11. SOIL SUITABILITY CLASSIFICATION FOR AQUACULTURE
11.0 The problem of soil classification
There are several systems of soil classification which are based generally
on particle size or on some additional soil properties such as plasticity
and compressibility. Soil classification
based on particle-size characteristics is widely used, especially
for preliminary or general descriptions (see Section 6.4). However,
any system based on particle size alone is likely to be misleading because
the physical properties of the finest soil fractions depend on many
factors other than particle size. This has led to the development of
the Unified Soil Classification (USC) which is considered today to be
the most useful of the engineering soil classification systems. The
USC permits reliable classification on the basis of relatively few and
inexpensive laboratory tests.
11.1 The Unified Soil Classification
The Unified Soil
Classification identifies soils according to their texture and plasticity.
The USC soil groups are based on:
For engineering use, four ranges of particle sizes are recognized.
They are:
- Cobbles: particles with a diameter larger than
75 mm;
- Gravel: particle sizes from 4.75 to 75 mm;
- Sand: particle sizes from 0.075 to 4.75 mm;
- Fines: particles smaller than 0.075 mm (silt
and clay).
The particle sizes used in the Unified Soil Classification are
somewhat different from the other classification systems shown
earlier in Table 2. The USC particle sizes correspond
to US Standard Sieves, 3 inches (76.2 mm), No. 4 (4.76 mm) and
No. 200 (0.075 mm) respectively, as follows:
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Soils in the USC are classified in three basic soil
types. They are:
- Coarse-grained soils (CGS) which contain 50
percent or less of fines;
- Fine-grained soils (FGS) which contain more than
50 percent fines;
- Highly organic soils which are peat, muck, humus
or swamp soil.
The coarse- and fine-grained soils are subdivided
according to either the particle-size frequency (for CGS)
or the plasticity of the soil (for FGS). In the fine-grained soils,
plasticity is determined from the liquid limit and the
Plasticity Index (see Section 8.6). These are plotted
in a modified plasticity chart (see Table
19) for each particular soil sample. Soils are then
classified into groups according to the zone of the chart
where their representative point (LL, PI) is located (see
Tables 20A and 20B).
Each soil group is given a descriptive name and a
letter symbol which indicates its principal
characteristics (see Table 21). The
letter symbol is made up of two capital letters. The first
letter defines the major soil component and the second
letter defines either the
plasticity-compressibility (Table 14), or the
particle-size frequency, as follows:
- C for clay, M for silt, S for sand, G for gravel,
or 0 for organic;
- L for low or H for high plasticity-compressibility;
- W for well-graded or P for poorly graded
materials; in well-graded materials, no particle
size is dominant, but in poorly graded materials,
some particle size is dominant;
- Borderline cases are given a double symbol such
as CL-ML or GW-GM.
Note:examples of soil
descriptions using such letter symbols are given in Tables 12 and 13.
11.2 Field classification of fine-grained
soils
In the field, fine-grained soils can be separated into
the USC groups using simple tests (see
Table 22). They are:
11.3 Field classification of coarse-grained soils
In the field, coarse-grained soils can be separated
into the USC groups as described in Table
23, remembering that:
- Fines are all soil particles which cannot be seen
individually by the naked eye;
- For plasticity, use the test to determine wet-soil
plasticity (see Section 8.1).
11.4 Correspondence between USDA textural classes and
the USC system
If your soil samples have been analysed and classified using the
USDA textural classes (see Table 4), you may use this as a basis
for defining the USC group to which your soil samples belong, as shown
in Table 24.
