Hydro-Mechanical Properties of an Unsaturated Frictional Material

7.4. UNSATURATED HYDRAULIC CONDUCTIVITY 153
Table 7.2: Constitutive parameters calibrated using the SWCC model by Pham et al. (2003)
Parameter
wu(%) 33.22
c(%) 1.24
bd/bw 64.57/0.83
dd/dw 6.29/3.27
7.4 Unsaturated Hydraulic Conductivity
For indirect calculation of unsaturated hydraulic conductivity statistical models (Eq. 2.20
to 2.22) introduced in Chapter 2 were used. As explained in detail in Chapter 2 the saturated
conductivity, a best fitted soil-water characteristic curve (in this case the best fit from Fredlund
& Xings (1994) model) as well as a statistical model are needed for indirect determination.
Direct the unsaturated hydraulic conductivity was derived from tensiometer and TDR sensor
measurements observed in the transient state sand column test I. However, the comparison
of the measured soil-water characteristic curves showed, that significant influences to their
shape is related to density and also flow path, but not to the flow condition (steady state
test, transient state test). Therefore exemplary drainage and imbibition results derived from
steady state modified pressure plate apparatus and transient state sand column tests I were
used for calculation of unsaturated hydraulic conductivity. Whereas the results from the
modified pressure plate apparatus were used for indirect method (statistical model) only, the
measurements observed from the sand column test I were used additionally for direct method
(instantaneous profile method).
Figure 7.12 gives the results of the unsaturated hydraulic conductivity functions for loose
and dense sand specimens derived from modified pressure plate apparatus. In the saturated
zone the hydraulic conductivity is only decreasing slightly till reaching the air-entry value.
After reaching the air-entry value (the point where air starts to enter the largest pores), the
unsaturated hydraulic conductivity decreases rapidly because the amount of water is dra-
matically decreasing due to the large and uniform pores of the Hostun sand. With increasing
suction the flow path where water can flow becomes more tortuous and as a result the unsatu-
rated hydraulic conductivity decreases. When reaching the residual zone where discontinuous
water phase is present only as a thin layer on the sand grains, the unsaturated hydraulic con-
ductivity tends to extremely low values and the remaining pore water is transported through
vapor phase. Similar to the drainage path, along the imbibition path no relevant change in the
unsaturated hydraulic conductivity could be observed in the residual zone. After reaching the
water-entry value (the point where water starts to enter the smallest pores of the sand sam-
ple) during imbibition path, the unsaturated hydraulic conductivity increases along a narrow