The nature of the ground where the pond is being dug can cause problems particularly if it is stony, or wet. At this stage, is worth spending time removing all pebbles, stones and sharp edges. Pond liner is expensive and if punctured obviously it will leak, which is exactly what you don’t want as that is a major headache to solve. Weight of water causes pressure pushing down on any stones which can then puncture the liner. Careful preparation at this stage is tedious but worthwhile in the long run. The other problem with flinty ground is that it is very difficult to shape the pond and shelves, rocky ground does not hold its shape and if this is a problem, it can be overcome by cementing the pond.
Wet ground is a challenge. If your garden is close to the water table or in wet ground, contrary to instincts, this is a problem when constructing a pond. You may guess that the land is close to the water table because of pooling water after rain, areas of wet or boggy ground in the garden, or the presence of water loving plants. Wet ground will causing pooling in the bottom of the pond during construction and this in itself is a problem. Once the hole is dug the fact it fills repeatedly with water can once the liner is in place, can cause a build up pressure under a pre formed or Butyl liner pushing the liner or parts of it upwards.
One way to deal with the problem is by a change in design to raise the pond above the water level. Use landscaping to raise the edges of the pond so that the liner does not have to be laid so deep. Alternatively, build a permanent pipe or channel to funnel the water out as in the image. When digging started on the pond in the image, it was unknown that the position was over a spring well and the problem could only be resolved with permanent drainage. By that time a very large hole had been dug, and so not everything can be planned for. If you look closely at the image (the one above the liner image) you will see a channel right in the centre lowest point of the pond excavation where a drainage pipe is being laid to take away the spring well water which otherwise would lie under the pond.
For all ponds, once the hole is dug out it needs to be lined with sand. This should be a firm layer of several centimetres, more if you are on stoney ground. This is needed to protect the liner, and the sand acts as a cushion. Remove all stones and sharp objects from the hole before adding a protective underlay of soft sand. A preformed liner will need to be placed on the ground where the pond is to be constructed, which is best drawn around to the shape of the liner. Use a cane to measure the depth of the preformed liner and then dig out to that level. If your pond is quite deep and large, consider how to make the maintenance easier. Pond plants require attention such as cutting back, especially lilies, which if growing in the centre of the pond will need to be accessed. This can be done either by getting into the pond or by constructing a Jetty. If you intend getting into the pond, make it easy and at the construction stage plan a set point of access into the pond with some step down free from planting.