Construction of a Road

The Romans are believed to have inherited the art of road construction from the Etruscans. No doubt the art grew as it went along and also incorporated good ideas from other cultures.

After the architect looked over the site of the proposed road and determined roughly where it should go, the agrimensores went to work surveying the road bed. They used two main devices, the rod and one called the groma, which helped them obtain right angles. The gromatici, the Roman equivalent of rod men, placed rods and put down a line called the rigor. As they did not possess anything like a transit, an architect tried to achieve straightness by looking along the rods and commanding the gromatici to move them as required. Using the gromae they then laid out a grid on the plan of the road.

The libratores began their work. Using ploughs and legionaries with spades, they excavated the road bed down to bed rock or at least to the firmest ground they could find. The excavation was called the fossa, “ditch”. The depth varied according to terrain.

The road was constructed by filling the ditch. The method varied according to geographic locality, materials available and terrain, but the plan, or ideal at which the architect aimed was always the same. The roadbed was layered.

Into the fossa was dumped large amounts of rubble, gravel and stone, whatever fill was available. Sometimes a layer of sand was put down, if it could be found. When it came to within a few feet of the surface it was covered with gravel and tamped down, a process called pavire, or pavimentare. The flat surface was then the pavimentum. It could be used as the road, or additional layers could be constructed. A statumen or “foundation” of flat stones set in cement might support the additional layers.

The final steps utilized concrete, which the Romans had exclusively rediscovered. They seem to have mixed the mortar and the stones in the fossa. First a several-inch layer of coarse concrete, the rudus, then a several-inch layer of fine concrete, the nucleus, went onto the pavement or statumen. Into or onto the nucleus went a course of polygonal or square paving stones called the summa crusta. The crusta was crowned for drainage.

It is unclear that any standard terminology was used; the words for the different elements perhaps varied from region to region. Today the concrete has worn from the spaces around the stones, giving the impression of a very bumpy road, but the original surface was no doubt much closer to being flat. These remarkable roads are resistant to rain, freezing and flooding. They needed little repair.

River crossings were achieved by bridges. Single slabs went over rills. A bridge could be of wood, stone, or both. Wooden bridges were constructed on pilings sunk into the river, or on stone piers. Larger or more permanent bridges required arches. Roman bridges were so well constructed that many are in use today.

Causeways were built over marshy ground. The road was first marked out with pilings. Between them were sunk large quantities of stone so as to raise the causeway 6 feet above the marsh. In the provinces, the Romans often did not bother with a stone causeway, but used log roads.

Outcroppings of stone, ravines, or hilly or mountainous terrain called for cuttings and tunnels. Roman roads generally went straight up and down hills, rather than in a serpentine pattern. Grades of 10%-12% are known in ordinary terrain, 15%-20% in mountainous country.