Ponds with a preformed liner, establish outer edge of pond, firm support for liner, check sand-covered bottom & shelves with a level, fill the space between the sidewalls with sifted soil or sand.
Pond with Preformed-liner
A. General Information.
B. Establish Outer Edge of Pond.
C. The Liner needs to have Firm Support.
D. Use a Level to Check that the Sand-covered Bottom and Shelves.
E. Fill the Space between the Sidewalls.
A. General Information
When you are ready to install a preformed pond liner, pick the biggest liner possible that goes with your site and your spending limit. Preformed units look smaller once installed, filled with water, and edged. If you want fish, the pond should be deep enough for them. If the pond has plant shelves, they should be wide enough to accommodate the containers.
B. Establish Outer Edge of Pond
When you figure where you want to install the pond, carry the preformed unit to the site. Orient the liner, topside up, on the spot where you want to install it. Use a carpenter's level and plumb bob or weighted string to establish the outer edge of the pond. Preformed rigid pond walls slope slightly inward from top to bottom, so the top perimeter is bigger than the bottom perimeter. The plumb bob drops straight down from the top perimeter, so you can outline the perimeter on the ground directly below it. Mark the perimeter using a rope or a hose. Enlarge the whole perimeter by 2 inches to allow working room within the excavation. The extra space will be filled later with sifted soil or sand. Finalize the outline by marking it with spray paint, flour, or sand.
C. The Liner needs to have Firm Support
The liner needs to have firm support under the shelves, bottom, and sides for it to be stable and resist buckling. Set the preformed pool aside while you excavate. If the pond has no shelves, dig straight down (or at a slight angle no greater than the inward slope of the preformed unit) to the bottom. The depth should match the depth of the pond form plus 2 inches. If the pond has shelves, measure carefully to make the excavation conform to their depth and width. In order to sustain the weight of water when the pond is filled, the shelves have to be supported by soil. Leave room to add 2 inches of damp sand under the shelves once the excavation is complete. If you want to protect the pond from surface runoff, measure and dig out only 1 inch extra; the 2 inches of added sand will put the pond top 1 inch above ground level.
D. Use a Level to Check that the Sand-covered Bottom and Shelves
Use a spirit level to check that the sand-covered bottom and shelves of the excavation are completely level. Avoid over digging the shelf areas. Figure out in advance what to do with the soil removed during excavation. You could use it to build up an area planned for a waterfall or to change the contour of the land somewhere else on your property. Ask friends or neighbors if they need the soil. Otherwise, look for a place wanting clean fill dirt.
Spread 2 inches of damp sand across the bottom of the excavation on the shelf surfaces.
Use a board or the straight edge of a garden rake to spread sand evenly over the entire bottom and the shelves.
Place the liner into the excavation; use a spirit level to make sure the pond is level. If it needs leveling, remove the preformed liner and rework the sand. You might need to remove it several times making adjustments. The pond's contact marks on the sand show where to remove high sand and where fill-in sand is needed. Keep working the sand until the pond comes within ¼ inch level. Add the edging. If you want your pond to be partially in-ground, dig down as already described, but only far enough so the top of the pond will be at the height you want it. Remember to add the 2 inches of sand to the excavation. If you want an aboveground pond, remove the top 2 inches of soil and replace it with sand. Rake and work the sand as needed so the top of the pond sits level.
E. Fill the Space between the Sidewalls
Preformed ponds have a little flexibility. Units built with rigid fiberglass and high-density polyethylene usually have sufficient structural integrity to hold water without outside support. But since they are somewhat flexible, they could become distorted when filled if the sidewalls are not supported.
You have to prevent this distortion by filling the space between the sidewalls and the excavation with sifted soil or sand as you fill the pond with water. As the first few gallons of water spread evenly across the bottom of the liner, the form will probably sit level in the ground, but the weight of added water might cause a slight shift. Backfill around the form as the pond fills Adjust the water flow so that the pond fills to about the same level that you are working the soil on the outside of the liner. Periodically check the level in all directions three or more times while the pond is filling with water. If it's not level by more than ¼ inch, remove the water and soil, make adjustments, and start over. Add the edging to complete installation.
