---
title: "Flexible pavement design: four lane, ADT 8000 trucks"
subject: "Transportation Engineering II"
chapter: 2
key: "semester_7/transportation_engineering_ii:flexible-design-adt-8000-four-lane"
source: https://www.bctengineer.com/q/semester_7/transportation_engineering_ii/flexible-design-adt-8000-four-lane
site: BCT Engineering
---

# Flexible pavement design: four lane, ADT 8000 trucks

Transportation Engineering II · Chapter 2: Highway Pavement

- Asked in: 2082 Bhadra, 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/flexible-design-adt-8000-four-lane
- Whole chapter: https://www.bctengineer.com/md/semester_7/transportation_engineering_ii/chapter-2

## Question

A new four lane divided highway is to be for the ADT of 8000 truck traffic based on the last count. The directional split of traffic is 55:45. Vehicle damage factor based on axle load survey was 4.0. Design a suitable pavement section for a design life of 15 years. The last traffic count was taken 1 year back and the project would be completed in 2 years from now. Growth rate of traffic = 7%.

The results of subgrade soil CBR (%) at seven locations obtained in a certain stretch of road are given below.

| S.No. | Chainage | Test CBR results in (%) |
|---|---|---|
| 1 | 0+50 | 12 |
| 2 | 0+350 | 8 |
| 3 | 0+500 | 7 |
| 4 | 0+650 | 8 |
| 5 | 0+800 | 6 |
| 6 | 0+950 | 5 |
| 7 | 1+500 | 5 |

## Short answer

Use 90th percentile CBR, giving design CBR 5%. Traffic after 3 years: 9800 CVPD; design lane 5390 CVPD; cumulative standard axles about 150 msa with VDF 4.0, LDF 0.75, r=7%, n=15. Pavement: 50 mm BC + 170 mm DBM + 250 mm WMM + 300 mm GSB, total 770 mm.

## Answer

#### 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):

| CBR (%) | No. of values equal or greater | Percent equal or greater |
|---|---|---|
| 5 | 7 | 100.0 |
| 6 | 5 | 71.4 |
| 7 | 4 | 57.1 |
| 8 | 3 | 42.9 |
| 12 | 1 | 14.3 |

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):

| Layer | Thickness (mm) |
|---|---|
| Bituminous concrete (BC) | 50 |
| Dense bituminous macadam (DBM) | 170 |
| Wet mix macadam (WMM) base | 250 |
| Granular sub-base (GSB, drainage + filter) | 300 |
| **Total** | **770** |

```text
  +-----------------------------+
  | 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.

## Check yourself

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

1. For the given CBR results, why is design CBR taken as 5 %?
   A. It is the mean of the results
   B. The 90th percentile value is used for a major highway and lies at the lower value 5 %
   C. It is the maximum value
   D. It is the median value 8 %
   Answer: B. At 5 % all 100 percent of values are equal or greater; at 6 % only 71.4 percent, so the 90th percentile gives 5 %. (Key point: Design subgrade CBR)

2. Traffic count was 1 year ago and construction takes 2 years. Traffic at completion is:
   A. 8000(1.07)^2
   B. 8000(1.07)^3 = 9800 CVPD
   C. 8000(1.07)^15
   D. 8000 without growth
   Answer: B. Growth acts for x = 1 + 2 = 3 years. (Key point: Years of traffic growth)

3. How is the design lane traffic A obtained?
   A. 0.55 x 9800 = 5390 CVPD, then multiplied by D = 0.75
   B. 9800 x 0.45
   C. 9800 x 0.75 only
   D. Half of 8000
   Answer: A. The heavier direction carries 55 percent, and the lane distribution factor for a dual two-lane carriageway is 0.75. (Key point: Directional and lane factors)

4. The design traffic for 15 years is about:
   A. 15 msa
   B. 50 msa
   C. 148 msa, adopted as 150 msa
   D. 1000 msa
   Answer: C. Ns = 365 x 25.129 x 5390 x 0.75 x 4.0 = about 148 msa. (Key point: Design traffic result)

5. For CBR 5 % and 150 msa the total thickness is:
   A. 770 mm: BC 50 + DBM 170 + WMM 250 + GSB 300
   B. 300 mm
   C. 520 mm
   D. 1000 mm
   Answer: A. The IRC:37-2012 plate gives 50 + 170 + 250 + 300 = 770 mm. (Key point: Resulting pavement)

## 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 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
- [Flexible pavement design with 8 CBR values](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Aflexible-design-eight-cbr-340-cvd) — Transportation Engineering II, chapter 2
- [Lane distribution factor and vehicle damage factor](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Alane-distribution-vdf-factors) — 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: 4-lane single carriageway, 2000 CVPD](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Aflexible-design-four-lane-single-2000) — Transportation Engineering II, chapter 2