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Discussion by Chester W. Jones5
The writer is not surprised that the authors arrived at filter criteria
different from the USBR (U.S. Bureau of Reclamation) criteria (7,16) which
they criticized. There were significant differences in: (1) The purpose for
which the criteria were developed (see discussion by W. G. Holtz); (2)
the laboratory test procedures; (3) the gradings of base and filter materials; and (4) the development of the criteria from the test results. The
authors did not reference the original USBR report (15) with the details
of the test procedures and analysis of data.
The USBR tests (9,15) were performed on soils placed at 50% of specified field compaction and in a near saturated condition to represent a
field condition after a filter becomes operable. With the SCS (Soil Conservation Service) method of applying a high hydraulic gradient on dry
soil compacted to an estimated 80-100% of relative density for a short
time, it would not be possible to achieve a very high degree of saturation. Saturation is necessary to reduce air voids and arrive at near-maximum flow conditions which would tend to increase movement of particles into the filter. An increase of 10% in the degree of saturation can
cause an increase of 50-70% in the permeability and resulting flow rate.
Because of the need for a high degree of drainability, particular attention in the USBR tests was paid to pressure measurements from piezometers in the base and filter, and flow rates through the base-filter
assembly. Any penetration of base material into the filter was determined by: (1) Observations through the clear plastic wall of the cylinder;
(2) settlement measurements of the filter surface; (3) the distribution of
any sink holes in the surface; (4) observations of penetration of the base
material into the filter as the assembly was dissected for samples; and
(5) gradation tests on third portions of the filter compared with the original grading. These steps would have revealed any significantly greater
penetration of fines at the cylinder wall, which the authors maintain
occurred, compared to the interior of the filter.
Although some of the USBR tests were labeled as failures, as determined by significant penetration of base into filter, the filter criteria were
not based solely on penetration of base into filter. From the measurement of flow rates through different combinations of base and filter materials, criteria in terms of DK/dK and D50/d50 (or D50/d50 only for uniform
grain-sized filter) were established arbitrarily for zones of the highest
flow rates. For graded filters, the D50/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 that of, the base
material. The author's suggestion for the filter to have a C„ < 10 serves
somewhat the same purpose in limiting the particle size distribution for
the filter. For uniform grain size filters, only the 50% mean size was used
because the 15% size was not greatly different from the 50% size.
The USBR filter criteria, questioned by the authors, have been used
in canal underdrain construction and for other purposes for about 35 yrs
without excessive costs for filter materials and, to the writer's knowledge, without failures. For filters in USBR earth embankment dams un5
Civ. Engr., U.S. Bureau of Reclamation, P.O. Box 25007, Denver, CO 80225.
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J. Geotech. Engrg. 1985.111:1469-1470.
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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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