Hydro-Mechanical Properties of an Unsaturated Frictional Material

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CHAPTER 7. ANALYSIS AND INTERPRETATION OF THE EXPERIMENTAL
RESULTS
as imbibition path. The imbibition curves start with an unsaturated hydraulic conductivity
close to a zero value. With a decrease in the suction, the unsaturated hydraulic conductivity
increases. The unsaturated hydraulic conductivity function increases first for the layers at
the bottom of the (loose and dense) sand specimens where the voids are faster filled with
water. In comparison to statistical models by Mualem (1976) and Fredlund et al. (1994),
the scanning imbibition hydraulic conductivity curves derived using Childs and Collis George
(1953) model begin at significant larger hydraulic conductivity values.
In a second analysis (i.e. the instantaneous profile method) the profiles of hydraulic head
gradient and water content are used to calculate the unsaturated hydraulic conductivity func-
tion directly for the drainage process. Figure 7.14 shows the unsaturated hydraulic conduc-
tivity for loose and dense specimens (1st drainage curve with a flow rate of 30 ml/min and
2nd drainage curve with a flow rate of 100 ml/min) calculated from both the instantaneous
profile method (i.e. direct method) and the statistical models (i.e. indirect method). The
instantaneous profile method is assuming that the water phase is continuous. The method is
applicable in the saturated zone and in the unsaturated zone where water transport occurs
in the wetting phase. The instantaneous profile method should not be applied in the residual
zone, where water is believed to be exclusively transported in the vapor phase (non continuous
water phase). The instantaneous profile method for the loose specimens shows unsaturated
hydraulic conductivities in a range of approximately 0.3 × 10 −4 to 0.007 × 10 −4 m/s for a
suction of 1.6 to 2.0 kPa respectively. For the dense specimen the hydraulic conductivities
are in the range of approximately 0.4 × 10 −4 to 0.005 × 10 −4 m/s for suctions ranging from
1.7 to 2.2 kPa respectively. Similar to the results calculated when using the indirect method,
the unsaturated hydraulic conductivity is decreasing rapidly along a narrow range of suctions.
Results derived using statistical model by Mualem (1976) and Fredlund et al. (1994) are close
to the results derived from direct method.
Unsaturated hydraulic conductivity functions resulting from modified pressure plate appa-
ratus (steady state tests) and sand column I (transient state tests) are compared in Fig. 7.15
for loose specimen and in Fig. 7.16 for dense specimen. The results are given for the indirect
as well as for the direct approach. The soil-water characteristic curve is strongly related to the
unsaturated hydraulic conductivity function and therefore their behavior is similar (including
the effect of hysteresis). The unsaturated hydraulic conductivity functions are close to each
other during the drainage path. The unsaturated hydraulic conductivity functions along the
scanning pathes are located within the main drainage and imbibition loop. This behavior is
consistent with drainage and imbibition soil-water characteristic curves.
The following observations are made with regard to the effect of void ratio on the unsat-
urated hydraulic conductivity function. For low suction values the hydraulic conductivity for
the loose specimen is higher due to the larger cross-sectional area available for water flow.
When reaching the air-entry value, the unsaturated hydraulic conductivity for the loose speci-
men becomes less than the one for the dense specimen. The larger pores of the loose specimen