Backfill on a fiberglass pool......... Gravel or Sand?

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Brianziggy64

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Aug 16, 2014
4
Canton
I am getting estimates on large fiberglass pools. I have had two builders come out so far and say they backfill with "wet sand". They say that's the only way to backfill all the voids. But most of what I have researched say that sand is not the preferred method anymore as they have found that two things happen with sand. 1. It causes more pressure on the side walls than small gravel 2. If it gets wet, it will settle. Think of a sandcastle on the beach and what happens when the sand gets water on it, it sorta melts down from the shape it was in before it got saturated with water.

My research on the net says that the preferred method is small gravel because it will lock into place, but if it gets wet, it will not change shape. The gravel also does don't exert the same pressure on the sidewalls as the sand does. So I am throwing this out there to see what experience you have had or what you know first hand as a builder. This pool will be in the 16x40 range in size and with the decking (850 sq ft pavers), fence, electrical, plumbing, filter, pump all included, I'm looking at about 45K to 50K and I don't want to make a big mistake on the install. Thanks!!
 
Our PB said he normally used sand for backfill. But, like you, my research indicated that gravel was much better than sand. So, at my insistence (and slightly higher cost) he used gravel for the backfill.
 
Thank You all for your responses!! I just don't understand why there are builders still doing it the way "they" want to do it instead of what's better for the pool and the client?? I guess these builders are not interested in a happy client for years to come and that might also get them more business down the road. Too bad there's not a way too weed out these type of builders who use sand!!!!
 
Work work in the pool industry and have 8 years experience. I can tell you that a combination of sand and rock work the best.

Rock works best placed on the bottom of the pool. The reason why is the weight of the pool can compact sand slightly. Imagine how many gallons there is in the pool and how much weight that would be. Also some pools are not perfectly flat spreading the weight evenly. Some of them on the bottom are very narrow because of seating or big ledges up near the top.

Sand however should be used everywhere else. You cannot compact Rock gravel or a 57 anything around the side of a pool. You will always have air gaps and therefore can have water and wheel shift. If you're installer digs the hole to specifications of the manufacturer it's going to be a tight fit. Sand will be the only medium that you can properly "wash" around the pool. Bear in mind that both products will not be perfect around any pool. There is definitely no way you can come pack rock around the sides and under decks completely. Sand will hold water and as it drains it will leave a gap. However, that Gap is minimal compared to any Gap that Rock will have.

On the question of weight of sand bulging the side of a pool, that won't happen if the installer digs it to specification, meaning, there should only be three or four inch Gap between dirt and pool when you install the pool. If an installer has a foot or two feet from the edge of the pool then where he dug, of course there's going to be a huge weight of sand and it could bulge out the side. But that means the installer has no clue to what they're doing and did not dig a hole according to specifications.

At this moment I has spent 3 days correcting a installer's pool which decided to use Rock in the back under a deck and in the front. As as my picture will show, the Rock they dumped in the back there was no way for it to fill under the deck perfectly. So there is a huge void back there and there is no way to wash sand now to fill up that void because of the Rock. Sand in those cases as I stated before you can wash with water and it fill up that void much better than Rock because you cannot wash it with water underneath the pool.
I have personally put in over 500 pools. I'm a supervisor for affordable pools in Louisiana and have over 8 years experience. We do close to 70 pools a year.Screenshot_20220531-181353.png
 
I have not seen one FG pool manufacturer recommend anything but gravel for the base. All the major brands have recommended installation methods, and all have different wording, but all use the word "gravel". Installing a pool different than the manufactures instructions could lead to warranty issues down the road and not sure a PB would want to deal with that, let alone the home owner.

Aside from the gravel, I have seen some PBs use a concrete like slurry to float under some of the sun ledges, but can't recall seeing anyone who has used sand to do that.
 
First make sure the base is compacted, before pool is placed. Rock/gravel is more work to get into voids, but it can be done pretty easily. I am sure there are some types of sand that would work but I doubt getting the sand in place with water is going to ideal. What is all the water going to do to all the subsoil? Also sand holds water better than gavel, so there more of a chance of frost heaving in the winter. Type 57 gravel is what I would use. The one down side to gravel is if you ever need to dig in it. But if done properly you never will need to.
 

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