CBR METHOD OF THE DESIGN OF PAVEMENTS HAS BEEN FOUND VERY RELIABLE AS WELL AS THE PRACTICAL

CALCULATION OF THE THICKNESS OF THE PAVEMENTS DESIGNED:--It is necessary to protect the road surface for which the pavement is designed by the competent authority.The factors  wheel load and it's impact effect, volume and the character of the traffic expected to use the road, bearing capacity of the soil on which the road is built.The CBR method of the pavement design has been found very reliable and practical for evaluating the strength of the sub grade (BCof soil) It is the most convenient and widely used where CBR is a measure of the load carrying capacity ie resistance to the direct penitration of the any soil and granular material which is expressed as a percentage of the load carrying capacity for which penetration test is required.A surface having a CBR of 100 percent may exert 1360 kg of load to drive in a cylindrical flat plunger with a base area 19.3 sq cm to a distance of 0.250 mm at the rate of penetration of 0.125 mm.Computation of the traffic is required for use of CBR  and it is determined inthe units of commercial vehicles.In case of the single lane 3.75 mt wide the design is based on the total number of commercial vehicles per day in both directions multiplied by the factor 2 In case of the intermediate width ie 5.5 mt wide the factor has been considered as 1.5 and in the case of two lanes single carriageway roads the design is based on 75 percent of the total number of commercial vehicles in both directions.In case of four lanes single carriageway roads the design should be based on 40 percent of the total number of commercial vehicles in both directions and in the case of dual two lanes carriage way roads the design should be based on 75 percent of the number of commercial vehicles in each direction.The distribution factor shall be reduced by 20 percent for each additional lane