Environmental Engineering Reference
In-Depth Information
where P
percentage of soil finer than 0.005 mm for the test without dispersant and
Q
percentage of soil finer than 0.005 mm for the test with dispersant.
Sherard et al. (1976a) indicate that soils with a percent dispersion greater than 50% are
susceptible to dispersion and piping failure in dams, and those with a percent dispersion
less than 15% are not susceptible. A percent dispersion of less than 30% is unlikely to test
dispersive in the pinhole test. Bell and Maud (1994) indicate that soils with
50% disper-
sion are regarded as highly dispersive, 30-50% moderately dispersive, 30% to 15% slightly
dispersive, and
15%, non dispersive.
7.6.1.3
Pinhole dispersion classification
Pinhole dispersion classification, also known as the pinhole test, or Sherard pinhole test
(Standards Australia 1997, test AS1289, 3.8.3 and ASTM 1998, test D4647-93).
This test was developed by Sherard et al. (1976b). A 1.0 mm diameter hole is preformed
in soil to be tested, and water passed through the hole under varying heads and for vary-
ing durations. The soil is sieved through a 2.36 mm sieve and compacted at approximately
the plastic limit to a density ratio of 95% (to simulate conditions in a dam embankment
with a crack or hole in the soil).
Soils which tested as D1 and D2 were found by Sherard et al. (1976a) to have suffered
piping failure in earth dams, and severe erosion damage by rainfall in embankments and
natural deposits while those with ND1 and ND2 classification had not.
As for the Emerson class number, the results are dependent on the chemistry of the
water used for the test (the standard test uses distilled water). The method is relatively
simple with moderate cost, and has the advantage that “it identifies soil erodibility
directly, rather than indirectly”. All dispersive soils are erodible to some degree - usually
highly - but erodibility depends on other factors than the clay fraction. Craft and Acciardi
(1984) describe some modifications to the Sherard et al. (1976b) method. These are incor-
porated in Standards Australia (1997) and ASTM (1998).
Gerber and Harmse (1987) found that some dispersive soils with free salts in solution
in the pore water were not identified as dispersive by the pinhole, SCS or crumb tests.
7.6.1.4
Chemical tests
Based on correlation with many dam failures and soil from dams which have leaked con-
tinuously (without any filters to control erosion) and not failed (Sherard et al., 1976a)
proposed
Figure 7.15
to determine the dispersivity of soil.
In this figure,
Na
Total dissolved salts
percent sodium
1
00
(7.7)
Na
or
100
(7.7a)
Ca
Mg
Na
K
Na
Sodium absorptio
nnratio(SAR)
(7.8)
1/2
1
2
(
Ca
Mg
)
in which Na
, Ca
, and Mg
are measured in milliequivalents per litre of saturation
extract.
Sherard et al. (1976a) indicate that the case histories and pinhole tests confirm that the
main factor governing dispersibility is the relative content of pore-water sodium, and
