A lot of people, even some who sell and build pools, don’t really understand how a pool wall works. Well, in this post we’ll cover that, which will require learning a little about the construction properties of concrete and rebar, illuminating the principle behind retaining walls, and some other fun stuff.
A standard pool wall is just a retaining wall. It’s meant to hold the dirt out of the pool. When the pool is full, that’s not usually a problem, but when it’s drained, the soil can push on the wall, sometimes causing it to crack. That’s not good. If it’s designed right, with the right amount of rebar and concrete in specific places, it can resist the push of the dirt so that it won’t crack.
Notice that verbal gymnastics in the last sentence? All that “If it’s designed right,” and “with the right amount . . . in specific places.” That pretty much describes what an engineer does. And if there’s something I’ve learned in the pool business, it’s this: let the experts do their jobs. Lawyers make sure everything’s legal, accountants watch your money, and engineers make sure your pool won’t fail. Build the pool the way your engineer specifies. Don’t cut corners, don’t do stuff on your own (this includes things like upgrading to #4 bars where #3′s are specified, or guessing how a waterfall footing should be done). Do it the right way. Believe me, you don’t want a cracked pool.
Reinforced Concrete Basics
Let’s start off by looking at the structural properties of reinforced concrete. It’s made of concrete-encased steel reinforcing bars (“rebar”). Both the concrete and the rebar have properties that are of interest to us.
First, they have similar thermal expansion properties. This is smart-person-speak meaning that they expand and contract at the same rate, whether the environment is hot or cold. If they weren’t the same, the concrete would start to break apart very soon after it was poured.
That thermal expansion stuff is nifty and all, but what we’ll really be looking at are how these materials behave under compression and under tension — two of the fundamental, common forces in engineering and construction. So, let’s first take a short detour and explain what compression and tension are.
Lets imagine that we have long concrete bar, say a couple of feet long, maybe three or four inches in cross section. If we were to put it in a machine that could push the two ends toward each other, the concrete would be in compression. On the other hand, if that machine could pull the two ends apart, the concrete would be in tension.
Concrete is very strong in compression, but weak in tension. You can place an extremely heavy weight load on concrete before it will fail. I once heard, by way of example, that you could take blocks of concrete and stack them up until they were out of the atmosphere before the weight of all that concrete would cause the bottom-most blocks to fail. But a small bar of concrete would be easy to break over your knee (more accurately, breaking it like that would be putting the concrete in tension and torsion, but, hey, I’m just trying to illustrate a point).
In pools, we usually use gunite or shotcrete, which are both types of concrete. By the way, gunite used to be a trade name, kind of like when someone says Kleenex they’re referring to facial tissue. Gunite is actually known as dry-mix shotcrete, and in turn shotcrete is pneumatically placed concrete. So, basically … gunite = shotcrete = concrete.
While concrete is weak in tension, steel is very strong. Think of a train pulling a long line of boxcars: all that weight of all those boxcars is pulling on the first steel coupling of the locomotive. That’s a tension application. On the other hand, steel is fairly weak in compression. It can flattened out pretty easily.
Rebar, of course, is made of steel. Actually, in smart-person speak, a stick of rebar is called a “deformed steel bar.” That deformed part refers to those ridges on rebar. They’re meant to “grip” the concrete so the materials stick together when placed under a load. In pools, engineers usually specify 40-grade rebar, which is the kind you see at Home Depot. 60-grade can also be used, but it’s a little more expensive, a bit stronger, and is harder for the rebar crew to work with.
When you put rebar inside the concrete you get something that takes the best properties of its components: it’s strong in compression and in tension. Now we’re talking.
A Pool Wall Is just a Retaining Wall . . .
What makes a pool work the right way is the proper use of steel and concrete. We want more steel where there’s more tension, more concrete in areas likely to be in compression. Clever, huh?
But what goes where, and how much, is dictated by site conditions. At the beginning I said that a pool wall is just a retaining wall: it holds the dirt back. So how the wall should be built depends on the qualities of the dirt it’s retaining.
In a lot of areas the soil is somewhat expansive. This just means that when it gets wet it expands. Imagine the dirt behind a pool wall getting wet. It will then expand, putting an additional load on the wall, pushing it inward, toward the pool. The back side of that wall, the part in contact with the dirt, is being stretched (tension), while the inside face of the pool wall is being mashed together (compression).
This is an important concept, so let’s look at it a little more closely. Put your fists together, knuckles on one hand against knuckles on the other, forearms in a line. Now move that knuckle/forearm line so that it’s vertical, right arm above, left below. Imagine that this represents a pool wall, soil to the left of your arms, pool to the right.
