Hydro-Mechanical Properties of an Unsaturated Frictional Material

18 CHAPTER 2. STATE OF THE ART
- Residual zone
The transition zone is followed by the residual zone. Whereas in the transition zone the
pore water is retained mainly by capillary forces, in the residual zone the pore water is
retained in form of thins films on the soils grains. This is caused by the surface area
and the surface charge density of the soil. The gas phase is the continuous phase and
the liquid phase is the non-continuous phase. Water is transported only by water vapor
diffusion.
Following Fredlund & Xing (1994) typical parameters of the soil-water characteristic curve
can be estimated. These parameters are:
- Air-entry value ψaev
The air-entry value ψaev is the value of soil suction, where air starts to enter the largest
pores of the soil during drainage process.
- Saturated volumetric water content θs
The saturated volumetric water content is the water content in a soil, where all pores
are filled with liquid S = 1.
- Residual soil suction ψr and corresponding residual volumetric water content θr
The residual soil suction ψr and corresponding residual volumetric water content θr is
the point, where the water starts to be held in the soil by adsorption forces (Sillers
1997).
- Water-entry value ψwev
The water-entry value is the soil suction where water starts to displace air in the soil
and enters the pores in the soil during imbibition process (Wang et al. 2000, Yang,
Rahardjo, Leong & Fredlund 2004a). When reaching the water-entry in a soil the water
content starts to increase significantly.
For sand the residual zone is generally very limited, because both the surface area of the
soil particles and the surface charge density are negligibly small. The pore-size distribution
mainly influences the shape and the slope of the soil-water characteristic curve. Uniform soils
have steep soil-water characteristic curves, because the majority of pores are drained at a
narrow range of soil suction. Well-graded soils have flatter soil-water characteristic curves,
because the pores are different in size and drained over larger range of soil suction. Controlled
by relatively large pores the air-entry pressure as well as the water-entry pressure are low,
when working with sand. The soil-water characteristic curve of a silt is characterized by a
larger range of soil suctions. Silts absorb a greater amount of pore water due to smaller pores
and a significant surface area as well as surface charge density. The smaller pores retain