Flexible pavement design: four lane, ADT 8000 trucks

Design method

Four-lane divided highway, so IRC:37-2012 (Guidelines for the Design of Flexible Pavements) is used with its CBR-based catalogue; Nepal DoR adopts the same approach (cumulative msa and subgrade CBR). The traffic of 8000 is taken as commercial vehicles per day (CVPD) in both directions.

Step 1 - Design subgrade CBR

For a major highway the 90th percentile CBR is used (the value that 90 percent of the results equal or exceed):

The 90th percentile lies between 5 percent (100 percent of values) and 6 percent (71.4 percent of values); interpolation gives about 5.3 percent, so the design CBR is conservatively taken as CBR = 5 %.

Step 2 - Design traffic

• The traffic count was one year ago and construction takes 2 years, so $x = 1+2 = 3$ years.
• Traffic at completion (both ways): $A_{total}=8000(1.07)^3 = 9800$ CVPD.
• Heavier direction 55 percent: $A = 0.55\times9800=5390$ CVPD.
• Lane distribution factor for a dual two-lane carriageway (IRC:37-2012): $D = 0.75$ of the directional traffic.
• VDF $F = 4.0$, design life $n = 15$ years, $r=7\%$.

$$\begin{aligned} N_s &= \frac{365[(1+r)^n-1]}{r}\,A\,D\,F\\ &= 365\times\frac{(1.07)^{15}-1}{0.07}\times 5390\times0.75\times4.0 \\ &= 365\times25.129\times5390\times0.75\times4.0 = 148.3\times10^{6}\ \text{standard axles} \end{aligned}$$

Design traffic is about 150 msa (the upper limit of the IRC:37-2012 catalogue).

Step 3 - Pavement composition (IRC:37-2012 plate for CBR 5 %, 150 msa)

Read the layer thicknesses from the design plate for CBR 5 % at 150 msa (use the chart supplied in the exam if one is given):

  +-----------------------------+
  | BC               50 mm      |
  | DBM             170 mm      |
  +-----------------------------+
  | WMM base        250 mm      |
  +-----------------------------+
  | GSB sub-base    300 mm      |
  +-----------------------------+
  | Subgrade (CBR 5 %)          |
  +-----------------------------+

Answer: design traffic $\approx 148$ msa (150 msa); pavement = 50 mm BC + 170 mm DBM + 250 mm WMM + 300 mm GSB = 770 mm total. The sub-base should be extended across the full formation width for drainage.