Fig. 1.10 Example 1.6.

      1.6.2 Description of soils

      Classifying and describing a soil are two operations, which are not necessarily the same. An operator who has not even visited the site from which a soil came can classify the soil from the information obtained from grading and plasticity tests carried out on disturbed samples. Such tests are necessary if the soil is being considered as a possible construction material and the information obtained from them must be included in any description of the soil.

      Further information regarding the colour of a soil, the texture of its particles, etc., can be obtained in the laboratory from disturbed soil samples but a full description of a soil must include its in situ, as well as its laboratory, characteristics. Some of this latter information can be found in the laboratory from undisturbed samples of the soil collected for other purposes, such as strength or permeability tests, but usually not until after the tests have taken place and the samples can then be split open for proper examination. Other relevant information such as bedding details, gravel particle shapes (e.g. angular, rounded, elongated), clay consistency (e.g. soft, firm, stiff) and site observations can also be included in the soil's description.

1.7 Soil properties

From the foregoing it is seen that soil consists of a mass of solid particles separated by spaces, or voids. A cross‐section through a granular soil may have an appearance similar to that shown in Fig. 1.11a.

      In order to study the properties of such a soil mass, it is advantageous to adopt an idealised form of the diagram as shown in Fig. 1.11b. The soil mass has a total volume V and a volume of solid particles equal to V_s. The volume of the voids, V_v is obviously equal to V − V_s.

      1.7.1 Void ratio and porosity

      From a study of Fig. 1.11, the following may be defined:

      Void ratio, e

      (1.7)

      Porosity, n

      (1.8)

Fig. 1.11 Cross‐section through a granular soil. (a) Actual form. (b) Idealised form.

      1.7.2 Degree of saturation, S_r

The voids of a soil may be filled with air or water or both. If only air is present the soil is dry, whereas if only water is present the soil is saturated. When both air and water are present the soil is said to be partially saturated. These three conditions are represented in Fig. 1.12a–c.

      The degree of saturation is simply:

      (1.9)

      (usually expressed as a percentage)

      For a dry soil, S_r = 0

      For a saturated soil, S_r = 1.0

      1.7.3 Particle density, ρ_s and specific gravity, G_s

      The specific gravity of a material is the ratio of the weight or mass of a volume of the material to the weight or mass of an equal volume of water. In soil mechanics the most important specific gravity is that of the actual soil grains and is given the symbol G_s.

      From the above definition it is seen that for a soil sample with volume of solids, V_s, mass of solids, M_s and weight of solids, W_s,

      (1.10)

      where ρ_w is the density of water (=1.0 Mg/m³ at 20 °C) and γ_w is the unit weight of water (=9.81 kN/m³).

      The density of the particles ρ_s is defined as:

Fig. 1.12 Water and air contents in a soil. (a) Dry soil. (b) Saturated soil. (c) Partially saturated soil.

      therefore,

      If ρ_s is measured in units of Mg/m³ and the water temperature is assumed to be 20 °C, it follows that ρ_s and G_s are numerically equal. G_s, however is dimensionless whereas ρ_s has the units of density, Mg/m³.

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