Clay mineralogy, soil properties, and dispersive soils - Geotechnical Engineering of Dams - page 312

Table 7.4. Clay mineral identification from the environment (Ingles and Metcalf, 1972).

Dominant clay component (1) Observation

Turbid water of strong yellow-brown to red-brown Montmorillonites, illite plus soil

colour salinity

Clear waters Calcium, magnesium or ironrich soil,

highly acid soil sands

Clear waters with a bluish cast Non-saline kaolins

Erosion gullies and/or field tunnelling in the natural soil Saline clays, usually montmorillonites

As above, mild erosion Kaolinites

Landslips Kaolinites, chlorites, montmorillonites, illites

Surface microrelief (gilgai) Montmorillonites

Country rock type granitic Kaolinites, micas

Country rock type basaltic, poorly drained topography Montmorillonites

Country rock type basaltic, well drained topography Kaolinites

Country rock type sandstones Kaolinites

Country rock type mudstones and shales Montmorillonites or illite, often soil salinity

Country rock type limestone Alkaline montmorillonites and of very

variable properties

Country rock type recent pyroclastics Allophanes

Note: (1) The dominant component of the soil may be silt or sand sized eg. for granites, sandstones.

- The presence of cloudy water in farm dams and puddles of water after rain indicates
dispersive soils.
One can infer the clay mineralogy from such observational techniques. Some guidelines
are given in Tables 7.4 and 7.5 reproduced from Ingles and Metcalf (1972).
The geology of the area can also be a guide to dispersivity.
Sherard and Decker (1977) indicate that:

- Many dispersive clays are of alluvial origin. (The authors' experience is similar but

there are many non dispersive alluvial clays. Some slopewash clays are also dispersive);

- Some soils derived from shales and claystones laid down in a marine environment are

also dispersive;

- Soils derived from weathering of igneous and metamorphic rocks are almost all non

dispersive (but may be erodible, e.g. silty sand derived from grandiorite);

- Soils with a high organic content are unlikely to be dispersive (this needs to be treated

with caution, since many “black cotton” soils are dispersive).

7.7 USE OF DISPERSIVE SOILS IN EMBANKMENT DAMS

7.7.1 Problems with dispersive soils

Dispersive soils are a major contributing factor to piping failure of embankment dams,
particularly for small homogeneous dams constructed without filters and often with poor
construction supervision. This view is widely held e.g. Sherard et al. (1976a, b, 1985).
However, failures do occur in structures which are reasonably well engineered. The main
contributory factors are:
- The presence of dispersive soils in the embankment or foundation;

- Poor compaction of the soil, i.e. to a low density ratio and/or dry of optimum water

content;

- Poor compaction of soil around pipes or conduits which pass through the embankment;