TABLE 20A
The Unified Soil Classification (definition of major
coarse-grained soil groups)
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TABLE 20B
The Unified Soil Classification (definition of
major fine-grained soil groups)
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TABLE 21
Typical names and group symbols of the Unified
Soil Classification System
USC group symbol |
Typical names for soils |
Coarse-grained soils |
GW |
Well-graded gravel, gravel and sand mixtures,
little or no fines |
GP |
Poorly graded gravel, gravel and sand mixtures,
little or no fines |
GM |
Silty gravel; gravel; sand and silt mixtures |
GC |
Clayey gravel; gravel; sand and silt mixtures |
SW |
Well-graded sands, gravelly sands, little
or no fines |
SP |
Poorly graded sands, gravelly sands, little
or no fines |
SM |
Silty sands, sand and silt mixtures |
SC |
Clayey sands, sand and clay mixtures |
Fine-grained soils |
ML |
Inorganic silts and very fine sands, rock
flour, silty or clayey fine sands, or clayey silts with
slight plasticity |
CL |
Inorganic clays of low to medium plasticity,
gravelly clays, sandy clays, silty clays, lean clays |
OL |
Organic silts and organic silty clays of low
plasticity. |
MH |
Inorganic silts, micaceous or diatomaceous
fine sandy or silty soils, elastic silts |
CH |
Inorganic clays of high plasticity, fat clays
|
OH |
Organic clays of medium to high plasticity,
organic silts |
Highly organic soils |
Pt |
Peat and other highly organic soils |
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TABLE 22
An example of the field classification of the USC
fine-grained soils
USC soil group
|
Plasticity
(wet soil) |
Dry consistency
|
Shaking test reaction
|
Plastic limit, toughness of thread
|
Odour
|
ML
|
0
|
0 - 1
|
Rapid to slow |
None |
Uncharacterized, often nil |
CL
|
2
|
2 - 4
|
None to very slow |
Medium |
Slight earth smell |
OL
|
1
|
1 - 3
|
Slow |
Slight |
Decomposed organic matter |
MH
|
1
|
1 - 3
|
Slow to none |
Slight to medium |
Uncharacterized, often nil |
CH
|
3
|
3 - 5
|
None |
High |
Strong earth smell |
OH
|
2-3
|
2 - 4
|
None to very slow |
Slight to medium |
Decomposed organic matter |
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TABLE 23
An example of the field classification of the USC
coarse-grained soils
Soil group
|
Total sample, except cobbles
over 12-cm
|
Part of the sample: particles
less than 3-mm diameter only
|
GW |
Relatively few fines |
Clean material; not enough clay to
agglomerate the sand particles |
GP |
One or several sizes of coarse
particles dominant |
Clean material; not enough clay to
agglomerate the sand particles |
GM |
Dirty material; good range of sizes
for coarse particles only; many fines |
Plasticity nil or very small |
GC |
Dirty material; good range of sizes
for coarse particles only; many fines |
Plasticity moderate to high |
SW |
All sizes of coarse particles well
represented; relatively few fines |
Clean material; not enough clay to
agglomerate the sand particles;
plasticity nil |
SP |
One or several sizes of coarse
particles dominant |
Clean material; not enough clay to
agglomerate the sand particles;
plasticity nil |
SM |
Dirty material; good range of sizes
for coarse particles only; many fines |
Plasticity nil or very small |
SC |
Dirty material; good range of sizes
for coarse particles only; many fines |
Plasticity moderate to high |
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TABLE 24
Soil properties for engineering use corresponding
to USDA textural classes and the USC system1
USDA textural
class |
USC
group |
Soil properties2 |
Fine sand
(0.25-0.1 mm) |
SP |
Fines less than 10 percent |
SP-SM |
Fines 5-10 percent |
SM |
Fines more than 10 percent |
Very fine sand
(0.1-0.05
mm)
|
SM |
Low plasticity |
ML |
Little or no plasticity |
Coarse sand
(1-0.5 mm) |
SP or GW |
Fines less than 5 percent |
SP-SM |
Fines 5-12 percent |
SM |
Fines more than 12 percent |
Loamy sand |
SM |
Non- to slightly plastic |
Sandy loam |
SM |
Slightly plastic |
SC |
Plastic |
Loam, silty loam |
ML |
Slightly plastic |
CL |
Plastic |
Silt |
ML |
Slightly plastic |
Clay loam, silty clay loam
|
CL |
Liquid limit less than 50; plastic |
ML-CL |
Liquid limit less than 50; slightly
plastic |
CH |
Liquid limit more than 50; high
shrink-swell clays |
MH |
Liquid limit more than 50; mica, iron
oxide, kaolinite clays |
Sandy clay loam |
SC |
Plastic; fines less than 50 percent |
CL |
Plastic; fines more than 50 percent |
Clay, silty clay |
CH |
LL > 50; high shrink-swell clays (for
example, montmorillonite clays) |
MH |
LL > 50; mica, iron oxide, low
shrink-swell clays (for example,
kaolinite clays) |
CL |
Liquid limit less than 50; generally
less than 45 percent clay |
1USDA textural classes as
defined in Table 4.
2 Fines: silt + clay particles smaller
than 0.075 mm; degree of plasticity as in Chapter 8.
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