Using a pre-cast pond shell
Positioning a pre-cast pond is best achieved by inverting the shell and laying it on the ground in the right place and marking its exact outline, but this is unfortunately suitable only for symmetrical pools. Irregular ones have to be placed right way up, and the edge marked by dropping vertical positions to the ground. Check the levels to allow for any building-up of soil around one end, otherwise you may have one end of the precast shell sticking out of the ground.
When a precast pond has to be set into the soil it is necessary to dig a hole to conform as closely to the shell as possible. This is quite an undertaking, particularly if the shell is large, and when the hole is completed there are usually some gaps around the edge that need to be back-filled. This is done with sand, ramming it well down to give the cast as much support as possible. There are difficulties where the gaps are under the planting shelves or the shell’s base; but if a layer of sand is spread on these flat surfaces and the cast wriggled into position, the sand will conform closely to the shape of the shell. Awkward areas are sometimes filled with sand by forcing down the sand with a jet of water from a hosepipe, which will wash the sand into the cavities that otherwise cannot be reached.
Once the pre-cast pond is in position and the edges again checked for levels it can be filled with water, which will make it settle a little. A semi-rigid pool cast will have its sides pressed out against the walls of the hole, but the fibreglass models are more rigid and will be more inflexible. Some ponds have the edge built into the cast and can be left exposed, whereas others need a covering to make them a little more unobtrusive. This can be done using, stone or paving slabs, though the latter need to be bedded onto a layer of mortar for stability. Remember to leave the water to settle for a reasonable length of time, so that it can reach the temperature of its surroundings and lose the and other harmful gases. Only then should you set about introducing plants and fishes into the pond.