Environmental Engineering Reference
In-Depth Information
soils, because of the large strains involved. The traditional one-dimensional Terzaghi the-
ory is not applicable and would result in overestimation of the time of consolidation and
underestimating pore pressures. It is also necessary to use special slurry consolidometers,
so tests can begin at the density of the tailings after sedimentation.
Schiffman and Carrier (1990) and Schiffman, Vick and Gibson (1988) give overviews
of this topic, and reference other papers which can be used to obtain the detail of the test-
ing and analysis techniques. They indicate that centrifuge testing can also be used for
more accurate modelling, but advocate monitoring and analysis of the behaviour of the
early phases of tailings deposition as the best means of predicting behaviour.
The reader is referred to these references rather than have the information repeated
here. However, the following practical points are based on the authors' experience:
- It is essential to carry out testing with tailings and water which are representative of
the actual tailings and water. It is also essential that the tailings have not dried out
before carrying out testing, particularly if the tailings have a significant clay content.
Drying completely changes the sedimentation behaviour (usually makes the tailings set-
tle more quickly and more densely) and affects the consolidation and permeability
properties;
- Sedimentation tests, where the tailings are mixed and placed in a 500 mm column and
allowed to settle, give a good indication of the minimum settled density. Figure 19.14
gives the results of such tests for the tailings in
Tables 19.2
and
19.3
in 500 mm
columns. Field measurements indicate that the 500 mm columns slightly underestimate
the field density of tailings;
- Field sampling of the very loose/soft upper layer of tailings is difficult. Ritcey (1989)
refers to the use of piston samplers. Fell et al. (1992) show a simple device, which was
developed by R. Hogg at the University of New South Wales, which has been used
to obtain disturbed samples of tailings. This allows determination of water content,
and hence dry density or void ratio, and give samples for particle size distribution
testing.
Figure 19.14.
Settling tests on mine tailings.
