Major Soil Classification Systems in the World

Soil classification is the arrangement of soils into different groups in a way that the soils in a particular group have similar behaviour. It is often described as the language of the geotechnical engineer in that an engineer anywhere in the world can understand the behaviour of any soil any other place based on the class the soil belongs to, for example, if a soil is classified as SW based on Unified Soil Classification System (USCS), a geotechnical engineer anywhere in the world would understand that the soil is well graded sand that has low compressibility and high shear strength. Classification is important to the geotechnical engineer because it enables him to know the suitability of a soil as a foundation or construction material.

Classification Tests

These are tests that are commonly used to classify soils. these tests are usually conducted on disturbed soils and may not be substitute for exact analysis based on engineering properties. This is because the properties determined from disturbed soil samples are approximate of the real engineering properties of soils which should be determined from tests on undisturbed soil samples.

However, due to the importance of soil classification systems and the challenges of obtaining undisturbed soil samples, the following tests called index properties tests or classification tests are used to classify soils:

1. Particle size distribution analysis or sieve analysis.
2. Specific gravity tests.
3. Atterberg limit tests (liquid limit, plastic limit, plasticity index).

Major Soil Classification Systems

Classification Systems Based on Particle Sizes Alone

Some systems developed earlier were based on particle size alone because the size of individual particles has an important influence on the behaviour of soils. however, these systems are misleading for fine-grained soils whose major behaviour are based on plasticity characteristics.

These systems include:

1. MIT System: This system was developed by Prof. G. Gilboy of Massachusetts Institute of Technology (MIT) in USA. In the system, the soil is divided into four (4) groups:

• Particles above 2 mm are classified as gravel.
• Particle sizes between 0.06 mm to 2 mm inclusive are classified as sand
• Particle sizes between 0.002 mm to 0.06 mm are classified as silt.
• Particles less than 0.002 mm (2µm) are classified as clay.

Sand and silt are further subdivided into:

• Coarse sand: 0.6 – 2 mm
• Medium sand: 0.2 – 0.6 mm
• Fine sand: 0.06 – 0.2 mm
• Coarse silt: 0.02 – 0.06 mm
• Medium silt: 0.006 – 0.02 mm
• Fine silt: 0.002 – 0.06 mm

Note: particles less than 0.06 mm are not visible to naked eye.

2. International Classification System: This system previously known as the Swedish classification system was proposed for general use at the International Soils Congress held at Washington in 1927. In the system, in addition to sand, silt and clay, a term Mo has been used for soil particles in the size range between sand and silt (see table below).

       3. US Bureau of Soils Classification: This system was developed by US Bureau of soils in 1895. In this                   system,.

• Particle sizes less than 0.005 mm – clay
• Particle sizes between 0.005 mm and 0.05 mm – silt
• Particle sizes between 0.05 mm and 1.0 mm – very fine, fine, medium and coarse sands
• Particle sizes between 1.0 to 2.0 mm – fine gravels

4. Textural Classification: This system coined by the US Bureau of public roads is similar to other classification systems based on particle sizes but here, a triangle is used to describe the system of soils. In the system, the term texture is used to express the percentage of the three constituents of soils namely; sand, silt and clay (see diagram).

(Credit: engineeringdiscoveries.com)

Classification Systems Based on Particle Sizes and Atterberg Limits

1. American Association of State Highway and Transportation Officials (AASHTO): This system was basically designed for classifying soils for highways. It uses both plasticity characteristics and particle sizes analysis to classify soils. In the system, the soils are divided into 7 types, A-1 to A-7 with further sub-division. The soil with the lowest number, A-1 is the most suitable as a highway material or subgrade. In general, the lower is number, the more suitable is the soil. example, the A-3 is better than the A-4.

Fine grained soils are further rated for their suitability for highways by the group index (GI).

GI = (F-35) [0.2 + 0.005 (w_l – 40)] + 0.01 (F-15) (I_p – 10)

Or GI = 0.2 (F-35) + 0.005 (F – 35) (w_l – 40) + 0.01 (F – 15) (I_p – 10)

Where F = % by mass passing sieve No. 200 (0.075 mm) expressed as whole number

w_l = liquid limit (%) expressed as whole number

I_p = plasticity index (%) expressed as whole number

If any term in parenthesis becomes negative, discard it. The maximum values of (F-35) and (F-15) are                    taken as 40 and that of (w_l – 40) and (I_p – 10) are taken as 20.

The group index is given to the nearest whole number. Negative value is reported as zero while the                         value is appended to soil type as A – 4 (12) which indicates soil type A – 4 having group index of 12. The                 smaller the group index, the better is the soil in that category.

2. Unified Soil Classification System (USCS): This system was first developed by Casagrande in 1948. It is the system adapted by American Society of Testing Materials (ASTM) and most popular system for use of all types of engineering problems involving soils. the system classifies soils using 15 groups. However, soils are first classified into two groups: coarse-grained soils and fine-grained soils and later sub-divided into smaller groups based on particle size analysis and plasticity characteristics.
3. Indian Classification System: This system used by Bureau of Indian Standards is similar to USCS except that fine-grained soils are sub-divided into low, medium and high compressibility against the low and high compressibility used in USCS. Still in contrast, it classifies soils into 18 groups instead of 15. Soils are sub-divided into 3 broad divisions against 2 in the USCS namely:

• Peat (Pt) – highly organic soil and contains large percentage of organic matter and particles of decomposed vegetation.
• Coarse-grained soils – more than 50% retained on No. 200 sieve.
• Fine-grained soils – more than 50% pass No. 200 sieve.

4. Boundary Classification: This may not be a separate type of classification system but where a soil does not fit in any of the groups above properly, they can be put into boundary using dual symbols. For instance; GW – GP, SM – SC etc. In the symbol nomenclature, the coarser soil are placed before the finer soil.

References

Arora, K.R. (2014): Soil Mechanics and Foundations. Standard Publishers Distributors, New Delhi, India.