Hydro-Mechanical Properties of an Unsaturated Frictional Material

2.3. HYDRAULIC FUNCTIONS 29
Determination of Soil-Water Characteristic Curve
Commonly the soil-water characteristic curve is determined by relating the suction to the
water content that are measured in cells under equilibrium condition and fitting a paramet-
ric suction-water content function to these data pairs. To achieve equilibrium condition in
a soil is extremely time consuming and laborious. Thus flow experiments were developed
and in combination with inverse modeling of experimental data the soil-water characteristic
curve was estimated among others. Several flow experiments are available for determining
hydraulic properties of an unsaturated soil. As proposed by Schultze et al. (1997) in this work
flow experiments are classified into: multistep flow experiments, one-step flow experiments
and continuous flow experiments. These experiments are performed either under steady-state
condition (equilibrium) or transient state condition (non-equilibrium) (Topp et al. 1967).
State of the art in inverse modeling of one-step, multistep and continuous flow experiments
is given in Durner et al. (1999). An overview of several equipments used for determination of
soil-water characteristic curve is given in Tab. 2.2. Some of the listed equipment is explained
in detail in Chapter 2.4.
- Multistep Method:
The multistep method (Gardner 1956, Klute 1986, van Dam et al. 1990) is used to per-
form steady-state experiments, where several suction steps are imposed to the specimen
and the next suction increment is not applied before reaching equilibrium condition
in the soil, i.e. no inflow (during imbibition process) or outflow (during drainage pro-
cess) of water is measured or observed. Multistep outflow experiments were performed
for instance by Eching & Hopmans (1993), Dam et al. (1994), Schultze et al. (1997),
Wildenschild et al. (2001), Fujimaki & Inoue (2003). Different equipment devices are
available to determine the suction-water content drainage/ imbibition relationship un-
der steady-state conditions. The relationship can be directly determined using a suction
cell apparatus (Haines 1930) or pressure cell apparatus (Richards 1941, 1947). Tempe
cell for application of matric suctions up to 100 kPa (Reginato & van Bavel 1962),
volumetric pressure plate for application of matric suctions up to 200 kPa and large
pressure plate for application of 500 and 1500 kPa (Fredlund & Rahardjo 1993b) were
developed. Pressure plate apparatus was developed also by Pham et al. (2004). The
change of water volume or mass of water in the specimens is measured during testing
procedure carried out in these cells. After finishing the test, the specimen is dismantled
and the gravimetric water content is measured by oven drying method. Using back-
calculation method the water content is related to each imposed suction step. By using
the axis-translation technique (Hilf 1956) suctions are applied to the specimen. The
approximate measurement range for the axis-translation technique is 0 kPa to 1500 kPa
depending on the air-entry value of the ceramic disk used. Additionally some modified
cells are equipped with tensiometers or pressure transducers for measuring the pressure