is it possible to us heavy duty weed control fabric [of which i have lots of surplus], as underly for a 9m x 5m pond ? anyone done this before?
I think the aim with the underlay is to provide a sort of thickish spongy layer to avoid stones poking through or wearing the butyl or whatever liner you have - not sure that weed control fabric would do this. I've always found a layer of old carpet serves the purpose admirably, or thick sand packed down.
If yoir soil is free of stones then it should o.k Depending on shape , sand is good but if sides are steep then it is likely to finish up in the bottom of the hole.