Finalizing the excavation

last updated: Dec 27, 2023

With several backbreaking days of pick and shovel work behind me, I think I can finally call the excavation complete. Things went mostly according to plan, with the exception of a possible miscalculation of the pool wall to wall width…

Dimensional abiguity

Comprehensive and accurate dimensions of the pool are essential for this project. Unlike a traditional installation, we have fixed walls on three sides of the shell. The concrete wall footings will be poured prior to the shell being put in place, and thus there is no possibility of moving the walls after the shell arrives.

All fiberglass pools have coping around the perimeter. Coping is the part highlighted in yellow below:

Fiberglass pool shell cross section showing coping

Fiberglass pool shell cross section showing coping

For a typical in ground installation, the precise width of the coping is not overly important. The hole is over excavated by around 100-150mm and the gap is filled. Given our fixed walls, the width is critical. The engineering diagrams for the pool provide the dimensions from the outer edges of the shell at 9300 x 4200mm. The top of the inner walls are therefore this size minus the coping part:

Pool shell cross section

The wall distance is the outer width minus the coping widths

One would hope that those same engineering diagrams would give details on the angle of the wall (for floor width) and coping width. Unfortunately they are silent on these measurements and actually show conflicting information in different diagrams - a vertical wall on the coping details section and a sloped wall on the pool cross section.

With such uncertainty, I contacted the pool manufacturer during the design phase to enquire as the coping width and was told that it is “approximately 150mm”. This gives a wall distance of 3900mm:

Wall distance
= width from outer edge to outer edge - 2 * coping width
= 4200 - 2 * 150
= 3900

The walls were designed and the site was excavated on this basis. Additional diagrams obtained later hinted that the total distance from the pool edge to the pool floor is 150mm. This includes both the coping, the safety ledge halfway up the pool wall, and any angle in the pool wall. If valid, the actual coping width is likely closer to 70mm, giving a wall->wall distance of 4060mm rather than 3900mm.

With this new information, we decided to excavate the uphill wall edge an additional 160mm. The shallow/deep end walls can only move in multiples of 200mm given the blocks used and so will remain unchanged. The pool shell slopes significantly at these ends and so is less likely to present a problem.

We will have the pool shell onsite for a few days before it is craned into place, so last minute adjustments (aside from moving the footings) will still be possible.

The completed excavation


Final shape prior to clean up.

Completed and cleaned up excavation

Completed and cleaned up excavation.

New trench for sewage and storm water pipes to run under the pool deck.
Trench continues down below pool edge retaining wall footings.

Envisaged vs reality

The below image compares the 3D dig plan representation to the actual result:

It’s difficult to see with all of the dirt on the pool deck, but the two match up quite well.

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