Central Africa tunnels tenders

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A tunnel is an underground passageway, dug through the surrounding soil/earth/rock and enclosed except for entrance and exit, commonly at each end. A pipeline is not a tunnel, though some recent tunnels have used immersed tube construction techniques rather than traditional tunnel boring methods. A tunnel may be for foot or vehicular road traffic, for rail traffic, or for a canal. The central portions of a rapid transit network are usually in the tunnel. Some tunnels are used as sewers or aqueducts to supply water for consumption or for hydroelectric stations. Utility tunnels are used for routing steam, chilled water, electrical power or telecommunication cables, as well as connecting buildings for convenient passage of people and equipment.

Central Africa is a subregion of the African continent comprising various countries according to different definitions. Angola, Burundi, Cameroon, the Central African Republic, Chad, the Democratic Republic of the Congo, the Republic of the Congo, Equatorial Guinea, Gabon, Rwanda, and São Tomé and Príncipe are members of the Economic Community of Central African States (ECCAS). Middle Africa is an analogous term used by the United Nations in its geoscheme for Africa. The main economic activities of Central Africa are farming, herding and fishing. Crop production based on rain is possible only in the southern belt. Slash-and-burn agriculture is a common practice. Flood recession agriculture is practiced around Lake Chad and in the riverine wetlands. Nomadic herders migrate with their animals into the grasslands of the northern part of the basin for a few weeks during each short rainy season, where they intensively graze the highly nutritious grasses. When the dry season starts they move back south, either to grazing lands around the lakes and floodplains, or to the savannas further to the south. Fisheries have traditionally been managed by a system where each village has recognized rights over a defined part of the river, wetland or lake, and fishers from elsewhere must seek permission and pay a fee to use this area. Oil is also a major export of the countries of northern and eastern Central Africa, notably making up a large proportion of the GDPs of Chad and South Sudan. Countries in Central Africa are Angola, Cameroon, Central African Republic, Chad, DR Congo, Republic of the Congo, Equatorial Guinea, Gabon, São Tomé and Príncipe, Rwanda, Burundi.