Longitudinal longhole retreat

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Longitudinal longhole retreat is a branch of the generic mining method known as sublevel stoping. For longitudinal retreat the long axis of the stope is along (or parallel) to the strike of the orebody. It may also be referred to as bench stoping or sublevel benching. [1] Three variations of this mining method are avoca, eureka, and creeping cone, where avoca is used the most. Using longitudinal mucking access requires that remotely operated LHDs are used </ref> to ensure workers are not at risk by being under a working back. This is depicted in the Figure below. This method is similar to Sublevel open stoping and Blasthole stoping methods. It is a branch of longhole mining differing from Transverse longhole stoping, where the long axis of the stope is perpendicular to the strike of the orebody. The width of these designs is almost always greater than 15 m, compared to the case for longitudinal retreat where the width of the design is very seldom greater than 20 m.[1]

To choose the appropriate variation for longitudinal longhole retreat mining, one must know the geotechnical conditions and the mine’s operational requirements, which vary for each mine. This includes faulting, degree of jointing and the failure method of these joints. The intact rock strength and local stress conditions are also important.

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Longitudinal longhole retreat mining[1]

Contents

Orebody Characteristics and Geotechnical Conditions

Longitudinal longhole mining may be used when there is a steeply dipping orebody with the inclination of the footwall greater than that of the angle of repose of the blasted muck. Also, the hanging wall and footwall, must be fairly good or better but group support in the form of cable bolts must be installed, and the mineralized zone must have regular ore boundaries. [2] The size and shape of the extraction block can be altered to fit the orebody geometry and the mine infrastructure. Any openings created during this process may be filled or left unfilled. [3] When longitudinal mining is used, most of the development necessary for this mining method may be kept in the orebody.[1] Rock mass quality is another variable to take into consideration when designing a stope. There are very few operations in Canada that have been able to successfully use a width greater than 20 m in their design. [1] Referring to the graph below, it is possible to see values for designed longitudinal mining. This width is dependent upon the orebody rock mass quality. These values were obtained from a study conducted in mines across Canada. [1]

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Longitudinal mining width with respect to orebody rock mass quality[1]

Several components relating to the success of mining with the longitudinal retreat method are:

  • Drilling and Blasting
  • Safety and Performance
  • Sequencing
  • Backfill
  • Dilution
  • Ground support

Drilling and Blasting

Sublevel stoping uses two main drill systems. They are in-the-hole hammer (ITH) and top hammer drills, where the former is used in longhole applications. For ITH hammer bits, the drill-hole diameter ranges from 75 to 150 mm. [3]It is very important to determine a stable span length for each bench. This span directs how many production rings can be blasted prior to the commencement of any backfilling processes. [4] It is also important to ensure that blast damage is under control. Blast induced damage will weaken a rock mass, and lead to stability problems, in particular when the size of the excavation is to be increased. [5] Hanging wall holes should be loaded more lightly as a result of this. [4] The hanging wall hole is usually scheduled to be the last hole fired in a ring, which reduces the blast vibrations in the wallrock [4]. Firing multiple rings in a blast allows for continuous mucking, and decreases the number of blasts needed in a bench. This helps to cut-down on the effect of vibration on the unsupported hanging wall.

Refer to the drilling and blasting section of the sub-level open stoping article for a more detailed explanation of the three main different drilling patterns including parallel drilling.

Safety and Performance

For longitudinal mining, keeping the operator away from the mucking area minimizes risk such as falling rock or remote operating hazards. Other hazards include heat, noise, vibration and dust. As well, drilling operations happen over top of mucking points. By operating remotely as opposed to directly in an LHD, these hazards are decreased significantly. Further, this mining method works on a retreat pattern where the equipment and operator work under a supported back.[3] [4] The diagram below is a typical performance factor model for bench stoping operations.

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Performance for bench stoping operations[6]

Sequencing

In regards to sequencing for longitudinal longhole mining, stopes may be retreated in the direction of the cross-cuts with a top-down or bottom-up sequence.[7] This is shown below.

