SMALL DAMS PETITS BARRAGES

It is generally accepted that cohesive non-dispersive clays are stable in some cases
where above-mentioned criteria are not satisfied. The standard is not well defined. The
explanation by the Corps of Engineers in 1955 is mostly accepted.
“The above criteria will be used when protecting all soils except for medium to highly
plastic clays without sand or silt partings, which by the above (basic) criteria may be required
multi-stage filters. For these clay soils, the D15 size of the filters may be as great as 0,4mm
and the above D50 (supplementary) criteria will be disregarded (This refers to the D50
(filter)/D50(base) < 25 rule). This relaxation in criteria for protecting medium to highly plastic
clays will allow the use of one-stage filter material; however the filter must be well graded, and
to ensure non-segregation of the filter, a coefficient of uniformity (ration of D60 to D10) of not
greater than 20 will be required”.
5.7.1.11 Criteria Regarding Dispersive Clay
First criterion is that the filter material must not be dispersive. Criteria stipulated in Table
5.8 are to be followed. Second practice to obtain less permeability is to compact to at least
98% of the Standard Proctor density at 0% to 2% above optimum. The paper by Melvill (1986)
describes further aspects regarding filtration of dispersive soils.
5.7.1.12 Organic Material Criteria
Less than 2% organic materials in filters are acceptable.
5.7.1.13 Criteria and Practice Regarding Synthetic Materials
Woven, needle punched polyester synthetic materials are available in South Africa, since
1972. Synthetic materials are to be compared to natural materials regarding filter criteria. The
exact size and void ratios, however, can only be determined accurately using experimental
data.
It is generally assumed that synthetic materials have an application. The following is
important regarding the application to dams:
� Stresses in embankment caused by differential settlement can cause the material to
be torn apart which influences the permeability and stability against piping.
� Synthetic materials can therefore be used where permeability is not a priority e.g.
downstream side of sand of a chimney filter.
� Access to synthetic materials must be possible. Therefore use as chimney filter
material is not acceptable as access is not possible. It must be remembered that the material
can clog.
� Construction problems are to be acknowledged. Synthetic materials can be damaged
under vehicle loads. When one hole develop, water and soil can move through with no control
regarding filter material. Ultra-violet rays of the sun can cause deterioration and exposure must
be prevented.
5.8 SLOPE STABILITY
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