Soil consistency is the strength with which soil
materials are held together or the resistance of soils to
deformation and rupture. Soil consistency is measured for
wet, moist and dry soil samples. For wet soils, it is
expressed as both stickiness and plasticity, as defined
below. Soil consistency may be estimated in the field
using simple tests or may be measured more accurately in
the laboratory.
Note: in each case, indications will be obtained
concerning the relative value of soil for fish-pond
construction, particularly when the wet-soil consistency
is determined.
8.1 Determination of wet-soil
consistency
Testing is done when the soil is saturated with water, as, for example,
immediately after a good rainfall. First, determine stickiness,
that is, the ability of soil materials to adhere to other objects. Then,
determine plasticity, that is, the ability of soil materials
to change shape, but not volume, continuously under the influence of
a constant pressure and to retain the impressed shape when the pressure
is removed.
Field test for stickiness of wet soil
Press a small amount of wet soil between your thumb and forefinger
to see if it will stick to your fingers. Then slowly open your fingers.
Rate the stickiness as follows:
0 Non-sticky, if no soil or practically no soil
sticks to your fingers;
1 Slightly sticky, if the
soil begins to stick to your fingers but comes off one or the other
cleanly and does not stretch when the fingers are opened;
**2 Sticky
**3 Very sticky
Field test for plasticity of wet soil
Roll a small amount of wet soil between the palms of your hands
until it forms a long, round strip like a wire about 3 mm thick.
Rate the plasticity as follows:
0 Non-plastic, if no wire
can be formed;
1 Slightly plastic, if
a wire can be formed but can easily be broken and returned to
its former state;
**2 Plastic
**3 Very plastic
8.2 Determination of moist-soil consistency
Field test for moist-soil consistency
Testing is done when the soil is moist but not wet, as, for
example, 24 hours after a good rainfall.
Try to crush a small amount of moist soil by pressing it between
your thumb and forefinger or by squeezing it in the palm of
your hand. Rate moist soil consistency as follows:
0 Loose, if the soil is
non-coherent (single-grain structure);
1 Very friable, if the
soil crushes easily under very gentle pressure but will stick
together if pressed again;
2 Friable, if the soil
crushes easily under gentle to moderate pressure;
**3 Firm
**4 Very firm
5 Extremely firm, if the
soil crushes only under very strong pressure, cannot be crushed
between the thumb and forefinger, but must be broken apart bit
by bit.
8.3 Determination of
dry-soil consistency
Field test for dry-soil consistency
Testing is done when the soil has been air-dried.
Try to break a small amount of dry soil by pressing it between
your thumb and forefinger or by squeezing it in the palm of
your hand. Rate dry soil consistency as follows:
0 Loose, if the soil is
non-coherent (single-grain structure):
1 Soft, if the soil is
very weakly coherent and friable. breaking to powder or individual
grains under very slight pressure;
2 Slightly hard, if the
soil resists light pressure, but can be broken easily between
thumb and forefinger;
3 Hard, if the soil resists
moderate pressure, can barely be broken between the thumb and
forefinger, but can be broken in the hands without difficulty;
4 Very hard, if the soil
resists great pressure, cannot be broken between the thumb and
forefinger but can be broken in the hands with difficulty;
5 Extremely hard, if the
soil resists extreme pressure and cannot be broken in the hands.
8.4 Determination of soil
consistency using the Atterberg Limits
As can be seen from the various wet, moist and dry
soil tests in Sections 8.1 to 8.3, the consistency of a
soil sample changes with the amount of water present.
Such changes in soil consistency may be accurately
measured in the laboratory following standard procedures
which determine the Atterberg Limits. These limits may
then be used for judging the suitability of the soil, e.g.,
for the construction of small earth-dams and pond dikes.
You should become familiar with the terminology related
to this process, as well as with its general
significance, the better to understand and discuss fish-pond
design and construction with specialized technicians.
An Atterberg Limit corresponds to the moisture content
at which a soil sample changes from one consistency to
another. Two of the Atterberg Limits are of particular
interest for aquaculture, the liquid limit and the
plastic limit, which are defined from three soil
consistencies:
Liquid consistency - fluid or liquid mud;
Plastic consistency - kneading and moulding
to shape are possible;
Semi-solid consistency - no more kneading is
possible and the volume decreases (shrinkage) as the sample
dries up.
Atterberg Limits - soil consistency
The liquid limit (LL)
The percentage moisture content at which a soil
changes with decreasing wetness from the liquid to the
plastic consistency or with increasing wetness from the
plastic to the liquid consistency.
The plastic limit (PL)
The percentage moisture content at which a soil
changes with decreasing wetness from the plastic to the
semi- solid consistency or with increasing wetness from
the semi-solid to the plastic consistency.
The plastic limit is the lower limit of the plastic
state. A small increase in moisture above the plastic
limit will destroy the cohesion* of the soil.
Note: these limits may be easily and cheaply
determined in the laboratory, using disturbed or
undisturbed samples. They provide very useful information
for classifying soils (see Chapter 11). In Tables
12 and 13, examples of Atterberg
Limits are given.
