12. SOILS AND FRESHWATER FISH CULTURE
12.0 What have you learned?
In the previous chapters of this manual you have been told:
- How soils develop with time;
- What the main soil characteristics are;
- How to evaluate these soil characteristics so that you can use this
information to your advantage.
On the basis of this new knowledge, you should be better able to:
- Select a suitable site for either small reservoirs or freshwater fish-ponds;
- Manage your ponds.
Site selection and pond management will be discussed in detail in later
volumes of Simple methods for aquaculture, but certain aspects
will be briefly discussed here so that you will have an idea of how the
results of your soil survey may apply directly to these two subject areas.
12.1 Soil suitability for the
building of earthen ponds
If you plan to build earthen fish-ponds, soil quality will be important,
both for the selection of the best materials for making the pond dikes
and for designing the fish-farm. The inlet canals and the pond bottoms
will have to be sufficiently impermeable to minimize water losses by seepage.
For good fish production, the fertility of your pond water should be maintained
to prevent the loss of nutrients through high permeability of the pond
bottom.
Note:the coefficient of permeability
of soils to be used for pond bottoms should preferably be smaller than
K = 5 x 10-6 m/s.
When a site is unsuitable for earthen ponds
A site may be considered unsuitable for earthen ponds if it contains:
- Rock outcroppings or big surface stones;
- Gravel beds or rocky soils;
- Sandstone soils;
- Organic soils such as peat soils which
should be avoided if possible because of their fast permeability and
their unsuitability as dike construction material. When ponds are built
in such soils, special construction techniques will have to be used.
When a site is suitable for earthen ponds
A site may be considered suitable for earthen ponds if the
soil will ensure:
- Good water retention such as clay or sandy clay soils;
- Good pond fertility such as clay loam or silty clay loams.
To be suitable, the texture of the soil should be fine grained
with the silt and clay particles representing more than 50 percent
of the total dry weight. The best soils for fish culture
are the sandy clay, silty clay loam or clay loam soils which
belong to the USC-CL group.
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Good pond fertility
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12.2 Soil suitability
for building embankments
A soil survey will help you to decide upon the suitability
of a particular site. If you are planning to have a small reservoir
to meet your water requirements, you will need to select a site
for building the dam. Among the main factors to consider is
soil quality, not only as construction material for building
the earthen dam itself but to avoid accidents later. For example,
if the dam is not well anchored to a deep impermeable soil layer,
water may percolate along this layer making it slippery enough
for the dam to slide away.
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Dam or pond dikes
The suitability of a soil as material for building a dam or pond dikes
decreases as the percentage of the clay particles decreases. This can
be seen from Table 25 where soils with different textures are compared
for permeability, compressibility and compaction characteristics.
The characteristics of different soil materials for compacted
embankments are summarized, for guidance only and according to USC
soil groups, in Table 26. The best compaction characteristics are achieved
with soils having a liquid limit equal to 35 percent and a plasticity
index equal to 16 percent. The moisture
content of the soil should also be as close as possible to the optimum
value (see Section 10.2).
Dikes without a clay core
For the construction of dikes without a clay core, look for the following
soil material properties:
- Particles with a diameter smaller than 0.1 mm, 20 to 70 percent;
- Particles with a diameter smaller than 0.05 mm, 10 to 40 percent;
- Plasticity index, 8 to 20 percent;
- Coefficient of permeability from 1
x 10-4 to 5 x 10-6 m/s.
Dikes with a clay core
For the construction of dikes with a clay core, you must use a good impermeable
material for the clay core with the following plasticity characteristics:
- Liquid limit greater than 60 percent;
- Plastic limit greater than 20 percent;
- Plasticity index greater than 30 percent.
Note:the relative position of the particle-size
frequency curve of your sample may also assist you in judging soil
suitability, as discussed later in Section 12.4.
TABLE 25
The relative suitability as dike material of
various types of soil
Texture
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Permeability
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Suitable as dike material
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Clay
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Impermeable
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Medium
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Fair to good
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Excellent
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Sandy clay
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Impermeable
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Low
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Good
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Good
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Loam
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Semi-impermeable to impermeable
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High
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Fair to very poor
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Fair
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Sandy loam
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Semi-impermeable to impermeable
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Medium to high
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Good to very poor
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Poor
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Sand
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Permeable
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Negligible
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Good
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Poor
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Peat
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Very poor
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TABLE 26
Characteristics of various soil materials for the construction
of dikes and dams1
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Suitability for dikes/dams
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Workability as a construction material
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GW
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Very stable permeable shells of dikes/dams
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K > 10-4
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Good crawler tractor, pneumatic or
steel roller
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Low
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Excellent
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High
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High
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Low
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About nil
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GP
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Reasonably stable permeable shells
of dikes/dams
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K > 10-4
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Good crawler tractor, pneumatic or
steel roller
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Low
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Good
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High
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High
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Low
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Very low
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GM
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Reasonably stable not particularly
suited to shells, but may be used for impermeable cores
or blankets
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K = 10-5
to 10-8
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Fair to good with very close control
of moisture, pneumatic or sheeps'-foot roller.
