12.12 Downstream Method
The design requirements for downstream method of construction are similar to conventional water storage dams. It begins with a starter dam constructed of compacted borrow materials. However, this starter dam may also be constructed of pervious sands and gravels or with predominately silts and clays to minimize seepage through the dam. If low permeability materials are used in the starter dike, internal drains will need to be incorporated in the design. A variety of tailings depositional techniques can be used in conjunction with the down stream construction method, but peripheral spigottting of tailings is very common. Coarse tailings can be spread in thin layers utilizing on-dam cycloning, or they can be hauled from a central cycloned stockpile, then spread and compacted. If the volume of coarse tailings is not sufficient to construct the dam, local borrow materials may be incorporated for part of the structure. If coarse rock is used, due to its porosity, a filter or impervious upstream membrane is required to prevent piping of the tailings through the rock.
The downstream construction method allows for incorporation of drains and impervious cores to control the phreatic surface. Drainage controls help to control the phreatic surface and minimize the chance for built-up of pore water pressures which reduce shear strength. Due to the ability to incorporate drains into the design, this method of construction is well-suited to conditions where large volumes of water may be stored along with the tailings solids. This method of construction provides a degree of stability not found in upstream construction due to the ability and ease of compaction, the incorporation of phreatic surface control measures and the fact that the dam raises are not structurally dependent upon the tailings deposits for foundation strength (figure 10).
Figure 10: downstream construction of retaining dam
A major disadvantage of this method is the large volume of fill material required toraise the dam. The increased volume of fill required can dramatically increase the cost of this method of construction if the tailings from the mill cannot provide a sufficient volume of sand.