Design of Flexible Pavements:
The design of flexible pavements is done by the CBR method. The CBR test, described further, gives an indication of the supporting power of the sub-grade soil. The other input needed for design is the traffic. Heavy commercial vehicles (trucks and buses) alone matter, and not the light vehicles like cars, two-wheelers and vans. Since the heavy commercial vehicles carry different loads, some heavy and some low, it is the practice to convert the axle loads to a standard axle load of 81.6 kN (8.16 T) by using the following formula :
Damaging power of an axle of weight W (in kN) = (W/81.6)^{4}
Thus, if an axle load of a vehicle is 100 kN, its damaging power is :
(100 /81.6)^{4 }= 2.26
This means that an overloaded vehicle with axle load of 100 kN causes damage to a pavement which is 2.26 times that caused by a standard axle load of 81.6 kN. If the damaging power of each vehicle is thus found and added up, and then divided by the number of commercial vehicles, one gets the Vehicle Damaging Factor (VDF).
VDF = Sum of damaging power of axles counted in a period/Number of commercial vehicles counted in the same period
Flexible pavements are designed for a life of 15 years. During the period, the traffic grows. A growth rate of 7.5 per cent per annum can be taken as a rough guide.
The distribution of traffic across a carriageway is needed for arriving at the traffic.