Tips for Building Backyard Pools | Water Gardens & Water Features from Preformed Ponds
1. Place the pool liner in position and mark out the shape on the ground around it.
2. Carefully remove any turf within the shape and excavate to a level soil surface slightly larger than the shape. A long spirit level, or a spirit level with a long straight edge is essential.
3. Excavate the shape of the pool liner down to the same depth as the marginal shelf. Then make it one inch deeper (25mm) and 4 inches (100mm) larger all around. Check the hole is level, particularly around the edge. If the pool is going to be raised or partially raised: The shape of the pool must be marked on the ground. From this line inwards 2 ins (5cm) and outwards by possibly 4 inches (10cm) a small 4 inch (10cm) deep trench for a footing must be excavated and filled with a semi-dry concrete 5:1 mix of All-in ballast and cement powder. The width of the footing depends ultimately on the thickness of the facing stone or brick that the water feature is intended to have.
Use 4 inch (10cm) concrete blocks on the inside edge of this footing to build up to below the rim of the pool. These will support the pool liner, the facing stone and the edging for the pool. Leave the vertical joints of the block-work partly open to begin with, in order to facilitate the backfilling with sand ( see 9. below).
4. Place the pool shaped liner in the excavation in its intended position and mark with a sharp stick or trowel the shape of the bottom of pool liner in the soil.
5. Remove the pool shape and excavate to the full depth of the liner plus an inch (25mm). With a bit of adjustment the liner will fit like a glove, albeit slightly too large. The liner should be sitting just below the level you had intended.
6. Remove the liner and spread a layer of sand one inch thick over the base and the marginal shelf areas.
7. Put the liner back in. Ensure the base is firm and level and there is solid support under the marginal shelves. Start to fill it with water. As the pond is filling, push sand into the gap between the liner and the soil. The level of this backfill must go in unison with the water filling the pool and the pressure at which it is pushed in should correspond with the pressure from the water in the pool. This is a balancing act in which the sand or water could distort the shape. One German manufacturer used to recommend washing the sand into place in order to consolidate it. Doing this presupposes that the excavation is in fairly well drained soil and that you do not to get carried away with the backwash, otherwise the liner just floats out of the hole.
8. Whatever method you use, leave it there full of water over night to do any settling that might latterly occur, before you attempt any edging. If it shifts - empty it and build up the sand support where it has sunk. Gently ramming down the backfill further at the lowest point can make up tiny fractions in subsidence. If you need to do this don't overdo it because it can buckle up the rim of the liner, which you want to keep as flat as possible to take whatever edging you might want to choose to go round it.