The Overburden of waste and uneconomic mineralized rock is required to be removed to mine the useful mineral resource in a surface mining operation. In this process a dump is formed by casting the waste material and dumping it in nearby area. The dump so formed is known as mine waste dump. Waste dump may be classified as internal and external dump. External dump is created outside the pit whereas internal dump is created inside back of the mining area.
These waste rock dumps are heterogeneous in terms of grain size and structure. The fragmentation of rock in a dump is a product of number of mechanical processes, like drilling, blasting, ripping, etc. Consequently, dump rock may range in size from clay particles to boulders (e.g. less than 0.1 mm to greater than 1 m in diameter). Natural gravity sorting of rock poured from a haulage truck onto a waste dump face may result in a vertical size distribution, finer materials tend to remain near the top and coarse materials tend to roll down the face toward the toe of the dump.
As most of the waste rock disposal facilities at open pit mines are constructed with run off mine materials using trucks, there is very little control over the exact size distribution of these materials. However, modern blasting technology allows considerable control on the size of largest particles. In-pit crushing of waste rock is also done at a few mines to reduce the size of material, to transport it by belt conveyors out of the pit to the disposal facility. Often overburden and waste rocks are end-dumped from the trucks and the excess material is bulldozed over the storage facility edge to construct slopes at the angle of repose, where the outer slope is just stable under the static loading conditions at the site and is typically 37–40°.
The manner in which a waste rock dump is designed and constructed can also result in significant differences in structure. Commonly, construction of a dump progresses by addition of material to its top at the face, allowing waste rock to form a continuously renewed veneer on the face. The dump progresses outward horizontally, as successive layers are added to the face. However, some dumps are engineered in other ways, resulting in significantly different internal structures. For instance, in order to enhance stability and minimize the release of fine sediments into the down-stream environment, some dumps are designed with a French drain, a layer of coarse and durable rock such as chert, placed at the base to allow unrestricted flow of stream through base of the dump. Waste rock dumps are also constructed in layers resulting in a sequence or stack of dumps.
The shape of a mine dump is mainly based on the nature and topography of the area, where they are emplaced. Mine dump can take the shape of one or a combination of many different configurations, such as valley-fill, cross-valley, side-hill, ridge, or heaped, depending on the topography of the area. The dumping method of material can be used to classify dumps into five basic types:
• End dumping-dumping rock over dump face resulting in some particle size segregation down slope towards the toe of the rock pile, with particle size generally increasing
• Push dumping - dumping from trucks, followed by leveling and pushing by tractors and shovels resulting in particle size segregation: finer at the top and coarser at the toe of the dump slope
• Free dumping or plug dumping - dumping in small piles on the surface of the rock pile, grading the material, and compacting in layers or lifts resulting in dense layers with no real particle size segregation
• Dragline spoiling-deposited on the surface without construction of lifts and minimal compaction resulting in dense layers with no real particle size segregation because of relatively low overall height of the spoil piles, typically used in coal mining
• Mixing of waste rock with tailings.