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: Percentages of selected ranges of particle sizes (see below); Shape of the particle-size frequency curve (see Section 6.7); Plasticity-compressibility characteristics (see Sections 8.5 and 10.3). 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:

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.

TABLE 19 Modified plasticity chart and Unified Soil Classification for fine-grained soils Note:For Unified Soil Classification designations,see Table 21

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:

• Plasticity: use the test to determine wet soil plasticity (see Section 8.1);
• Dry consistency: use the test to determine dry soil consistency (see Section 8.3);
• Shaking test: use the shaking test (see end of Section 6.4);
• Plastic limit: use the thread method (see end of Section 8.4);
• Odour: smell it ...

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)

TABLE 20B The Unified Soil Classification (definition of major fine-grained soil groups)

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.