SOIL AND GROUNDWATER - Introduction to Soft Soil Geotechnique: Content, Context and Application - page 51

distinguishes the unsaturated zone and the saturated zone (pores are completely
filled with water). The water pressure in the soil, the pore pressure, is affected by
atmospheric pressure, gravity and pore water flow. The theoretical surface where
the pore pressure equals the atmospheric pressure is called the phreatic surface or
free water table, above which is the so-called capillary zone (height h c ), a semi-
saturated zone supported by water-grain attraction (molecular adhesion). The
capillary zone in sand or peat is in the order of some decimetres, and in clays it can
be metres. In the unsaturated zone isolated areas of hanging saturated soil may
occur, so-called pendular pore water, with an independent pressure field. These
zones do not affect the coherent groundwater area, but may affect local soil
behaviour.
In geohydrology, formations (sand) that provide for groundwater flow are called
aquifers, layers (clay) that allow minor groundwater flow are called aquitards, and
impermeable formations (rock, salt) are called aquicludes. Natural processes, such
as roots, worms and leaching, cause soil formation changes. Hence, clays at the
surface may have a permeability equal to sand. Sands may suffer internal erosion
or internal migration of their finer particles. Suffosion or suffusion refers to the
process of extracting of fine soil components and increasing permeability.
Colmation is the opposite process where fines are blocking the pores and
permeability decreases.

Groundwater head
When placing an open-end pipe (piezometer) in the saturated zone at a certain
depth z , the groundwater will penetrate in the pipe up to a level where its weight
counterbalances the actual pore pressure u at the pipe tip. This rise expressed as
pressure head h p reads in formula

h p = u/ w (4.6)

The position of the pipe tip is measured from a certain reference and it is called
elevation head h e . The water level in the piezometer is measured from the same
reference and called potential head or total head h (often written as potential

).
During field measurement the reference is the ground surface. Here, the z -direction
is usually taken positive pointing down. Thus, the potential head is equal to the
elevation head h e minus the pressure head (Fig 4.1a), in formula

= h = h e + h p = z + u/ w (4.7)

Since the potential head contains information about local pore pressure and
includes stagnant hydrostatics 16 , it is suitable to detect with it groundwater flow
behaviour in terms of local pressure and flow. Lines (or surfaces in 3D) of equal

16 If the potential does not vary in z-direction, i.e. / z = 0, it follows ( z u/ w ) / z = 0, or
u =

c ). Here constant c is related to the position where u = 0, the groundwater table.
Thus, / z = 0 reflects hydrostatic pressure.

w ( z