9.0 Why is it important to determine soil permeability?
Soil permeability is the property of the soil to transmit water
and air and is one of the most important qualities to consider
for fish culture.
A pond built in impermeable soil will
lose little water through seepage.
The more permeable the soil, the greater
the seepage. Some soil is so permeable and seepage so great that
it is not possible to build a pond without special construction
techniques. You will learn about these techniques
in a later volume in this series.
Soils are generally
made up of layers and soil quality often varies greatly
from one layer to another. Before pond construction, it is important
to determine the relative position of the permeable and impermeable
layers. The design of a pond should be planned to avoid having a
permeable layer at the bottom to prevent excessive
water loss into the subsoil by seepage.
The dikes of the pond should be built with soil which
will ensure a good water retention. Again, soil quality will have to
be checked with this in mind.
9.1 Which factors affect soil permeability?
Many factors affect soil permeability. Sometimes they are extremely localized,
such as cracks and holes, and it is difficult to calculate representative
values of permeability from actual measurements. A good study of soil profiles provides an essential
check on such measurements. Observations on soil texture, structure, consistency,
colour/mottling, layering, visible pores and depth to impermeable layers
such as bedrock and claypan* form the basis for deciding
if permeability measurements are likely to be representative.
Note: you have already learned that soil is made up of a
number of horizons, each of them usually having different physical and
chemical properties. To determine the permeability of soil as a whole,
each horizon should be studied separately.
9.2 Soil permeability relates to soil texture and
The size of the soil pores is of great importance with regard to the
rate of infiltration (movement of water into the soil) and
to the rate of percolation (movement of water through the
soil). Pore size and the number of pores closely relate to soil texture
and structure, and also influence soil permeability.
Permeability variation according to soil texture
Usually, the finer the soil texture, the slower the permeability, as
From very slow to very rapid
Example Average permeability for different soil textures in cm/hour
Structure may greatly modify the permeability rates shown above, as follows:
- Greatly overlapping
From very slow to very rapid
- Slightly overlapping
1 This may vary according to the degree
to which the structure is developed.
It is common practice to alter the soil structure to reduce
permeability, for example, in irrigated agriculture through
the puddling of rice fields and in civil
engineering through the mechanical compaction* of
earthen dams. Similar practices may be applied to fish-ponds to
reduce water seepage.
9.3 Soil permeability classes
Permeability is commonly measured in terms of the rate of water
flow through the soil in a given period of time. It is usually
expressed either as a permeability rate in centimetres
per hour (cm/h), millimetres per hour (mm/h), or centimetres per
day (cm/d), or as a coefficient of permeability k
in metres per second (m/s) or in centimetres per second (cm/s).
For fish culture, two methods are generally used to determine
soil permeability. They are:
The coefficient of permeability;
The seepage rate.
For the siting of ponds and the construction of dikes,
the coefficient of permeability is generally used to qualify
the suitability of a particular soil horizon:
Dikes without any impermeable clay core may be built from soils having
a coefficient of permeability less than
K = 1 x 10-4 m/s;
Pond bottoms may be built into soils having a coefficient of permeability
less than K = 5 x 10-6 m/s.
For pond management, the seepage rate is generally
In commercial pond culture, an average seepage rate of 1 to 2 cm/d
is considered acceptable, but corrective measures should be taken to
reduce soil permeability when higher values exist, particularly when
they reach 10 cm/d or more.
9.4 Measurement of soil permeability in the laboratory
When you take an undisturbed sample to a testing laboratory,
to measure permeability, a column of soil is placed under specific conditions
such as water saturation and constant head of water. The result will be
given to you either as a permeability rate (see Table 15),
or as a coefficient of permeability (see Table 16).
TABLE 15 Soil permeability classes for agriculture and conservation
Soil permeability classes
Less than 0.13
Less than 3
0.13 - 0.3
3 - 12
0.5 - 2.0
12 - 48
2.0 - 6.3
48 - 151
6.3 - 12.7
151 - 305
12.7 - 25
305 - 600
More than 25
More than 600
1 Saturated samples under a constant water head
of 1.27 cm
Soil permeability classes for civil engineering
Soil permeability classes
Coefficient of permeability (K in
2 x 10-7
2 x 10-1
1 x 10-11
1 x 10-5
1 x 10-11
5 x 10-7
9.5 Measurement of soil permeability in the field
To measure soil permeability in the field, you can use one of the following
The visual evaluation of the permeability rate of soil horizons;
A simple field test for estimating soil permeability;
A more precise field test measuring permeability rates.
The visual evaluation of the permeability rate of soil horizons
The permeability of individual soil horizons may be evaluated by the
visual study of particular soil characteristics which have been shown
by soil scientists to be closely related to permeability classes. The
most significant factor in evaluating permeability is structure:
its type, grade, and aggregation characteristics, such as the relationship
between the length of horizontal and vertical axes of the aggregates and
the direction and amount of overlap.
Although neither soil texture nor colour mottlingalone are reliable
clues, these soil properties may help to estimate permeability when considered
together with the structural characteristics. To evaluate
visually the permeability of soil horizons:
Examine a fresh soil profile in an open pit;
Determine the soil horizons present;
Using Table 17A, evaluate the permeability
class to which each horizon belongs, carefully studying the structural
characteristics of the soil;
Confirm your results through the other soil properties shown in Table
Ranges of permeability rates may then be found inTable
TABLE 17A Visual indicators of permeability: structural characteristics
TABLE 17 B Visual Indicators of permeability: texture, physical behaviour
and colour of soil
A simple field test for estimating soil permeability
Dig a hole as deep as your waist;
Early in the morning, fill it with water to the top;
By the evening, some of the water will have sunk into the soil;
Fill the hole with water to the top again, and cover it with
boards or leafy branches;
If most of the water is still in the hole the next morning,
the soil permeability is suitable to build a fish-pond here;
Repeat this test in several other locations as many times as
necessary, according to the soil quality.
