(b) Saturated soil


(c) Partially saturated soil

Fig. 1.10 Water and air contents in a soil.

The degree of saturation is simply: Volume of water Vw Volume of voids Vv

(usually expressed as a percentage)

1.7.3 Particle specific gravity (Gs)

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 Gs.

From the above definition it is seen that, for a soil sample with volume of solids Vs and weight of solids Ws,

Gs = ^ Vs'yw where 7W = weight of water and, if the sample has a mass of solids Ms, Ms

VsPw Vs'Yw The density of the particles ps is defined as: Ms

Vs therefore,

BS 1377: Part 2 specifies methods of test for determining the particle density. For fine, medium or coarse soils the Standard specifies the use of a one litre gas jar fitted with a rubber bung and a mechanical shaking apparatus which can rotate the gas jar, end over end, at some 50rpm (Fig. 1.11).


-Rubber bung

, Soil Cover plate

Electric end over end shaker

Fig. 1.11 Determination of particle density.

The test consists briefly of placing oven dried soil (approximately 200 g for a fine soil and 400 g for a medium or coarse soil) into the gas jar along with some 500 ml of water at room temperature. The jar is sealed with the bung and shaken, first by hand and then in the machine for some 20 to 30 minutes.

From various weighings that are made the specific gravity of the soil can be calculated. (See Example 1.6.)

If ps is measured in units of Mg/m3 and the water temperature is assumed to be 20°C, it follows that ps and Gs are numerically equal. Gs is dimensionless.

Soil contains particles of different minerals with consequently different specific gravities: Gs therefore represents an average value for the particles.

Generally sands have an average value of Gs = 2.65 and clays an average value of 2.75. The particle specific gravity of organic soils can vary considerably. An organic clay can have a Gs value of about 2.60 whereas a bog peat can have a value as low as 1.3.

For coal spoil heaps Gs can vary from about 2.0 for an unburnt shale with a high coal content, to about 2.7 for a burnt shale.

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