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der 15 m in height, the criteria 5 < D15/d15 < 40 and D15/das < 5 have
been published (8). The current tendency for all USBR embankment dams
is to continue to use the D15/dS5 < 5 criteria to prevent piping and to
specify uniformly graded filters to provide sufficient permeability for
gradation ranges discussed by the authors. The USBR has other less conservative criteria for granular envelopes around drainage pipe in agricultural land drainage systems.
The writer agrees with the authors that it is not necessary for the filter
and base materials to have approximately the same shapes. As to the
USBR separate criteria for filters with angular particles, as stated in the
report the authors referenced (9), these were tentative because they were
based on only six tests. The writer would like to see more tests before
discarding the separate criteria for use in canal underdrains.
The writer believes that any filter criteria developed from laboratory
tests should be used only as a guide and the selection of filter gradings
should be subject to modification to suit the purpose of the drain and
the peculiar conditions at the structure site. Primary interrelated factors
involved in filter design are: (1) The expected hydraulic gradient; (2) the
range of grading, erodibility, and plasticity of the base material to be
protected; (3) the filter thickness; (4) the availability of filter material; (5)
the quality of the filter material; and (6) the probable extent of damage
in case of failure. Laboratory filter tests on existing base materials from
the structure site and on proposed filter materials are helpful, but the
results should be used judiciously since field conditions and construction practices cannot be exactly duplicated by laboratory tests.
Closure by James L. Sherard,6 F. ASCE, Lorn P. Dunnigan,7
and James R. Talbot,8 Members, ASCE
The discussions of Holtz and Jones are welcome because they focus
attention on several main conclusions of our filter research which deserve clarification and emphasis.
We also inform the reader that the paper under discussion was written
at the conclusion of the first two years of a research program which
finally was continued for a total of four years. A summary of the main
results of the completed program is given in Ref. 2.
One main conclusion of our paper was that the filtration properties of
a sand or gravelly sand were dominated by the minimum pore diameter
(or maximum size of particle which can pass through the pores) of the
compacted filter. This minimum pore diameter in a normal sand or gravelly sand (not gap-graded) is directly related to the sizes of the finer sand
particles, such as to the D15 size. Hence, normal (reasonably well-graded)
sand and gravel filters are well-defined by the single parameter (D15),
'Consulting
Engr., San Diego, CA.
7
Head, Soil Mechanics Lab., National Technical Center, Soil Conservation Service, U.S.D.A., Lincoln, NE.
"National Soil Engr., Soil Conservation Service, U.S.D.A., Washington, DC.
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in millimeters. It was concluded that the D50 size is not a good measure
of the pore diameter or of the filter ability of a sand or gravel and, therefore, that existing filter criteria using the D50 size should be abandoned.
The publication describing the filter criteria of the U.S. Bureau of Reclamation (USBR) are among the more influential pieces of literature advocating the use of the D50 size in current filter practice. The USBR criteria were adopted after an extensive laboratory research program, in
which Holtz and Jones were active participants.
The Holtz discussion does not address the reasons for proposing filter
criteria utilizing the D50 size. Jones states, "For graded filters, the Dm/
d50 relationship, which the authors decry, was chosen to prevent the
filter material from becoming too broadly graded and possibly too low
in permeability relative to the base material," and "For uniform grain
size filters, only the 50% mean size was used because the 15% size was
not greatly different than the 50% size."
For reasons presented in the paper (p. 696), we believe it is clear that
the D5o size is not a good measure of the basic properties of a filter. At
the same time it is clear from our research results that the single, old
and widely applied filter design criterion (D15/d85 s 4 or 5) is not adequate by itself to define conservative filters for many impervious soils,
such as sandy (and gravelly) clays and silts, so that other filter criteria
are needed (18). Hence, the limitation imposed by the current, fairly
widely used filter criterion D50/d50 < 40 (or 58) was generally working
in the right direction to require finer filters for these coarse impervious
soils, in spite of the fact that it was not founded on a sound theoretical
or experimental base.
