The Importance of Choosing the Right Backfill for Fiberglass Pools

I read this on the River Pools and Spas website recently, and I thought it should be shared with our potential clients living in Aiken, SC.

There’s an old story that’s been passed down for generations about a wise builder who chose to construct a house on solid rock, while another chose to build on sand. When storms came, the house on the rock stood firm, while the house on the sand collapsed. It’s a simple but powerful lesson: a strong foundation matters. Yet even today, some in the pool industry insist on using sand as the base and backfill for fiberglass pools.

Now, setting sarcasm aside for a moment, let’s talk about why this matters. When you break down how these materials behave under pressure—literally—the difference between sand and gravel becomes impossible to ignore.

Why Gravel Is Better Than Sand for Fiberglass Pools

Just think about it for a second. For sand to come close to compaction, it must be saturated with water and tamped. But gravel? That's right, gravel only needs to be tamped to achieve compaction. It can even reach 95% compaction upon placement! This is why it's used as the base and footer for incredibly heavy and large structures all over the world.

When was the last time you saw concrete, asphalt, basements, or block walls built on sand? For example, my home has a basement constructed with large concrete wall panels. These incredibly heavy panels line up vertically and interlock to form our basement. Do you know what they are sitting on? That's right; you guessed it—a gravel footer. In other words, my entire home is sitting on a 12" gravel base.

Yet, unfortunately, many in the industry still insist on using sand as a base and backfill material.

The Science Behind Backfill Choices

But enough of common sense. Let's look at the scientific reasons why gravel is better as a base and backfill for fiberglass pools. One of my business partners, Jason Hughes, wrote an incredibly respected and popular article about fiberglass pool problems, which discussed sand versus gravel.

He noted, "If you talk to enough people or scroll through enough online forums, you'll probably come across someone who has developed a bulge in the wall of their fiberglass pool. We encountered this problem years ago with several of our pools. This caused us to scrutinize our installation process and thoroughly inspect the pools where these problems occurred."

What we found flies in the face of the standard and accepted method of installing fiberglass pools across the country. The problem in our case stemmed from the backfill material that we used—namely sand. Tens of thousands of fiberglass pools have been installed with sand backfill without incident, and most of our sand backfilled pools were fine as well. But when the right (or wrong) combination of circumstances align, fiberglass pool walls do bulge.

The Risks of Using Sand

What happens to sand when it becomes saturated with water? It liquefies. That would be fine if the liquefied sand were resting against something other than fiberglass, which is renowned for its flexibility. Fiberglass pools are structurally engineered to remain full of water. The outward pressure from the water works in conjunction with the structure of the pool to stabilize the vessel.

When sand liquefies, it becomes heavier than the water inside the pool. If the wall is not strong enough to maintain its shape, a bulge develops. High water tables and soils that hold water lead to situations where a significant amount of water surrounds the pool structure and saturates the backfill material.

So does this mean that fiberglass pools are not suitable for installations with high water tables or certain soils? Quite the contrary; I would submit that there is no better pool to have in such circumstances. You just need to ensure two things: first, that your pool is strong enough to prevent bulges, and second, that you eliminate the sand backfill and use gravel.

Choosing the Right Backfill Material

Is there a difference between the strength of fiberglass pools? Yes, but from the consumer's perspective, it will be virtually impossible to rate the "bulge-ability" of the various pools on the market because all manufacturers claim to be the strongest and most stable.

The one thing you CAN control is the backfill material. Based on our experience, using gravel backfill is the only insurance against wall bulges in any situation with any pool. Clean crushed blue stone, ¾" or smaller, is the gravel used in our installations. It has several distinct advantages over sand or even round pea gravel.

First, its properties do not change when it becomes saturated with water, so it performs the same wet or dry. This makes it ideal for any situation, especially in areas with a high water table or difficult soil conditions.

Second, the point-on-point friction that occurs between the pieces of gravel makes it a very stable material on the sides of the pool. This is much better than saturated sand that liquefies or pea gravel that acts like little ball bearings.

Third, gravel compacts upon placement, unlike sand, which needs to be placed in lifts and compacted with water. This is a major advantage as well.

Opponents of gravel backfill argue that it’s a cheaper material used to cut costs. In reality, it actually costs us significantly more to use gravel. We feel it is well worth the investment because you can't put a price tag on peace of mind.

The Impact of Settling on Plumbing

Why would the plumbing settle? Good question! The plumbing doesn't settle; the backfill material around the fiberglass pool settles and takes the plumbing with it. Yes, we're back to our sand vs. gravel discussion again.

The problem is that it's very difficult to fully compact sand around a fiberglass pool during installation. Many times, despite a fiberglass pool installer's best efforts to bring the sand up in 6"-12" lifts and fully saturate the sand with water, the sand still settles over time. The plumbing is encased in this sand, and as the sand settles, it exerts downward pressure on the plumbing of the swimming pool.

Many installers tie their plumbing up to help hold it in place, but notwithstanding, that's still a lot of pressure. Settled plumbing manifests itself in one way: leaks. "Leak" is a dirty word! We want to avoid saying that word, and we do so by using gravel backfill that just doesn't settle. If the backfill doesn't move, the plumbing doesn't move. It's as simple as that!

The Importance of a Solid Foundation

Let’s suppose you have a 6' deep fiberglass pool to install. When excavating for a fiberglass pool, the over-dig is typically about 12". Therefore, the deep end of the pool will be backfilled with either 6' of sand or 6' of gravel to fill in this 12" over-dig. Keep in mind that your pool's patio (concrete deck) will be resting on top of that first 12" of backfill.

It's crucial that this first 12" of concrete decking is on solid ground—ground that does not settle over time. Many pool builders will backfill the deep end of the pool with 6' of sand and then add about 3" of gravel on top of that sand to form the base of the concrete decking.

Are you starting to see just how little sense this makes? Would you rather have the first 12" of your concrete decking resting on 6 feet of gravel or 6 feet of sand with 3" of gravel on top? I can guarantee that 3'-8' of sand backfill (depending on the depth of the pool) will settle at least slightly over time. It doesn't take a genius to realize this is not healthy for the structural integrity of a pool's concrete decking.

Plumbing Leaks and Their Consequences

The other point I'd like to add deals with sand's ability to follow moving water. Jason mentioned this a little bit, and I want to delve further into the subject. Let's say that after 6 years of owning your fiberglass pool (that was backfilled with sand), you fall victim to a plumbing leak at one of the pool's return fittings. This means that huge amounts of water are blasting away behind the pool wall.

Before long, because sand so easily follows moving water, the force of the water creates a tunnel effect, and an empty void is created behind the wall. Voids in the backfill of a fiberglass pool can have major negative ramifications, with the fiberglass bending to the shape of the void. As you can imagine, this is not a good thing, but it can be almost entirely prevented with the use of gravel. Unlike sand, gravel will not wash and follow moving water very easily. Therefore, if leaks do occur, the pool's backfill will likely remain intact.

In conclusion, when it comes to the backfill for fiberglass pools, choosing gravel over sand is a decision that can save you from future headaches. The benefits of gravel are clear: it compacts better, provides a solid foundation, and minimizes the risk of plumbing issues. Make the smart choice for your pool installation and enjoy peace of mind for years to come.