Hydro-Mechanical Properties of an Unsaturated Frictional Material

22 CHAPTER 2. STATE OF THE ART
DrainageImbibition
b) c) d) a)
Figure 2.11: Capillary rise in tubes different in diameter and drainage as well as imbibition
process in tubes with varying diameters (Haines 1930, Miller & Miller 1988)
- Snap-off effect (Chatzis & Dullien 1983)
The inclusion of the non-wetting phase (i.e. gas) during the wetting cycle is known as
snap-off effect. In Fig. 2.12 can be seen that it depends on the diameter of the pore
bodies and the pore necks.
- Effect of by-passing (Chatzis & Dullien 1983)
Consider the combining of two capillary tubes different in diameter. The system is fluid
saturated and starts to drain. Then the non-wetting phase (gas) is replacing the wetting
phase (fluid). Due to less capillarity the larger tube is first filled with gas. In the smaller
tube fluid is still retained and separated from the fluid continuum causing entrapped
fluid (see Fig. 2.13). Vice versa during imbibition process entrapment of gas occurs.
Yang, Rahardjo, Leong & Fredlund (2004b) discussed the hysteretic behavior and correlated
it to grain size parameters (e.g. grain size parameter d10).
The relevance of the hysteresis mechanism is still a topic of discussion. However, in the
context of coarse-grained granular material, such as sand used in this study, hysteresis is most
phase
Figure 2.12: Influence of the pore geometry on the inclusion of a fluid caused by snap-off
effect (Chatzis and Dullien 1983)
neck body phase Non-wetting Wetting phase
Non-wetting phase Wetting Pore Pore