As discussed in Ref. 18, the application of the filter criterion D50/d50
< 40 frequently defines the same general type of filter for these coarse
impervious soils as that shown to be desirable from our research; but
for some base soils the D50/d$o < 40 criterion requires filters which are
much finer than needed.
The use of the D5o/(f5o criterion "to prevent the filter from becoming
too broadly graded . . ." is primarily for the purpose of minimizing undesirable segregation of the coarser particles during construction. The
primary need to minimize segregation occurs in sandy gravels used for
filters for fine-grained silts and clays. As proposed in Ref. 18, and discussed in more detail in Ref. 19, such filters with 40%, or more, of sand
sizes (finer than the No. 4 sieve) can be placed without significant segregation using reasonable construction care.
Another main conclusion of our research, not addressed specifically
in either discussion, was that the d15 size of the base soil has no general
influence on the needed filter and that the USBR filter criterion D15/d15
< 40 is not reasonable and is unduly restrictive. For example, many tests
have been made (17,18) to show conclusively that a sand or gravelly
sand with D15 = 0.5 mm is a conservative filter for most clayey soils with
d15 < 0.001 mm, or with a D15/rfi5 ratio larger than 0.5/0.001 = 500.
APPENDIX.—REFERENCES
17. Sherard, J. L., Dunnigan, L. P., and Talbot, J. R. (1984). "Filters for Clays
and Silts." /. Geotech. Engrg., ASCE, 110(6), 701-718.
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18. Sherard, J. L., and Dunnigan, L. P. (1985). "Filters and Leakage Control in
Embankment Dams," Proc. ASCE Symp. on Seepage and Leakage from Dams and
Impoundments, Denver, CO, May, 1985, pp. 1-30.
19. Sherard, J. L. (1985). "The Upstream Zone in the Concrete Face Rockfill Dam,"
Proc. ASCE Symp, on Concrete Face Rockfill Dams, Detroit, MI.
POST-CONSTRUCTION DEFORMATION OF ROCKFILL D A M S 3
Discussion by Nelson L. de S. Pinto, 2 M. ASCE
and Pedro Lagos Marques Filho3
The advantages of comparing d a m s of similar characteristics for prediction of settlements a n d deflections of n e w dams as stressed b y the
author, have induced the writers to present some additional data a n d
comments on deformations observed in the 525 ft (160 m) high Foz do
Areia Dam built in Southern Brazil, 1975-1980 (68).
Crest settlements per unit height versus log time are plotted in Fig.
10, in the same fashion as in Fig. 2. Foz do Areia settlements are compared with recently published data on the 459 ft (140 m) high concrete
face Alto Anchicaya Dam in Colombia (69). The higher deformation of
Foz do Areia basaltic rockfill as compared with Alto Anchicaya hornfels/
diorite embankment is evident from the unit settlements after four years
of operation: 0.18% at Foz d o Areia as against 0.095% at Alto Anchicaya.
Comparison of the observed data with results from proposed formulas
such as Soydemir a n d Kjaernsli's, even with coefficients a n d exponents
derived from the author's best fit analysis, only confirm the precariousness of predictions based on the height of the d a m and time of service
(Table 8).
Certainly, much better estimates should result from comparison with
dams having similar characteristics. However, such a comparison is far
from being a clear and straightforward process due to the lack of h o mogeneity of t h e generally available data o n the behavior of existing
dams.
Probable deformations of Foz do Areia D a m were estimated by comparison with 360 ft (110 m) high Cethana D a m in Australia (1971) (70)
and re-evaluated from early information on the performance of Alto Anchicaya Dam (1979). Some estimates were fairly accurate such as the
maximum deflection of the concrete slab after reservoir filling. Prediction
of crest settlements however turned out to be a more involved process
due to the history of Cethana Dam construction. The d a m was built to
90% of its height in about one year. Rockfill construction was interrupted for 11 m o n t h s . The last 39 ft (11.9 m) of the d a m were placed in
"July, 1984, by Ronald P. Clements (Paper 18988).
Consulting Engr., Companhia Paranaense de Energia, COPEL, Brazil.
"Consulting Engr., COPEL, Brazil.
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