Macadam road construction method

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

        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.