Your Dam Your Responsibility (PDF~1.2MB)
Your Dam Your Responsibility (PDF~1.2MB)
Your Dam Your Responsibility (PDF~1.2MB)
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5 Construction of a <strong>Dam</strong><br />
Mixing topsoil with the bank material to save costs or because of convenience is<br />
asking for trouble! The organic matter in the topsoil will decay in due course, causing<br />
leakage paths to develop later, and may even lead to difficulty in obtaining adequate<br />
impermeability in the embankment in the short term.<br />
A core (cut-off) trench should be excavated along the centreline of the bank to provide<br />
good protection against under-bank leakage. The cut-off trench should extend the full<br />
length of the bank including up the abutments. It must be wide enough to allow the<br />
construction equipment to achieve the required standard of compaction; its depth will<br />
depend on site soil conditions.<br />
In most cases, it is not difficult to find suitable foundation materials relatively close to<br />
the surface, if unsuitable upper soils (including topsoil) are first removed. The founding<br />
material must be sufficiently stiff and impermeable, and must extend to sufficient depth<br />
to allow the storage behind the dam to be retained without significant leakage, but also<br />
to avoid any appreciable settlement of the constructed embankment. In some locations,<br />
upper soils below topsoil are soft, weak or contain gravel or other inclusions (such as<br />
calcareous materials) which can form a leakage path. This is one of the reasons for<br />
recommending the inclusion of a cut-off trench below the embankment, no matter how<br />
low, so that the content of the next layers below the embankment can be checked out<br />
before construction of the embankment begins.<br />
A large number of dams are still being constructed using a method whereby the topsoil is<br />
mixed into, or left under the clay material that forms the bank. These dams are more likely<br />
to leak or fail. This is because the organic matter in soil layers rots and allows seepage to<br />
occur. It is also because topsoil is likely to be more permeable than clay in the first place.<br />
5.4 Embankment Materials and Construction<br />
5.4.1 Selection of Materials<br />
Most dam walls (whether on or off waterways) are constructed of earth materials. If your<br />
dam is constructed from other materials, then much of the following sections will not<br />
apply to you, but expert advice will be required in any case.<br />
While construction of dams has to be practical, and is limited by available materials<br />
within economical distance, selection of appropriate materials is vital for dam safety and<br />
performance. This applies not only to the materials used in the embankment, but also to<br />
the materials on which it is founded, as noted above.<br />
The embankment must be capable of securely retaining water. This is generally done<br />
by (a) ensuring that the materials for the<br />
embankment contain sufficient clay, and (b)<br />
ensuring that the materials are adequately<br />
compacted.<br />
Piping failure due to excess rock,<br />
gravel, silt and no clay being used.<br />
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