OK, so now we’ve got our pool wall illustration going. Let’s say that there has been a lot of rain and that the surface soil is now saturated. The soil expands, pushing the top of the wall (your right elbow) inward, toward the pool. Keep your wrists locked, fists and forearms aligned. As your right arm rotates slightly, note what’s going on at your fists. The soil side is opening up, while the pool side is being pressed together more.
That’s tension and compression. Get it?
What we want is more steel on the soil side of the pool wall where there’s more tension. And more concrete on the pool side where there’s more compression. Note, too, that the more expansive the soil is, the greater the push against the wall. More and/or heavier rebar on one side, thicker and/or stronger concrete on the other. Same for other types of additional loads on the wall: foundations, slopes, large piles of heavy rock, stuff like that.
Simple, once you get a handle on it. Now you have some tools with which to examine standard structural engineering plans. Look for the ones that use more steel and concrete in situations that apply an additional load. These engineers that use additional materials in the right places are designing their pool structures to go beyond the minimum requirements. And, hey, steel and concrete are cheap. Use a little more — sleep well at night.
. . . Except When It Isn’t
Sometimes a pool wall isn’t meant to hold soil out, it’s meant to hold water in. Like a pool on a hillside with nothing but air behind the wall. Or near a downslope, which will — sooner or later — start moving so that it is no longer supporting the pool wall. After all, that’s what slopes do: move downhill. Gravity and all that.
Anyway, this type of pool wall is called “freestanding.”
This type of wall reverses the placement of steel and concrete: more steel near the surface of the concrete on the water side, more concrete on the dirt side. I’ll leave you to ponder how the weight of all that water will push at the wall; and in turn, which parts of the wall will be in tension, which parts in compression.
Most gunite pools use a structural engineering plan called a standard plan. The engineer doesn’t draw it up for just your pool; it’s meant to cover most situations in most of the pools built. It will cover most of the common pool site conditions: flat or sloping ground, surcharge conditions, height of raised bond beam, stuff like that. It will show how thick walls, coves and floor should be, the placement and size of the rebar. It will also show details like how the light niche or skimmer niche should be built, how a bench should be constructed. In other words, it’ll show how to build the guts of the pool that’s “bread and butter” of this business: the standard backyard pool.
The unusual or uncommon applications may require additional details, a sheet or sheets that are used with the standard plan. Like if you want to have a waterfall on your pool, you’ll most likely need a special detail showing how it must be built. How the excavation should look, how the rebar and gunite should be done.
These standard plans can be obtained from engineers that are familiar with pool construction. They cost about $100-200, at least around here (the Los Angeles area in California). The special details for stuff like waterfalls, grottoes — you know, the fancy stuff — cost a lttle bit more.
Know the Site Conditions
What all this means is that you (or your pool builder) need to understand the conditions at the pool site. Depending on the situation, more (or less) rebar and shotcrete will be needed in specific places. And its placement inside the wall may change as surcharges and stuff come into play for that section of pool wall. Make sure — it’s critical — that the right details are being used for the particular situation. You’ll need to know stuff like:
- What type of soil? What are its properties? For example, high sulphur content in the soil means that you’ll need to use Type V cement, because sulfates will eat up the regular old Type II.
- How expansive is the soil? If it is significantly expansive, tests need to be done to find out exactly what loads will be placed on the pool, so that it can be engineered to withstand those pressures. Engineers generally want a number expressed in pounds per cubic foot of equivalent fluid pressure.
- Will the floor of the pool be in two different types of soil? For example will it cross a soil/bedrock or natural/fill line? If so, you need to get ahold of a soils engineer.
- Are there any additional loads that will be placed on the pool? These are called surcharges, and require a beefier structure.
- Any future construction planned nearby that will impact the pool (a patio cover,a pool house, waterfalls)? Surcharge time.
- Any slopes nearby? If so, are they upslopes or downslopes? One uses freestanding, the other the standard wall. By the way, make sure you know what you’re doing when building near a slope. An engineer once told me that he believed that over 90% of pool failures involved poor construction near a slope.
- Any retaining walls nearby? If so, will the pool be on the side being retained, or the lower side? One uses freestanding, the other the standard wall and an appropriate surcharge.
- Will a deep-end ramp need to be used to dig the pool? If so, you’ll need to use the freestanding wall detail for that section.
And on and on.
You get the idea … know what you’re doing before you start. And, no, even though you now have a handle on some of basics of reinforced concrete construction, it doesn’t really count for much in the know-what-you’re-doing department. Bottom line — if you’re uncertain which details apply or what your site conditions are, talk to your engineer.