Farhad Salour Doctoral Thesis

Figure 8
. Temperature and suction gradient along pavement profile during the 
formation of an ice lens. (modified after Mokwa, 2004). 
The frost penetration process is a relatively long procedure that results in continuous 
accumulation of moisture content in the pavement system in the form of ice lenses. In 
addition to the frost heave related problems that might occur if frost susceptible 
materials are present in the pavement structure, the thawing of the frost zone becomes 
a major consequent problem. The large quantity of moisture that is accumulated in the 
pavement system during the freezing period is released over a much shorter period as 
the thaw penetrates the pavement in the spring and the ice lenses convert back to liquid 
moisture. This usually leads to loss of bearing capacity of the pavement structure during 
the spring-thaw period. 

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14 
3.
 
Emergence of Unsaturated Soil Mechanics in Pavement 
Engineering 
As discussed in the previous chapter, the unbound layers in pavement structures and 
the upper part of the subgrade can experience considerable variation in the moisture 
content throughout the year. The moisture content of the unbound layers can fluctuate 
seasonally from the natural equilibrium condition due to several reasons, i.e. winter frost 
action and spring-thaw in cold regions, infiltration during rain events and variation in 
the groundwater table. 
Even though the underlying materials in pavement structures are generally in partially 
saturated conditions, the impact of an unsaturated state on the mechanical behaviour of 
the unbound materials and subgrade soils is generally not taken into account. The 
performance of the pavement structures can to a large extent be affected by the 
behaviour of the underlying unbound materials and more reliable performance 
prediction of pavement systems require consideration for the unsaturated state. 
The mechanical behaviour of the soil is generally governed by the stress variables that 
control the equilibrium of the soil structure (Fredlund and Morgenstern, 1977). An 
unsaturated soil element consists of four different phases: soil particles or solid phase, 
water phase, air phase and the contractile skin. The total equilibrium equation of this 
four-phase soil structure can be formulated using the force equilibrium equation for the 
air phase, water phase, contractile skin phase and the total equilibrium equation for the 
soil element (Fredlund et al., 2012). 
From the equilibrium equation of the unsaturated soil structure, three independent sets 
of normal stresses can be extracted to form the stress state variables. These three 
normal stress state variables are (
a
u


), (
w
a
u
u

) and 
a
u
, where 

is the total normal 
stress and 
𝑢
𝑎
and 
𝑢
𝑤
are pore-air and pore-water pressures, respectively. The difference 
between the pore-air and pore-water pressures (
w
a
u
u

) within the unsaturated soil 
structure is known as the matric suction (
m

). Assuming that the soil particles are 
incompressible, the stress variable 
𝑢
𝑎
can be eliminated and therefore the stress state 
variables for the unsaturated soil structure can be defined as (
a
u


) and (
w
a
u
u

) 
(Fredlund et al., 2012). As the moisture content of the soil increases and the soil 
structure goes from an unsaturated state to a fully saturated state (
S
=
100%) the 
pore-water pressure approaches the pore-air pressure and therefore the matric suction 
of the soil structure (
w
a
m
u
u



) goes toward zero. Matric suction is therefore known 
to be the state variable with the highest relevance to unsaturated soil mechanics. The 
matric suction of the subgrade is highly dependent on the moisture content of the soil 
structure, commonly defined by the soil-water characteristic curves. 

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15 
Since matric suction of the soil structure is directly related to its moisture content, and 
moisture is the main environmentally driven factor affecting the behaviour of unfrozen 
unbound layers, matric suction can be incorporated into the material response 
characterization models (i.e. the stiffness) for capturing the seasonal moisture variation 
effects. Accounting for suction effects in the mechanical behaviour of unbound 
materials is particularly important in materials with a high fines content (i.e. in subgrade 
soils) (Khogali et al, 1991; Zapata et al., 2009).