Hydro-Mechanical Properties of an Unsaturated Frictional Material

210 CHAPTER 11. SUMMARY AND OUTLOOK
for specimens under loose condition. Two different methods of sample preparation related to
variation of initial conditions are described. It was found that the collapse potential is very
small for the stress paths analyzed. Investigations on preconsolidation pressure lead to the
conclusion, that no influence of suction on the soil history was observed.
To underline the importance of unsaturated soil behavior, bearing capacity of surface
model footing was carried out for saturated and unsaturated specimens. A meaningful influ-
ence of suction was found to the bearing capacity of strip foundation. The bearing capacity
significantly improved for unsaturated specimen. The bearing capacity of the unsaturated
sand specimen was found to be approximately 2.5 to 4 times higher than for the saturated
specimen (S = 0 and S = 1). Similar to the influence of suction on the stiffness behavior of
sand, the bearing capacity increases with increasing suction and again decreases when reach-
ing dry condition. Using the equation proposed by Vanapalli & Mohamed (2007) bearing
capacity was predicted. Measured and predicted values are in good agreement, but further
experimental and theoretical investigations are necessary.
Calculations of unsaturated bearing capacity as well as unsaturated hydraulic conductiv-
ity showed, that the soil-water characteristic curve is an important tool when dealing with
unsaturated soil behavior. It is the basic relation used for prediction of unsaturated hydro-
mechancial behavior.
11.4 Numerical Simulation using MUFTE-UG
A transient state test was conducted on loose Hostun sand specimen in the column testing
device I for several alternating drainage and imbibition events. TDR and tensiometer sensors
are used to measure saturation and pore-water pressure at different depths over the time.
The derived soil-water characteristic curves demonstrate significant hysteresis and air
trapping effects. Numerical simulations of the transient state test are carried out using the
two-phase flow module of the MUFTE-UG simulator. The simulations incorporate informa-
tion on the soil-water characteristic curve determined directly on the column. The scaling
hysteresis concept from Parker & Lenhard (1987) was used, that accounts for entrapped
non-wetting phase saturation.
The simulation and experimental results were compared in terms of saturation versus time
as well as pore-water pressure versus time for the entire sequence of alternating drainage and
imbibition conditions. The comparison showed the necessity of the inclusion of a hysteresis
concept in the numerical simulator, especially related to the effect of air trapping and the for-
mation of residual air saturations during imbibition. Using the hysteresis concept from Parker
& Lenhard (1987), only small deviations in the predicted and measured suction-water content
curves for the second imbibition have been observed. Despite this limitation, it can be stated
that a positive prediction of the flow process in the transient state test has been obtained
with a conceptually simple scaling approach for hysteresis.