Roman Construction and Infrastructure Part 2: Roads and Bridges 3.0 The Roads and Bridges of Rome In slightly over 400 years, Roman legions built 53,568 miles of roads. Since one Roman mile equals 1,620 yards, this translates into about 50,000 standard miles. This mileage was spread over 34 countries through a network of 372 highways. Originally, these roads were financed by the land-owners through which these roads passed. Later, they were financed by private donations. Eventually, taxes were collected at city gates for new construction as well as maintenance of existing roadways. 3.1 Road Construction Road construction was based primarily on the importance of the road and the material found in the vicinity. These roads were directionally straight, meaning they proceeded in a generally straight line with few curves. Considerable attention was paid to the subgrade, resulting in a high quality road system which required little maintenance. A typical roadbed would be built by first removing the upper layers of soil until firm material was found. The resulting ditch was filled with four to six inches of sand covered over with about one foot of flat or squared stones laid in cement or bound in clay. This layer was known as the statument. Following this layer was the rudus, a ten inch thick bed of concrete made from small stones, gravel, crushed brick, or tile. Over the rudus was the nucleus, an eighteen inch layer of concrete which was firmer than the rudus. This layer was rolled out in successive layers until the desired thickness was obtained. Finally, the summa crusta, or top layer, was laid. This layer varied according the materials found nearby. In Italy, the summa crusta was silex shaped in irregular polygons from one to three feet in diameter. Roads in Britain were sometimes surfaced with stones of flint laid in cement. Less important roads, such as those found in Africa, were surfaced with gravel. Some roads were not even surfaced at all. Roads within city limits sometimes even had curbstones. The main Roman roads were usually fifteen to sixteen feet wide with a depth of three to four feet. In the Alps, the roads narrowed to six feet. The road surface was generally crowned to allow water to run off of the surface. It was not uncommon for roadways to have gradients of ten to twelve percent. Sometimes slopes of as much as fifteen to twenty percent were used, not only in the mountains, but for bridge approaches as well. 3.2 Travel This network of roads allowed the Romans to move rapidly over their domain. Ox drawn vehicles could cover eight miles a day in summer and ten in winter. Horse relays were able to cover fifty Roman miles in twenty-four hours. Since there were no customs barriers at that time, travel was always open and free. Police guarded against highwaymen. With this unprecedented mobility, it became necessary for travelers to not only know how far they had gone, but also made necessary the building of lodging along the way. The first objective was achieved by constructing milestones along the roadways. These markers were eight feet high, twenty inches in diameter, and weighed two tons. They gave the distance to all the principle cities in the region. As for lodging, this was achieved as early as 2 BC An average of one stop every ten miles was constructed. These way stops were of three basic types. Mutationes were spaced about ten to twelve miles apart. They provided fresh horses for the emperor's messengers, similar to the Pony Express stations. Then there were mansiones, places where travelers could spend the night or procure supplies. They were located about thirty to forty miles apart. Finally, civitates were located at the junctions of major roads and in principle cities. The civitates were open to all who traveled and provided all the comforts of a modern Roman home. 3.3 Notable Roadways Within this vast network of roadways, several roads are of particular interest. The first is the Via Appia, constructed in 312 BC, by Appius Claudius. This roadway is Rome's oldest road. Another interesting road is the Via Hadriana. This road led from Carthage to Thugga. After digging a ditch wider than the proposed road, heavy human operated rams were used to compact the soil. Nine inches of stone was added, and then topped with a thick course of gravel. This road when finished was eighteen feet wide. Along the road can still be seen the remains of seven large cities, ten dams and bridges, seven milestones, and three triumphal arches. One final route of interest is the Kazanpass along the Danube river. This roadway was carved out of the rocky cliffs. This roadway was only wide enough for one wagon due to the difficulty in removing the rock. The usual method of excavation was to pry out pieces of rock following natural fault lines. If this didn't work, the rock was heated and vinegar was poured over it. This reaction caused the rock to split open, facilitating the removal of the rock. 3.4 Bridges Another interesting aspect of Roman construction was bridge building. The Romans constructed over two thousand bridges, several of which can still be seen today. Since comparatively little was known about bridge construction at that time, most bridges consisted of short spans on heavy piers. This is the primary reason many structures are still in existence. The primary reason behind most bridge failures was with the piers. These piers were generally shallow since pumping facilities were inadequate to go very deep below the water table. Instead of pumping, they usually tried to divert the stream. If this was impossible, a foundation of rip-rap was used. This had two disadvantages, it did not serve as a adequate foundation for the bridge, and also decreased the size of the waterway. Therefore, the Romans tried to avoid the need for piers in midstream by selecting the shortest crossing points. 3.5 Notable Bridges There are several bridges that are worth mentioning. The first is the Pons Aelius, finished in 153 AD This bridge had eight arches. The three middle arches each had a fifty-nine foot span. The center of this bridge was fifty feet above the water, and had an eleven percent approach gradient. The construction of this bridge was fairly common as Roman bridge construction went. It was imitated throughout Europe during the early middle ages. Another bridge of interest was the wooden arched bridge built by Trajan in 106 AD. Three parallel courses of timbers which were tied together by braces composed the arched members. These members continued upwards to form the supports for the floor of the bridge. The piers were made by sinking barges filled with cement. Unfortunately, the ends of the arches were not adequately secured, and the long timbers buckled, resulting in the collapse of the structure. A single arch bridge was constructed to cross a fifty foot wide gorge. This bridge, built in 3 BC, and known as the Pondel, had its abutments founded on solid rock one hundred and thirty feet above the bottom of the gorge. Given their technology, this had to be quite a feat. Julius Caesar also was quite a bridge builder. He supervised the construction of a bridge at Confluentes on the river Rhine during the Roman conquest of Gaul. This structure was particularly difficult since it spanned a river which was 1300 feet wide, 6 feet meters deep, and flowed at 6 feet per second. To overcome this, Caesar used parallel sets of piles. One set of piles was inclined in the direction of the current. Forty feet downstream, a second set of piles was inclined in the opposite direction. These piles were held together by iron braces. The result was a structure that was so rigid that the stronger the current, the more tightly the piles held in place. Another interesting fact about this wooden bridge was that it was constructed in only ten days. The other interesting feat of Caesar was the construction of two parallel bridges across the swamps of Oise. Each bridge was 2000 feet long and 11 feet wide. The first step was to cover the swamps with brushwood. In deeper areas of the swamp, whole logs were sunk. Over this brushwood was placed a horizontal framework of planks. These planks were held in place by long wooden pegs. On this framework was smaller planking over which foliage was placed. Finally, a layer of sandy gravel was placed. A unique aspect of these bridges was that it was divided into units 10 feet long and prefabricated. Perhaps the greatest bridge in all of the Roman world was the Alcantra, in Spain. This bridge was constructed by Trajan in 109 AD over the river Tagus. This structure was six hundred feet long and rose one hundred and seventy five feet above the water. It was carried on six cut stone arches without mortar. The middle two arches have spans of one hundred eleven and one-half feet and one hundred eighteen feet. As can be seen from the above picture, this bridge is still in excellent shape even after 1900 years of use.