VDF; four-lane flexible design from catalogue

Vehicle Damage Factor

Vehicle Damage Factor (VDF) is the number of standard axle load repetitions (80 kN single axle) that cause the same damage as one pass of a commercial vehicle. It converts mixed traffic into equivalent standard axles.

• By the fourth power law, the damage factor of one axle is

$$\text{EALF}=\left(\frac{\text{axle load}}{80\ \text{kN}}\right)^{4}$$

• For a vehicle, VDF is the sum of the EALFs of all its axles; for a traffic stream, VDF = $\dfrac{\sum n_i\,\text{VDF}_i}{\sum n_i}$ (weighted average). Values are obtained from an axle-load survey at the site. If no survey is possible, IRC:37-2012 gives indicative values (for plain terrain 1.5, 3.5 and 4.5 for 0-150, 150-1500 and over 1500 CVPD respectively).
• Design traffic: $N_s=\dfrac{365\,[(1+r)^n-1]}{r}\,A\,D\,F$, where $F$ is VDF, $A$ the initial CVPD, $r$ the growth rate, $n$ the design life and $D$ the lane distribution factor.
• Overloading in Nepal is common, so a site axle-load survey is preferred to default values.

Design of the 4-lane divided pavement (IRC:37-2012)

Assumptions: each commercial vehicle is represented by its quoted axle load on the critical axle; directional split 50:50; the 4000 CVPD is the traffic in the year of completion, so no construction-period growth is needed.

Step 1 - Design CBR.

The 90th percentile lies between 4 % (100 percent) and 5 % (85.7 percent): $4+\dfrac{100-90}{100-85.7}\times1=4.7$ %. Only the 4 % and 5 % catalogues are provided and 4.7 % is below 5 %, so the design CBR is taken as 4 % (safe side).

Step 2 - VDF (80 kN standard axle):

• 100 kN: $(100/80)^4=2.441$; 80 kN: $1.000$; 70 kN: $(70/80)^4=0.586$
• $VDF=0.30\times2.441+0.50\times1.000+0.20\times0.586=1.350$

Step 3 - Design traffic. Dual two-lane carriageway: directional share 0.5 and lane factor 0.75 (IRC:37-2012), $r=8\%$, $n=15$ years:

$$N_s=365\times\frac{(1.08)^{15}-1}{0.08}\times4000\times0.5\times0.75\times1.350=365\times27.152\times1500\times1.350=20.06\ \text{msa}$$

Step 4 - Section from the CBR 4 % catalogue. The design traffic of 20.06 msa is just above the 20 msa row, so the next higher row (30 msa) is adopted on the safe side. (Linear interpolation between 20 and 30 msa would give practically the 20 msa section: DBM about 110 mm, total 730 mm.)

Answer: VDF = 1.350, design traffic $\approx$ 20.06 msa (adopt the 30 msa row); pavement = 40 mm BC + 130 mm DBM + 250 mm base + 330 mm sub-base = 750 mm.