How Geography Can Impact Your Fiberglass Pool Installation

When you make a significant purchase, you want reassurance that it will last. You want to be sure your house is built on a solid foundation, and the same goes for your pool. While many factors impact your fiberglass pool installation, your soil type plays a big part in the installation cost of your fiberglass pool. But you still may be asking, just what goes under a fiberglass pool? 


How Long Does Pool Installation Take?To get to that answer, you must first consider the topography in your locale. What’s the land like? Where you live and where your installer will dig are crucial factors that will affect your inground pool installation and what they put under your pool. You may envision a scenario where installers remove dirt in your backyard, drop the pool shell in the hole, and then the water goes in. The soil in your backyard may make your pool installer’s job a dream or a big challenge. What that means to you is that your overall pool cost may change depending on the situation. Let’s dig into the three most common issues that installers find when putting in a pool.


Rock is known for causing problems early in the installation process. Underground rock, such as limestone, is more prevalent in some areas and can play a big part in your inground pool installation. Excavating rock often requires specialized equipment to break the rock into manageable pieces so that it can be removed and hauled away. Many swimming pool installation contracts have a “rock clause,” so read your agreement carefully. Rock clauses usually state that the homeowner must pay for the expense of removing any rock from the project site.


Soils with a high clay content are “expansive soils.” Expansive means that they have a high capacity to absorb moisture and can swell and contract with moisture. It’s the type of soil that can cause basement or foundation problems. Your installer must replace this type of soil with less absorptive fill soil during your pool installation – too much clay and the likelihood of groundwater pressure under your pool increases, and it can damage your inground fiberglass pool shell if it’s not properly installed. 


If your soil is sandy, the installers will have to amend the soil to include some sturdier fill dirt. Sand can shift, and shifting soil under your pool will not provide a sufficient support system. Sand that liquifies outside a pool can upset the balance between the water inside the pool and outside the pool. The liquified sand is heavier and can cause a bulge. Your pool builder can eliminate these problems by amending the soil and using gravel. 

Patented Solutions

Will soil type determine the type of pool that you get? It doesn’t need to be a factor. There are innovations within the fiberglass pool industry that can help anchor your pool to the earth and give you peace of mind, such as the Geo-Anchoring Pool Wall® System and the Backfill Eliminator®.

geo-anchor fiberglass pool wall

The Geo-Anchoring Pool Wall System helps secure fiberglass pools during installation and protects them against groundwater pressure. It has revolutionized the fiberglass pool industry by providing a solution that prevents wall movement or bulging. We infuse a specially formulated geotextile material into the pool wall during manufacturing, then roll it up and secure it for delivery. During installation, it’s rolled out into the backfill and covered, thus anchoring the pool to the earth. As a result, the fiberglass pool walls will resist movement or bulging.

Backfill Elminator


Another installation technology from Thursday Pools is the Backfill Eliminator®, which works in tandem with the Geo-Anchoring Pool Wall. The backfill eliminator is added during the manufacturing process and encases the tanning ledge on the pool’s underside. It permits free groundwater flow and provides access to plumbing or wiring. Most importantly, the backfill eliminator provides lifetime structural support to the tanning ledge and one continuous slope for excavation, making a pool with this feature every bit as easy to install as one without!

The Bottom Line

Your pool builder will be familiar with the types of soil in your area and should be able to give you a good idea of what you might be up against even before they start to dig. Just be sure to ask about any additional costs that might crop up once they’ve had a chance to assess your soil and build those into your pool finance budget as contingencies, just in case. If your project turns out to be a more straightforward dig than they thought, you can use that money for fun stuff, like ledge loungers for your tanning ledge or other add-on goodies, like water features or specialty lighting.