Volume 6 – Geotechnical Manual, Site Investigation and Engineering ...

Chapter 10 SEEPAGE
a) The filter material should be more pervious than the base material in order that no
hydraulic pressure will build up to disrupt the filter and adjacent structures
b) The voids of the inplace filter material must be small enough to prevent base material
particles from penetrating the filter and causing clogging and failure of the protective filter
system.
c) The layer of the protective filter must be sufficiently thick to provide a good distribution of
all particles sizes throughout the filter
d) Filter material particles must be prevented from movement into the drainage pipes by
providing sufficientlyy small slot openings or perforations or additional coarser filter zones if
necessary. This requirement could also be fulfilled by using some of the non-woven and
woven fabric materials developed recently.
The gradation requirements for protective filters are given in Table 10.2. The first ratio, R 15 ,
ensures that the small particles of the material to be protected are prevented from passing through
the pores of the filters; the second ratio, R 50 , ensures that the seepage forces witin the filter are
reasonably small. If the criteria in this table cannot be met by one layer of filter material, then a
zoned or multilayered filter can be designed and specified.
Table 10.2 Gradation Requirements For Filter Materials (after USBR, 1974)
Filter Materials Characteristics R 15 R 50
Uniform grain size filters, C u = 3 to 4 - 5 to 10
Graded filters, subrounded particles 12 to 40 12 to 58
Graded filters, angular particles 6 to 18 9 to 30
R15 = D15 of filter material
D15 of material to be protected
R50 =
D50 of filter material
D50 of material to be protected
Notes:
Maximum size of the filter material should be less than 76 mm. Use the
minus No. 4 fraction of the base material for setting filter limits when
the gravel content (plus No. 4) is more than 10%, and the fines (minus
No. 200) are more than 10%. Filters must not have more than 5%
minus No. 200 particles to prevent excessive movement of fines in the
filter and into drainage pipes. The grain size distribution curves of the
filter and the base material should approximately parallel in the range of
finer sizes.
10-6 March 2009