blasting, and the resulting space is packed with fill material. With drift and fill mining, after completion of the first drift, a second drift is driven adjacent to the first. Additional drifts are developed until the ore zone is mined out to its full width, after which a second cut is started on top of the first cut.
Shrinkage stoping is a mining method that can be used for steeply dipping orebodies. Ore is extracted in horizontal slices, starting at the stope bottoms and advancing upward. Most of the blasted rock remains in the stope to provide a working platform for the miner drilling holes in the roof, and and it also serves to keep the stope walls stable. Because blasting increases the volume of the rock by about 60 percent, some 40 percent of the ore is drawn at the bottom during stoping in order to maintain a working space between the top of the blasted rock and the roof. The remaining ore is removed after blasting has reached the upper limit of the stope. Shrinkage stoping allows mining that is very selective, but one disadvantage is that there is a delayed return on capital investment because most of the ore stays underground until mining of the stope is completed.
Room and pillar mining is commonly done in flat or gently dipping ore-bodies. Room and pillar mining accesses an orebody by horizontal drilling advancing along a multifaced front, forming empty rooms behind the producing front. “Pillars” of rock are left between the rooms for support to keep the roof from caving. The usual result is a regular pattern of rooms and pillars, with their relative size representing a compromise between maintaining the stability of the rock mass and extracting as much of the ore as possible. In some room and pillar mines, once the rooms are mined out the pillars can be mined, starting at the farthest point, allowing the roof to collapse. This allows the ore contained in the pillars to be accessed.
Block caving is a large-scale mining method that is used to mine massive orebodies with specific characteristics that enable gravity to do part of the work. Preparation for block caving requires long-range planning and extensive initial development involving a complex system of excavations beneath the orebody. An “undercut” is mined under the orebody, and cavities are excavated to serve as repositories for caving rock to be collected. The orebody is drilled and blasted above the undercut, and ore is removed through the accessway. Because of the characteristics of the orebody, material above the first blast area falls into the collection areas. As ore is removed from the collection areas, subsequent caving provides steady availability of ore. Extensive rock bolting and concrete lining are required to keep the openings intact, and if caving stops and removal of ore continues, a large void may form that can have the potential for a sudden and massive collapse.
Ground Control in Underground Mining
Ground control—the prevention of rock collapse into a mined cavity—is an integral part of mine design to ensure a safe underground working operation.