Hydro-Mechanical Properties of an Unsaturated Frictional Material

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CHAPTER 7. ANALYSIS AND INTERPRETATION OF THE EXPERIMENTAL
RESULTS
7.2.2 Steady State Test Results
For interpretation and comparison of the results typical soil-water characteristic curve param-
eters as the air-entry value, the saturated volumetric water content and the residual volumetric
water content with the corresponding residual suction were estimated. According to Fredlund
and Xing (1994) these parameters were graphically determined.
For the soil-water characteristic curve determined in the modified pressure plate apparatus
(see also Fig B.1 and B.2 for loose and dense specimen in Appendix B) an air-entry value of
approximately ψaev = 1.4 kPa was found for the loose specimen. This value is smaller then
that for the dense specimen which is about ψaev = 1.9 kPa. After reaching the air-entry value
the water content decreases along a narrow range of suction for both sand specimens. The
transition zone is between ψaev = 1.4 kPa to ψr = 2.8 kPa (θr = 5%) for the loose specimen
and between ψaev = 1.9 kPa to ψr = 3.3 kPa (θr = 2%) for the dense specimen. The residual
zone starts at a relatively low suction value for the drainage cycle for the dense specimens
and the loose specimens. Along the imbibition path there is no significant change in water
content measured in a range from approximately 50.0 kPa to 3.0 kPa. The transition zone for
loose specimen starts at a water-entry value ψwev = 2.8 kPa and for the dense specimen at a
water-entry value ψwev = 3.3 kPa. For the loose specimen the transition zone extends up to
0.27 kPa and up to 0.5 kPa for the dense specimen. The saturated zone falls in a relatively
narrow range for both dense and loose specimens.
The experimental results of the steady state column tests I and best fitted curves are
shown for loose and dense specimens in Fig. B.3 and B.4. The specimens are initially water
saturated (θs,loose = 46%, θs,dense = 40%). After imbibition process both specimen do not
reach complete saturation due to occlusion of air in the specimens voids. The saturated
volumetric water content after imbibition process is θ ′ s = 39% for the loose specimen and
θ ′ s = 37% for the dense specimen. While the loose specimen starts to drain approximately
at an air-entry value ψaev = 1.2 kPa the dense specimen starts to drain at ψaev = 1.9 kPa
(saturated zone). The residual suction is equal to ψr = 2.8 kPa with a corresponding residual
volumetric water content θr = 4% for the loose and ψr = 3.2 kPa and θr = 6% for the dense
specimen. In a narrow range of suction the volumetric water content is decreasing beginning
from the air-entry value to the residual suction (transition zone). The residual zone starts at
low suction value of ψr = 2.8, θr = 4% for the loose and ψr = 3.2, θr = 6% for the dense
specimens. Influenced by occluded air after imbibition process apparent saturated volumetric
water content of θ ′
s = 39% for loose and θ ′
s = 37% for dense specimen were achieved.
Influence of void ratio
Loose and dense soil-water characteristic curves are compared in Fig 7.4, that includes the
main drainage (here the initial drainage curve is equal to the main drainage curve because
after imbibition process fully saturated specimen was derived again) and imbibition loop