Fibreglass pools are almost never fully drained; the water serves as the counterweight to external forces. Without an equal amount of force pushing from within the pool, the hull may either pop out of the ground or form bulges where they shouldn't.
To prevent this problem, pool installers fill the area surrounding the pool with backfill to act like a shock absorber. The amount of backfill needed depends on the pool's dimensions, but experts generally suggest adding 5 to 10 tonnes of backfill for every 5 feet of pool area. For instance, a 30-foot pool may require 50 tonnes of backfill.
The type of backfill material also matters. Some contractors use gravel since it doesn't undergo physical change when saturated with water, whilst others use sand since it doesn't move easily when subject to regular tectonic movement. Other backfill materials include clay and dirt, but these are never suitable for the wet conditions associated with pools.
Perth's geography gives installers access to one of the most hydrophobic sands in the world, however. It means sand from this part of the world won't get wet as easily as sand from other areas, making it less prone to liquefaction, which has caused pool bulges for years. 89 and 57-stone gravel, however, is great for draining runoff, which can saturate the soil.