You need a geotextile cushion under the lining if you have any sharp angular rocks or round rocks bigger than ¾”. Commonly you will use PermeaTex 4080 (8ounce nonwoven product) as a cushion geotextile up to 1” minus rock. A 4 -6” lift of sand or select soil can also be used for a proper sub grade cushion as well. On small decorative pond you could use carpet remnants as a cushion.
