This will help protect the butyl liner being punctured. A layer of pond underlay or old carpet above the sand will provide additional protection. Get a friend to assist in unfolding the butyl liner.

If the water level in a lined pond drops or the flow of water decreases, check that no part of the liner has slipped below the water level. If a pond is leaking, it needs to be drained and inspected.

Whilst ponds might not be at the forefront of this ... for an awkward-shaped space then you can use the flexible liner option ...