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Low
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Good
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High to medium
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Medium to low
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Medium to low
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Very low
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GC
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Fairly stable may be used for impermeable
cores
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K = 10-2
to 10-5
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Good to fair, pneumatic or sheeps-foot
roller
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Low to medium
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Good
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Medium
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Low
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Medium to low
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Low
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SW
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Very stable permeable sections slope
protection required
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K > 10-5
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Good, crawler tractor or pneumatic
roller
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Low
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Excellent
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High
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High
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Medium
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About nil
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SP
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Reasonably stable may be used
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K > 10-5
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Good, crawler tractor
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Low
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Fair
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Medium
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High
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Medium to low
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About nil
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SM
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Fairly stable not particularly suited
to shells, but may be used for impermeable cores or dikes
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K = 10-5
to 10-8
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Fair to good with very close control
of moisture, pneumatic or sheeps'-foot roller.
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Low to medium
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Fair
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Medium
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Medium to low
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Medium to high
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Very low
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SC
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Fairly stable; used for impermeable
core for flood control structures
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K = 10-8
to 10-10
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Good to fair; pneumatic or sheeps'-foot
roller.
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Low to medium
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Good
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Medium to low
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Low
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Medium to low
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Low to medium
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ML
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Poor stability; may be used for embankments
with proper control
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K = 10-5
to 10-8
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Fair to poor with very close control
of moisture, pneumatic or sheeps'-foot roller
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Medium
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Fair
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Medium to low
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Medium to low
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High
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Low to medium
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MH
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Poor stability; core of hydraulic fill
dams not desirable in rolled fill construction
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K = 10-6
to 10-8
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Poor to very poor; pneumatic or sheeps'-foot
roller
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High
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Poor
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Low
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Low to medium
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Medium to low
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High
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CL
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Stable; impermeable cores and blankets
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K = 10-8 to 10-10
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Fair to good; pneumatic or sheeps'-foot
roller
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Medium
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Good to fair
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Medium to low
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Low
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Low to medium
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Medium to high
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CH
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Fair stability with flat slopes; thin
cores, blankets and dike sections
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K = 10-8
to 10-10
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Fair to poor; pneumatic or sheeps'-foot
roller
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High
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Poor
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Medium to low
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Low
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Low
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High
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OL
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Suitable for low embankments with very
low hazard only
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K = 10-6
to 10-8
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Fair to poor; sheeps'-foot roller
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High
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Fair
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Low
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Low to medium
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Medium to high
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High
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OH
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Suitable for low embankments with very
low hazard only
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K = 10-8
to 10-10
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Poor to very poor; sheeps'-foot roller
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High
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Poor
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Low
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Low
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Medium to low
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High to very high
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1 This information given for
guidance only.
2 This equipment will usually produce
good compaction with a reasonable number of
passes if moisture
conditions and thickness of layers are
properly controlled.
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A simple test for soils to be used in building embankments
It is very important to know the ability of a soil to resist
water saturation* when you are selecting soil material for embankment
construction. Here is a very simple test that you can perform to determine
this soil quality:
- Take a sample of the soil and wet it well;
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- Knead it with your hands until it becomes a stiff plastic
mass;
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- Make several balls, each about 10 cm in diameter;
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- Put the balls in still water about 45 to 60 cm
deep. You can use a hole dug in the ground and lined with a
plastic sheet or a large container such as a 200 l metal drum;
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- Look at the balls of soil every few hours at first, and later
several times a day
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- If the balls fall apart within a few hours (E), the soil is
not good for embankment construction;
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- If the balls do not fall apart but remain intact for at least
24 hours (F), the soil is good for embankment construction.
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12.3
Soil suitability for water canals
The relative stability of various USC soil groups
for water canals, such as inlet and outlet canals for fish-farms, varies,
as shown in Table 27, which gives estimates of resistance
to water erosion and of suitability as compacted earth
lining. When digging water canals, you should also consider the
permeability characteristics of the soil, giving preference to those
with a coefficient of permeability smaller or equal
to 10-5 m/s.
