How to build a SSD?

Checklist

Before the construction work is started, the following things need to be checked.
1. The building location is decided on after a feasibility study as described at the site selection page.
2. The community, implementing NGO and water committee have signed a written agreement on the ownership of the dam site, access road, usage of water from the dam and conservation of the catchment.
3. The memorandum of understanding and communtiy action plan are agreed on an signed by the water committee and the implementing partner (NGO). Design drawings and bill of quantities for the dam are ready.
4. Any legal requirements have been addressed.
5. Funds for survey, design and construction of the dam have been secured.
6. All construction material and tools are available.
7. The building process should be done during dry seasons. The sand storage dam in the constructing stage will be easily damaged or swept away by a thunderstorm or heavy rainfall.

Building the sand storage dam

If all of the above conditions are met, the actual building process can begin. The first step is marking the position and size of the dam, taking in to account the size of the wing walls and working space during construction (see below figure).
To do so, the following steps should be taken:
• At the decided building location, a peg is fixed into the riverbank.
• A wire is drawn across the river exactly perpendicular to the river course, where another peg is fixed.
• In the riverbed along the wire pegs are fixed into the ground every meter.
• At a distance of 60 centimetre from the row of pegs, another row of pegs is made in the same manner as described above.

Example of setting a trench with pegs.

Setting and excavating the building trench

The marked trench is dug guided by the building line, as is shown
in the photo (Rain, 2007). The depth of the trench is determined by the depth of impermeable layer in the ground which will obstruct seepage below the sand storage dam. The dug out soil should be placed downstream of the building location to avoid it filling the aquifer. If the dam is build into bedrock material, a trench should be cut into the rock to ensure secure jointing of the rock and mortar. Care should be taken to make sure no fractures or weathering zones are present in the basement rock. If suspected, this can be tested by poring water on the suspected weathered zones. If the water leakes away, the rock surface should be cleaned from the weathered rock or fractured rock. If clay forms the impermeable layer, the trench should be dug in for about 0.5 m to avoid seepage. After these conditions are met, the trench is ready for dam setting and construction (Munyao et al, 2004).

Construction of the dam

Construction starts with putting in place the reinforcement columns vertically in the trench followed by the construction of the foundation blinding slab. Reinforcement is only required if a very large or high dam is constructed. After this the second horizontal reinforcement layer is placed, followed by the second foundation blinding slab and then the actual masonry structure (of hard core and mortar) starts (Munyao et al, 2004). In the attachement to this page, you will find a detailed guideline for construction of a sand dam. Intensive technical supervision and monitoring is the major activity that should be attained during the construction process of a sand dam.