Building your own pond is a challenging and rewarding project. Concrete block walls allow you to sit on the edge and enjoy your fishy friends. By building the pond partially below the surface of the soil, it is protected from the temperature extremes of hot and cold weather. In a shale-filled clay soil, the concrete blocks also protect the pond liner from the rocks, preventing punctures, tears and leaks.
Select the pond's location. A level space visible from your patio, porch or living areas makes the pond a focal point in the garden. Dig a 36-inch-deep, 60-by-60-inch square hole, removing all vegetation and tree roots.
Sprinkle water onto the dirt and loose shale in the bottom of the hole. Using a plate compactor, tightly compact the dirt and rock. Rake 6 inches of crushed gravel over the compacted soil. Sprinkle with water and compact it tightly.
Check the level of the compacted gravel. Lay a 6-foot-long board on top of the gravel with a spirit level on the board. The bubble in the spirit level should be centered in the glass window. If the base is not level, the water in the pond will not be even with the tops of the blocks. Mark the corners of a 56-by-56-inch square with stakes. Mix the mortar in a bucket. Start the first row of blocks in the corner, aligning them in a straight line. Apply mortar to each block before butting it to the previous block. At each corner, set the block at a 90-degree angle and continue installing the blocks until the square is complete.
Apply mortar to the top of the first two blocks in the bottom row. Start the second row of concrete blocks with the first block set at a 90-degree angle over the bottom row. Continue installing the blocks with mortar until the pond wall is four rows high. Check with the spirit level to ensure that the blocks remain perfectly horizontal.
Insert a piece of rebar into every block, placing it against the inner wall of the core hole. Work the rebar down through the core of all four rows of blocks, then pound with a hammer or sledge until it is slightly below the top of the blocks. Repeat all the way around the block wall.
Pour the dry concrete mix into a wheelbarrow, one or two bags at a time. Mix the dry concrete with water to the consistency of pancake batter. Scoop the concrete into the holes in the concrete blocks. Continue mixing bags of concrete with water and pouring into the blocks until they are all filled with concrete. Smooth the concrete over the holes, then allow to dry for three days.
Add 4 inches of sand to the bottom of the pond. Sprinkle lightly with water and compact until firm. The sand cushions the pond liner, preventing the gravel from tearing it. Unfold and spread the pond liner over the inside of the pond. Extend the liner up and over the concrete walls. Fold and press into the corners, keeping the bottom as flat and unwrinkled as possible.
Fill the pond slowly with water, adjusting the liner as needed to prevent excessive wrinkling. When the water reaches ground level, begin backfilling with soil around the outer edge of the pond wall. Tamp the soil firmly. Turn the water off when it is within 2 inches of the top of the wall.
Trim the excess liner so it overlaps the pond wall by 4 inches. Mix and apply mortar over the top edges of the concrete blocks. Place wall caps on top of the wall, pressing firmly onto the mortar. Allow the mortar to dry completely.
Install the pond pump according to the manufacturer's instructions. Add pond plants and fish to complete your pond.