A more precise field test for measuring permeability rates
Carefully examine the drawings you have made when studying your
Note: you could also use the visual method (see Tables
17A and 17B) to estimate
On the basis of texture and structure, determine which soil
horizons seem to have the slowest permeability;
Mark the soil horizons on your drawings which seem to have the
slowest permeability. Use a coloured pencil;
Note: water seeps into the soil both horizontally
and vertically, but you need only be concerned with the vertical
water seepage because this is mainly what happens in ponds.
Dig a hole approximately 30 cm in diameter until you reach the
uppermost least permeable horizon;
Thoroughly smear the sides of the hole with heavy wet clay or
line them with a plastic sheet, if available, to make them waterproof;
Pour water into the hole to a level of about 10 cm;
At first, the water will seep down rather quickly, and you will have
to refill as it disappears. When the pores of the soil are full of water,
seepage will slow down. You are then ready to measure the permeability
of the soil horizon at the bottom of the hole;
Make sure that the water in the hole is about 10 cm deep as
before. If it is not, add water to reach that level;
Put a measuring stick into the water and record the exact
water depth, in millimetres (mm);
Check the water level in the hole every hour for several hours.
Record the rate of seepage for each hourly period. If the water
disappears too rapidly, add water to bring the level up to 10
cm again. Measure the water depth very carefully;
When your hourly measurements become nearly the same, the rate
of permeability is constant and you may stop measuring;
If there are great differences in seepage each hour, continue
pouring water into the hole to keep the level at 10 cm until the
rate of seepage remains nearly the same;
Note: a soil horizon with suitable permeability for
a pond bottom should also be at least 0.7-1 m thick, unless lower
horizons exist with suitable permeability and thickness.
Now compare your results with the following values:
Permeability rate in mm/h
Suitability of horizon for a pond
Slower than 2
Acceptable seepage: soil suitable
Fast seepage: soil suitable ONLY if seepage due
to soil structure which will disappear when pond is filled
Excessive seepage: soil unsuitable unless seepage
can be reduced as described below
If the permeability rate is faster than 5 mm/h, this may
be owing to a strongly developed structure in the soil. In such cases,
you try to reduce the permeability rate by destroying the structure,
Puddle the bottom soil of the hole as deep as you can;
Repeat the more precise permeability test until you can measure
a nearly constant value for seepage.
Check this with the following test
Dig a new hole 30 cm in diameter through the uppermost
least permeable layer (A) to the top of the next least permeable
Repeat the permeability test until you measure a nearly
constant value for seepage;
If this permeability rate does not exceed 3 mm/h, you
may consider this soil horizon as suitable for a pond bottom.
However, remember that such slow permeability should be found
in a layer at least 0.7-1 rn thick to ensure limited seepage
through the pond bottom.
Note: when building your pond, you do not necessarily need
to remove a shallow permeable layer if there is a deeper layer of soil
which is not permeable and will serve to hold the water. You must, however,
build the pond dikes down to the deeper non-permeable layer to form an
enclosed basin and to avoid horizontal water seepage (see Section 9.0).
9.6 Determining coefficients of permeability
To obtain a more accurate measurement of soil permeability, you can perform
the following test in the field which will give you a value for the coefficient
Using a bucket auger, drill a hole about 1 m deep in the soil
at the location where you wish to determine the coefficient of
Fill the hole with water to the top;
Every five minutes, for at least 20 minutes, refill the hole
to the top to be sure that the soil is fully saturated;
Top the water in the hole and start measuring the rate at which
the water surface goes down, using a watch to measure time and
a centimetre-graduated ruler to measure the distance P between
the water surface and the top of the hole. Stop measuring when
the rate becomes nearly constant;
Rate becomes constant
Measure exactly the total depth of the hole (H) and its diameter (D).
Express all measurements in metres (m): for example
H = 1.15 m and D = 12 cm or 0.12 m
For each of the above two consecutive measurements of time/distance,
calculate the coefficient of permeability K using the following
K= (D÷2) x In (h1÷ h2) / 2
where (D ÷ 2) is the radius of the hole or half its
diameter in metres; In refers to the Napierian or natural
logarithm; h1 and h2 are the two consecutive depths
of water in metres, h1 at the start and h2 at the
end of the time interval;
(t2 - t1 ) expresses the time interval between two
consecutive measurements, in seconds;
Note: the h-values may be readily calculated as the differences
between the total depth of the hole H and the successive P values. Be
careful to express all the measurements in metres and seconds so as to
obtain K in m/s.
Now compare your K values (in m/s) with those in Table 16.
If (D ÷ 2) = 0.12 m ÷ 2 = 0.06 m and H = 1.15 m, calculations of
the various K values are made progressively according to the formula (see
Note: for obtaining the natural logarithm of
(h1 ÷ h2), you will have to use either a logarithmic
table or a pocket calculator.
Remember that 10 - 6 = 0.000001 and 6.8 x 10-6
= 0.0000068, the negative exponent of 10 reflecting the decimal place
to be given to the multiplicant.
If you wish to compare a K value (m/s) with permeability rates
(cm/day), multiply K by 8 640 000 or 864 x 104 such
as for example:
K = 1 x 10-5 m/s = 86.4
TABLE 18 Successive steps for the calculation of coefficients of
permeability on the basis of field measurements (for a test hole with H = 1.15 m and D =
NOTE: The formula for calculating coefficients of
permeability is K = [(D ÷ 2) x In (h1 ÷ h2)]
/ 2 (t2 - t1) or A ÷ B (see Section 9.6).