the aggregate of operations performed by a dredge for extraction of submerged mineral deposits. In the dredging of placers, a distinction is made according to the system used: with a single face (single longitudinal and single transverse), with adjoining faces (adjoining longitudinal and adjoining transverse), and with residual pillars (longitudinal and transverse). Systems consisting of various combinations of the main types are widely used. In dredging, the methods of excavation differ according to the sequence and procedure of extraction of rock in the vertical and horizontal planes of the face. The layer method of excavation is the most widespread; it makes possible the control of the filling of scoops depending on the nature of the rock and its content of valuable components and the separate extraction of barren rock and productive sands. The length of the extracting season for dredging depends on the climatic conditions of the deposit area. It generally ranges from 150-160 days in the extreme northeast to 340-350 days in the Urals and Western Siberia.
Multiscoop dredges are used to a limited extent for working placers on the ocean floor. They differ from their continental prototype in terms of the distinctive design features of their maneuvering, transportation, and dumping equipment. During operations in the open sea, the dredge is usually navigated and maneuvered along the face by means of a cable-anchor device. The availability of sufficient dumping capacity (since the surface of the deposit is located significantly below the water level) makes possible the gravity-flow transportation and free disposal of washing wastes in worked-out areas. Cable-bucket dredges equipped with one or more (as many as four to six) grapples or draglines are considered to be promising for dredging placer deposits on the ocean floor. A great deal of attention is being devoted to the possibilities of using pumping and hydraulic and pneumatic suction dredges in working placers. Pumping dredges work placers most successfully at depths of 9-27 m; however, they are also built for operations at depths up to 80 m. As of 1971, work was being conducted on the construction of a suction dredge with deep-well pumps that would make economically feasible the working of ocean-floor deposits lying in deep-water areas (1,200 m and deeper).
Efficient working of heavy gravel in conditions of deep-water excavation is implemented by dredges equipped with airlift working members. The use of an ejector working member, which makes possible a further increase in the efficiency of working rock through the addition of hydraulic attachments for loosening, thus significantly intensifying the process of excavation, is promising under such conditions.
V. G. LESHKOV