---
title: "Flexible pavement design with 8 CBR values"
subject: "Transportation Engineering II"
chapter: 2
key: "semester_7/transportation_engineering_ii:flexible-design-eight-cbr-340-cvd"
source: https://www.bctengineer.com/q/semester_7/transportation_engineering_ii/flexible-design-eight-cbr-340-cvd
site: BCT Engineering
---

# Flexible pavement design with 8 CBR values

Transportation Engineering II · Chapter 2: Highway Pavement

- Asked in: 2079 Bhadra, 8 marks
- Video: [Lec-32_Flexible Pavement Design by C.B.R. Method | PDHC | Civil Engineering](https://www.youtube.com/watch?v=9BL6rl6hPQ0) — Civil Engineering Department_LJIET
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/transportation_engineering_ii/flexible-design-eight-cbr-340-cvd
- Whole chapter: https://www.bctengineer.com/md/semester_7/transportation_engineering_ii/chapter-2

## Question

Design a flexible pavement for a two lane highway using provided tables and catalogues. Subgrade CBR values obtained from tests conducted at eight locations are: 11%, 7%, 6%, 7%, 4%, 4%, 8%, and 5%. Total both directional traffic as per last count is 340 CV/day with 60% vehicles of 10000 kg axle load and 40% vehicles of 9000 axle load. Use design life of 10 years, expected completion period of 2.5 years from the date of last count and a traffic growth rate of 7%. State your assumption for estimating the design traffic.

## Short answer

Design CBR from 90th percentile conservatively taken as 4%. Weighted VDF = 1.95; design traffic after 2.5 years to opening and 10-year life with 7% growth and 75% lane factor: Ns = 2.97 msa. Section adopted from 10-msa catalogue: 40 mm BC, 80 mm DBM, 250 mm WMM base, 330 mm GSB (total 700 mm).

## Answer

#### 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
| CBR (%) | No. equal or greater | Percent equal or greater |
|---|---|---|
| 4 | 8 | 100.0 |
| 5 | 6 | 75.0 |
| 6 | 5 | 62.5 |
| 7 | 4 | 50.0 |
| 8 | 2 | 25.0 |
| 11 | 1 | 12.5 |

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).

| Layer | Thickness (mm) |
|---|---|
| Bituminous concrete (BC) | 40 |
| Dense bituminous macadam (DBM) | 80 |
| Wet mix macadam (WMM) base | 250 |
| Granular sub-base (GSB) | 330 |
| **Total** | **700** |

**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 %.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. How is the design subgrade CBR chosen from the eight test values?
   A. The average of all values
   B. The value at which 90 percent of results are equal or greater, taken conservatively as 4 %
   C. The highest value, 11 %
   D. The median, 7 %
   Answer: B. The 90th percentile value lies between 4 and 5 % (about 4.4 %), and 4 % is adopted on the safe side. (Key point: Design CBR selection)

2. What is the damage factor of a 10 000 kg axle by the fourth power law (standard axle 80 kN)?
   A. (98.1/80)^4 = 2.26
   B. 98.1/80 = 1.23
   C. (80/98.1)^4 = 0.44
   D. (98.1/80)^2 = 1.50
   Answer: A. 10 000 kg = 98.1 kN, and the damage ratio is raised to the fourth power, giving 2.261. (Key point: Vehicle damage factor)

3. How is traffic at the opening of the road found?
   A. Use 340 CVPD unchanged
   B. A = 340 (1.07)^2.5, growing the last count over the 2.5-year construction period
   C. A = 340 (1.07)^10
   D. A = 340 x 2.5
   Answer: B. The count is taken before construction, so it is grown over the construction period to get 402.7 CVPD. (Key point: Traffic at opening)

4. Design traffic is about 2.97 msa but the catalogue starts at 10 msa. What is done?
   A. Extrapolate a thinner pavement without checking
   B. Reject the design
   C. Adopt the 10 msa section for CBR 4 % (safe for 3 msa)
   D. Use the 150 msa section
   Answer: C. The lowest catalogue traffic is 10 msa, so that section is adopted as it is on the safe side. (Key point: Choosing catalogue section)

## Related in Transportation Engineering II

- [VDF; four-lane flexible design from catalogue](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Avdf-flexible-design-catalogue-4000-cvpd) — Transportation Engineering II, chapter 2
- [Flexible pavement design: four lane, ADT 8000 trucks](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Aflexible-design-adt-8000-four-lane) — Transportation Engineering II, chapter 2
- [Flexible pavement life with 140 cvpd traffic](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Aflexible-design-life-years-140-cvpd) — Transportation Engineering II, chapter 2
- [Flexible design: four-lane, AADT 12,000](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Aflexible-design-aadt-12000-four-lane) — Transportation Engineering II, chapter 2
- [IRC pavement design: 5640 CVPD, CBR 6%](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Airc-design-5640-cvpd-cbr-6) — Transportation Engineering II, chapter 2
- [IRC CBR-method design: 1200 CVPD, 4% subgrade](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Airc-cbr-method-design-1200-cvpd) — Transportation Engineering II, chapter 2

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