Hydro-Mechanical Properties of an Unsaturated Frictional Material

60 CHAPTER 2. STATE OF THE ART
When using van Genuchtens model for inverse simulation of hydraulic unsaturated soil
properties (Eching et al. 1994, Dam et al. 1994, Zurmühl & Durner 1996) demonstrated
that the multistep method is superior to the one-step method.
- Inverse procedure using experimental results derived from the continuous flow method
Alternatively to the one step and multistep method Durner (1991) suggested the con-
tinuous flow method. A comparison of inverse modeling of one-step, multistep and
continuous flow method experiments by Durner et al. (1999) showed that the multi-
step method and the continuous flow method are the preferred methods to identify
unsaturated hydraulic parameters.
A free-form parameterization approach for estimation of soil hydraulic properties was sug-
gested by Bitterlich et al. (2004) to avoid often experienced problems of ill-posedness of
inverse problems. Mathematical and numerical modeling concepts for simulation of unsatu-
rated/multiphase flow were discussed in detail by Helmig (1997).
2.7 Volumetric Behavior of Partially Saturated Soils
In the review below it is focused on one-dimensional behavior of sand that is derived from
one dimensional compression and rebound tests performed in oedometer cell.
2.7.1 Stress-Strain Behavior
Several tests are available to measure the stress-strain behavior of a soil, e.g. the isotropic
compression test, the one-dimensional compression test, the triaxial test or the direct shear
test. In the present study the stress-strain behavior of unsaturated soil was investigated using
one-dimensional compression and rebound tests. This type of experiments are performed in
conventional oedometer cells. The test includes the application of stress to a soil specimen
along the vertical axis, while the strain in the horizontal direction is restricted. Typical
results are shown in Fig. 2.28. For determination of stress-strain behavior one-dimensional
compression and rebound test is often used, because it is simple to perform and the strain
condition in the soil specimen is approximately similar to the situation of the soil in civil
engineering problems as settlements of an embankment. The loading of a soil specimen can
be applied either under drained or undrained conditions. Under drained condition the pore-air
and pore-water pressure are allowed to drain during testing procedure while under undrained
condition the total stress is applied to the specimen resulting in an excess of pore-water and
pore-air pressure (Rahardjo & Fredlund 2003). Major problem when dealing with oedometer
is the friction between the soil sample and the oedometer ring, where shear forces develop. The
presence of side friction disturbs the one-dimensional state of strain and prevents the applied
axial force to reach to 100% the bottom part of the specimen. For minimizing this effect the