Cut and fill

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Cut and Fill mining method overview[1]

Cut and fill mining is a highly selective open-stope mining method considered ideal for steeply dipping high grade deposits found in weak host rock. Many variations of the general cut and fill technique exist, however this article will focus on overhand cut and fill. Overhand cut and fill evolved from square set stull stoping to provide stronger support. In this method, mining begins at the bottom of the ore body or block and progresses upward. During the mining sequence, the back of the excavation is temporarily supported using rock bolts before the stope is back filled to form the floor of the next level of development. Backfill is designed to provide mild excavation support as well as to provide a strong working floor for personnel and equipment. Backfill selection is dependent on the quality of the host rock and the size of equipment operating a top of the backfill. Progression between stopes is achieved through the construction of raises driven upward through the ore body. A schematic of a typical stope development is displayed below.[2]
In order to better understand this mining method, a link to a video has been provided here.

 

Contents

Selection Considerations


There are a number of ore body features that should be considered when evaluating the applicability of a cut and fill mining technique. These features include:
• Orebody geometry
• Ore grade/method cost
• Rock quality
• Waste Disposal


The ore body must be narrow and steeply dipping as the method relies on gravity to draw ore. Cut and fill is a very expensive mining method, due in large part to the high costs of backfill. This cost can be justified if the ore body is of a particularly high grade. (Cut and fill, Drift and fill) The rock quality designation is a further critical factor. Though the surrounding country rock may be weak, and require support following excavation, the ore should exhibit strength qualities that make it safe enough to be worked under or allow it to be supported sufficiently. In general, cut and fill can be an attractive option surface tailings disposal must be minimized since much of the waste rock can be returned underground for use as backfill.[2]


Planning

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Stope Planning[3]


The first step in the planning process for a cut and fill operation is to determine the type of backfill that will be used. Past fill and hydraulic fill draw additional consideration in terms of planning due to the infrastructure requirements such as paste plants, pumping systems, and piping networks. The chemical characterisitcs of the waste rock must also be considered since these effect its efficacy as a filler element. Floor dilution generated from excessively uncosolidated or weak fill must also be considered. Secondly, access selection (ramp or shaft) may be of importance in the planning process since a ramp may offer the flexibility of continuous mining by reducing the cyclical effect of mining. Thirdly, a preliminary selection of material removal equipment can be made. The geometry of the ore body dictates the size and type of such equipment including slushers, drills, and LHD's. Cut and fill operations with large stope geometry may permit the use of drill jumbos and large LDH equipment whereas smaller stopes may preclude the use of stopper drills and smaller LHD's. This selection is also dependent on the desired productionr ate and ore-pass capacity. In cut and fill mining all work is done inside the stope therefore proper ventilation must be provided to the worker. To accomplish this, adequately sized ventilation raises must be constructed. Given the selective nature of cut and fill, the stope geometry is largely dictated by the ore body shape, thus the design of each stope will be unique and may be subject to great variation even within a designated zone.[3]

     Ground Control and Rock Mechanic Consideration in stope planning

Given the high degree of worker expsoure in cut and fill operations, rock mechanics properties must be extensively known in order to provide adequate protection to workers. In the case of deep mining, where stresses are high, the most economical strategy for ground control is proper stope planning. A staggered stope face advance is used to reduce the stress envelope experience at the mining face. The figure below demonstrates this staggering technique used to reduce stress concentrations. In addition, temporary support may be required to secure rock along near the excavation surface while workers are present.[3]
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 Development

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Cut and Fill Development[3]


The development sequence described below pertains to a highly mechanized over-head cut-and-fill operation where the stope is mined from the bottom upward. This technique is illustrated in the figure below.  

1. An undercut is constructed beneath the stope along its entire strike length. This undercut will form the transport drift from which ore will be removed by LHD’s or rail cars and will also provide access for construction of draw points
2. A ramp is constructed at the side of the stope connecting the transport drift and the first production level. This ramp will provide access for drill jumbos and LHD’s to the production level.
3. A cut is made beginning at the intersection of the ramp and stope to form the first production level. The width of the pillar between the roof of the transport drift and the floor of the production levels is highly dependent on rock mechanics characteristics of the ore.
4. An ore pass is constructed through the floor pillar of the production level to connect the transport drift to the production level.
5. A manway is constructed at a location near to the ore pass to connect the production level to connect the transport drift to the production level.
6. Additional ore passes and manways are constructed at regular intervals along the strike length of the stope as the production level advances horizontally.
7. Auxiliary ventilation fans are installed in the access ways as required delivering fresh air to the working face. These fans may be in the range of 5-15HP depending on the size of the face and the number and size of equipment in the stope.
8. Using a raise boring machine, a raise is constructed connecting the production level to a point where backfill may be fed; either an upper level or surface. This raise may also act as a ventilation raise.
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Drill patern for Cut and Fill[3]

