To keep nut grass from ruining your pool liner, you must take precautions prior to installing the liner. Nut grass and others weed that grow under the pool liner eventually cause the liner to become weak and develop holes. If a liner is already installed, remove the liner and prepare the surface, or wait until it tears and fix the surface before replacing it.
Mark the circumference of the pool if this is a new installation. Dig to a depth of 4 inches both within the pool area and 12 inches out from the pool wall edge. If the pool is already installed, dig 4 inches deep inside the pool walls and 12 inches outside the pool walls. Digging to a depth of 4 inches ensures that you have removed the roots to grasses and weeds.
Layer newspaper--at least four layers--around the base of the hole. If the pool is already installed and you are replacing the liner, add the newspaper outside the pool walls in addition to inside the pool walls.
Layer weed barrier on top of the newspaper. Liberally spray the weed barrier with weed killer. Spread 3 inches of sand on top of the weed barrier, both inside the pool walls and outside the pool walls.
Install the liner according to the manufacturer's instructions. Install plastic garden edging along the edge of the prepared sand barrier outside the walls of the pool. Add wood chips or your favorite landscaping material, such as rubber mulch or stone, on top of the sandy area outside the walls of the pool.