EMBANKMENTS. in engineering. masses of earth, rock, or other materials artificially formed and rising above the natural surface of the ground. They are built chiefly either to carry railways, common roads, canals, etc., over depressions or for hydraulic purposes, such as the formation of dams and reservoirs, or for de fenses against the overflowing of rivers and the encroachments of the sea or lakes. form and dimensions of embankments and the materials and methods of construction adopted depend upon the purpose for \Odell the embankment is to be used. In all cases except where they are confined by external constructions, such as re taining walls, embankments are trapezoidal in section with a broad base and a narrow top con nected by sloping sides. The material employed in constructing embankments varies from almost any available mixture of rocks and earth or other fragmentary material used in railway em bankments to very carefully selected finely divided earths used in forming water-tight em bankments for dams, reservoirs, dikes, levees, and canals. The method of construction likewise
varies from a mere dumping of the material in place to he roughly leveled of and allowed to settle under action of the elements, to a careful building up of the selected material in layers a few inches thick which are carefully compacted by ramming or rolling. Embankments for dams and reservoirs are instances where the more care ful methods are employed. Embankments for railways are examples of less careful methods of embankment construction. In constructing embankments it is usually necessary, as a basis upon which to determine the cost, to calculate their volume. This calculation is made by means of the prismoidal formula for determining the volumes of prismoids. For a full discussion of the prismoidal formula and its application to earthworks, the reader should consult : Johnson, Theory and Practice of Surreying (New York, 1900) ; and Taylor, Pri•mobial Formula- and Earthwork (New York, 1898). See DAMS AND RESERVOIRS: RAILWAYS.