Both the liquid and plastic limits depend upon the amount
and type of clay present in the soil:
A soil with a high clay content usually has high LL and PL;
Colloidal clays have higher LL and PL than non-colloidal clays;
Sand, gravel and peat have no plasticity, their PL= 0;
Silts have plasticity only occasionally, their PL being equal to
or slightly greater than 0.
Examples Typical laboratory tests showing average
LL and PL
Soil type
LL
PL
Sands
20
0
Silts
27
20
Clays
100
45
Colloidal clays
399
46
Some critical values of the Atterberg
Limits for aquaculture
For constructing a pond dike without a clay core*, the
liquid limit of the soil material should be equal to 35
percent for best compaction*results.
For constructing the impervious clay core* of a pond
dike, you should use soil material with an LL greater
than 60 percent and a PLgreater than 20 percent.
Field determination of the plastic limit - the
thread method
Take a soil sample and let it dry;
Add a little water to the soil sample and roll it on a flat
surface like a small glass plate. Try to form a thread
3 mm thick and 10 cm long without breaking it;
If you do not succeed, add a little more water and try again;
Repeat this process, adding a little more water each time,
until you can roll a thread. The water content will then correspond
to the plastic limit and can be expressed as a percent of
the weight of the sample.
8.5
Calculation of the plasticity index and its significance
On the basis of the liquid limit and the plastic
limit, the plasticity index (PI) can be defined as the
numerical difference between them:
PI = LL - PL
The plasticity index is expressed in percent of the
dry weight of the soil sample. It shows the size of the
range of the moisture contents at which the soil remains
plastic. In general, the plasticity index depends only on
the amount of clay present. It indicates the fineness of
the soil and its capacity to change shape without
altering its volume. A high PI indicates an excess of
clay or colloids in the soil. Its value is zero whenever
the PL is greater or equal to the LL.
The plasticity index also gives a good indication of compressibility (see Section
10.3). The greater the PI, the greater the soil compressibility. Examples
of plasticity index values are given below in Tables
12 and 13.
Examples Plasticity of various
silt/clay soils
Category
Soil
PI (percentage)
Degree of plasticity
I
Sand or silt
traces of clay
little clay
0-1
Non-plastic
1-5
Slight plasticity
5-10
Low plasticity
II
Clay loam
10-20
Medium plasticity
III
Silty clay
Clay
20-35
High plasticity
>35
Very high plasticity
Some critical values of the plasticity index for
aquaculture
To construct a pond dike without a clay core*, the
plasticity index of the soil material should have a value
between 8 and 20 percent. For best compaction, the PI
should be as close to 16 percent as possible (see Section
10.3).
To construct the impermeable clay core* of a pond
dike, you should use soil material with a plasticity
index greater than 30 percent.
TABLE 12 Selected results of mechanical soil analyses (analyses
of disturbed soil samples collected from open
pits)1
1 Soil survey for the construction
of the Soraon Pati Hatchery (Uttar Pradesh, India).
2See Section 11.1.
Comments:
Predominantly silt with a good clay percentage;
Textural class from silty clay loam to silt and
silty clay;
The Atterberg Limits show that the soil is fairly
plastic and suitable for embankment construction
(good stability and low seepage losses).
TABLE 13 Selected results of mechanical soil analyses (analyses
of disturbed soil samples collected from auger
bores)1
1 Soil survey for a fish-farm site
(Majargahi Gaura, India). 2 See Section 11.1.
Comments:
Predominantly sand with a good silt percentage
and little clay;
Textural class is sandy loam throughout;
The Atterberg Limits show that the soil is fairly
plastic and suitable for pond construction.
8.6 The plasticity chart for fine-grained
soils
Many properties of clays and silts (the cohesive soils)
such as their compressibility*
(reaction to the shaking test and consistency near the
plastic limit) can be correlated with the liquid limit
and the plasticity index. This correlation has been
expressed in Casagrande's plasticity chart for fine-grained
soils. It is based on the following observations:
As the liquid limit of soils increases, the
plasticity and compressibility of soils also
increase;
The values LL = 30 percent and LL = 50 percent
differentiate between the various degrees of
plasticity of inorganic soils;
At equal LL values, the dry strength of inorganic
soils increases generally with an increasing
plasticity index.
The plasticity chart for fine-grained soils (see Table 14) is divided into six sections by
the oblique line A drawn so that the PI = 0.73 (LL - 20)
and two vertical lines drawn at LL = 30 percent and LL =
50 percent.
Each section of the chart characterizes a group of
soils with well-defined mechanical characteristics. The
three sections above line A are inorganic clays of low,
medium or high plasticity. The three sections below line
A are inorganic silts of varying compressibility, organic
silts and organic clays. These form the basis of a useful
soil classification system
(see Chapter 11).
Note: soils with a plasticity index lower than 10
percent and a liquid limit lower than 20 percent are
cohesionless soils. These appear in a separate section of
the plasticity chart and the above considerations do not
apply.