SMALL DAMS

Chapter IV
Fig. 2 - Drainage system for a homogeneous clayey fill
In general those thicknesses are very generous versus infiltration flows but give a
safety margin over the long term against partial clogging by fines and/or carbonates.
The nature of the fill materials may result in choosing even greater thicknesses.
The available bucket widths must also be considered.
H² V < 30 30 to 100 100 to 300 300 to 700 700 to 1 500
Thickness 0.50 m 0.80 m 1.00 m 1.20 m 1.50 m
Table 6 - Minimum thickness of a sand chimney drain
(H and V correspond to the altitude being considered)
79
Drainage system for zoned dams
The drainage system separates the watertight zone from the downstream shoulder. Its
thickness will depend in particular on the number of layers required to meet filter
conditions.
Use of geotextiles as filters or drains 1
Geotextiles consist of fibres woven into strong, flexible permeable mats. There are several
such products on the market, each ensuring one or several functions (filter, drain, protection
from puncturing, etc.). Some applications have already been mentioned.
Of course, it is essential to avoid considering a geotextile as a general-purpose product (a
geotextile that can support rockfill placement without tearing is not at all similar to a geotextile
used as a filter).
In a dam, geotextiles are most often used to help in discharging percolation flows by acting
as a protective filter for a granular drainage material (see photo 9 p. IV). However, installation
of a geotextile to protect a chimney drain is no easy matter. In this case, an alternative
solution consists in building the fill dam in two stages, in order to place the geotextile
surrounding the drain on a sloped face that is stable in the short term (see figure 3).
1. See Bibliography, references 3 and 4, p.111.