Expandable liners are not expandable in the sense that they stretch. Expandable liners have a specific fixed maximum detph. They are made of vinyl material that is only made to slightly stretch due to its pliability. The term expandable liner originates from the concept of a flat bottom above ground pool. Taking a flat bottom pool mindset and expanding it to have a slightly deeper swim area.
Several years ago there were only flat bottom above ground pools. A fixed depth if you will. Then a liner was designed that would allow the flat bottom concept to be expanded to include a deeper area for swimming. Thus the birth of expandable liners. Today, the expandable liner depths now vary from 60" to 72" depending upon the liner manufacture.