While a plastic preformed pond from the garden center seems like a quick way to get a pond up and running, it requires careful installation. If you don't make sure the pond is level and well supported, it's likely to crack under the weight of the water. These rigid liners create a durable pond that should last for years -- they won't tear like a flexible liner might. When installing a preformed liner, each of its "shelves" must sit level.
Place the plastic molded pond liner upside down on the installation site. Lay a garden hose around the outside edge of the pond liner or trace around the liner with powdered chalk to mark its perimeter. Move the pond liner out of the way. Mark a second line 2 to 4 inches outside of the first line. Remove the first perimeter line.
Place a tarp on the ground 6 to 12 inches away from the marked perimeter. Measure the distance from the liner's top edge to the first shelf's bottom with a tape measure. Add 2 inches to the measurement.
Dig the entire area inside the perimeter line with a shovel to a depth equal to your measurement. Pile the soil you dig out onto the tarp.
Measure the width of the top shelf's bottom edge. Draw a line on the bottom of the excavated area, positioning it the same distance away from the sides as the measurement. Shape the line to reflect the outer perimeter. For example, if the shelf is 4 inches wide, then draw the line 4 inches away from the hole's sides to mark the next perimeter line for digging.
Measure the distance from the top shelf's bottom to the next shelf's bottom. Add 2 inches to the measurement. Dig the area within the perimeter line to a depth equal to this measurement. For example, if the distance between the top and second shelves equals 6 inches, you would add 2 inches to get to get 8 inches. You would need to dig to a depth of 8 inches, staying within the perimeter line.
Lay a two-by-four lengthwise across the edges of the layer. Place a level on the board to ensure the two edges sit level with each other. Move the board so it once again rests on the edges of the top layer, but runs in a crosswise direction. Place the level on the board to ensure that the edges match. Remove soil from the edges as needed to make the layer level.
Repeat the processes from Step 4 and 6 to create as many levels in the ground as there are shelves in the pond liner. End with the last excavated layer 2 inches below the liner's bottom.
Spread a 2-inch-deep layer of fine sand in the bottom of the deepest layer. Smooth the sand's surface with a rake. Place the level on the layer of sand to ensure it sits level. Add or remove sand to make the layer even and level. Check the layer with the level again.
Place the pond liner in the hole. Lay the board across the edges of the pond liner. Place the level on top of the board and check for level. Pull the liner from the hole if it isn't level. Add or remove sand from the hole's bottom, as needed, to level the liner. Replace the liner in the hole and check it with the level again.
Add water from a garden hose slowly to the liner. Pour sand into the hole around the edges of the pond, pushing it down around the sides of the bottommost layer while the water runs. Fill the space around the sides of the pond, staying at the same level as the water. Turn the water off, if needed, to catch up to its position. Repeat this process until the water level sits 2 to 4 inches below the liner's top and the space around the liner’s sides holds a tightly packed layer of sand.