---
title: "Macadam road construction method"
subject: "Transportation Engineering I"
chapter: 1
key: "semester_6/transportation_engineering_i:macadam-construction"
source: https://www.bctengineer.com/q/semester_6/transportation_engineering_i/macadam-construction
site: BCT Engineering
---

# Macadam road construction method

Transportation Engineering I · Chapter 1: Introduction to Transportation Planning and engineering

- Asked in: 2078 Chaitra, 4 marks
- Video: [4.2.4 Water Bound Macadam Road (WBM) | CE603(D) |](https://www.youtube.com/watch?v=e8q8rbgs9os) — RGPV Official
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/transportation_engineering_i/macadam-construction
- Whole chapter: https://www.bctengineer.com/md/semester_6/transportation_engineering_i/chapter-1

## Question

Discuss the Macadam road construction method with a neat sketch.

## Short answer

Macadam construction uses layers of angular broken stone compacted on a cambered, well-drained subgrade without a heavy foundation. Stones lock under rolling with water and fines, creating a strong, flexible pavement; later improved with bitumen.

## Answer

**Macadam construction** (water-bound macadam, WBM) is a method of building roads from compacted layers of clean, angular broken stone laid directly on a well-drained, cambered subgrade, described by John McAdam about 1820.

#### Principles
- The road does not need a heavy stone base; the **subgrade supports the load if it is kept dry**.
- Stones are **angular, of uniform size** and compact so that they lock together; the fine dust and water bind them.
- The surface is **cambered** (about 1 in 36) with side drains so water drains away quickly.

#### Sketch
```text
        crown (camber 1 in 36)
         _______.-------._______
   ~~~~ |  top layer 25 mm stone  | ~~~~
        |  layer 2: 50 mm stone   |
        |  layer 1: 50 mm stone   |
   ======= compacted subgrade ========
   drain                          drain
```
Total thickness about 250 mm (in layers of 75-100 mm in later practice).

#### Method
1. Prepare and compact the subgrade to a camber, with drains.
2. Spread the first layer of broken stone (size 50-75 mm), roll it and let traffic or rollers lock the stones.
3. Spread the next layer of smaller stone (25-40 mm) and roll with water; fine screenings fill the voids.
4. Complete the top layer, sprinkle dust or screenings, water and roll again.
5. Later improved with tar or bitumen (bituminous macadam), which is today's base for flexible pavements.

## Check yourself

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

1. What allows a Macadam road to work without a heavy stone base?
   A. Thick concrete
   B. The subgrade supports the load if kept dry
   C. Rounded stones
   D. Heavy rollers only
   Answer: B. The subgrade carries the load provided it is kept dry through drainage and camber. (Key point: Macadam principle)

2. What type of stone is used and how does it bind?
   A. Rounded stone bound by cement
   B. Rounded stone bound by clay
   C. Angular broken stone of uniform size, locked together and bound with fine dust and water
   D. Large boulders bound by bitumen
   Answer: C. Angular stones interlock and are bound by fine dust and water. (Key point: Stone shape and binding)

3. What is the purpose of the camber of about 1 in 36 and side drains?
   A. Decoration
   B. Reduce vehicle speed
   C. Increase thickness
   D. Shed water quickly to keep the subgrade dry
   Answer: D. Camber and drains remove water so the subgrade remains dry and strong. (Key point: Camber)

4. In the construction method, how do the stone sizes of layers change going up?
   A. Smaller stone in upper layers, with screenings filling voids
   B. Larger stone in upper layers
   C. Same size throughout, no screenings
   D. Only dust is used
   Answer: A. The first layer is 50-75 mm stone and the next layers are smaller (25-40 mm), with screenings filling voids. (Key point: Construction layers)

## Related in Transportation Engineering I

- [Macadam versus Telford construction](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Amacadam-vs-telford) — Transportation Engineering I, chapter 1, asked 2×
- [Tests on road aggregates](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Aaggregate-tests-list) — Transportation Engineering I, chapter 6, asked 8×
- [Special considerations in hill road design](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Ahill-special-considerations) — Transportation Engineering I, chapter 5, asked 5×
- [Desirable properties of road aggregates](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Aaggregate-desirable-properties) — Transportation Engineering I, chapter 6, asked 5×

## Related in other subjects

- [Water bound macadam materials and construction](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Awbm-construction-procedure) — Transportation Engineering II, chapter 3, asked 4×
- [Road construction equipment and mass haul characteristics](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Amass-haul-diagram-equipment-plants) — Transportation Engineering II, chapter 3, asked 3×
- [Short note: construction procedure of gravel road](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Agravel-road-construction-procedure) — Transportation Engineering II, chapter 3, asked 2×
- [Report on road construction in remote Nepal](https://www.bctengineer.com/question.md?key=semester_5%2Fcommunication_english%3Areport-road-construction-remote-nepal) — Communication English, chapter 9