SMALL DAMS PETITS BARRAGES

If the moisture content of the soil in the field is greater than the optimum, the soil should
be given an opportunity to dry out in storage after spreading.
If it is less, water should be added in the borrow pit or by sprinkling before compaction.
The addition of water can be undertaken by:
� Mixing water into the soil by cultivation with a disc plough or rotary hoe;
� Irrigating as the soil is spread out on the embankment;
� Deep ripping and irrigating the soil before excavation, also known as “borrow pit
irrigation”.
Borrow pit irrigation is usually more economical than adding water directly to the
construction surface. It results in more even distribution of water, and saves time by avoiding
the necessity to water the construction surface between each layer.
The water content at which a soil is compacted has an effect on all the physical
properties of the compacted soil, including the permeability. Experience indicates that an
increase in initial water content from a value somewhat below the optimum to a value
somewhat above is likely to cause a large decrease in the coefficient of permeability.
5.5.2 Water Content Variation and Effect on Geomechanical Properties
For the core of an embankment, sealing and therefore elasticity and cohesion are
dominant and strength is second in line. For the outer zones of an embankment, stability and
therefore strength is most important and elasticity not a priority.
The above provides the motivation for specification of water variation 0% to 2% of
optimum for core impervious materials and –1% to +2% or +3% for semi-pervious materials.
The strength and elasticity values obtained in the laboratory at the specified density and
optimum moisture content of a material can be taken as design parameters if water content
and density control were done during construction in accordance with the mentioned criteria.
5.5.3 Layer Thicknesses
Layer thicknesses are to be tested with the placement and construction machines to be
used during construction. Special care for proper mixing with water and effective compaction
through the complete layer must be investigated. Test sections must also be constructed,
opened and checked visually.
If layer thickness cannot be determined experimentally then layer thicknesses must be
limited to 20 cm after compaction, depending on the material. Maximum thickness after
compaction must always be limited to 30 cm.
5.5.4 Quality Control During Compaction
This is very important. The frequency of testing per layer and per volume placed must be
specified. A test section is normally specified with more frequent testing by Troxler and sand
replacement methods. Troxler machines must be calibrated. When soil materials in borrow
areas change a new Standard Proctor maximum dry density and optimum water content
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