SOIL ENGINEERING (EENV 4300)

Chapter 5

Classification of Soil

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Why Classification?

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Classification systems provide a common language to concisely express the general characteristics of soils, which are infinitely varied, without detailed descriptions.

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Two Main Groups of Classification

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(A) Textural

classification

(B) Classification by engineering behavior

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• The textural classification systems developed by the U.S. Department of Agriculture (USDA) will be used here.

• This classification method is based on the particle-size limits as described under the USDA system in Table 2.3;

• Table 2.3 divided soils into gravel, sand, silt, and clay categories on the basis of particle size.

• What is texture? In a general sense, texture of soil refers to its surface appearance. Soil texture is influenced by the size of the individual particles present in it.

• Figure 5.1 (next slide) shows the textural classification systems developed by the U.S. Department of Agriculture (USDA)

A- Textural Classification

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• Textural classification of soil is relatively simple (it is based entirely on the particle-size distribution).

• The amount and type of clay minerals present in fine-grained soils dictate to a great extent their physical properties. Hence, the soils engineer must consider plasticity, which results from the presence of clay minerals, to interpret soil characteristics properly.

• Because textural classification systems do not take plasticity into account and are not totally indicative of many important soil properties, they are inadequate for most engineering purposes.

• AASHTO and USCS systems are used.

B- Classification by Engineering Behavior

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AASHTO Required parameters are grain-size analysis, LL, PI. Using the known parameters & the AASHTO table, one

proceeds to classify the soil into type A-1 or A-2 or … A-7

A soil can be further described using a group index (GI) This index utilizes the % of soil passing No. 200 sieve,

LL & PI

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…

To classify a soil according to Table 5.1, one must apply the test data from left to right. By process of elimination, the first group from the left into which the test data fit is the correct classification.

General Guidance for AASHTO Method 8 major groups: A1~ A7 (with several subgroups) and

organic soils A8 The required tests are sieve analysis and Atterberg

limits. The group index, an empirical formula, is used to further

evaluate soils within a group (subgroups).

A4 ~ A7 A1 ~ A3

Granular Materials

≤ 35% pass No. 200 sieve Silt-clay Materials

≥ 36% pass No. 200 sieve

Using LL and PI separates silty materials from clayey materials

Using LL and PI separates silty materials from clayey materials (only for A2 group)

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The original purpose of this classification system is used for road construction (sub-grade rating).

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• It is used to evaluate the quality of a soil as a highway subgrade material.

• This index is written in parentheses after the group or subgroup designation.

Group Index (GI)

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Rules for Determining the Group Index (GI)

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• The original form of this system was proposed by Casagrande in 1942 for use in the airfield construction works undertaken by the Army Corps of Engineers during World War II.

• In cooperation with the U.S. Bureau of Reclamation, this system was revised in 1952.

• At present, it is used widely by engineers (ASTM Test Designation D-2487).

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Fig. 5.4: Flowchart group names for gravelly soil

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Figure 5.4: Flowchart group names for sandy soil

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