How to Install a Rigid Pond Liner
With this type of pond, the hole into which the unit will sit is crucially important. If your chosen pond is a regular or symmetrically shaped unit (square, circle, rectangle, oval, etc.), turn it upside down, and position it so that it is directly over the precise place you want the finished pond to be. Depending on the shape and size of the unit, this may or may not be a two-person job.
With a stick, spade, or handfuls of sand, mark an outline right around the rim of the unit, but bringing the mark out from the edge by about 15cm (6in). This extra space will allow for miscalculations in digging and, just as importantly, will permit backfilling later with softor sand.
If your chosen pond is not symmetrical, but of an informal style (such as the most popular ‘kidney-shaped’ style), then upturning and marking out as just described above, will not work. Instead, place it upright over its intended position, and level it out with supports. Insert stakes or canes vertically into the ground at intervals all the way around the unit.
Then remove the unit, and measure the depth and size of any shelves that are moulded in to it. Now with your spade, dig a level, flat-bottomed hole as marked out, but only dig to a depth that is just a little deeper than the level of the first shelf. Rake over the bottom of the hole, and then stand the pond unit inside the hole and press down so that once it is removed again you will see that the deeper section of the unit has left an impression.
Remove the unit again and excavate the newly marked section down to about 10cm (4in) below where the bottom of the pond will be. Always check that the pond is sitting horizontally by using a straight-edged board and a spirit level. Check in six or seven directions so that the hole is being dug with a level base —there is little that is worse than a lopsided pond. Water will always find its level, even if the pond is not, and it will look hideous.
Now check the hole thoroughly, removing any large or sharp stones. Compact the soil, and then spread a 5cm (2in) layer of builder’s sand on to the base. Lower the pond gently into position, and add or take away sand until you are happy with the level.
Woven fibre underlay, similar to carpet underlay but thicker, is available and can be used with pre-formed liners. They are, however, more ideally suited for use with flexible liners.
Keep checking with the spirit level. If you have measured correctly, the lip of the pond should end up lying just below ground level.
Next comes the satisfying bit — running water into the unit for the first time. But don’t fill the whole pond. Run just a few inches into the bottom to stabilize the unit, and to bed the bottom of it on to the sand base.
Then fill in some of the larger gaps around the edge of the pond unit with sieved soil or builder’s sand. To backfill, pour the sand or soil into the space and then ram it down with a large piece of wood. This will help to ensure that there are no air pockets.
Add more water and backfill as you go. By doing the filling and backfilling at the same time, you are exerting even pressure on to the sides of the unit— front and back — and so avoiding buckling of the walls.
Fill to about 10cm (4in) from the lip of the pond. This will allow you room to complete your job, which may be to plant perennials or bog plants along the edge of the pond, or to lay paving stones.