Fiberglass in-ground pool shells are thin walled and made to be fitted into holes specifically excavated for each one. All fiberglass in-ground pool shells take advantage of the structural support the ground around them creates. Without sufficient structural support, an in-ground fiberglass pool shell sitting above ground and filled with water would soon bulge outward and collapse. The immense weight of water in an in-ground fiberglass pool shell that's above ground would be too much for an unsupported shell to withstand.
A fiberglass in-ground pool shell is made to be very strong, but it's a thin-walled creation. Most in-ground swimming pool fiberglass shells are 3/16 to 3/8-inch thick and are flexible as well. But a 40-foot long by 20-foot wide by 4-foot deep pool is 24,000 gallons and nearly 200,000 lbs. (100 tons) of water weight. One hundred tons of water pressing outward on a pool this size sitting completely out of the ground will rupture the shell.
To use an in-ground fiberglass pool shell for above ground pool use, you'll need to buttress and brace it properly. Unfortunately, in-ground fiberglass pool shells aren't actually engineered to be used completely out of the ground. Most often, only about 18 inches of fiberglass pool shell is allowed to be out of the ground without strong structural support backing it. All fiberglass in-ground swimming pools, though, can be built partially out of the ground as long as they're adequately supported.
Typically, fiberglass pool shells placed above ground have a maximum of 3 feet of shell exposed, with 18 inches of that supported by a retaining wall. One reason to partially expose a fiberglass in-ground swimming pool shell is so that elevated pool decking can be constructed around it. If you want a completely above ground fiberglass pool, you'll need a specially fortified shell. Fortified fiberglass pool shells for above ground use are thicker and have more extensive support built into them by their manufacturers.