Detailed design

The detailed design of our self-built swimming pool.

last updated: Apr 10, 2024

Fiberglass pool or concrete pool?

Concrete pools are built directly in the ground onsite. Concrete (or more often shotcrete) is set on a framework of steel reinforcement and then plastered or tiled. Concrete pools are endlessly customizable and can be built to suit any site.

Fiberglass pools are built in a factory and delivered onsite as a single piece with an already finished surface. The fiberglass shell alone is not structurally capable of retaining the pool water, and requires a hole in the ground, a concreate beam around the perimeter, and an incompressible material packed between the shell and the earth.

Price-wise they are similar.

We chose fiberglass primarily for warmth. We’re located in a temperate climate where the average ground temperature ranges from around 13°C to 18°C. We will be using solar heating to heat the pool water and the fiberglass shell is a good insulator. Other benefits include lower maintenance for the pool surface and lower likelihood of leaks.

While not simple, a typical fiberglass pool installation is a relatively straight forward process:

  • Dig a pool-sized hole and a trench for the plumbing.
  • Pack the base with some relatively non compressible material and shape it to fit the base of the pool.
  • Plumb the pool with suction and return water fittings and associated piping.
  • Place the pool in the hole, piping in the trench, and connect the two.
  • Backfill the gap between the hole and the pool with sand/gravel/crusher dust heavily debated) while filling the pool with water.
  • Lay steel rebar and mesh around the coping (edge) of the pool and create a bond beam by filling the last 300mm or so of pool/earth gap with concrete and extend around the pool.

This approach won’t work with our site, which is far from flat.

Gentle slopes can be dealt with via site work or a turndown slab, but serious slopes require retaining walls.

Retaining walls

Retaining walls retain soil. Deceptively simple in appearance, they hide surprising engineering complexity.

Soil weighs a lot, particularly when wet. The force exerted by tonnes of wet soil sitting behind the wall is considerable. Poorly designed walls fail quickly.

Typical retaining wall types

Typical retaining wall types

There are a variety of designs for retaining wall construction, but for our purposes, cantilevered walls are most suitable. As can be seen in the diagram above, the cantilevered wall typically uses the soil pressure on a large heel (footing extending uphill) to assist keeping the wall upright. Given there is a pool on the retaining side of our walls, a large heel is unsuitable and so a large toe is required instead. This a somewhat less efficient design as we don’t get the benefit of the soil pressure on the footing. Consequently, the toe needs to be larger.

The combined toe/heel is referred to as the footing, while the vertical part of the wall is the stem.

The walls need to be built of steel reinforced concrete. A professional would likely pour solid concrete walls and may pour the footing and stem concrete in one go (a mono pour). This would require building full size forms (i.e. molds for the concrete) for the footing and stem. Something like this.

Wet concrete is very heavy and with a maximum wall height for our project of 1.6 (1.8 including the footing) meters, the forms would require substantial bracing and support, not to mention a large amount of expensive wood that I would have little use for when the project was complete. Building such complex forms also requires tremendous skill and experience.

Instead of poured walls, we’ll be building the walls using hollow concrete blocks (AKA cinder blocks, AKA besser blocks), and then filling them with concrete. The blocks themselves act as the formwork, forming a mold for the poured concrete. The footing will be poured first (into simpler forms), the walls will be built on top of the footing, and then filled with concrete.

Concrete is crazy strong, compressively speaking

Concrete is very very strong, but it’s strength is compressive strength (i.e. strength when getting squeezed). Standard concrete is around 25 MPa. That’s 25 million Pascals. What’s a Pascal? It’s a unit of pressure, named after the French mathematician Blaise Pascal who once wrote:

"Since we cannot know all that there is to be known about anything, we ought to know a little about everything."

Now that’s an idea I can get behind. One Pascal equals one Newton per square meter. What’s a Newton? That’s a little more complicated, but suffice to say, when you hold a one kilogram weight, the force you feel pushing down is around 10 Newtons.

So 25 MPa concrete means it can withstand the pressure of a 2.5 million kilogram weight sitting on one square meter. That’s equivalent to 3,600 pounds per square inch (PSI). Like I said, strong!

Compression vs. tension

Where concrete doesn’t fare so well is when you try to pull it apart. It is virtually without tensile (pulling apart) strength. A child could easily snap a thin concrete rod in two.

Steel however, has excellent tensile strength. By embedding steel rods into concrete, we create reinforced concrete; a material that has both tensile and compressive strength. No doubt you’ve seen complex steel rod frameworks like this on construction sites before concrete is poured:

Rebar framework

Rebar for foundations and walls of a sewage pump station.

Image credit: Argyriou

Since I’m not a bricklayer, I’d like to avoid laying bricks. The concrete blocks I’ll be using are ADBRI Versaloc. The Versaloc blocks slot together and aside from the first course (i.e. row), don’t require mortar.

Versaloc construction

Typical Versaloc construction

Image credit: ADBRI Masonry

In addition to the retaining walls around the pool, we need an additional wall on the uphill side behind the pool deck. The completed wall and footing design looks like this:

Walls, footings, and the bond beam

Walls, footings, and the bond beam

Bond beam

The pool coping (i.e. the top rim) needs to be structurally connected to the walls. For a normal in ground installation, a wide, steel-reinforced concrete beam is poured along the ground around the pool edge. In our case, we’ll need a concrete bond beam that connects the coping to the top of the walls. We’ll extend the steel reinforcement in the walls up to the coping level and wire it to the coping. The bond beam will also have steel reinforcement running around the perimeter. The design for the lower wall looks like this:

Walls and footings

Walls and footings

Item 1 (wall detail) is a cross section of the lower pool retaining wall and item 2 (beam detail) shows the reinforcement details for the concrete beam that will connect the pool shell to the top of the wall. Some terms from the diagram for the uninitiated:

  • N12 - 12mm diameter steel bar
  • N16 - 16mm diameter steel bar
  • SL72 - Steel mesh (6.75mm diameter wire in 200x200mm square pattern)
  • L8TM200 - 200mm wide trench mesh (3x 8mm parallel steel bars 100mm apart each with a thin wire connecting them)
  • lap - Overlap
  • road base - crushed rock including fines (max 20mm pieces)
  • crusher dust / 5mm dust - crushed rock including fines (max 5mm pieces)

This is my design, but the reinforcement details are as recommended by the block manufacturer. The beam reinforcement is my own (conservative) interpretation based on the pool manufacturer’s recommendation for a normal in-ground installation.

All of these designs will be reviewed by a structural engineer prior to being given approval to build as I have no training in this area and all of my designs are based on what I’ve managed to teach myself in the last couple of months…

Speaking of pool manufacturers, that brings us to the next topic - choosing a fiberglass pool shell.