Design and construction of Roman roads; the case of Via Egnatia in the Aegean Thrace, northern Greece
Romans, the first real road designers, designed and constructed the first organized road system in Europe. This system was in use for almost 2,000 years with some parts still in use as secondary roads. Via Egnatia, the first highway to cross the Balkan Peninsula, was the first road built by Romans outside Italy. It was constructed in the second century B.C. The road began in Dyrrachium (modern Durres), by the Adriatic sea, and passed through Serbia, Macedonia (Thessaloniki) and Thrace terminating at Cypsela (east of Evros river) and later extended up to Konstantinoupolis. The total length of the road from Dirrachium to Cypsela was about 750 km. From the study of sections of the road surviving in the Thrace region, Greece, it appears that the design and the construction of the road (in the area under discussion) was based on well-known specifications. Specifically, the road was always adapted to local topography, geomorphology and ground conditions. Thus the road avoided the difficult and unstable ground, the close curves and the steep grades. The thickness and the layering of the pavement varied according to the foundation conditions. In stable, rocky ground, the pavement consisted of only one layer of well-fitted cobble stones; whereas, in soft and unstable ground the soft soil was excavated and replaced by several layers of cobbles, gravels and rubbles held together with compacted sandy soil or lime mortar. Up to four stone layers have been found in an archaeological excavation in the road pavement in the Thrace area. Some layers were made waterproof by well-compacted clay soil. The thickness of the pavement varied from 25 cm to more than 150 cm. The materials used were mainly of local origin. The width of the road ranged from 4 m to 8 m, depending on the ground conditions and the traffic demand. In cities its width reached up to 20 m to accommodate the increased traffic. The horizontal curvature (curves) of the road was usually more than 100 m (R>100 m). Only in a few cases in mountainous areas curvatures of R = 10-20 m were found. The grade of the pavement normally was 1 to 2 percent, but in mountain regions gradients of 16 to 18 percent were observed. Gradients up to 20 percent were measured in a stretch of the road 2 km west of Kavala. The cross section of the pavement was convex, with grades perpendicular to its axis from 5 to 10 percent, for rapid drainage. Large rock blocks were placed at the sides of the pavement, raised above the surface, to prevent lateral spreading of the pavement and deterring carts and wagons from sliding off the road. A series of elongated rock blocks were constructed in the middle of the pavement, possibly for separating the opposite traffic. It is suggested that the remnants of this Roman road be preserved not only for their historic value, but also for their engineering significance.