Because of their elasticity and moisture absorbency characteristics it is possible for liners to crease even when full of water.
Whilst heavy bathing loads, steep slopes, felt or foam underlay and high water temperatures can contribute to liner movement, the fundamental cause is almost certain to be an equalisation of water pressure inside and outside of the liner, i.e.: water trapped beneath the liner.
Should liner movement caused by foot traffic take place at any time during the life of the liner always initially look for water build up behind the liner as this is most likely to be the cause. It is essential that the shell of a liner pool is totally permeable and that the pool site is generally free draining. It is essential that a facility for de-watering the pool shell is incorporated into the pool structure at the build point. If this facility is provided, then high water table and clay sites for liner pools cease to be a problem. Should creases in the liner need to be removed, this can be accomplished by pouring boiling water over the area of liner that has been stretched by foot traffic (not the crease). This operation can only be carried out with water removed from the pool and a small amount of air introduced behind the liner to enable it to move back to its original unstretched form. It is recommended that the above procedure is carried out by a professional pool engineer and the water lowered by the minimum amount and for the shortest period of time.
Liner material is affected by its environment and absorbs moisture and water treatment chemicals. In normal swimming pool conditions, absorption will not exceed 5% of the original weight of the liner but with high temperatures and low pH, total absorption could reach much higher levels.
The huge weight of pool water that is standing on the liner ensures that it remains in place even though it has a tendency to grow. only when the absorption levels within the liner have become so great that they exceed the water pressure will the liner move and start to crease or bubble.