Macadam Method of Construction

John Macadam (1756-1836) put forward an entirely new method of road construction as compared to all the previous methods. The first attempt to improve the road condition was made by him in 1815. Macadam was the Surveyor-General of Roads in England and his new concept of road construction became known by the year 1827.
The important modifications made in Macadam's method with respect to the older methods and the main features of the proposed method are given under:
  • Macadam realised that the soil subgrade being the lowest portion of the pavement should be prepared properly and kept drained so as to carry the load transmitted through the pavement. Therefore the subgrade was compacted and was prepared with a cross slope of 1 in 36.
  • He was the first engineer to suggest that heavy foundation stones are not at all necessary to be placed at the bottom layer of construction. Macadam suggested that instead of placing large foundation stones, small size broken stones shall be spread over the prepared soil subgrade and compacted.
  • Similarly, the next layer of the pavement also was constructed above this layer with small broken stones.
  • Though the total thickness of construction was less than previous methods, this technique could serve as a carriageway in a better way due to improved load dispersion characteristics of compacted broken stone aggregates of smaller sizes.
  • The size of broken stones for the top layer was decided based on the stability under animal-drawn vehicles. The pavement surface was also prepared with a cross slope of 1 in 36 for drainage of surface water
A typical cross-section of Macadam's construction is shown in the following figure.

This was the first method based on scientific thinking. It was realised that the stresses due to the wheel load of traffic get decreased at the lower layers of the pavement and therefore it is not necessary to provide large and strong boulder stones as 'foundation or soling course' at the lowest layer of the pavement. This method became very popular far and wide.
The construction steps are:
  1. The subgrade is compacted and prepared with a cross slope of 1 in 36 up to a desired width (about 9 meters).
  2. Broken stones of a strong variety, all passing through 5 cm size sieve were compacted to a uniform thickness of 10 cm.
  3. The second layer of strong broken stones of size 3.75 cm was compacted to thickness of 10 cm.
  4. The top layer consisted of stones of size less than 2 cm compacted to a thickness of about 5 cm and finished so that the cross slope of pavement surface was also 1 in 36

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