Understanding Deepened Foundations in Pool Construction

When it comes to building a swimming pool, what happens below the surface is just as important as what’s visible above it. In areas with unstable, expansive, or soft soils—or on sloped lots—standard foundations simply won’t cut it. That’s where deepened foundation systems come into play. These structural supports, often hidden underground, are crucial for ensuring the long-term stability and safety of a pool. Depending on site conditions and engineering requirements, contractors may rely on a variety of pier types—each with its own strengths and limitations. In this article, we explore the most common deep foundation options used in pool construction: friction caissons, driven piles, helical piers, and end-bearing caissons.

Deepened Foundations in Pool Construction: Caissons, Piles, and Piers Explained

Friction Piers/Caissons

Friction caissons are drilled holes, filled with reinforcing steel and concrete. The engineer calculates the friction between the earth and the outer surface of the concrete. These rely upon the surface area of the outside of the caissons and the coefficient of friction of the soil for support. Therefore, their diameter and depth are designed to develop sufficient surface area and soil friction. If the earth is removed they lose their ability to support the structure.

Driven Piles

Driven piles work in much the same way as friction caissons, except that they are pounded into the ground. Precast concrete, wood or steel I-beam piles are driven into the ground by a pile driver to the point of refusal. They are not often used in residential construction, due to the noise and ground vibration they cause.

The vibrations can actually damage nearby structures. They also require large machinery that often cannot access residential lots. Driven piles can be “end bearing,” friction or a combination of both. They are most often used in swimming pools adjacent to water (lake front, beach front, swamp land), that have soft or incompetent soils.

Helical Piers

Helical Piers are giant soil “screws” that are drilled into the soil. They have significant limitations, due to their size and method of installation. Helical piers are limited in their load bearing capacity to the torque that the installation machinery can deliver. Helical piers also do not perform well in loose or soft soils. Their development depth is limited by the length of the shaft.

For pools, helical piers are most often used in areas of expansive soils or high water tables to hold down the structure. They are the most economical and weakest of the available options. To resist point loading on the tip of the shaft, grade beams or mat foundations are often utilized to spread the load across the underside of the pool floor.

End-bearing caissons

As the name implies, end-bearing caissons (seen above – Featured Image) are drilled or driven until the end of the pile contacts supporting bedrock or soils. They often develop support through a combination of skin friction and end load bearing. Bell caissons, flare out on the bottom to increase the contact footprint of the shaft. They require specialized augers to create the flared end.

The portion of a pier above grade is a “free standing” pier (aka: column). The tops of the columns are tied together by beams or a mat foundation to resist movement. They are often seen on bridges, overpasses and above grade swimming pools.

You can see why a soils report is required for pier supported structures. Without a soils report, the structural engineer will not know the soil’s coefficient of friction, it’s load bearing capacity or the expansion index for their design.

Conclusion

When it comes to pool construction, building a structurally sound pool on challenging terrain requires more than just a good understanding of design—it demands a deep understanding of soil behavior and the right foundation system to match. Whether it’s friction caissons, driven piles, helical piers, or end-bearing caissons, each solution serves a specific purpose based on site conditions and load requirements. Because these systems involve critical calculations and specialized installation methods, they’re never a DIY task. A thorough soils report and the expertise of a qualified structural engineer are essential. For pool contractors working in uncertain ground conditions, relying on professional guidance isn’t just smart—it’s a matter of long-term safety and liability.

Photo Credits: Swimming Pool Expert Witness