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Installing a Fiberglass Pool Properly: Sand vs. Gravel, Which is Better?

By: Marcus Sheridan on October 3rd, 2009

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Installing a Fiberglass Pool Properly: Sand vs. Gravel, Which is Better?

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There is a famous bible verse that applies to this important subject:

 House built on and"Therefore whoever hears these sayings of Mine, and does them, I will liken him to a wise man who built his house on the rock: and the rain descended, the floods came, and the winds blew and beat on that house; and it did not fall, for it was founded on the rock.

"But everyone who hears these sayings of Mine, and does not do them, will be like a foolish man who built his house on the sand: and the rain descended, the floods came, and the winds blew and beat on that house; and it fell. And great was its fall."

(Matthew 7:24-27)

I have to tell you, it's a funny thing to me that man was told thousands of years ago that building structures upon rocks was much more intelligent than building upon sand, notwithstanding, there are still people, some of which claim to be ‘industry experts', that insist upon the use of sand as the base and backfill for a fiberglass pool. Alas.......I guess some of these 'foolish' people never learn.  Just look at what Todd Stahl, the owner of Viking Pools, says about the manner:

 "The fact of the matter is that in most situations gravel is not the correct material to be placed around your pool. It is difficult to compact and is typically only used because it is inexpensive."

Let's analyze this statement for a second. First of all, in Virginia and Maryland, gravel literally costs twice as much as sand per ton. Apparently, Mr. Stahl has been living in the beautiful hills of West Virginia too long to know the price of pool materials along the ENTIRE EAST COAST. Secondly, to say that gravel "is difficult to compact" is like saying "fire is hard to get warm." How anyone in the world could view their own vast and profound knowledge of geology greater than that of proven science is beyond me. Just think about it for a second- In order 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 and it's compacted, which is why it's used as the base and footer for incredibly heavy and large structures all over the world. In fact, when was the last time you saw concrete, asphalt, basements, block walls, etc, etc built on sand??  For example, my home has a basement constructed with large concrete wall panels (Superior Walls). These incredibly heavy panels line up vertically and interlock to form our basement and 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 people like Todd Stahl in the fiberglass pool industry still subscribe to sand..... OOOOO-KEEEE-DOOO-KEEEE

 

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

"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.  Question:  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 is heavier than the water on the inside of the pool and if the wall is not strong enough to maintain its shape a bulge develops.  High water tables as well as soils that hold water both 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 table or certain soils?  Quite the contrary; I would submit that there is no better pool to have in such circumstances.  You just need to make certain of two things.  First, that you're pool is strong enough to prevent bulges.  And second, that you eliminate the sand backfill and use gravel.  Is there a difference between the strength of fiberglass pools?  Yes, but from the perspective of the consumer 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, and 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 because 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 and 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 make it a very stable material on the sides of the pool; much better than saturated sand that liquefies or pea gravel that acts like little ball bearings.    Third, the gravel compacts upon placement unlike sand which needs to be placed in lifts and compacted with water.  This is further addressed in the discussion on plumbing problems below.  You will see that this is a major advantage as well.  Opponents to gravel backfill pitch against it by saying that it's a cheaper material and used in order to cut cost.  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........... 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.  This is great because this is a topic that needs attention.  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 some 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 not withstanding, 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!"

There are a couple of other points I'd like add to Jason's explanation. 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 here that your pool's patio (concrete deck) will be resting on top of that first 12" of backfill. Obviously, it's very important that this first 12" of concrete decking is on solid ground, ground that does not settle over time. Notwithstanding, 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? Tell me, which 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 quickly realize this is not healthy for the structural integrity of a pool's concrete decking.

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 further delve 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. Obviously, 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 well 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 ever do unfortunately occur, at least the pool's backfill will likely not be altered.

So there you have it folks. I could go on and on about this topic but I'll stop there. Everything written here is simply the result of having installed our first 300 pools with sand and our last 300 pools with gravel. The reality is that anyone who insists sand is a better base for fiberglass pools is simply living in an antiquated state of denial and refusing to build a fiberglass pool to a modern standard. Such people are likely still using rotary phones in their homes and 8-tracks for easy listening in their vehicles. This is also why I give credit to cutting edge companies like Trilogy pools of Tennessee because they have taught their dealers that gravel should be the base and backfill for a fiberglass pool. Mark my words: Within 10 years of this little blog, over 90% of the fiberglass pool industry will be using some type of gravel as the base and backfill for their pools. Until then though, I, along with my business partners Jim and Jason (and anyone else who likes to mix common sense with pool education), will continue to do everything we can to be a source for positive change in the fiberglass pool industry.

Marcus Sheridan, http://www.poolschool.us/