Design of flexible pavement by CBR method (IRC : 37-2001)

$\rightarrow$ (MU - Dec. 15)

Q. Explain in brief the importance of CBR test in design of flexible pavement.

Q. Explain the CBR method of pavement design as per IRC : 37-2001.

Q. Explain CBR method of testing of soil subgrade.

Dec. 15, 8 Marks.

• The pavement design in IRC : 37-1984 were applicable for the design traffic upto 30 million standard axles (msa).

• Day by day, traffic goes on increasing and also with incidence of overloading, hence it is more essential to design arterial road for traffic for greater than 30 msa.

• As empirical method have limitations for their applicability and extrapolation, the analytical method of design are used to reanalyze the existing designs and develop a new set of design for designs traffic upto 150 msa by making use of the

Design of Flexible Pavement.

$\rightarrow$ Design considerations.

Following are the various factors, are to be considered in the design of flexible pavement.

1] Initial traffic after construction in terms of number of commercial vehicles per day (CVPD)

2] Traffic growth rate during the design life in percentage.

3] Design life in number of years.

4] Vehicle damage factor (VDF)

5] Distribution of commercial traffic over the carriageway.

$\rightarrow$ Design procedure.

Design procedure of flexible pavement by CBR method by IRC : 37-2001 is given in following steps.

1. Lane distribution factor.

The following distribution may be assumed for design until more reliable data on placement of commercial vehicles on the carriageway lanes are available.

$\rightarrow$ [1] Single lane roads.

Design should be based on total number of commercial vehicles in both directions.

$\rightarrow$ [2] Two lane single carriage way roads.

The design should be based on 75% of the total number of commercial vehicles in both direction.

$\therefore$ Distribution factor = 0.75

$\rightarrow$ [3] Four lane single carriage way roads.

The design should be based on 40% of the total number of commercial vehciles in both direction.

$\therefore$ Distribution factor = 0.4

$\rightarrow$ [4] Dual carriageway roads.

• The design of dual lane carriageway roads should be based on 75% of the number of commercial vehicles in each direction.

$\therefore$ Distribution factor = 0.75

• For dual 3 lane carriageway distribution factor will be 60% and that for dual 4 lane carriageway, it will be 45%.

$\therefore$ Distribution factor for dual 3 lane carriage way = 0.6

$\therefore$ Distribution factor for dual 4 lane carriage way = 0.45

2. Computation of design traffic.

• The design traffic is considered in terms of the cumulative number of standard axles (in the lane carrying maximum traffic) to be carried during the design life of the road. This can be calculated by using the following equation:

$N = \frac{365 \times [ (1+r)^n – 1]}{r} \times \ ADF$

Where,

N = The cumulative number of standard axles to be catered for in the design in terms of msa (i.e. million standard axles)

A = Initial traffic in the year of completion of construction in terms of the number of commercial vehicles per day.

D = Lane distribution factor as explained in step 1.

F = Vehicle damage factor (VDF)

n = Design life in years.

r = Annual growth rate of commercial vehicles (for 7.5%, annual growth rate is 0.075 i.e. r = 0.075)

• The traffic in the year of completion is estimated using the following formula:

$A = P (1+ r)^x$;

Where,

P = Number of commercial vehicles as per last count.

X = Number of years between the last count and the year of completion of construction.

r = Annual growth of commercial vehicles.