Hydro-Mechanical Properties of an Unsaturated Frictional Material

26 CHAPTER 2. STATE OF THE ART
drainage process, where following the soil-water characteristic curve the amount of water filled
pores is greater, than during imbibition process.
The effect of hysteresis in the relationship between unsaturated hydraulic conductivity
and volumetric water content or saturation is supposed to be minor. Experimental results
of drainage and imbibition hydraulic conductivity versus water content k(θ) or hydraulic
conductivity versus saturation k(S) can be grouped in two converse categories:
- Hysteresis:
Youngs (1964), Staple (1965) found differences between imbibition path and drainage
path for slate dust and silt loam. Poulovassilis (1969) studied sand giving significant
hysteresis in the intermediate range of volumetric water content but vanishing for both
low and high values of volumetric water content. For high degrees of saturation in a
clay Pavlakis & Barden (1972) found differences up to a factor of 10. Dane & Wierenga
(1975) found no hysteresis for a sand but significant hysteresis for intermediate and low
ranges of volumetric water content for a clay loam. Based on the independent domain
theory it is assumed, that the pore water is contained in pores not common during
drainage and imbibition process. While during drainage process the water volume is
contained in pores with larger radii, the radii of the pores containing the water volume
at imbibition path are smaller (Poulovassilis 1969).
- No significant hysteresis:
Most of the studies which concluded no significant effect of hysteresis on hydraulic
conductivity function are related to uniform granular materials such as sand (Klute
et al. 1964, Collis-George & Rosenthal 1966, Talsma 1970, Poulovassilis 1970, Topp
1969) and glass beads (Topp & Miller 1966). Little evidence is available for clay and
silt loam (Nielsen & Biggar 1961, Topp 1971a). Under the assumption that no volume
changes occur in the tested specimen the effect of hysteresis is supposed to be negligible.
Differences only can be found between initial drainage and main drainage curve due
to consolidation associated with the applied suction (Nielsen & Biggar 1961, Enderby
1955).
When analyzing the results from the research literature, the following aspects are noteworthy.
Most of the studies are missing precise information on volume change behavior during drying
and wetting cycles. Derivation of suction-hydraulic conductivity functions is performed using
different methodologies, including both direct and indirect procedures. Experimental devices
include different scales ranging from oedometer type cells up to soil column devices. Applied
sensors show different precision depending on the installation procedure and sample geometry.
Hysteresis in the suction-hydraulic conductivity function seems to be dependent on the type
of soil and the range of volumetric water content: Clay soils seem to have a tendency for
hysteresis, particularly in the intermediate range of volumetric water contents. Most of the