Pond construction - Putting in the concrete base

A Guide to Modern Koi Pond Construction

Putting in the concrete base

Now that you have decided on the layout for the bottom drains you can now think about the concrete base. This is very important for it has to support the structure of the pond during the whole of its lifetime.

[Pond base]

The sketch above shows the general layout. Once the hole has been dug out, and is reasonably level then you can start preparing to pour the concrete.

The soil in the bottom of the hole must be well compacted, and in most cases it is necessary to put down chippings or scalpings, and then compact them down with a mechanical vibrator. The reason for this is to prevent the liquid part of the poured concrete seeping into the soil, and thus weakening the mix.
On building sites they use polethene sheeting on the ground before pouring the concrete, but as we are not worried about dampness then this is not necessary with a koi pond.

	The next consideration is reinforcing for the concrete base. This photo shows the reinforcing bars in position. This is the base for a filter, and although straight rods have been used on this occasion it is now more common to use 8 ft x 4 ft sheets and cut them to shape. The reinforcing must be suspended above the base, roughly about half way up . In the case of six inch thick concrete the spacers should be three inches, If the base is nine inches thick then two layers of reinforcing should be used.
	These photos show bricks being used to support the reinforcing, but when you purchase the sheeting you can also get a pack of spacers which will hold the bars in place, and not affect the concrete once it is poured. When the concrete is poured then it should be spread out evenly, but it should not be tamped down too hard to make a smooth surface as this brings the liquid to the surface and weakens the mix.
The weight of water will exert pressure on the outside walls, and this must be borne in mind when back filling after the walls have been partly built. The most important section is the lower part above the concrete base and this should be filled with concrete and well tamped down. It is better do this when you have laid about five blocks high as it is much easier than trying to do it when the walls are six feet high especially as rubbish is likely to fall down the space and is very difficult to remove. The remainder of the space can be filled with concrete when the wall is being built up, or it can be back filled with stone dust or something similar. Never backfill with clay soil or other similar material as this cannot be compacted down sufficiently.
	This photo shows a case where reinforcing bars have been installed vertically so that hollow concrete blocks as shown in the photo can then be used for the first two rows, and then you can continue with the 18" x 9" x 4" blocks. This method is not usually used, but some people do prefer it. Back filling with concrete should still be carried out as described above.
Now we can look at the type of base we wish to construct.

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