The type of soil at the site is a critical part of determining whether the dam will eventually hold water. In particular the embankment must be structurally stable and able to hold water. This means that the soil used to build the embankment must also be structurally stable or impermeable and preferably both.
The soil type that usually satisfies both these criteria is clay. However, not all clays are suitable for dam embankments. For example, some clays disperse when wet and result in tunnels forming in the embankment. Thus, you must test the soil to determine its behaviour.
To do the tests, samples of the soil must be obtained from the excavation area. The samples can come from auger holes or backhoe trenches. The sampling depth should extend at least to the depth of anticipated excavation. Sampling the site will also give you an idea of whether there is enough soil in the excavation area to build the embankment. The samples should be then tested by someone experienced with soil behaviour in dam construction.
Often, if there is not sufficient clay in the dam vicinity, it is worthwhile carting bentonite or clay in from elsewhere and applying it in a thick layer across the whole of the dam to ensure that you obtain a satisfactory seal. If no suitable clays are available locally, you may have to consider a plastic membrane to line the inside of the dam if you wish it to reliably store water.
How to look after your dam
To allow your dam storage to fulfil its function it is important to carry out regular maintenance. It also helps to observe the following:
- Ensure topsoil cover of about 150 mm.
- Establish a grass cover on the embankment and spillway as soon as possible. A grass that mats, such as kikuyu, is preferable to a grass that tufts.
- Do not let trees or shrubs grow on the embankment, spillway or spillway outlet slope. Roots might disturb the compacted soil and provide a seepage path for water, while trees or shrubs in the spillway area will restrict the flow of flood water.
Some further maintenance tips for looking after your dam:
- Top-dress areas that become bare of topsoil as soon as possible.
- Fertilise and water grass cover regularly.
- Slash grass regularly on the spillway and outlet slope to promote a dense groundcover.
- Avoid using the spillway and outlet slope for vehicular access, to reduce potential erosion.
- Maintain a sealed catchment to minimise the potential silting-up of the dam. A permanent pasture is ideal.
