Hydro-Mechanical Properties of an Unsaturated Frictional Material

7.2. SOIL-WATER CHARACTERISTIC CURVE 145
Influence of net stress
In contrast to cohesive soils the influence of net stress on the shape of the soil-water charac-
teristic curve is small for Hostun sand as can be seen in Fig. 6.4. Changes in void ratio are
negligible small during the whole experimental procedure. After saturation process and after
application of net stress main changes in void ratio take place. While wetting the sand spec-
imen with water small changes in settlements were measured and thus no significant changes
in void ratio were observed.
7.2.3 Transient State Test Results
The experimental results of sand column tests I and the best curve fits respectively are given
in in Appendix B in Figs. B.5 to B.8 for loose and dense specimens. Soil-water characteristic
curves θ(ψ) and S(ψ) are presented for the tests performed under transient state condition
with a flow rate of 30 ml/min as well as 100 ml/min. The saturated volumetric water content
is θs = 46% for the loose specimen and θs = 39% for the dense specimen under initially
saturated condition. Caused by the occluded air after first imbibition process the saturated
volumetric water content reduces to θ ′ s = 39% for the loose specimen and θ ′ s = 35% for the
dense specimen. For the experimental results of the transient state test an air-entry value
of approximately ψaev = 1.6 kPa was found for the loose specimen and ψaev = 2.1 kPa for
the dense specimen. Up to the air-entry value the soil-water characteristic curve is located
in the saturated zone. After reaching the air-entry value, the water content decreases rapidly
for both sand specimens and reaches the transition zone. The transition zone is between
ψaev = 1.6 kPa to ψr = 2.7 kPa (θr = 5%) for the loose specimen and between ψaev = 2.1 kPa
to ψr = 3.0 kPa (θr = 6%) for the dense specimen. The residual zone starts at a relatively
low suction value in the drainage cycle for both sand specimens.
In Fig. B.9 and B.10 the experimental results and the curve fits of the transient state
tests performed in the column testing device II are shown. A saturated volumetric water
content of θs = 47% was measured for the loose specimen and θs = 41% for the dense
one. The volumetric water content is not changing till reaching the air-entry value, that is
ψaev = 1.7 kPa for the loose specimen and ψaev = 1.9 kPa for the dense specimen (saturated
zone). When passing the air-entry value the volumetric water content is rapidly decreasing
and the soil reaches unsaturated condition. The transition zone extends from the air-entry
value up to residual suction and the residual volumetric water content of ψr = 3.6 and θr = 6%
for the loose specimen and ψr = 3.8 and θr = 8% for the dense specimen. Similar to all derived
results the residual zone begins at low suction value.