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Roads
Roman Period


 

Road Construction on Trajan's Column.

The Process of Road Construction

There is no single standard construction for Roman roads but there are some rules. The Romans varied the road construction to accommodate local materials and the terrain.

Roman road consists of three layers:

  1. A bottom foundation layer, often of stone
  2. A middle layer of softer material such as sand or gravel
  3. A surface, or "metalling," usually a gravel, but sometimes paving stones. The upper layers of the road are always laid carefully, "of finer material well-rammed down" possibly in several, successive layers.  The road surface itself consists of layers of finer material with a total thickness of between 2-3in (5-7.5cm) and 1-2ft (30-60cm). Additional layers are added by re-surfacings.
  4. The total depth of a road, from surface to the bottom of the base, could reach 1 to 1.5 meters steeply sloped to each side from the centre.
  5. Most roads were defined by curb stones on each side.

The road was built on a well-constructed embankment to give it a properly drained base. The Romans called this embankment an agger. The agger is a ridge supporting the road’s surface. The Agger was constructed with material dug from lateral ditches. On important routes, the agger can be 4 to 5 feet high and 45 to 50 feet wide. Along less important routes the road is occasionally set directly on the levelled ground surface with stones laid to provide drainage with the lateral ditches barely visible.

Material was derived locally, though if no suitable stone was available it might be brought from a distance. Margary says that the material for the agger was usually dug out of ditches on the side of the road, which he calls "scoop-ditches" which served as storm drains. In stony areas, "there are often well-laid layers of big stones as a foundation for the surfacing," which he says must have entailed quarrying along the way.

These ditches also served to define the road in areas where the surrounding terrain might offer cover for ambush.

On marshy land, roads were given "a proper causeway, and not just an earthen ridge"

Steep ground required a different solution. The roads followed a path such that major natural obstacles were avoided, but in following a direct path "it is inevitable that some local obstacles such as steep-sided valleys will be encountered." To cross these, the road is turned along the side of the valley and continues in a zigzag pattern up the steep slope

According to Chevallier, sand is a common part of the middle layer, serving to lend the road resilience. This is sometimes called the rudus, a layer of "sand or gravel and sand, sometimes mixed with clay".

The ancient Roman roads are not always paved, especially along difficult stretches, but were paved at least with gravel.

There is great variation in the thickness of this upper layer. There are sections of road where the surface layer is only two to three inches thick, while some are one to two feet in the centre and thin to a few inches at the sides.

Romans also classified their roads in order of importance. The important roads were viae publicae (public roads). These were the widest roads, called decumanus maximus, and could be 40 feet (12m) wide.  Secondary roads were viae militares (military roads) built and maintained at the expense of the army. Local roads (actus), and finally privatae (private roads) were built and maintained by the landowner.

Roman roads are generally laid out in a straight line as it was easier to lay out the road given their simple, surveying techniques. But roads frequently follows ridges, rivers or valleys still laid out in straight lengths rather than curves because it reflected their surveying and work practices.

Marcus Vitruvius Pollio (c. 90-20 B.C.), more commonly known as Vitruvius, was a Roman architect and military engineer who served both Julius Caesar and Caesar Augustus. His treatise, De Architectura, consisting of ten books on architecture and engineering, drew from his own experience and that of other architects. The books discuss town planning, building materials, temples, civic and private structures, pavement techniques, water supply, geometry, astronomy and machinery (both civil and military), and is still used as a reference in modern times.

Vitruvius described the process of Roman road construction as follows:
  1. The field engineer, assisted by a stake man aligned the road with a groma and ran levels with chorobates. A plow was used to loosen the soil and mark the trench (fossa) margins. Workmen dug trenches for a roadbed with a depth of 6 to 9 feet, carrying away the dirt in baskets.
  2. The earthen bed was tamped firm. The foundation of lime mortar or sand was laid to form a level base (pavimentum). Next came stones of about 4 to 5 in. in diameter, cemented together with mortar or clay (statument). This layer could be anywhere from 10 inches to 2 feet deep.
  3. The next course (rudus) was 9 to 12 inches of concrete filled with shards of pottery or stone. Atop this layer was the nucleus, a concrete made of gravel or sand and lime, poured in layers with each layer compacted with a roller. This layer was one foot at the sides and 18 inc. at the crown of the road. The curvature was to allow good drainage to the finished road.
  4. The top course was the summum dorsum, polygonal blocks of stone that were 6 inches or more thick and carefully fitted atop the still moist concrete. When a road bed became overly worn, this top course was removed, the stones turned over and replaced. A road was 9 to 12 feet wide which allowed 2 chariots to pass in each direction . Sometimes the road was edged with a high stone walkway. Milemarkers indicated the distance. A cart, fitted with a hodometer was used to measure distances. Later maps detailed routes, miles towns, inns, mountains and rivers. The first roads were quite straight going over hills rather than around them.

How to Build a Roman Road

Sources:

  • Oxford Classical Dictionary, Oxford University Press, 1970 - J.Jahnige, June 1999
  • http://www.teachingideas.co.uk/history/romanrd.htm
  • http://www.dl.ket.org/latin3/mores/techno/roads/construction.htm

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Created: Sunday, September 27, 2009; Last Updated: Wednesday, January 09, 2013  
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