Hydro-Mechanical Properties of an Unsaturated Frictional Material

58 CHAPTER 2. STATE OF THE ART
method. The authors examined the parameter identification procedure on two hy-
pothetical soils (parameters correspond approximately to those of a sandy loam and
a clay loam) in combination with van Genuchten’s model. Unique identifications only
were performed when the applied suction increment was large enough to yield a low
final water content under an adequate period of time and thus a broad range in water
contents was examined. Initial parameter estimates close to their true values as well
as precise experimental data are necessary for the approach. In a second paper Kool
et al. (1985b) used data of cumulative outflow versus time from one-step outflow ex-
periment for calculation of the van Genuchten parameters by inverse simulation of the
flow problem. Experiments were carried out for drainage on 4 different soils (sand loam,
silt loam, sandy clay loam, clay) by applying a pressure step of 100 kPa to the initially
saturated specimens. 3 methods were compared: i) a method, where the cumulative
outflow with time was used, ii) a method, that includes additionally measured data of
the water content at a pressure head of 1500 kPa and iii) a method, which uses data
from conventional equilibrium experiments. Predicted and measured cumulative out-
flow data, diffusivity as well as predicted and measured soil-water characteristic curves
were compared. Using the 2 nd method yields the best fit in cumulative outflow data and
hydraulic diffusivity. While the 3 rd method gave the best description of the soil-water
characteristic curve. Although good results were achieved for all methods, pressure head
data were required to get unique solutions.
Similar to the work of Kool et al. (1985b) the work by Toorman & Wierenga (1992) is
concerned with the inverse approach for estimating unsaturated flow parameters from
one-step method on a synthetic soil. Response surfaces of van Genuchten parameters
(i.e. one of the three parameters was kept constant during optimization procedure,
while the other two were changed) were plotted to show the non-uniqueness and ill-
posedness of the solutions. Non-uniqueness was proven by wide valleys for instance in
the α − n plane indicating that there are many solutions for providing reasonable good
predictions of one outflow problem. To improve the inverse approach additionally water
content as well as pressure head measurements were carried out during the one-step
outflow test. However, measurements of water content did not improve the parameter
estimates. The authors found, that combining outflow data and pressure head data
appears most promising for estimating unsaturated flow parameters.
Dam et al. (1992) performed an extensive work on the reliability of inverse estimation
using one-step outflow data on loess, silt loam, sand and loam. They used different
parameter combinations and input data to estimate the unsaturated hydraulic con-
ductivity functions of the investigated soils and compared the results with unsaturated
hydraulic conductivity function measurements. The results showed, that it is insufficient
to carry out inverse simulation only on cumulative outflow data. Independent data of