The permeability of soils is a material parameter that relates the rate of water flow to the hydraulic gradient in the soil and, therefore, determines the material’s suitability for drainage layers. An embankment usually consists of compacted materials. The compaction often results in anisotropy such that the vertical and the horizontal permeability properties are not equal. For road construction layers, water movements below the ground water table are almost entirely horizontal and thus it is the horizontal permeability that should be measured. Above the groundwater table in the unsaturated zone the movement of water is much more complex, involving vertical as well as horizontal components depending on material parameters such as temperature, water content and matric suction.
Some typical values of the coefficient of permeability for saturated soils are shown in Table 3.1.
The permeability of soils can either be estimated in saturated conditions or for partially saturated conditions. If the permeability of soils is estimated from saturated
|
Soil |
Coefficient of perm. K (m/s) |
Degree of permeability |
|
Gravel |
> 10-3 |
Very high |
|
Sandy gravel, clean sand, fine sand |
10-3 > K > 10-5 |
High to medium |
|
Sand, dirty sand, silty sand |
10-5 > K > 10-7 |
Low |
|
Silt, silty clay |
10-7 > K > 10-9 |
Very low |
|
Clay |
< 10-9 |
Virtually impermeable |
|
Table 3.1 Typical values of the coefficient of permeability of saturated soils |
samples and the unsaturated permeability is sought, the Soil Water Characteristic Curve (SWCC — see Chapter 2) can be used to predict the permeability for a specified volumetric water content (Fredlund & Rahardjo, 1993).
