Environmental Engineering Reference
In-Depth Information
Figure 7.14.
Determination of the Emerson class number of a soil (Ingles and Metcalf, 1972).
7.6.1.1
Emerson class number
In this method, the soil is sieved through a 4.75 mm sieve and collected on a 2.36 mm
sieve and tested as outlined in Figure 7.14. The test procedure is detailed in Standards
Australia (1997), test AS 1289, 3.81 and USBR (1979).
The test is carried out in distilled water, but may be repeated in water from the dam, or
groundwater. This often gives significantly different results due to the presence of dis-
solved salts in the water (higher salt content gives less dispersive results). The soils
are graded according to class, with Class 1 being the highly dispersive, Class 8 non
dispersive. Soils with Emerson Class 1 to 4 need to be treated with extra caution in dam
construction.
Sherard et al. (1976b) indicate that if a soil tests as dispersive in the crumb test, it will
also test as dispersive in the pinhole test, but 40% of soils testing as dispersive in the pin-
hole test, test as non dispersive in the crumb test (note that Sherard et al., 1976b, use dif-
ferent classes to the Australian standard).
The Emerson test allows identification of dispersive soils, but does not provide a meas-
ure of their erodibility. It is inexpensive, and a useful first check on dispersivity.
7.6.1.2
Soil conservation service test
Soil Conservation Service test, also known as the double hydrometer test, or percent
dispersion test (Standards Australia, 1997, test AS1289, 3.8.2; and ASTM 2001 test
D42291-99).
This involves two hydrometer tests on soil sieved through a 2.36 mm sieve. The
hydrometer tests are carried out with dispersant and without. The percent dispersion is:
P
Q
100
(7.6)