Once you’ve decided on the layout of the bottom drains/piping, you can now move on to the concrete base. This has to be planned out carefully as it is the structure that supports the pond in its lifetime.
Laying The Base – Reinforcement
Make sure that the soil in the bottom of the pond is well compacted. You can add chippings or scalpings, and then compact them down with a mechanical vibrator. This is al prevent the poured concrete from seeping into the soil, and weakening the mix.
Reinforce the concrete base with reinforcing bars. Use 8 ft x 4 ft sheets and cut them to shape. Suspend the reinforcing bars above the base, roughly about half way up from concrete thickness i.e., for six inch thick concrete the spacers should be three inches. Two layers of reinforcing should be used if the base is 9 inch thick.
When you purchase the reinforcing bar sheeting, get a pack of spacers which will hold the bars in place. You can pour the concrete in without having to remove the spacers.
Spread the concrete out evenly when you pour it in. Do not tamped down too hard to make a smooth surface as this brings the liquid to the surface and weakens the mix.
Concrete back fill
Once the concrete base has been poured in and leveled, you can start building the walls.
You will need to back fill after the walls have been partly built. This should be done once you’ve laid about five blocks high of wall. The weight of the water in the pond will exert pressure on the outside walls, especially the lower part above the concrete base. So this should be filled with concrete and well tamped down.
Doing it while the walls are partly built makes it easier for you than to back fill when you’ve got 6 feet high walls, especially when you have rubbish going down the sides and they would be hard to get rid of. Once you’ve done this, fill the remainder of the wall with concrete while the wall is being built up. You can opt to back fill with stone dust or something similar. NEVER backfill with clay soil or other similar material as this cannot be compacted down sufficiently.
Another method is to use hollow concrete blocks at the wall at the bottom of the pond. For this you will need to install the reinforcing bars vertically so that these blocks can be slotted on to the first two rows, then continued with the 18″ x 9″ x 4″ blocks.
You should then back fill with concrete as described above.
Types of Base
You would have decided on which type of base you will be using during the planning of your pond. Basically there are three types of concrete base.
Pond base – type 1
1. Lay reinforcing bars that are cut and fitted in place. Raise it off the base by about three inches with spacers
2. Pour in six inch deep layer of concrete and level out.
3. Allow the concrete to cure. Once its cured, start building the walls up to five or six blocks high, then back fill between the wall and the excavation with concrete.
4. Right now would be the time to put in place the bottom drains and all the associated pipe work as described earlier.
5. Once completed put steel reinforcing in place for the second layer, again cut to size and suspended with 3 inch spacers .
6. Pour in the second layer of concrete.
Pond base – type 2
This base is a one layer base. When you dig out the pond you will need to also dig trenches to support the drains.
1. The bottom drains are six inches in depth with four inch diameter pipes. Make sure that there is sufficient strength in the concrete base in a way of these items a trench is dug underneath them.
2. Put in place the bottom drains and all the associated pipe work in the trenches then place cement at certain intervals, with the pipes bedded into it.
3. Cover the area with reinforcing bars that are cut and fitted in place. Raise off the base by about three inches with spacers.
4. Pour in six inches of concrete and level off.
5. When the concrete has had time to cure then you can start building the walls the same way as type 1 base.
NOTE: It is common that you will need a depth of more than 6 inches with these type of concrete base. Use nine inches and remember to use two layers of reinforcing steel.
Pond base – type 3
Before you lay the concrete base for this method, you will need to dig a trech out for the foundation of the walls.
You should make this trench 3 inches wider than the wall on each side, ie., for a 9″ wall the trench must be at least 15″ wide. The depth of the trench should be from 9 to 12 inches.
Place pegs in to show the correct level, then pour the concrete in.
Allow the concrete to cure, then start building the walls
The concrete base is built the same way at method 2.
NOTE: You should only use this method if you have knowledge of the building trade, or someone who can advise you.
Pouring the concrete base
Pouring in concrete is not as easy as one thinks. There is an amount of work that is required, so that you can do it successfully with the minimum amount of stress. To do this, you will need to plan the process well.
A standard mix of concrete consists of 3 chippings, 2 sand, and 1 cement by volume. If you are using ready mix concrete, inform the company that you are buying it from what you are using it for and ask them to recommend the mix you should use.
A cement mixer may work well if you’re pouring a small area and have help from friends or neighbour. But if you’re covering a large area that is more than ten feet square, you may not be able to maintain the same consistency over the whole area. This will weaken the pond base.
Hiring a truck
You can hire a truck to hold the ready mix concrete, and the driver will then park the truck fairly close to the pond. These trucks are very heavy, and cannot come down a drive normally designed for private cars. You will have to cart the concrete in wheel barrows. A small army of helpers would do quite well with this method.
You can also hire a truck with a pump. The ready mix truck will pour the concrete into the truck and it will then be pumped via the gantry to the base of your pond.
For truck and pipes to be put in position and stowed away again takes an hour, the actual process of pouring the concrete into the base of the pond only takes 10 minutes.
The only downfall to this method is that it is costly. The truck has to be hired for a minimum of six hours. So decide on which way you would want to go about pouring concrete into your pond.