Environmental Engineering Reference
In-Depth Information
For any dam, including pumped storage dams, where the drawdown could be as high
as several metres within hours, or on a daily or even twice daily cycle, it would be wise to
use conventional filters designed according to Section 9.2.4, at least in the zone of daily
drawdown.
9.2.5.2
Filters under rip-rap
There are two requirements for these filters:
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That they are coarse enough not to wash out of the rip rap;
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They are fine enough to prevent erosion of the soil beneath the filter.
Two layers of filters may be required to satisfy these requirements. If proper protection
is required, no-erosion filter criteria should be satisfied. This is what is recommended by
US Corps of Engineers (1984a).
It is fairly common to use more relaxed criteria in non-critical conditions where some
damage can be tolerated. ICOLD (1993, 1994) suggest that, for bedding filter under rip
rap, filters which are reasonably well graded, between a maximum of 80-100 mm and
coarse sand sizes, are satisfactory for the great majority of dams, but warn that segrega-
tion will be a problem and that such filters will not protect against frequent surging from
wave action.
The authors' view is that the design of these Zone 2D filters (Figure 13.5a and c) is not as
critical as for Zone 2A filters in that, if damage does occur, it can usually be repaired, and
so a relaxation of the strict no erosion rules may be appropriate. If there is a reasonably
well-graded sandy gravel/ gravelly sand from 0.075 mm to 50 mm or 75 mm available
either naturally or with a minimum of processing, this should be satisfactory in most
cases. This is also the experience of Sherard et al. (1963). Filters which consist only of fine
or medium (or even coarse) sand are unlikely to be satisfactory as they will not self-filter
against the rip-rap.
The authors take the view that for thin layers of rip-rap, on important dams, with sig-
nificant wave action, properly designed filters should be provided. Advantage may be
taken of the knowledge of the work described above, or simply standard filters designed
using Section 9.2.4.
For smaller dams, where the risk of deterioration and the need for repairs may be
acceptable, geotextiles may be used under rip-rap. If so, the following points should be
noted:
(a) Use non-woven needle punched heavy quality fabric e.g. Bidim A44;
(b) Use a maximum size of 150 mm cobbles or gravel-cobble mix directly onto the fabric
to avoid developing folds in the fabric between the particles, below which soil could
be eroded.
9.3
ASSESSING FILTERS AND TRANSITION ZONES IN EXISTING DAMS
9.3.1
Some general issues
When assessing the likelihood of internal erosion and piping failure of dams, it is neces-
sary to consider the whole of the piping process - initiation, continuation, progression to
form a pipe and breach (See Chapter 10). This section considers only the filters or transi-
tion zones, which mainly affect the continuation of erosion.
The starting point for assessing existing dams for internal erosion and piping is to
determine, as reliably as is practical, what materials were used to build the dam and their