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Longitudinal view of top-down and bottom-up sequences of extraction[7]

In order to control dilution between individual stopes along the strike, a top-down sequence normally requires the installation of permanent rib pillars. [7] A crown pillar also needs to be used in order to control dilution and maintain stability. This also means LHDs are operating under backfill which is usually weaker than the rock. As well, it is used to set apart any unconsolidated backfill that is used in the upper stopes as the mining sequence continues. For the bottom-up sequence, fill is required so that there is a working floor as the extraction of the ore continues upwards. If rib pillars are used along the strike of the orebody along with backfill then a crown pillar is not usually used. Two access crosscuts will cost more, but increase the tonnage allowing for the best stress re-distributions. The initial stopes may be located in the center of the mining block with retreats to follow towards the abutments.[7] A schematic of this is seen below:

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Longitudinal view of extraction sequence with two access points[7]

Backfill

The use of backfill is a key role in longitudinal stoping, as it may occur at the same time as mucking does, depending on the variation that is being used. Backfill is any material that is placed underground to fill the voids created by the extraction process. [6] Backfill serves as a working floor when the bottom-up mining sequence method is used. Backfill also helps to reinforce exposed spans by minimizing deformation and any dynamic loading of the excavated walls due to blasting. [6] Once the ore has been extracted, the void from the bench stope is to be filled with either hydraulic or waste backfill. This new layer is now the new level for the next stage of the extraction process. [6] When blasting, it is important to ensure that extra dilution does not occur. This is due to the fact that the muck from the blast could potentially backfill to become lose, and thus contaminate the ore during future mucking. There are three main methods involving the use of backfill in longitudinal longhole mining.

Hydraulic fill and the recovery of pillars

This involves the extraction of a bench to a maximum stable unsupported strike length. Backfill is used with brick bulkheads. This is shown below.

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Shows the use of hydraulic fill with pillar recovery[6]

Full avoca extraction method

The bench is extracted along the entire length of the stable span. There is no free space for blasting that is to follow on the next level, as seen in the figure.

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The use of the avoca longitudinal mining method[6]

The use of permanent pillars with backfill

This method uses pillars in between hanging wall spans for the length of the bench. It is thus imperative to calculate the optimum distances between the pillars so that the numbers of pillars required is kept to a minimum. [6]

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Shows the use of hydraulic fill with pillar recovery[6]

Dilution

A good approach to the control of dilution is to examine its potential different sources during the design and extraction stages, as opposed to just measuring it once mining has commenced. [6] Unplanned dilution is material that is not within the engineered stope boundary. It is backfill or waste rock that was introduced into an open stope unexpectedly. [8] An illustration of this is depicted in the diagram below on the left.

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(Left)Vertical section of stope depciting planned and unplanned dilution, (Right)Vertical Stress Distributions around a shallow dipping stope, [8]

In regards to the amount of dilution, it is determined by dividing the value of the waste mined by the value of the ore mined. Another way to calculate it is taking the amount of waste and dividing it by the waste plus the ore. This will give the percentage of muck in each LHD bucket. Unplanned ore dilution is a result of stope properties, rock mass characteristics, blasting patterns, and the geometry of the orebody. [8] In considering stope stability in longitudinal mining, the hanging wall is the face that will be most affected by unplanned dilution. The chance of failure is increased by gravity because the hanging wall is usually the largest open span. Also, since the effect of gravity is affected by the dip of the stope, more stress will occur around the hanging wall for shallower dipping stopes. This is observed in the above diagram on the right.

Longitudinal Longhole Retreat Variations

If a longitudinal longhole mining method is to be used, then there are three variations to choose from. None of these methods are very common in Canada. Avoca is the primary method, with the eureka and creeping cone methods being secondary and tertiary in common usage.

Avoca

Avoca mining consists of an undercut level and an overcut level. The cross cuts are in waste and connects the main transport drift to one end of the stope, where there is ore development along the length of the stope. The overcut level contains a backfill drift which goes to the opposite end of the stope. With this method, there is no limit to the length of the stope, and two tasks are occurring at once. First, ore is drilled by longhole and then drawn off in retreating vertical slices. Second, unconsolidated backfill is placed over the bench through the back of the stope from the drift of the footwall. [9] Referring to the figure below, it is possible to understand the avoca mining sequence. Initially, slices 1 and 4 are removed, as seen in the first stage. Next, a series of drill holes are drilled connecting slices 1 and 4. These holes are blasted and the muck is extracted via the use of an LHD in slice 1, the undercut. There are no draw points for this method. Hence, the ore is mucked from the undercut. [10] While retreating occurs, so does backfilling, at the other end of the stope which is the second stage of this mining process. As long as a gap is kept between the extraction and the backfilling, dilution is minimized. [3] Once the backfilling is done for this level, stage three is started and the cycle starts over again. [3]

