9. SOIL PERMEABILITY

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 structure

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 shown below:

Permeability variation according to soil structure

Structure may greatly modify the permeability rates shown above, as follows:

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).

Example

For agriculture and conservation uses, soil permeability classes are based on permeability rates, and for civil engineering, soil permeability classes are based on the coefficient of permeability (see Tables 15 and 16).

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 used:

• 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).

9.5 Measurement of soil permeability in the field

To measure soil permeability in the field, you can use one of the following tests:

• 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 mottling alone 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 17B;
• Ranges of permeability rates may then be found in Table 15.

TABLE 17A Visual indicators of permeability: structural characteristics of soil

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 soil profiles; Note: you could also use the visual method (see Tables 17A and 17B) to estimate permeability.		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:

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, as follows:

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.

• If this new permeability rate does not exceed 4 mm/h, you may consider this soil horizon as suitable for a pond bottom. However, the entire bottom of the pond will have to be puddled before filling it with water;

• If this new permeability rate exceeds 4 mm/h, this may be owing to the presence of a permeable soil horizon under the horizon you have tested. Such a permeable layer is often found between layers of soil which are semi- permeable or even impermeable;

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 layer (B); 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 of permeability:

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 permeability;

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; Example 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 formula:

K= (D�2) x In (h₁� h₂) / 2 (t₂- t₁)

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.

Example

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 Table 18).

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 cm/day

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 = 0.12 m) 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).