Although it may be rather tiresome to excavate more soil than is necessary for the final shape and size of a pond, making an oversize excavation does provide a much easier and more accurate installation for a pre-formed pond. By digging a hole that embraces the greatest dimensions of the pond and allowing sufficient space for backfilling, you will ensure complete support of the moulding, while levelling will also be greatly simplified.
Before starting to excavate for a pre-formed pond, knock pegs into the ground at regular intervals around the edge of the site. Level their tops, using a board on edge and a spirit level. The peg that projects most from the ground indicates the lowest point, and the surface soil should be removed from the entire area so that the distance between the top of each peg and the soil is exactly the same. Only by starting the excavation with a level soil surface will you be able to accommodate the pond properly.
Having ensured a level from which to work, mark out an area with sand or lime that embraces the maximum length and width of the pond. Add an extra 15cm (6in) at each end and side to provide manoeuvring room.
Unless a major part of the pond is either shallower or narrower than the rest, dig out the entire marked area to the required depth. This makes installation so much simpler than trying to create a hole that matches the shape of the pre-formed structure.
The hole should be about 2.5cm (1in) deeper than the pond to allow for a cushioning layer of sand on the bottom and for the fact that if the pond is allowed to remain empty, it will inevitably rise a little while it is being backfilled. A mechanical digger can be hired to do the job if the pond is particularly large.
In most cases, the soil that is excavated will not be suitable for backfilling once the pond is in place, nor is it likely to be of much use elsewhere in the garden, so it should be disposed of. If, however, the structure of the soil is such that it flows easily around the pond shape, it is probably too good to waste and should be saved for another gardening project.
Once the excavation is complete, spread a generous layer of builders’ sand over the base of the hole on which the pond can be rested. Set the moulding in place, where appropriate propping it up on bricks, especially if part of the floor is higher than the rest. Loose bricks provide much better support than any backfilling mixture and can be left in situ.
The pond must be supported completely all round and underneath if it is to remain level and resist the damage that may be caused by a heavy footfall when cleaning out. The latter can cause an immediate fracture in a fibreglass pond. Sand or pea gravel are both excellent backfilling materials, pea gravel flowing beautifully to fill all the niches and providing excellent all-round support.
Add the backfill evenly, otherwise there is a risk of the pond structure tilting. Walk around the pond, systematically pouring the gravel into the space between the moulding and sides of the excavation. Keep a constant check on the levels across and along the pond structure to ensure that they remain true. Inevitably, the backfilling process will necessitate adjustments if the pond is to achieve the desired level.
If the pond is of vacuum-formed plastic, the weight of the gravel pushing against the flexible walls will cause them to buckle. In this case, the only way to guarantee a neat fit in the hole is to make sure that the edge of the pond is in the correct final position and add water at the same time as backfilling. This will balance the pressure against the sides and also prevent the pond from rising during the process.
Run water into pond, making sure that its level keeps pace with the backfilling. Too much backfilling will cause an inward buckling; too much water will result in bulging outwards. Remember that you can shut off the water flow completely if necessary. As the process continues, make sure that the pond remains level from side to side and end to end.
1. Mark out an area that will embrace the maximum width and length of the pre-formed pond shape. Excavate this area to the maximum depth of the preformed pond.
2. Place a generous layer of builders’ sand evenly over the bottom of the excavation. This provides a cushioning layer and ensures that the pond shape rests evenly in the hole.
3. Place the pond in position and, where necessary, use loose bricks to provide temporary support beneath the marginal shelves before backfilling. These can remain in situ, if desired.
4. Make sure that the pond is level from side to side and end to end using a spirit level. It should just be resting below the surrounding ground level, as backfilling will cause it to rise slightly.
5. Backfill with sand, pea gravel or crumbly soil, taking care to eliminate any air pockets. Ensure that the preformed pond does not rise above the surrounding soil level.
6. Add water, at the same time watching the levels carefully in order to avoid subsidence. For vacuum-formed plastic ponds, water must be added from the beginning to give stability.
