fiberglass pool installation is a very important and delicate process. It has to work in cross relation to the water fill making sure that the pool walls are not bowing in or bowing out. The fiberglass pool integrity depends on the backfill done correctly. fiberglass pool backfill requires proper backfill material and should never use excavated dirt, clay or any fill. Depending on the ground condition, whether it is clay, sock shell, sand or has a higher water table, the back fill corresponding aggregates must be selected. Some manufacturers require to use sand at all times making sure that their pool shells will not have any stress points after installation, yet the sand is unacceptable for higher water table grounds or ground with underground waters. Some really mushy grounds require 1 inch ballast as a first layer, 1/2 inch as a 2nd layer and 3/4 of an inch as a final pad.