Hydro-Mechanical Properties of an Unsaturated Frictional Material

32 CHAPTER 2. STATE OF THE ART
by Wildenschild et al. (1997), Plagge et al. (1997), Schultze et al. (1997), Wildenschild et al.
(2001). Plagge et al. (1997) investigated both the influence of flow rate and the influence
of boundary condition on the hydraulic conductivity function. They found an increase in
unsaturated hydraulic conductivity with increasing water potential gradients. Wildenschild
et al. (2001) analyzed the flow rate dependency of unsaturated hydraulic characteristics us-
ing steady-state and transient flow. One-step and multistep experiments were performed
on a sandy soil and a loamy soil. Several processes causing the observed phenomenon were
suggested and discussed on the basis of their experimental and theoretical results.
Theoretically the dynamic effect was investigated first by Stauffer (1977), who analyzed
the rate of change of saturation and the rate of change of suction for both transient state and
steady state experiments. Hassanizadeh & Gray (1990, 1993), Hassanizadeh et al. (2002) de-
veloped and applied successfully an approach including macroscopic balance laws, constitutive
relationships for interfacial as well as phase properties of the porous medium. They extended
the relationship between suction and saturation as proposed by e.g. Bear & Verruijt (1987).
As a result a model describing two-phase flow in a porous medium based on thermodynamic
theory has been proposed. Detailed investigation on dynamic effects in porous media is given
by Manthey (2006).
Determination of Hydraulic Conductivity Function
The unsaturated hydraulic conductivity can be measured directly and several indirect methods
are also available to determine this relationship. The hydraulic conductivity of an unsaturated
soil can vary with respect to soil suction by several orders of magnitude and is time-consuming
to measure. Direct methods for measuring the unsaturated hydraulic conductivity can be
generally classified as either laboratory techniques or field techniques and can be performed
under steady state condition or transient state condition. Outflow methods are not only used
to measure the soil-water characteristic curve of a soil but also to measure its unsaturated
hydraulic conductivity. These methods are already explained above. Detailed review of
outflow methods used for measuring hydraulic conductivity was given by Benson & Gribb
(1997).
- Direct Method performed under Steady State Condition:
Under steady state conditions, a constant hydraulic head or a constant flow rate is
imposed to the specimen under predetermined matric suction. The most commonly used
laboratory technique is the constant head method, where a constant head is maintained
across a soil specimen and the corresponding flow rate through the specimen is measured.
Numerous authors described the constant head method (Corey 1957, Klute 1972, Klute
& Dirksen 1986a, Barden & Pavlakis 1971, Huang, Fredlund & Barbour 1998). The
constant flow method is a laboratory technique, where a flow rate (hydraulic gradient)