As an alternative to making a large excavation based on the maximum length and width of the pond, it is possible to limit the amount of digging required by excavating a hole that matches the shape of the pre-formed pond. Although this demands a much greater degree of skill, it may be a preferable method of installation if the pond is being inserted into an established garden landscape or is to be linked to an existing feature such as a rock garden.
Carefully position the pre-formed pond, supporting it approximately 8cm (3in) above the ground on bricks or pieces of wood and making sure that the rim is level in both directions. Transfer the shape of the outer edge to the ground at regular intervals by placing a spirit level vertically between the rim and the ground. Mark the points by scratching the soil with a stick, then remove the pond and scratch a line through the points to complete the outline. Once the overall shape has been marked out, the hole can be dug.
As with the previous method, it is vital to begin with a level site. This can be achieved in the same manner by driving a series of pegs into the ground at 30cm (12in) intervals around the pond’s outline, levelling their tops and digging away the soil until they all project by an equal amount.
For accurate excavations such as this, hand digging is the only option. Begin by digging a very accurate trench around the inside of the pond’s outline. Its depth should match that of the marginal shelf in relation to the rim. Remove all the soil to the same depth across the pond, taking frequent measurements from the tops of the pegs to ensure that you never remove more soil than necessary. It is not desirable to have loose backfilling beneath the shelves or base.
Make sure that the shelf is level by knocking in pegs and levelling their tops in the same way as when originally setting out the site. It is surprising how quickly levels can go adrift if you depend solely upon your eye.
As the correct level for the shelf is reached, you can mark out the inner deeper area to be excavated, using sand or lime to indicate the digging line. To determine the size of the floor area, place the pond in position and press it down firmly. This will leave an imprint that shows the exact outline of the lower part of the pond. If the soil is not sufficiently fine for an imprint to be clearly shown, scatter a thin layer of sand over the top first. If the pond has sides that slope gently inwards, the initial hole will be slightly smaller than that ultimately required. However, it is not difficult to pare away the sides to achieve a neat fit.
It is much more difficult to install a pond in a tailor-made hole than in a large excavation, even if provision has been made in the upper part to allow for backfilling. The pond will have to be lifted in and out several times before an exact fit is achieved, but the support offered will be excellent.
In most cases, however, some backfilling will be necessary near the top. The pre-formed shape may not sit accurately in the hole and, therefore, levels must be checked from end to end and side to side. As the backfill is added, these will need constant monitoring, although it is possible to secure and weigh down the pond by adding some water once it has been levelled accurately.
This method of installation is not suitable for vacuum-formed plastic ponds because of their tendency to buckle.
1. Secure the preformed pond with bricks and mark out the exact shape, using a spirit level in a vertical plane. Mark points at short regular intervals to ensure accuracy.
2. When marking out is complete, check that all the pegs across and for the length of the excavation are level with each other. Absolute accuracy is essential with this sort of excavation.
3. Take the first spit of soil out from around the marked out area in order to define it clearly, then remove the remainder of the soil to level of the marginal shelf.
4. Mark out the area for the marginal shelves within the excavation and, using pegs, check carefully that the shelves are level before you undertake any further digging.
5. Excavate the deep area of the pond, once again using a spirit level at regular intervals to make sure that the excavation is even and level from side to side and end to end.
6. The pond shape can be placed into the accurately dug excavation. Continue to monitor levels, while carefully backfilling around the edges of the pond with sand or pea gravel.
Unless a sectional pond is of a simple configuration, it is better not to attempt to dig a hole that matches the slope of its sides. A much larger excavation makes for easier manipulation.
Assemble the pond according to the manufacturer’s instructions, screwing the units together and sealing the joints with the specially prepared sealant provided. Once the excavation has been dug, spread a 10cm (4in) layer of damp builders’ sand across the floor, levelling it carefully. Mark off the position of each joint and place a length of 10x10cm (4x4in) timber along each mark. Push these down into the sand until they reach the earth floor, then remove them to leave narrow recesses.
When the structure is lowered into the hole, the joints will fit snugly into the spaces left by the timber. Stabilize the pond by adding sufficient water to hold it in position while backfilling.
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