ENVIRONMENTAL ENGINEERING - Introduction to Soft Soil Geotechnique: Content, Context and Application - page 227

cannot be sharply specified due to saturation, capillarity, and pore structure of the
soil. The groundwater table usually follows the soil surface relief. Capillarity in
sands is in the order of centimetres. In fine soils (clay) it can be metres. The
groundwater table forms the border below which normally no plant roots grow
(lack of oxygen). Groundwater extends in the subsoil in different layers, since the
pore structure in coarse and fine soils are interconnected. The flow of water in
various soil layers may differ significantly (permeability).
For a practical characterisation of the presence of water in the soil, i.e. the
humidity, the pF -curve is used. It is the logarithm of the suction-tension (in mBar),
i.e. a measure of the force of binding of water to the soil. In Fig 14.1a, some pF -
curves are presented as a function of the saturation

for various soil types.

pF pF pF

7 7 7

5 5 5

drying drying drying

withering point withering point withering point

3 3 3

field capacity field capacity field capacity

wetting wetting wetting

fine sand fine sand fine sand

loam loam loam

peat peat peat

1 1 1

clay clay clay

sand sand sand

0.4 0.4 0.4 0.8 0.8 0.8

(a) (b)
Figure 14.1 pF-curves

Two characteristic values are distinguished: the field capacity and the withering
point, in between which the soil provides humid for plants. The field capacity is
related to the absorption capacity of the soil for heavy rain. Usually at pF = 3
(equal to 1 Bar) all capillary water has vanished. The field capacity varies normally
between 1.8 and 2.5. The withering point is around pF = 4.2 (equal to 16 Bar),
where plant roots cannot obtain water anymore, although some arid and alpine
plants species can exert suctions up to 60 Bar. The behaviour for wetting
(adsorption) or drying (desorption) is hysteretic (see Fig 14.1b). It causes a larger
pF value at drying than wetting for the same saturation.

Gas phase
The most common gas in soil is air (oxygen, nitrogen and carbon dioxide). The
amount of gas is determined by the porosity and the humidity (saturation). The
composition of the gas in soil is controlled by biological activities and the
atmospheric air composition. Usually the CO 2 concentration in soil is higher than
in the atmosphere, and the opposite holds for oxygen. A diffusion process triggered
by the concentration gradient will cause transport from and to the soil. Diffusion in