Soil index properties are properties which facilitate identification and classification of soils for engineering purposes. The nature of some properties differs for coarse- and fine-grained soils.

Coarse-grained (non-cohesive) soil index properties are:

  • particle-size distribution
  • particle shape
  • relative density
  • consistency
  • clay and clay minerals content

Fine-grained (cohesive) soil index properties are:

  • consistency
  • clay and clay minerals content
  • water content

One of soil index properties which describe non-cohesive soils is particle size distribution. Soil that contains wide range of particle sizes is named well-graded. The opposite type of soil, which contains narrow range of particle sizes, is categorized as poorly graded. Well-graded soils can be more densely packed. Particle shape also influences how closely particles can be packed together. The density of soil (especially of coarse-grained) is the indication of strength and stiffness. The relative density is the ratio of the actual bulk density and the maximum possible density of the soil. Relative density is a good indicator of potential increases in density, and thus deformations that may occur under the different loads.

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Consistency is the resistance of soils to deformation and rupture. The unconfined compression strength is often used as an indication of consistency. In practice, the terms soft, medium, stiff, very stiff, and hard are applied to rate consistency of soil. This soil index property describes both cohesive and non-cohesive soils. Consistency at non-cohesive soil depends primarily on particle shape and size distribution, while at cohesive soils this property primarily depends on water content.

Clay and clay minerals content is important soil index characteristic for both coarse- and fine-grained soils. Clay minerals are fine-sized platy silicates which are highly plastic. Therefore, depending on percentage and type of clay minerals, clayey soils are less or more plastic.

Water content is very important soil index property of fine-grained soils since their behaviour largely changes with water concentration variations. According to Atterberg there are four states: liquid, plastic, semi-solid and solid. Marginal water contents that separate these states are known as Atterberg limits and these are: shrinkage (SL), plastic (PL) and liquid limit (LL). These limits have different values for different types of fine-grained soils.

The Unified Soil Classification gives each soil type a two-letter designation. For coarse grained soils, the first letter, either G for gravel or S for sand, refers to the dominant particle size in the soil. The second letter is either W, for well graded or P, for poorly graded. The second letter can also be M for silt or C for clay if coarse-grained soils contain more than 12% of silt or clay. The first letter of the designation for fine-grained soils is M or C (silt or clay). The second letter, either H (high) or L (low), refers to the plasticity of the soil.


GRT products enable improvement of all soil types and make them suitable for various engineering purposes. The natural soils with highest level of unconfined compression strength (UCS) are categorized as hard soils when UCS is more than 0.4MPa. Through use of GRT stabilizers in concentration 0.5% UCS results ranging from 2 – 5.8MPa can be achieved. GRT7000 showed outstanding results when implemented in both highly coarse and highly plastic soils, and thus is suitable for stabilizing of soils with wide variety of soil index properties.

Testing of type 2 gravel treatment with 1% GRT7000 showed a UCS result at 5 days 8.9MPa. Comparative UCS results in cement stabilized type 2 gravels, require in excess of 5 and 6%, and up to 8% cement by mass for the lower sub-types. When it comes to fine-grained highly plastic soils GRT7000 is needed in much lower concentrations than lime and cement. In the case of soils with plasticity index higher than 20 (for which is lime considered very efficient stabilizer), general recommendation is 3-5% of lime or 3-7% of cement, while GRT7000 is needed in concentration as low as 1%.


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