Geoscience Reference
In-Depth Information
Fig. 8.12 Illustration of hysteresis in capillary tubes with
expansions. The water levels fall to positions 1 if the
tubes are initially already filled with water when
inserted into the water bath; the water levels rise to
positions 2 if the tubes are initially empty when
inserted into the water.
1,2
1
1
2
2
Fig. 8.13 Illustration of hysteresis in an aggregate of
soil particles. The menisci indicated by 1
are obtained during drying, and those
indicated by 2 during wetting. All menisci
in the sketch have the same curvature, and
therefore the fluid pressure at each of these
interfaces is the same, even though the
water content, when the fluid is at position
1, differs from that at position 2.
1
1
1
2
2
2 22
been related to permeability by, for example, Childs and Collis-George (1950), Marshall
(1958), Mualem (1978) and many others (see Brutsaert, 1967, 1968a,b). The basic idea
of a pore size distribution of a soil obtained in this way clearly has its shortcomings. As
shown in Figure 8.4, the water-air interfaces, or menisci, tend to occur at the narrower
openings or “necks” of the pores. It is thus incorrect to assume that at a given suction
all pores with effective sizes larger than that given by Equation (8.5) are filled with air.
Nevertheless, in the past the concept has been useful for comparative purposes and in
order-of-magnitude estimates.
8.2.3
Hysteresis
The relationship between water content and capillary pressure exhibits hysteresis. This
means that this relationship depends on the sequence of events of wetting and drying, by
which the current water content of the soil is attained. It also means that single-valued soil
water characteristic curves, such as those shown in Figures 8.5 and 8.6, are applicable
only either for sustained drying events, or (but with a different shape) for sustained
wetting or infiltration events, but not for situations involving alternate wetting and drying.
The word hysteresis is derived from the ancient Greek word
´
, meaning slow,
lagging behind or delayed. Figure 8.12 shows that the water-air interface can be found
at different levels in capillary tubes with expansions, depending on the manner, in which
these tubes have been filled. Similarly, Figure 8.13 illustrates schematically how for the
same water pressure, with menisci occurring at pore necks of the same size, it is possible
to have a different water content, depending on whether the pressure is achieved by filling
the pores upward or by emptying them downward. In Figure 8.14 some examples are
υστερ
o
ς
Search WWH ::




Custom Search