A Raised Pre-cast Garden Pond
A raised pond needs a substantial foundation under the cast to prevent the pond tilting or sinking, and plenty of reserve strength around it to contain the weight and stop the shell distorting and splitting. After placing the shell in position, carefully measure the height of the pond and mark it with canes and string so that you can be satisfied with the overall positioning. Remove the top, spread a 10cm (4in) layer of hardcore over the site and firm it down well. Place a further 10cm (4in) of concrete over the top and smooth off.
If the overall dimensions are in excess of 2.4m (8ft) in either direction it is wise to include some steel reinforcing. This can be welded mesh or a grid of rods wired together to form 10-15cm (4-6in) squares. Large ponds should have the thickness of concrete increased to 15cm (6in) to give a stable base, and this thickness should also be used where the soil is soft or unstable, whatever the size.
The cast can be set onto the concrete if both surfaces are smooth, but if either is irregular then a bed of mortar should be spread for the shell to be vibrated into place; check the levels in both directions. The perimeter wall needs a foundation similar to the pond base, with the hardcore finishing some 45cm (18in) below ground level and 45cm (18in) wide to provide adequate support for a wall of 23cm (9in) thickness.
The wall can be constructed of brick, concrete block or stone to fit into the garden scheme. As it rises a weak mixture of concrete of eight parts of ballast to one of cement can be packed between the wall and the cast; the concrete should be only just moist. If the cast appears too flexible it can be part filled with water to compensate for the exterior pressure.
Continue the wall and back-filling until level with the top of the cast, and smooth off to give a fine finish on the top surface. An alternative is to cover the top with stone or tiles, which should be set into a mortar bed and pointed between the stones or tiles to give a frost-proof finish. The pond should be filled with water and allowed to settle before planting.