Farhad Salour Doctoral Thesis

3.3.
 
Soil-water characteristic curve 
The soil-water characteristic curve (SWCC) is one of the most important functions used 
in assessment of unsaturated soil properties. SWCCs combine the water content 
measurement (mass or volume) of the unsaturated soil structure to the energy state of 
the water phase (matric suction). In other words, the relationship between the soil 
moisture content and the matric suction is usually represented by the SWCC. The 
SWCC of the soil depends on the water distribution in pores as well as the soil texture 
and gradation (Fredlund et al., 2012). 
Measurement of SWCCs can be done using numerous types of test equipment and 
procedures. The most common devices are the Tempe cell, pressure plate extractors, 
Fredlund pressure plate cell and modified triaxial cells (described later). In addition to 
direct measurement, the SWCCs can also be estimated. Since the SWCC is mainly 
related to the voids in the soil structure and the water content in the voids, many studies 
have been conducted to estimate the SWCC from the grain-size distribution curves of 
the soil as well as their plasticity index. Figure 9 shows SWCC prediction models for 
both plastic and non-plastic materials (Zapata, 1999). 
Figure 9.
SWCCs for both plastic and non-plastic materials (modified after Zapata, 
1999). 
The constitutive equations for unsaturated soil properties can be rewritten by 
incorporating the matric suction into the modelling using the SWCCs. As unsaturated 
soil properties are primarily a function of the moisture content of the soil and moisture 
is the main environmentally driven variable that directly affects material mechanical 
properties of unfrozen unbound materials, suction effects can therefore be coupled to 
the material response characterization. 

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