There are many ways to design and construct a concrete base for a pond. The specific design will depend on various factors, but mainly on the skill and experience of the pond builder. The base can be constructed in layers or as a single cast base. A more advanced method includes the curved bottom. All these will be demonstrated individually in this article. Benching the floor will be discussed elsewhere in this series.
Regardless of the choice of base that will eventually be constructed, it must be remembered that where the brickwork and base meet, it is normally the weakest bond in the pond, as well as the area that will be subjected to the highest water pressure. It is therefore not uncommon for ponds to start leaking at this point when it is filled with water unless it is reinforced. The most common practice is to drive reinforcing rods into the ground where the pond wall will be build. These reinforcing rods must be long enough to accommodate the depth of the base/foundation and also to allow the first few layers of bricks to be built around them.
The second aspect to remember is that the base will eventually be as strong as the weakest point, so consistency is the key word. There should be consistency of the concrete mix and consistency in the thickness of the base over the entire floor of the pond.
The third piece of advice is to lay normal builder’s plastic sheets at the bottom of the excavation before putting the reinforcing in place and pouring the concrete. This is done to contain as much as possible moisture in the concrete for as long as possible. While concrete is setting, the water content assists in the process of forming interlocking crystals in the mixture. These crystals are the determining factor in the successful bonding/curing of concrete. If the moisture leaches out of the mixture, and it dries too quickly, the result will be a weak and brittle pond base that will not withstand pressure.
Reinforced Double Base
The reinforced double base consists firstly of a level reinforced base of at least 15 cm (more for the base of a large pond). Once this base has cured sufficiently, the bottom drains and plumbing for these drains are laid on top of this base. The walls are then build up a few blocks high. A second reinforced base is then poured on top of the first base to cover the bottom drain pipes up to about two centimetres from the top rim of the bottom drain. The reason for allowing these two centimetres is the need for rendering/plastering/benching at a later stage.
The reinforced double base is popular because it is easy to build. The bottom drains the bottom drains can be laid on top of the first layer, making the layout and connecting of pipes simple. This base also provides a “floating platform” in unstable soil conditions like clay that tends to shrink and expand, depending on the moisture content of the soil. The fact that the walls are build on the first layer of the base, and then encased between the second layer of the base and the concrete backfill, makes cracking less likely.
Reinforced Single Base.
The reinforced single base consists of a single reinforced base that encompasses the drainage pipes and bottom drain. Because the diameter of the pipes and the height of the bottom drain may be at least 10 cm and 15 cm respectively, trenches should be dug underneath the bottom drains and pipe work. To support these in the trenches, concrete should be placed at certain intervals and the pipes and bottom drain embedded into it. Steel reinforcing is then placed into position and the concrete for the base is poured. The pipe work and bottom drains are therefore encased in concrete. The pond walls are then built on this single base.
The single base is commonly used in South Africa but poses some challenges when constructing the pond. The trenches should be deep enough to form a strong support underneath the bottom drain and the 10 cm pipes. If the trenching is not done properly this will become the weakest point and it will not support the tons of water when the pond is filled. You should also make sure that the concrete is worked well into the trenches and then vibrated to ensure that no air pockets under the pipe work remain. When done properly, it offers the same advantage that the reinforced double base offers.
The Reinforced Hanging Base
This kind of base is similar to building a house. The hole for the pond is excavated as normal. When completed, a trench is dug for the foundation on which the wall will be build. Make sure the foundation is wide enough to offer a lip on which the base will “hang”. This method is clearly demonstrated in the drawing.
At this stage you can choose between a single and a double reinforced base. The single reinforced base will require you to dig trenches for the bottom drains and piping before the base is poured, as explained above. It is obvious that this method will not be as effective in unstable soil, but under normal stable conditions it is acceptable and the bonding between the foundations, the walls and the base will be sufficient to ensure that leaking will be rare.
The Reinforced, Shaped Pond Base
The reinforced, shaped pond base is a variation if either the reinforced double base or reinforced single base. The principle behind this method is to minimise the cost of benching the eventual floor of the pond. Benching a flat base may require huge amounts of concrete to shape the eventual pond bottom to the desired 30 degrees towards the various bottom drains. You can imagine the amount of concrete necessary to slope the floor of an eight meter by 4 meter pond. It is for the above reason that some Koi keepers prefer to shape the excavation to plan, reinforce it and then pour concrete. This method is demonstrated below.
There are also certain challenges to this method of pond construction. When planning and preparing for a reinforced shaped pond base, make sure the soil is adequately compacted before pouring the concrete. Concrete should also be mixed to the correct texture to prevent it from sliding/running down the sloped areas. Secondly, it is very easy when pouring/packing concrete in a shaped excavation, to end up with an inconsistency in the thickness of the slab. This may create weak points which will eventually crack under water pressure. A way to overcome this threat is to drive reinforcing rods into the shaped excavation at various pints to assist with the levels while packing the concrete. These pegs can remain in the completed base when finished.
Last Updated on Saturday, 14 February 2009 23:02