From the word flexible, it is very clear that in this category pavement that come are those that deflects under the applied load. Flexible pavements are very dependent on the underlying layers, these underlying layers ( surface layer, base , subbase, subgrade layer) are composed of different materials like silt, clay, gravels, stones etc…
When load acts on the pavement then load spreads on every underlying layer. Type of material used to construct different layers depends on the load bearing capacity of each separate layer. Layer with the highest bearing capacity material will be on the top most layers and the layer with the lowest load bearing capacity will be on the bottom of all layers.
In flexible pavements, cost of every layer also depends on the load bearing capacity. Top most layer will the most expensive one, because it is carrying the maximum load and the bottom layer will the least expansive, because of minimum load bearing capacity.
Mechanism Of Load Transfer
Top most layer that is the pavement layer with highest bearing capacity and most expensive layer takes the load and bears load up to the bearing capacity limit. When load crosses the limits, then a deflection occurs in the top most layer and as a result load transfers the base layer which is underlying the top most layer.
Now in the same manner base layer bears only that much load of which it is being capable and when limit exceeds then remaining load transfers to the subase layer. In the same manner subbase layer carry loads and transfer loads to the subgrade layer if any load left to distribute. After that finally load reaches the underlying earth where remaining load distribute.
In this way load transfer mechanism occurs. Almost 90% pavements are flexible pavements. Therefore, flexible pavements are only placed where underlying strata has good load bearing capacity properties. In case when underlying strata lacks load-bearing capacity then we go for rigid pavement, because rigid pavements not depend on the properties of underlying strata.