Hydro-Mechanical Properties of an Unsaturated Frictional Material

11.5. FUTURE WORK 211
In a further investigation an additional soil-water characteristic curve was determined from
the steady state experiment on Hostun sand specimens and consequently used for a numerical
simulation. This is a common procedure in applications involving unsaturated flow. However,
the soil-water characteristic curve determined with this approach demonstrates absence of air
residual saturation in imbibition, in contrast to the transient state test where a strong trapping
effect was observed. The numerical simulation carried out here incorporates the Parker and
Lenhard hysteresis. However, the simulation cannot correctly predict the flow process in the
transient state experiment due to the inappropriate input suction-water content curve.
It is demonstrated that the implementation of a simple scaling approach for hysteresis
can provide a good prediction of hysteretic unsaturated flow including phase trapping effects.
However, this needs to be combined with reliable information of the input soil-water charac-
teristic curve parameters that is commonly measured in the laboratory. It is shown that this
is possible by using direct saturation and pressure measurements from the sand column test I.
The investigation showed that a single pair of TDR and tensiometer sensors in combination
with the implemented scaling concept for hysteresis provides good prediction of the flow in
the entire 0.54 m high porous medium column, for all drainage and imbibition events. If TDR
and tensiometer data were not available, the traditional steady state method would be used
for the determination of the suction water content relationship. Although the specimens used
in the steady state outflow/ inflow experiments correspond to the same spatial scale as the
TDR-T pairs, they provide different information on the amount of air trapping and conse-
quently a misleading prediction of flow in the transient state test. This observation suggests
that the amount of phase trapping strongly depends on the methodology used and the time
scales linked to it. When predicting hysteretic flow by means of numerical simulations, it is
therefore necessary to use hysteretic suction-water content curves that are determined under
conditions (i.e. fast or slow imbibition) similar to those of the application they are purposed
for.
11.5 Future Work
Due to the reason that the majority of civil engineering problems are related to unsaturated
soils, there is a strong motivation to understand their hydro-mechancial behavior and the im-
pact of suction vice versa water content on the stress-strain, shear strength and flow behavior.
To support the investigations carried out in the present study and to complete the present
study following tests are proposed to carry out:
- Performance of triaxial tests for investigation of the influence of suction on the shear
strength.
- Performance of biaxial tests for investigation of the influence of suction on shear
strength.