Raises can be used to connect different vertical mine elevations with each other (Figure 3.40). Raises are used for a number of purposes such as ventilation, ore passes, and travelways. Ore passes are usually designed with an angle exceeding 55° to the vertical in order to allow the broken rock to flow
by gravitational means. The modern mechanical methods of raising in open stoping include raising by longhole drilling and raise boring.
Raising by longhole drilling consists of drilling holes in a suitable pattern, through the full depth of the ground, up to 60 m long in some cases. Drilling is usually carried out from the top level using conventional longhole drilling equipment. In some cases, such as in a top-down bench extraction, uphole drilling is utilized. Downhole raises are blasted in sections of approximately 5–10 m, while uphole raises are blasted over their entire length, usually less than 25 m.
Raise-boring machines are capable of reaming raises with a sufficient diameter and height to match any stoping or mine development requirements. Although the cost per cubic meter of rock removed is higher, this type of development offers speed in advance, compared with conventional drill and blast methods. Raise boring is of particular importance for mine ventilation. Decline development can be undertaken blind and with increasing depths due to exhaust ventilation by raise-bored ventilation shafts. A major advantage is their smooth-walled finish, which reduces air resistance.
Fill masses are required to provide large-scale ground support, as well as localized stability for pillar recovery. The key stages of a fill operation for sublevel stoping are material and stope preparation, fill delivery or reticulation, placement, and drainage. Development for fill delivery and reticulation is usually addressed during a stope block design. The options may include fill delivery from a surface material station using raise holes or boreholes, trucked to stopes via ramp access or from underground sources.
Underground fill reticulation is achieved by means of gravity feed or pumping to stoped-out areas. Conveyor belts, pipeline distributions, or standard or ejection tray trucks can be used. Fill reticulation for massive orebodies usually requires long-term development within the crown of an orebody. In such cases, crown subsidence may threaten the stability of the development associated with a fill system above an orebody. To minimize this likelihood, progressive tight filling of stope voids is required, as the combined effect of unfilled stope crowns can result in regional subsidence. Geological and operational factors such as delaying filling can influence the rate of subsidence (Logan et al., 1993).
Large unfilled voids as well as progressive stoping may cause dilation of geological discontinuities, which in turn can be linked to rotation and slid ing of large blocks within the crown of a deposit (Logan et al., 1993). This localized block behavior may produce significant changes in the relative elevations along the strike of an orebody. Continued monitoring using precise level-surveying techniques can be used to obtain an understanding and to manage subsidence.
Long-Term Production Scheduling
Production scheduling is the highest level of scheduling and provides a long term view of the mining process by focusing on issues such as ore grade, extraction sequences, and production quantities. Production schedules typically extend over a number of years and are expressed in terms of ore sources relating to stoping blocks (Trout, 1997). These schedules can extend through to the life of a mine, depending upon which event comes first. The items included in a scheduling exercise are long-term production targets, fill, development, raising, and diamond-drilling requirements. Annual estimates for equipment replacement, capital, and operating expenditure may also be determined. The most common restrictions imposed on scheduling may include capital availability, expected life of the mine, infrastructure, and equipment life.