Hydro-Mechanical Properties of an Unsaturated Frictional Material

2.5. CONSTITUTIVE MODELS FOR HYDRAULIC FUNCTIONS 39
which is calibrated (Johnston 1942, Phene et al. 1971). Dane & Topp (2002) give an
overview of sensors and devices related to the measurement of soil suction. Electrical
conductivity sensors measure the electrical conductivity using two embedded electrodes.
Main disadvantage of electrical conductivity sensor measurements is their inherent sen-
sitivity to changes in electrical conductivity which is not related to the water content
of the porous medium but to dissolved solutes. Thus thermal conductivity sensors have
found a greater amount of use in geotechnical engineering practice.
- Contact Filter Paper Method:
Following the contact filter paper method a filter paper is placed in direct contact to the
specimen and matric suction is measured indirectly, by measuring the amount of water
transferred to the dry paper. Both non-contact and contact filter paper method are
discussed and analyzed in Fawcett & Collis-George (1967), Al-Khafaf & Hanks (1974),
Houston et al. (1994).
Measurement of Osmotic Suction
The osmotic suction can be determined by measuring the electrical conductivity of the pore-
water in the soil, which is related to the osmotic suction. With increasing dissolved salts in
the pore-water the electrical conductivity is increasing and thus the osmotic suction. The
pore-water, that is used for the measurement of the electrical conductivity can be extracted
using several methods, e.g. the saturation extract method, the centrifuging method, leaching
method or the squeezing method (Iyer 1990, Leong et al. 2003). The pore fluid squeezing
method has shown to be the most reliable measurement technique of osmotic suction (Krahn
& Fredlund 1972, Wan 1996, Peroni & Tarantino 2005). This technique consists of squeezing
a soil specimen to extract the fluid from the macropores, that is used for measuring the
electrical conductivity. The authors showed, that the method appears to be influenced by the
magnitude of applied extraction pressure.
2.5 Constitutive Models for Hydraulic Functions
As already mentioned in the previous chapter the soil-water characteristic curve and hydraulic
conductivity function must be known in order to model flow through unsaturated soils. Origi-
nating from classical studies in soil science, a large number of related models can be found. As
mentioned before the soil-water characteristic curve describes the relation between water con-
tent or saturation and soil suction (θ(ψ), S(ψ)) and the unsaturated hydraulic conductivity
function describes the relation between suction and hydraulic conductivity (k(ψ)) as well as
water content or saturation and hydraulic conductivity (k(S), k(θ)). These unsaturated soil
functions are subsequently used when modeling hydro-mechancial behavior of unsaturated
soils (Donald 1956, Bishop & Donald 1961, Fredlund et al. 1996a, Vanapalli et al. 1996).