9. The drill pattern is dependent on the drilling equipment employed. A typical jumbo may drill holes 3m deep spaced in a 1m x 1m grid. The drill pattern can be tailored to meet the desired production rate by adjusting the volume of broken rock produced by each blast sequence. The figure below shows a cross-section of a typical drill pattern. 10. Using LHD equipment, the broken rock is dumped down the ore-pass travelling down to the transportation drift where it is gathered at a collection point
11. The drill, blast, and muck sequence is repeated until mining has progressed along the entire strike of the stope.
12. Cribbing is placed at the top of the existing manways and ore passes to form a lining in preparation for backfilling. Old ventilation tubing may be used for this purpose.
13. Backfill enters the stope through the previously constructed raise in the production level roof. The level is filled and allowed to dry. Drying time is dependent on the backfill type and moisture content.
14. The ramp at the side of the stope is extended to the next production level to provide entry for drill equipment. The cycle is repeated until the vertical extent of the stope is reached.[3]


Backfill Requirements

There are a number of options available for the backfill to be used in cut and fill mining, the choice of which is dependent on the support requirements of the area. These options include:

• waste fill
• pneumatic fill
• hydraulic fill with dilute slurry
• high-density hydraulic fill (paste fill)[4]

The highest strength option available is paste fill, followed by sand fill, and finally unconsolidated rock fill. Beyond the support requirements, the fill must be able to support any equipment that is necessary for stope development, as it will become the working floor for the next stope.


Costing Information

When calculating the cost of a cut and fill operation, both primary and secondary development must be considered. Primary development work includes development of the main access shaft or ramp, secondary/escape shaft or ramp, level development, pump rooms, and hoist rooms. The secondary development costs consists of developing the sublevels, footwall ramps, raise preparations, raises to ore, access crosscuts, and ore passes; these are considered operating costs because they are ongoing. Once developments have been accomplished, mining operating costs can differ considerably from overhand cut and fill, conventional cut and fill, and highly mechanized cut and fill. For a highly mechanized cut and fill operation major operational costs include drilling, blasting, raising, mucking and slushing, ground support, cleanout, preparation for backfill, and backfilling material. Cost savings may be achieved through increased mechanization. Subsequently, increased mechanization provides a safer work environment as worker exposure to dangers at the working face is limited. [3]


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Costing information for Cut and fill
[3]


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Costing information for cut and fill
[3]


Infrastructure Requirements

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Paste plant flow sheet


Cut and fill mining requires a number of unique infrastructural requirements mostly pertaining to bakcfill preparation and delivery. These requirements are dependent on which back fill method is chosen. For paste or hydraulic backfill, the mine site must include a backfill plant and the underground network used to deliver the backfill to the working stopes. The underground network will include piping down to each level, and moveable pipes on each level to reach the individual stopes. Many mines use gravity to drive the delivery system, however sometimes it is necessary to introduce pumps into the system. The nature of the backfill means that wear will occur on the pipes, and their condition must be monitored in order to ensure that the system does not encounter down time when it is most needed.[5] An example of the backfill infrastructure required for paste or hydraulic backfill is displayed below.[2] The infrastructure required for rock fill is somewhat different, typically requiring mechanical dumping access so that LHDs or trucks can transport and deliver crushed rock fill.


Advantages and Disadvantages

Advantages

• High selectivity and low dilution ma achieved
• Minimal development is required; low capital cost
• Versatile for mining method; can follow irregular orebodies
• Flexible; mining method can be easily modified
• Low equipment investment relative to other methods
• Minimizes ground movement

Disadvantages

• Cyclical ore production
• Labour and skill intensive
• Dangerous working conditions; work conucted a top freshly blasted rock
• High degree of ground control required
• Expensive and costly ventilation system
• Need for backfill infrastructure (piping and paste plant)
• Not suitable for low grade ore due to high mining cost


Case Study

Lucky Friday Mine

References

  1. Cut and fill schematic. (2008, October 22). Retrieved 01 31, 2011, from Wikipedia: http://en.wikipedia.org/wiki/File:Cut_and_fill_schematic.png
  2. 2.0 2.1 2.2 Brackebusch, F. W. (1992). Cut and Fill Stoping. In H. L. Hartman, SME Mining Engineering Handbook Volume 2 (pp. 1743-1778). USA: Society for Mining, Metallurgy, and Exploration, Inc.
  3. 3.0 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 SME. (1998). Techniques in Underground Mining. Society for Mining, Metallurgy, and Exploration Inc.
  4. (2002). In H. L. Hartman, & J. M. Mutmansky, Introductory Mining Engineering (pp. 365-372). Hoboken, New Jersey: John Wiley and Sons, Inc.
  5. Moerman, A. (2011, 01 27). Chief Planning Engineer, Kidd Creek Mine. (N. Cook, M. MacPhail, & E. Giguere, Interviewers)
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