By Brian Van Bower
From its very first issue, this magazine has made one key point over and over again: Soil conditions determine the way a watershape’s shell is constructed; to achieve success in construction, the approach must be established by a competent engineer and followed on site.
Through the years, numerous contributors to the magazine have described the process of placing watershapes on hillsides or dealing with soil conditions that lead to differential settlement. So far, however, relatively little attention has been paid to the challenges of working in locations where high water tables prevail.
In my career, I’ve had the good fortune of working in southern Florida, which is about as flat as can be but is a place where it’s possible (or even likely) to encounter water just a few inches below grade. Building anything with these soil conditions – whether it’s a home, a pool, a deck or a shade structure – means dealing with a host of issues and potential problems in both design and construction.
Some other time, I’ll take a closer look at what’s involved in bringing structural integrity to watershapes built in water-rich conditions and the systems of piles they often require. Here, however, I’ll focus on the water itself and the effort it often takes simply to start the construction process.
It’s ironic in the extreme, but watershapes designed to hold water in aren’t necessarily prepared in ways that make them good at keeping water out.
We’re actually quite good at the former and have for generations lined the insides of our structures with plaster, exposed-aggregate or tile finishes to create what’s known as a “positive-pressure,” water-tight seal. To cinch the deal, many of us use waterproofing agents – coatings or admixtures – to add layers of protection against water penetration of the concrete. (Personally, I’m a big advocate for and often specify use of admixtures from Xypex Corp. (Richmond, British Columbia, Canada), which fill the voids in a porous concrete matrix with expansive crystals.)
Basically, in other words, we’ve figured out how to make our vessels hold water – definitely an important skill! In general, however, we as an industry are less concerned about building shells that keep water from moving into shells from outside as a result of what some call “negative pressure.” What almost anyone who builds in areas where there’s a high water table knows is that concrete – shotcrete, gunite or poured-in-place – is not necessarily waterproof.
As an example, I recently designed a project in Birmingham, Mich., where there’s a high water table. The way things worked out, a storm rolled through and halted construction while the completed shell was empty. When the weather broke and the contractor came back on site, he discovered that water had leaked through the shell in several places. While it had been properly installed, the shell definitely wasn’t impermeable.
This and similar stories have piqued my interest in discussions that have arisen during several recent Genesis 3 schools. Bill Drakeley, one of our instructors and the only American Shotcrete Association–Certified Nozzlemen instructor in the pool industry – is a vocal proponent of elevating basic specifications for concrete shells so that all become effectively watertight.
He’s a sharp guy, and when he says watertightness can be achieved if concrete is properly specified, mixed, applied and cured, I tend to take him at his word. He doesn’t go so far to say that shells prepared this way will be waterproof, conceding that “watertight” may involve minimal (if any) leakage.
There are a number of factors at play in his approach, one of them being the concrete’s density or strength. Drakeley recommends application of concrete at strengths exceeding 4,000 pounds per square inch – in direct contrast to industry standards that call for concrete that is 3,000 psi or better and common practice that deems concrete at densities of 2,500 psi (or even 2,000 psi in some cases) to be acceptable.
Without getting into the particulars of the debate, it’s fair to say that when you’re dealing with a waterfront installation or one located in an area with a high water table, it cannot hurt to specify concrete that tests out at 4,000 psi. (This may become the new industry standard if some of our Genesis 3 compatriots have their way.) Nor is it ever a bad idea to use a coating or admixture to increase watertightness.
UP AND OUT
Where water penetration is an annoyance and can lead to a variety of issues and maladies, the main thing to watch for when you build in areas with high water tables is the hydrostatic pressure that bears down on the outside of a pool shell.
Not only might the presence of this groundwater require you (at your engineer’s behest) to thicken the shell to resist the strain, but it can also influence the type of reinforcing bars you use in a shell, particularly on waterfronts where there’s concern about saltwater penetration and the rapid corrosion it can cause.
Many times, I’ve even seen engineers call for epoxy-coated rebar under these circumstances – and that’s despite the fact that there are questions about the integrity of structures in which the epoxy prevents a good mechanical bond between concrete and steel and the issues involved in penetrating the coating (and thereby compromising its integrity) to create an electrical bonding system.
(It’s interesting to note that, when I work on waterfronts in Bermuda, local codes call for the use of galvanized steel rebar in all concrete structures!)
The real concern with hydrostatic pressure, of course, has to do with its ability, under certain conditions, to pop a shell right up out of the ground. If you have a shell that is watertight (but isn’t on a piling system of some kind) and leave it empty for a period of time without having provided a way for water on the outside of the pool to pass inside or otherwise be evacuated from the vicinity, a shell will do all it can to float on the water.
