Environmental Engineering Reference
In-Depth Information
Figure 7.17.
Estimation of dispersivity from exchangeable sodium percentage (McDonald, Stone and
Ingles, 1981).
work. McDonald et al. (1981) suggested the use of the pinhole test for situations where
water would be flowing (eg. a crack in a dam) and Emerson test for quiescent conditions,
e.g. reservoir. They also point out that the ESP can vary a lot for soils in any dam and
make use of cheaper tests such as Emerson and pinhole tests more attractive. It is also
pointed out that where soils which are rich in bicarbonates are allowed to dry out, pre-
cipitation of relatively unsoluble CaCO
3
may occur, resulting in a lower Ca
ion con-
centration and hence an increased potential for dispersion.
It should be noted that Sherard et al. (1976a) believe that such transformation of a sta-
ble, flocculated soil into a dispersive soil by the seepage water is unlikely, citing that most
failures occur on first filling, pinhole tests eroding at the start of a test, or not at all (even
after long periods of flow), and that there is no reason why the ESP should reduce (their
experience being the contrary).
7.6.1.5
Recommended approach
Most authors consider that it is necessary to use more than one test to ascertain the dis-
persivity of a soil. Sherard and Decker (1977) suggest that four tests should be used: Soil
Conservation Service, pinhole, Emerson and chemical test. They were of the opinion that
the pinhole test was best. Moore et al. (1985) indicate that in their experience:
-
similar results were obtained using the pinhole and Emerson tests;
-
the pinhole test was not particularly sensitive to compaction water content;
-
only approximate agreement was found between the SCS test and the pinhole and
Emerson tests;
