Flexible pavement design with 8 CBR values

Design method and assumptions

IRC:37-2012 (also followed by Nepal DoR for road pavements) is used.

• Two-lane single carriageway: design lane carries 50 percent of the two-way commercial traffic as per IRC:37-2012; a more conservative 75 percent (IRC:37-2001) is used here to cover heavier lane loading on a two-lane road.
• Each commercial vehicle is taken to impose its quoted load on one critical axle (10 000 kg and 9000 kg) and the damage of that axle is counted; standard axle = 80 kN (8160 kg); $g=9.81$ m/s2.
• Traffic is counted now; the road opens after 2.5 years; design life $n=10$ years; $r=7\%$.

Step 1 - Design CBR

The CBR value at which 90 percent of results are equal or greater lies between 4 % (100 percent) and 5 % (75 percent), about 4.4 %. The design subgrade CBR is taken as 4 % (conservative; on the safe side).

Step 2 - Vehicle damage factor (fourth power law)

• 10 000 kg axle: $(10000\times9.81/1000\,/\,80)^4=(98.1/80)^4=2.261$
• 9000 kg axle: $(88.29/80)^4=1.483$
• Weighted: $VDF=0.6\times2.261+0.4\times1.483=1.950$

Step 3 - Design traffic

Traffic at opening: $A=340(1.07)^{2.5}=402.7$ CVPD.

$$N_s=\frac{365[(1.07)^{10}-1]}{0.07}\times A\times D\times F=365\times13.816\times402.7\times0.75\times1.950=2.97\ \text{msa}$$

Step 4 - Pavement composition

The lowest design traffic in the IRC:37-2012 catalogues is 10 msa, so for the subgrade CBR of 4 % the 10 msa section is adopted (it is safe for 3 msa; a thinner section can be designed using the IRC:37-2001 plates or IRC:SP:72 if economy is required).

Answer: design traffic $\approx 2.97$ msa; section = 40 mm BC + 80 mm DBM + 250 mm WMM + 330 mm GSB over subgrade of CBR 4 %.