Environmental Engineering Reference
In-Depth Information
100
Fine limit
Average grading
Coarse limit
80
Typical range
of fine limit
60
Songa Dam
coarsest grading
40
Typical range of
coarsest limit of
gradings
20
0
0.1
1
10
100
1000
Particle Size (mm)
Figure 9.22.
Gradings of filters which have experienced poor filter performance.
Details of the tests and the results of testing are given in Foster (1999), Foster and Fell
(1999a) and Foster and Fell (2001).
The excessive erosion boundary was determined from consideration of the performance
of dams which had resulted in damage to the dam in the form of sinkholes and large leak-
ages, but none had resulted in failure (i.e. breaching) of the dam (Foster et al., 1998).
Figure 9.22 presents the gradations of the filters of some of the dams with poor filter per-
formance. The case histories generally involved piping of core materials into coarse or seg-
regated downstream filters in zoned earthfill or central core earth and rockfill dams. The
dams were generally constructed in the 1960s to 1970s which coincides with a period when
there was a trend away from the use of uniformly graded multiple filters and towards the
use of a single filter of substantial width and broad gradation (Response by Ripley in
ICOLD, 1994). The filter gradings shown in Figure 9.22 have wide gradings and low pro-
portions of sand sizes which would tend to make them susceptible to segregation during
construction and also potentially make them internally unstable.
There are also several reported cases of concentrated leaks that have developed
through the cores of dams but which have evidently sealed due to the effectiveness of
the downstream filter, as evidenced by observations of near hydrostatic piezometer
levels in the downstream section of the core and “wet seams” in the core (Sherard,
1985a). Peck (1990) also describes several examples from the literature of dams, which
have shown evidence that some form of filtering action has taken place at the core-filter
interface.
Only two dams, Rowallan Dam and Whitemans Dam, were found in the literature
which have experienced poor filter performance involving piping of fine grained core mate-
rials with D
95B
2 mm. In both cases, the finest core material and coarsest filter combina-
tion fall into the continuing erosion category as defined by the laboratory tests, i.e.
D
15F
/9
D
95B
. At Rowallan Dam where very fine abutment contact earthfill was protected
by the filter provided for the bulk of the earthfill, the filter with the coarsest grading has a
filter opening size (D
15F
/9) of 11/9
1.2 mm. This is larger than the D
95B
of the finest grad-
ing of contact clay soil of 0.9 mm. At Whitemans Dam, core materials were eroded into the
