Civil Engineering Reference
In-Depth Information
swelling and shrinkage characteristics. Bentonite is a member of this mineral group and is usually formed
from weathered volcanic ash. Because of its large expansive properties when it is mixed with water it is
much in demand as a general grout in the plugging of leaks in reservoirs and tunnels. It is also used as a
drilling mud for soil borings.
Readers interested in this subject of clay mineralogy are referred to the publication by Murray (
2006
).
1.3.2 Structure of a clay deposit
Macrostructure
The visible features of a clay deposit collectively form its macrostructure and include such features as fis-
sures, root holes, bedding patterns, silt and sand seams or lenses and other discontinuities.
A study of the macrostructure is important as it usually has an effect on the behaviour of the soil mass.
For example the strength of an unfissured clay mass is much stronger than along a crack.
Microstructure
The structural arrangement of microscopic sized clay particles, or groups of particles, defines the micro-
structure of a clay deposit. Clay deposits have been laid down under water and were created by the
settlement and deposition of clay particles out of suspension. Often during their deposition, the action
of Van der Waals forces attracted clay particles together and created flocculant, or honeycombed, struc-
tures which, although still microscopic, are of considerably greater volume than single clay particles. Such
groups of clay particles are referred to as clay flocs.
1.4 Field identification of soils
Gravels, sands and peats are easily recognisable, but difficulty arises in deciding when a soil is a fine sand
or a coarse silt or when it is a fine silt or a clay. The following rules may, however, help:
Fine sand
Silt
Clay
Individual particles visible
Some particles visible
No particles visible
Exhibits dilatancy
Exhibits dilatancy
No dilatancy
Easy to crumble and falls off
hands when dry
Easy to crumble and can be
dusted off hands when dry
Hard to crumble and sticks to hands
when dry
Feels gritty
Feels rough
Feels smooth
No plasticity
Some plasticity
Plasticity
The dilatancy test involves moulding a small amount of soil in the palm of the hand; if water is seen to recede when the soil
is pressed, then it is either a sand or a silt.
Organic silts and clays are invariably dark grey to blue-black in colour and give off a characteristic odour,
particularly with fresh samples.
The condition of a clay very much depends upon its degree of
consolidation
. At one extreme, a soft
normally consolidated clay can be moulded by the fingers whereas, at the other extreme, a hard over
consolidated clay cannot. Consolidation is described in Chapter
11.
Common types of soil
In the field, soils are usually found in the form of a mixture of components, e.g. silty clay, sandy silt, etc.
Local names are sometimes used for soil types that occur within a particular region. e.g. London clay.
