Q. Purpose of using capping layers in pavement construction?

When California Bearing Ratio of subgrade is checked to be below a specific percentage (for example 5%), a capping layer is generally provided to decrease the effect of weak subgrade on the structural performance of the road. It also provides a working platform for sub-base to be created on top in wet weather condition since the compaction ofwet subgrade is difficult on site. Effect of interruption by wet weather can be decreased significantly and progress of construction works wouldn't be hindered. Most significantly, cost of cappinglayers is low since the material can be readily attained locally.