Water wise farmers build earth dams: Part 7
The core of the dam and construction methods T
he core must consist of the densest available material at the site, such as clay. The material must be moist and well compacted or rolled in thin layers of uniform thickness. If the clay is too wet, the wall will be unstable. The different layers must be built up consecutively as quickly as possible. The core is built up in the centre of the wall for maximum compression, symmetrical sagging and for the best connection with a possible pit wall. Cross sections must remain constant over the entire length of the main wall to prevent unequal sagging. The core must be of sufficient length as the site dictates. Treating a permeable dam basin • Lay a thick blanket over the entire basin as indicated in Figure 1. • If the basin is extremely porous, lay a layer of soil beneath the blanket. • If the soil in the basin consists of sand, clay and silt, plough 250 mm to 300 mm deep when moist
Figure 1: Blanket of impervious soil.
Figure 2: Drainage from toe of wall.
•
and consolidate with animals or mechanically. Compact naturally by means of silt deposits.
Drainage of the toe of the wall The seepage line through the wall is
intercepted by a filter drain. It reduces the danger of failing (Figure 2). The construction method General guidelines • The topsoil must remain uniformly moist at optimum moisture quality
ProAgri Zambia 63
•
during construction. It must neither be too wet nor too dry. It must be 2% below optimal moisture condition for good compaction. To obtain an equal moisture distribution, the use of a sprinkler irrigation system is recommended. 11
