Environmental Engineering Reference
In-Depth Information
Table 7.2.
Activities of various minerals (Holtz and Kovacs, 1981).
Mineral
Activity
Na-montmorillonite
4-7
Ca-montmorillonite
1.5
Illite
0.5-1.3
Kaolinite
0.3-0.5
Halloysite (dehydrated)
0.5
Halloysite (hydrated)
0.1
Attapulgite
0.5-1.2
Allophane
0.5-1.2
Mica (muscovite)
0.2
Calcite
0.2
Quartz
0
Note: Activity
(plasticity index)/(clay fraction).
7.5
ENGINEERING PROPERTIES OF CLAY SOILS RELATED TO THE
TYPES OF CLAY MINERALS PRESENT
The engineering properties of clay soils depend on compositional factors (Mitchell 1976,
1993):
- type of clay minerals present;
- amount of each mineral;
- type of adsorbed cations (and anions);
-organic content;
-
shape and size distribution of particles;
-
pore water composition.
and on environmental factors:
-water content;
- density;
- confining pressure;
- abric;
-
availability of water;
-
temperature.
Virtually all clay soils in nature are mixtures of clay and silt size particles (and some-
times sand), not just clay size particles. The silt and sand size particles are usually rounded
or sub-rounded and are derived from the parent rock. The most abundant mineral pres-
ent is usually quartz, followed by feldspar and mica.
7.5.1
Dispersivity
Soils in which the clay particles will detach from each other and from the soil structure
without a flow of water, and go into suspension are termed dispersive clays.
The dispersivity of a soil is directly related to its clay mineralogy. In particular soils with
a high exchangeable sodium percentage such as montmorillonite present, tend to be dis-
persive, while kaolinite and related minerals (halloysite) are non dispersive. Soils with
illite present tend to be moderately dispersive.
