|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:
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.
Vitruvius described the process of Roman road construction:
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.
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.
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.
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.
(Note: The use of paving stones was rare on country roads and this probably describes construction in Rome itself)
‘The first task here is to trace furrows, ripping up the maze of paths, and then excavate a deep trench in the ground. The second comprises refilling the trench with other material to make a foundation for the road build-up. The ground must not give way nor must bedrock or base be at all unreliable when the paving stones are trodden. Next the road metalling is held in place on both sides by kerbing and numerous wedges. How numerous the squads working together! Some are cutting down woodland and clearing the higher ground, others are using tools to smooth outcrops of rock and plane great beams. There are those binding stones and consolidating the material with burnt lime and volcanic tufa. Others again are working hard to dry up hollows that keep filling with water or are diverting the smaller streams.’
A poem by Statius praising the via Domitiana
Adam, Jean-Pierre. Roman Building: Materials and Techniques. 1994.
Margary, Ivan. Roman Roads in Britain. 1973.