Geotechnical Engineering

DHARM

134 GEOTECHNICAL ENGINEERING

Degree of Saturation

The higher the degree of saturation, the higher the permeability. In the case of certain sands
the permeability may increase three-fold when the degree of saturation increases from 80% to
100%.

Presence of Entrapped Air and Other Foreign Matter

Entrapped air has pronounced effect on permeability. It reduces the permeability of a soil.
Organic foreign matter also has the tendency to move towards flow channels and choke them,
thus decreasing the permeability. Natural soil deposits in the field may have some entrapped
air or gas for several reasons. In the laboratory, air-free distilled water may be used a vacuum
applied to achieve a high degree of saturation. However, this may not lead to a realistic esti-
mate of the permeability of a natural soil deposit.

The importance of duplicating or simulating field conditions is emphasised by the pre-
ceding discussion on the factors affecting permeability, when the aim is to determine field
permeability in the laboratory.

5.7 VALUES OF PERMEABILITY

Table 5.1 Typical values of permeability (S.B. Sehgal, 1967)

Soil description Coefficient of permeability mm/s Degree of permeability

(After Terzaghi and Peck, 1948)

Coarse gravel Greater than 1 High

Fine gravel—fine sand 1 to 10–2 Medium

Silt-sand admixtures,

loose silt, rock flour, and

loess 10 –2 to 10–4 Low

Dense silt, clay-slit

admixtures,

non-homogeneous clays 10 –4 to 10–6 Very low

Homogeneous clays Less than 10–6 Almost impervious

5.8 PERMEABILITY OF LAYERED SOILS

Natural soil deposits may exhibit stratification. Each layer may have its own coefficient of
permeability, assuming it to be homogeneous. The ‘average permeability’ of the entire deposit
will depend upon the direction of flow in relation to the orientation the bedding planes.

Two cases will be considered—the first one with flow perpendicular to the bedding planes
and the next with flow parallel to the bedding planes.

Flow Perpendicular to the Bedding Planes

Let the flow be perpendicular to the bedding planes as shown in Fig. 5.10.