So-called “popped pools” are among the most notorious and catastrophic of all structural failures of inground concrete vessels. We’ve all heard these stories, and doubtless some of us are amused (in a sick sort of way) because it’s tough to imagine a concrete structure weighing several tons bobbing up out of the ground. Safe to say, this isn’t a phenomenon you ever want to encounter in your own working life.
Unfortunately, I’m the source of one of those terrifying tales. Many moons ago when I didn’t know any better, I took on a renovation job – not near the shore, mind you, but in an area with a high water table. (I was dumb, but not an idiot.) We drained the pool in preparation for our work and left it empty overnight.
The next morning, I received an urgent call from the homeowner letting me know that something had gone horribly wrong. It was no exaggeration: When I showed up on site, I was confronted by a pool that, at one end, had risen fully five feet out of the ground and in doing so had destroyed both its plumbing and the surrounding deck.
The homeowner asked, “Is this going to be okay?” I told her, “Sure, eventually,” expanding that in my mind as, “Sure, I may go out of business, but eventually, you’ll have a new pool.” It was a misstep born of ignorance, but the result was that we tore out the old pool and built her a new one from scratch.
The moral of the story is, of course, that you can run into areas of high water in the most surprising places (you just never know unless you happened to be the original builder) and that you need to make certain water outside the pool has someplace to go when you remove the weight of the water inside it.
From that disastrous point forward, whenever I worked anywhere with even a possibility of high groundwater, I’d cut weeper holes in my shells in strategic positions to allow water to flow in and out of the pool during construction. (In some cases, it was actually fascinating to watch the water rise and fall with the tides.) Nowadays, of course, it’s pretty much standard practice to install hydrostatic-relief valves in main-drain sumps: These devices open under pressure to prevent spectacular catastrophes.
All of this, of course, begs the question: With so much water in the ground, how do you build the shell in the first place?
Force of Nature
The simple truth is, water has an amazing ability to work its way into places despite our best efforts to resist it.
Not long ago, for example, I designed a project for an island off Miami Beach that included a bay-side pool as well as, at one end, a Balinese-style structure with a thatched roof. Hidden beneath this structure was the equipment room, placed well below the water level. It was a large, air-conditioned space occupied mainly by three big collector tanks we set up for various features that were part of the overall landscape program.
By design and earnest intention, the general contractor meant for this big vault to resist water intrusion. On one side of the staircase leading down into the space, however, a minor leak appeared – despite the fact that the contractor had used an external waterproofing system and had in fact hired a waterproofing company to build a bathtub-like surrounding structure to keep water from reaching the sub-grade room.
To everyone involved, it looked as though the plan was waterproof – but it didn’t work out that way.
(Back in my early days in pool construction and renovation, we remodeled a number of these pools, usually just cutting down the edges and creating shallower pools that were more in keeping with modern trends anyway. I haven’t seen any recently, but I’m certain some of these odd pools still exist to this day.)
A better approach has since emerged in the form of de-watering systems that simply clear water away from construction sites. Our preferred method has been to overexcavate the shell, place perforated piping to act as an under-drain and then fill the hole to the desired level with a substrate of gravel on which the shell is installed.
We’d tie these under-drain lines to sump pumps that ran constantly during construction to keep groundwater at bay. Once a pool was complete, we’d cap the pipe, making sure to highlight its location on the plans so that, in the event the pool ever needed to be drained, the contractor doing the work could insert their own pump and de-water the area around the shell.
That’s just one approach, and there are many others. The point is, when you know a vessel is subject to water intrusion during construction (and thereafter), you must be aware of the level of the challenge and develop a de-watering program that gets the job done. You also need to warn clients of these situations and let them know what must be done if the pool ever needs to be emptied.
When you are designing or building on waterfronts or in areas with high water tables, you have enormous creative possibilities within your grasp – but you also have extreme responsibility to make certain your watershapes have what it takes to stand the tests of water, pressure and time.
One last note: As suggested briefly above, when you’re working in the vicinity of bodies of saltwater, electrical safety becomes an issue of special concern. I’ve done renovations in which, for example, bonding wires are attached to grounding stakes that no longer exist because salinity has completely eaten them away.
It’s not hard or terribly expensive to remedy such situations, but it’s worth mentioning that it doesn’t make sense to put a new stake in where the old one failed. If you can, find a place where corrosion isn’t an issue – or tie the bonding system to the home’s plumbing.
As is true of so many aspects of watershaping, if you enter the fray armed with knowledge and can communicate effectively with your clients about costs and possible hazards, you’ll usually come out ahead. If, by contrast, you decide not to survey the site and don’t bother to commission soil studies because your “experience” tells you to leave well enough alone, you may end up with a disaster on your hands!