Causes of moisture variation in subgrade

The moisture content of subgrade soil changes with time, and a wet subgrade has a lower CBR and bearing strength. The main causes of moisture variation are:

1. Rain-water infiltration through the pavement surface, cracks, joints, permeable shoulders and unlined side drains.
2. Seepage of ground water laterally from higher ground, springs and leaking canals or ponds, especially in cuttings and hill roads.
3. Fluctuation of the ground-water table, seasonal rise during the monsoon, and floods.
4. Capillary rise of water from the water table into fine soils such as silt and clay, which can reach 1 m or more above the water table.
5. Vapour movement and condensation. Temperature differences between day and night, and between the pavement and soil, drive water vapour upward or downward. It condenses beneath the impermeable pavement and builds up moisture.
6. Ponding and flooding of the road side because of poor surface drainage, blocked culverts or low embankments.
7. Evaporation and drying at the shoulders, which makes the edge zone drier than the centre. This differential moisture causes cracking and edge failure.
8. Moisture trapped during construction (compaction on the wet side of optimum) and the effect of tree roots and vegetation.

The moisture content under the centre of a covered pavement slowly reaches an equilibrium value, while the edges vary seasonally. The design therefore uses the soaked CBR (worst moisture state) of the subgrade (IRC 37, NRS 2070).