--- title: "IRC pavement design: 5640 CVPD, CBR 6%" subject: "Transportation Engineering II" chapter: 2 key: "semester_7/transportation_engineering_ii:irc-design-5640-cvpd-cbr-6" source: https://www.bctengineer.com/q/semester_7/transportation_engineering_ii/irc-design-5640-cvpd-cbr-6 site: BCT Engineering --- # IRC pavement design: 5640 CVPD, CBR 6% Transportation Engineering II · Chapter 2: Highway Pavement - Asked in: 2070 Chaitra, 8 marks - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/transportation_engineering_ii/irc-design-5640-cvpd-cbr-6 - Whole chapter: https://www.bctengineer.com/md/semester_7/transportation_engineering_ii/chapter-2 ## Question Design the pavement for an existing two lane single carriageway road with the following details. a. Initial traffic in both direction in the year of completion of construction = 5640 CVPD b. Design life = 10 years c. Design CBR value = 6% d. Axle load using the road (CV) = 118 KN (The paper attaches the CBR chart for IRC flexible pavement design and the CBR 6% catalogue table.) ## Short answer Using IRC:37-2012, VDF = (118/80)^4 = 4.73. Design traffic = 365×[(1.075^10-1)/0.075]×5640×0.5×4.73 ≈ 68.9 msa. For CBR 6%, adopt BC 50 mm, DBM 150 mm, WMM 250 mm, GSB 260 mm (total 710 mm). ## Answer #### 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: | Layer | Thickness (mm) | |---|---| | Bituminous concrete (BC) | 50 | | Dense bituminous macadam (DBM) | 150 | | Wet mix macadam (WMM) | 250 | | Granular sub-base (GSB) | 260 | | **Total** | **710** | ```text +------------------------------+ | 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 %). ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How is VDF found for the 118 kN axle load? A. 118 / 80 = 1.475 B. (118/80)^2 = 2.18 C. (118/80)^4 = 4.73 D. (80/118)^4 = 0.21 Answer: C. The fourth power law gives VDF = (118/80)^4 = 4.733 relative to the 80 kN standard axle. (Key point: Vehicle damage factor) 2. Why is D = 0.5 used in the design traffic calculation? A. Because growth is halved during construction B. Because half the axle loads are empty C. Because the design life is only 10 years D. A two-lane single carriageway design lane carries 50 percent of commercial traffic in both directions Answer: D. IRC:37 takes half of the two-way commercial traffic in the design lane on a two-lane road. (Key point: Lane distribution) 3. Why is there no (1+r)^x growth term for construction in this solution? A. The 5640 CVPD is already the traffic in the year of completion B. Growth is ignored in IRC:37 C. Because CBR is 6 % D. Because the design life is 10 years Answer: A. The traffic is given for the year of completion, so only growth over the 10-year design life is included via [(1.075)^10 - 1]/0.075. (Key point: Growth factor) 4. Design traffic is 68.9 msa. Why is the 100 msa column adopted? A. The 50 msa column is always too thick B. 68.9 lies between 50 and 100 msa, and the next higher value is taken on the safe side C. 100 msa is the standard for all national highways D. Because CBR 6 % needs 100 msa Answer: B. Catalogue sections exist for discrete msa values, so the next higher column is chosen so the pavement is not under-designed. 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