Under the pond liner you can use building sand (not sharp sand!) and / or some kind of material liner. I have even seen some recommend old carpet or carpet underlay but the point is you need something that will cushion and protect the liner from sharp bits of root or stones that lie under the surface around the hole. It’s amazing how many hidden bits of sharp ‘stuff’ you can find and remove and then still find more waiting there to puncture small holes in the liner if you’re not careful. So it’s best not to take the chance and add an underlay of some sort. We used a commercially made fleece liner. When you place it in the hole you will find that in ‘corners’ and around curves it can be a bit awkward because you have to try and fold it and pleat it to fit into the shape of the pond. This is the same problem when you add your pond liner on top of the underlay and it can be very fiddly. Once the water is in it, however, the various folds etc tend to become less visible as folds. They start to look like the ‘muddy’ bottom and sides of the pond. There are lots of liner materials but we went with the recommendation of most books and articles and chose butyl rubber. It was thick and pretty heavy to man-handle into the final shape etc but with another pair of helping hands it got done!
The overlap of the butyl liner around the edges of the pond must be at least 15cm (6in.) but I cut it to allow 30cm overlap. Of course you do not trim it to size at all until there is water in the pond up to approximately 25 to 50mm from the final level. This is to allow the liner to settle smoothly into its shape and depth so add water in stages and watch the liner does not suddenly slip into the hole. You can place bricks at strategic places around the hole to weigh the liner down while filling with water. We have a rainwater butt and used a lot of that but over half of the pond was filled from a hosepipe. This means leaving the water to settle for a day or so to let any chemicals from tap water evaporate from the pond as they could affect plants or creatures you wish to have in it.
Notice that after I was sure the water had settled to almost its final depth I folded the butyl overlap under so that it was only 15cm wide and therefore double thickness. This allowed me to pin it to the ground with 4inch plastic ‘nails’ about 30cm apart to keep the liner still. I made the holes through it with a 4inch steel nail and hammer. Over the edges I planned to put grass turves and cobbles and pebbles in various places.
To be continued…