Hydro-Mechanical Properties of an Unsaturated Frictional Material

12 CHAPTER 2. STATE OF THE ART
the net stress and suction stress variables for development of a constitutive model describing
the behavior of unsaturated cohesionless soils under complex loading conditions and loading
history.
The stress state variable must be measurable or predictable. Thus total stress state,
pore-water pressure and/or pore-air pressure are determined and measured in the laboratory,
when testing unsaturated soils. Using high-air entry disk enables to measure both pore-water
pressure and pore-air pressure independently in unsaturated soils. Measurement devices as
for instance tensiometer sensors enable to obtain pore-water pressure. The total stress is
controlled by applying load to the specimen.
2.2.2 Phases in Unsaturated Soils
An unsaturated soil is a multi-phase system comprising of more than two phases (as in sat-
urated soils), namely the solid phase, the liquid phase including the contractile skin and the
gas phase. The solid phase consists of the soil grains. Depending on the grain sizes the soils
may range from fine grained soils as silts and clays to coarse grained soils as sands and grav-
els. The liquid phase consists of any liquid or a miscible or immiscible combination of two
or more liquids (e.g. water, oil, light non-aqueous-phase-liquid LNAPL, dense non-aqueous-
phase-liquid DNAPL). Additionally the liquid phase may include the contractile skin, which
may act as a tensile force on the liquids surface. In this work the solid phase, the liquid phase
and the gas phase of the unsaturated soil are assumed to be the soil solids, water (with the
contractile skin) as well as air, that leads to a 3-phase system. The phases of unsaturated soil
are schematically given in Fig. 2.3.
2.2.3 Soil Suction
The total soil suction is referred to the free energy state of the soil pore water (Edlefsen &
Anderson 1943, Fredlund & Rahardjo 1993a). The thermodynamic relation between the total
suction or free energy of the soil water and the relative humidity of the soil air is given in the
following equation:
ψ = − RT
· ln
vw0 · ωv
RH
100
Gas
Fluid including
contractile skin
Soil
Figure 2.3: 3 Phase system representing unsaturated soils
(2.3)