Environmental Engineering Reference
In-Depth Information
7
Clay mineralogy, soil properties, and dispersive soils
7.1
INTRODUCTION
The properties of soils are determined by the properties of the constituent soil particles,
the nature and quantity of water in the soil, the past consolidation history of the soil and
soil structure. In this chapter the influence of the mineralogy of clays present in fine
grained soils is discussed, with a particular emphasis on dispersive soils, and their use in
the construction of embankment dams.
Dispersive soils are those which by the nature of their mineralogy, and the chemistry of
the water in the soil, are susceptible to separation of the individual clay particles and sub-
sequent erosion of these very small particles through even fine fissures or cracks in the soil
under seepage flows.
This is distinct from erodible soils, such as silt and sand, which erode by physical action
of the water flowing through or over the soil.
For many years it has been recognised (e.g. Aitchison, Ingles and Wood, 1963; Aitchison
and Wood, 1965) that the presence of dispersive soils either in the soil used to construct a
dam, or in the dam foundation, greatly increases the risk of failure of the dam by “piping
failure”, i.e. development of erosion to the extent that a hole develops through the
embankment, with rapid loss of water from the storage. Figure 7.1 shows such a case.
An understanding of the basic concepts of clay mineralogy is essential to the under-
standing of identification and treatment of dispersive soils in dam engineering. The
Figure 7.1.
Examples of piping failure of a dam due to the presence of dispersive soils (Soil
Conservation Service of NSW).
