IRC pavement design: 5640 CVPD, CBR 6%

Method and assumptions

IRC:37-2012 (flexible pavements, CBR catalogue), which is the basis of the Nepal DoR guideline for highways. Not stated, so assumed: growth rate $r=7.5\%$ (a commonly used value; IRC:37-2012 suggests 5 percent when no data are available, which gives about 61 msa and the same section), all commercial vehicles carry the 118 kN axle load, and the 5640 CVPD is the traffic in the year of completion (so no growth during construction is added). Two-lane single carriageway: design lane carries 50 percent of the total commercial traffic in both directions.

Step 1 - Vehicle damage factor

By the fourth power law with the 80 kN standard axle: $VDF=\left(\dfrac{118}{80}\right)^4=4.733$.

Step 2 - Design traffic

$$N_s=\frac{365[(1+r)^n-1]}{r}\,A\,D\,F=365\times\frac{(1.075)^{10}-1}{0.075}\times5640\times0.5\times4.733$$ $$N_s=365\times14.147\times5640\times0.5\times4.733=68.9\ \text{msa}$$

Step 3 - Pavement section

Subgrade design CBR = 6 %. Design traffic $68.9$ msa lies between 50 and 100 msa in the CBR 6 % plate, so the 100 msa column (next higher) is adopted on the safe side:

  +------------------------------+
  | BC                    50 mm  |
  | DBM                  150 mm  |
  +------------------------------+
  | WMM base             250 mm  |
  +------------------------------+
  | GSB sub-base         260 mm  |
  +------------------------------+
  | Subgrade (CBR 6 %)           |
  +------------------------------+

Answer: design traffic $\approx69$ msa; adopt BC 50 mm + DBM 150 mm + WMM 250 mm + GSB 260 mm = 710 mm. The values for the 6 % CBR column should be checked against the catalogue sheet attached to the paper (the sub-base for CBR 6 % is thinner than the 300 mm for CBR 5 %).