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Subgrade is considered to be the most significant component of the pavement structure. This is because of its function where it acts as a foundation for other pavement components that are constructed upon it and have to withstand detrimental influences of weather and traffic loads.

Varying water content affects the performance of subgrades hugely, therefore, it becomes important to study the behaviour of subgrade under different conditions. In this regard, wet, moist, and damp subgrade can be recognized based on the water quantity in the subgrade material.

Added to that, the Engineer-in-charge should know the differences between wet, moist, and damp condition and how they affect the stability of the pavement structure. The most obvious features of subgrade which are affected by moisture content are bearing capacity, shrinkage, and swelling. The following table provides differences between wet, moist, and damp subgrade.

Table 1 Differences between wet, moist, and damp subgrade

Wet subgrade Moist subgrade Damp subgrade
According to ACI 116R-90, wet condition occurs when the material is covered with visible free moisture According to ACI 116R-90, moist condition happens when the material is moderately covered with moisture According to ACI 116R-90, damp condition occurs when the material is slightly dump but not quit dry to the touch
It is the sign of highest level of moisture It is moderate moisture between wet and dump Indicates lowest level of moisture
Wet subgrade defects severely under loads. Bearing capacity performance is better than wet subgrade but inferior to damp subgrade Damp subgrade provide the best performance in terms of load bearing capacity in comparison with wet and moist subgrade.
Wet subgrade undergoes significant swelling and shrinkage. As a result, pavement deforms and cracks Swelling and shrinkage of moist subgrade and defects of pavement above is somewhat between damp and wet subgrade It does not suffer from swelling and shrinkage considerably, and hence the pavement does not suffer greatly.
Wet subgrade is not suitable for pavement construction, so it needs to be improved using suitable enhancement technique It might be used for pavement construction but a slight improvement may be needed. It is suitable for pavement construction
If roads are constructed on such subgrade without proper improvement, then future maintenance would be needed. Less costly improvement can be used to avoid future maintenance It does not require improvement and future maintenance is not costly.
Wet Subgrade
Fig. 1: Wet Subgrade
Damp Subgrade
Fig. 2: Damp Subgrade

About Madeh Izat HamakareemVerified

Madeh is a Structural Engineer who works as Assistant Lecturer in Koya University. He is the author, editor and partner at theconstructor.org.