once. Instead, the engineers divided the entire construction site into individual building areas. Through archaeological research, the boundaries of these building areas have been determined. For the Eifel aqueduct, they were each 15,000 Roman feet long (4,400 m or 2.7 miles in modern units). It has further been demonstrated that the surveying took place separately from the building, as is in fact the rule today in large construction projects. For each metre (3.3 ft) of aqueduct, approximately 3–4 m³ (100–140 ft³) of earth had to be dug up, followed by 1.5 m³ (50 feet³) of concrete and bricklaying, along with 2.2 m² (24 feet²) of plaster sealant. The complete labour expense is estimated at 475,000 man-days: with about 180 possible construction days in the year due to weather conditions, 2,500 workers would have worked 16 months to complete the project. The actual construction time appears to have been even longer, since this estimate leaves out the question of surveying and production of the building materials. After construction, the building trenches were filled in, the surface flattened, and a maintenance path built. The maintenance path also served to delimit areas where farming was not permissible. Other Roman aqueducts show similar facilities. One of the aqueducts to Lyon (France), the Aqueduct of the Gier, was marked with the following inscription: Roman surveying After a good location for the aqueduct was selected, it was necessary to guarantee a constant slope downwards in its course. Using devices similar to modern levels, the Roman engineers were capable of maintaining a slope as small as 0.1 percent—one metre of fall for every kilometre of aqueduct. In addition to the slope, it was necessary for the various building sections to be able to join up, while still maintaining a constant downward slope. The Roman constructors of the Eifel aqueduct carefully made use of the natural fall of the land. If the work from one segment arrived too high for the next segment, they built a small pool into the course to calm the falling water. Roman concrete The concrete used for the Eifel aqueduct was a combination of lime, sand, stones, and water. Boards were used to make a form into which the concrete was packed. Modern tests of the quality of the concrete show that it would pass current standards. This particular concrete is called opus caementicium in Latin. Operation of the aqueduct For the 180