Farhad Salour Doctoral Thesis

4.1.
 
Mechanism of moisture related material deterioration and degradation
 
In coarse-grained aggregate materials, the pores between the particles are relatively large. 
Under draining conditions, the load bearing mechanism of the material is mainly based 
on particle contact stresses and their inter-particle friction. Introduction of water to the 
material can usually result in reduction in the inter-particle frictional strength and 
therefore larger resilient and accumulated permanent deformations under traffic 
loading. If water fills in all the voids between the aggregate particles (non-draining 
condition) the pore water may be pressurised under traffic loading. The water pressure 

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counteracts the particle contact stresses, which can result in considerably reduction in 
the material strength (Dawson and Kolisoja, 2004). 
In fine-grained materials and subgrade soils, the undesirable effect of excess moisture 
presence is more significant. Besides the water lubrication effect, an increase in moisture 
content results in a decrease in the soil mass suction and therefore reduction in the 
effective stress (Sawangsuriya et al., 2008; Yang et al., 2008; Cary and Zapata, 2010 and 
2011). In the unsaturated state where pores are no longer completely filled with water, 
the soil suction parameter and its contribution to the effective stress can be introduced 
to describe the mechanical behaviour of the material. In this case, the effective stress, 
which is the function of external stresses and internal forces including the contractile 
water skin effects, is taken into account. This approach converts a four-phase porous 
medium to a mechanically equivalent single phase using nonlinear mathematical models 
that allow for considering moisture variation effects on the mechanical behaviour of 
materials. These constitutive models include the effect of suction and moisture pressure 
within the soil and aggregate mass (Laloui et al., 2009).