TABLE 27
Relative suitability of soils for water canals
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Erosion resistance
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Compacted earth lining
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GW
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1
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-
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GP
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2
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-
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GM
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4
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4
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GC
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3
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1
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SW
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6
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-
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SP
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7, if gravelly
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-
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SM
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8, if gravelly
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5 (Erosion critical)
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SC
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5
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2
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ML
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-
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6 (Erosion critical)
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CL
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9
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3
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OL
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-
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7 (Erosion critical)
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MH
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-
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-
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CH
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10
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8 (Volume change critical)
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NOTE: Number 1 indicates the best
soil
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12.4
Determination of soil suitability with the PSF-curve
If you have the particle-size
frequency curve for your soil (see Section 6.7), you
may compare it with the
particle-size frequency curve for your soil (see
Section 6.7), you may compare it with reference curves
and determine the relative suitability of the soil for
pond construction or for building dikes. This method,
however, is normally used only for the planning and
design of relatively large fish-farms by technicians
specialized in civil engineering. This is an example of
how it is used:
- You have taken a disturbed soil sample from the B-horizon
of one of your soil profiles to the testing
laboratory. It has been analysed for a series of
particle sizes and the results have been given to
you, either as percentages of occurrence by
weight or as a particle-size frequency curve;
- If you have the results as percentages of occurrence,
plot this information in pencil on a photocopy of the
graph given in Table 28 and trace your sample PSF-
curve;
- Now compare your PSF-curve with the two reference
curves on the graph ...
- If your sample curve falls within Zone A, the soil
is suitable for a pond bottom, providing that its coefficient of permeability
K is less than 5 x 10-6 m/s (see Table 16);
- If your sample curve falls within Zone B,
the soil is suitable for building dikes
without an impermeable clay core;
- If your sample curve falls within Zone C,
you will require further
studies concerning the soil
characteristics (see Sections 12.1
and 12.2). You may find that the soil may
be used but only under particular
conditions such as, for example, by
puddling the pond bottom and using an
impermeable clay core in the dikes.
Note: before making a final decision on the
suitability of your soil, you should carefully check the
other important soil characteristics such as soil
structure and permeability. These should confirm your
diagnosis (see Tables 17A and
17B).
Examples
- Two soil horizons have been sampled in an
open pit and mechanical analyses in the
laboratory have provided particle-size
frequencies;
- Cumulative frequencies are calculated;
.
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Particle sizes in mm
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SAMPLE 1 |
1
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0.2
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0.075
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0.04
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0.025
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0.02
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0.01
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0.005
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0.0035
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0.002
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Frequency (percent) |
0.3
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1.7
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17
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13
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17
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9
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8
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3
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0.5
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0.5
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Cumulative f�requency (percent) |
0.3
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2
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19
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32
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49
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58
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66
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69
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69.5
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70
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Particle sizes in mm
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SAMPLE 2 |
2
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1
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0.5
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0.2
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0.1
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0.05
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0.02
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0.01
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0.0075
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0.045
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0.002
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Frequency (percent) |
1
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2
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4
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7
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12
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30
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26
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9
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3
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3
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2
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Cumulative f�requency (percent) |
1
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3
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7
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14
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26
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56
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82
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91
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94
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97
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99
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- The PSF-curves are plotted on a photocopy
of the reference
graph given in Table 28, using
the right vertical scale to plot the cumulative
frequencies;
- Compare the position of the sample curves
with the position of the reference curves ...
- Sample 1 is a clay loam with 28 percent
sand, 42 percent silt and 30 percent clay and its
PSF-curve falls within Zone A of the reference
graph, which suggests that this is a suitable
soil for use as a pond bottom;
- Sample 2 is a sandy loam with 56 percent
sand, 43 percent silt and 1 percent clay and its
PSF-curve falls within Zone B, which suggests
that this is a suitable soil for the construction
of pond dikes without an impermeable clay core.
TABLE
28 Graph and reference curves for
the determination of soil suitability
from the particle-size frequency curve of the
soil sample
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12.5 Soils and pond management
A soil survey will help you to plan and practice better pond
management by reducing seepage losses and by improving pond
fertility.
Reduction of water losses by seepage
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If the pond has a sandy bottom
soil, the seepage will be unusually high, 10 cm per day
or more, and particularly during the first year after construction.
To reduce seepage, you can
block the soil pores by spreading organic matter such as compost
and animal manure on the pond bottom and mixing it well with the
top 10-15 cm of soil. |
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If the soil of the pond bottom has a strongly developed
structure and high seepage losses, 10 cm per day or more,
it may also be necessary to destroy the structure either through
mechanical compaction, using a crawler tractor or a sheeps-foot
roller, or through puddling
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If the percentage of clay present in the pond
bottom is high, more than 60 percent, when draining the pond
you should not allow the pond bottom to become too dry. If this
happens, deep cracks may develop and this may increase seepage
losses later when you refill the pond.
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Improvement of pond fertility
If the pond bottom is acid,
with a low pH, you can improve pond fertility by adding lime to
neutralize the acidity. This is a technique which
will be discussed further in a later volume of Simple
methods for aquaculture.
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