out of the active zone. Oftentimes, we’ll dig to the depth of the adjacent pool to avoid differential settlement. We’ll take these measures even if the spa is elevated halfway out of the ground and might normally only go 18 to 24 inches in the soil.
When building decks in expansive soil, we typically dig the top 2 to 4 inches of earth out, then replace it with sand, gravel or another non-expansive material that won’t move. We also build a deepened footing around the perimeter.