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Avoca mining sequence[3]

Eureka

This method in inspired from avoca. The eureka mining method involves eliminating the waste development to the end of the stope on the overcut. This method uses consolidated backfill, and an artificial slot raise is used for the start of the next stope.[10]This method was used at Bousquet Mine, Quebec. To mine a lift, a primary stope of 15 m long was blasted with a raise bore of 1.07 m in diameter. Once the mucking was completed, styrofoam blocks were installed to create an opening for the next stope. This was followed by backfilling with 3% cement content. The mining sequence used for the upper block was longitudinal retreat towards the center of a lift to the different sub-levels. Blasting on several sub-levels increases flexibility in the different working areas. The sub-level spacing used was 30 m. This mining sequence is seen below.

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Eureka mining sequence at Bousquet Mine, Quebec[11]

For this particular mining sequence, there are several advantages and disadvantages to be considered:

Advantages

  • Mining Cost Reduction.
  • Faster Stope Production Start.
  • Hanging wall and footwall stability is increased by the stope length.
  • Uneconomic ore slices are left in place and a new primary stope is made.

Disadvantages

  • Cemented Backfill increases production costs.
  • Production is decreased from long mucking distances.
  • Backfill cycle may be halted if a working area is not available.
  • Operational difficulty with drilling and filling from the same access point.

Creeping Cone

The creeping cone method is a combination of sublevel and shrinkage stoping methods, and involves the use of broken ore in the shape of a cone within the open stope. This cone helps support the walls of the stope, which allows for more ore extraction from the stope before movement along the bedding planes occurs. Ore is drawn from the open stope before there is sidewall failure from sloughing. This reduces dilution in the stope.

Case Study

References

  1. 1.0 1.1 1.2 1.3 1.4 1.5 1.6 Open Stope Mining in Canada. Australasian Institute of Mining and Metallurgy.
  2. W. Hustrulid, R. Bullock. (2001). Underground Mining Methods - Engineering Fundamentals and International Case Studies. Society for Mining, Metallurgy, and Exploration (SME).
  3. 3.0 3.1 3.2 3.3 3.4 3.5 Darling, P. (2011). SME Mining Engineering Handbook (3rd Edition). Society for Mining, Metallurgy, and Exploration (SME).
  4. 4.0 4.1 4.2 4.3 G. Tucker, D. Herbert, J. Robinson. (1998). Bench Stoping at Mount Isa Mines Limited Isa Lead Mine, Mount Isa, Queensland. Publication Series - Australasian Institute of Mining and Metallurgy, 3/98, 135-147.
  5. E. Villaescusa, I. O. (2010). Blast Induced Damage and Dynamic Behaviour of Haningwalls in Bench Stoping. Fragblast: International Journal for Blasting and Fragmentation.
  6. 6.0 6.1 6.2 6.3 6.4 6.5 6.6 6.7 6.8 E. Villaescusa, K. K. (1998). Backfill for Bench Stoping Operations.
  7. 7.0 7.1 7.2 7.3 7.4 E. Villaescusa. (2003). Global Extraction Sequences In Sublevel Stoping. Twelfth International Symposium on Mine Planning and Equipment Selection, (1) 9-17.
  8. 8.0 8.1 8.2 H. S. Mitri, R. Hughes, E. Lecomte. (2010). Factors Influencing Unplanned Ore dilution In Narrow Vein Longitudinal Mining. SME Annual Meeting, Phoenix, AZ.
  9. Vergne, J. d. (2003). Hard Rock Miner's Handbook. McIntosh Engineering.
  10. 10.0 10.1 De Souza, Euler. "Longhole Stoping Applications". Mine 244 Lecture Slides. Kingston: The Robert M. Buchan Department of Mining, 2010. Print.
  11. Natural Resources Canada. (2004). Barrick Gold Corporation - Bousquet Mine (Québec). Retrieved from http://mmsd1.mms.nrcan.gc.ca/DEV/narrowvein/longhole_print-e.asp?mineid=30.


This article was designed and created by John Forster, Stephen Soock, Nyree Grimes, Jihoon Ryan Hong, and Jordan Cooper as part of the MINE 448 Underground Mine Design Course taught by Professor Stephen McKinnon at Queen's University.

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