Civil Engineering Reference
In-Depth Information
Fig. 2.15
Example wetting and drying water retention curves.
2.13.4 The water retention curve
If a slight suction is applied to a saturated soil, no net outflow of water from the pores is caused. However,
as the suction is increased, water starts to flow out of the larger pores within the soil matrix. As the suction
is increased further, more water flows from the smaller pores until at some limit, corresponding to a very
high suction, only the very narrow pores contain water. Additionally, the thickness of the adsorbed water
envelopes around the soil particles reduces as the suction increases. Increasing suction is thus associated
with decreasing soil wetness or water content. The amount of water remaining in the soil is a function of
the pore sizes and volumes and hence a function of the matric suction. This function can be determined
experimentally and may be represented graphically as the
water retention curve
, such as the examples
shown in Fig.
2.15.
The amount of water in the pores for a particular value of suction will depend on whether the soil is
wetting or drying. This gives rise to the phenomenon known as hysteresis, and the shape of the water
retention curve for each process is shown in Fig.
2.15.
A full descriptive text on water retention curves
and hysteresis is given by Fredlund, Rahardo and Fredlund (
2012
).
2.13.5 Measurement of soil suction
From a geotechnical point of view there are two components of soil suction as follows:
(1)
Matric suction
:
that part of the water retention energy created by the soil matrix.
(2)
Osmotic suction
:
that part of the water retention energy created by the presence of dissolved salts
in the soil water.
It should be noted that these two forms of soil suction are completely independent and have no effect
on each other.
The total suction exhibited by a soil is obviously the summation of the matric and the osmotic
suctions.
If a soil is granular and free of salt there is no osmotic suction and the matric and total suctions are
equal. However, clays contain salts and these salts cause a reduction in the vapour pressure. This results
in an increase in the total suction, and this increase is the energy needed to transfer water into the vapour
phase (i.e. the osmotic suction).
