Soil texture and soil structure are both unique properties of soil that have profound effects on their behavior. The index properties commonly used for coarse-grained soils are grain size distribution and relative density. Index properties of fine-grained soils include consistency and sensitivity. These properties of a soil indicate the type and conditions of the soil and provide a relationship to its structural properties such as strength, compressibility, permeability, swelling potential, etc. Brief descriptions of some of these properties are given in this chapter. Towards the end, the chapter shows how these properties can be used for the classification of soils. The Soil Classification Systems considered include the following: Geological and Pedological Classification Systems (Classification by Origin and by Pedology), Morphological Classification Systems (Classification by Appearance and Textural Soil Classification System [USDA]), and Classification by Use (American Association of State Highway Transportation Officials System [AASHTO] and Unified Soil Classification System [USCS]).
Top3.1 Index Properties Of Soils
Since soils are formed in nature due to agents over which there is no human control, the properties of the resulting soils differ over a very wide range. Therefore, it has been found convenient to group together soils, which exhibit similar properties within a certain specified range and assign them names. The tests carried out in the laboratory to classify a soil are known as classification tests and the numerical results so obtained are termed Index Properties of soils.
Index properties such as density are used in the analysis of geotechnical problems while others give a general indication of how a soil is likely to behave under loading.
The index properties commonly used for coarse-grained soils are grain size distribution and relative density. Index properties of fine-grained soils include consistency and sensitivity. These properties of a soil indicate the type and conditions of the soil, and provide a relationship to its structural properties such as strength, compressibility, permeability, swelling potential, etc. Brief descriptions of some of these properties are given in this chapter. Towards the end of the chapter, it will be shown how these properties can be used for the classification of soils.
Soil texture and soil structure are both unique properties of soil that have profound effects on their behavior. These effects are dependent on the type of soil as shown in Table 1.
Table 1. General characteristics of granular and cohesive soils*
| Soil Type | Particles and Fabrics | k | Practical Implications |
| Granular | Large equi-dimensional Large voids
Very low SSA
Only mass forces Grain size | High | max uc is very low Engineering properties from in-situ Penetration tests
Drained loading
Sieve analysis |
| Cohesive | Small platy shaped Very small voids High SSA ˃ 10 m2/g Surface forces Grain size | Very Low | max uc is very high Engineering properties from in-situ and lab tests Un-drained loading Sedimentation analysis |
*Collated from many sources. SSA = Specific Surface Area (m2/g); uc = ua – uw = capillary pressure (soil sunction).