Stope sequencing

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[1] [2] [3]

[1] [2] [3]

The stope sequencing is one of the most important tools for controlling the behaviour of the rock mass in an orebody undergoing extraction. In order to determine an appropriate extraction sequence, an iterative approach must be taken. Once the prospective location of the pillars is established, the preliminary layout design for the set of stopes is planned. This can be conducted using the boundary element method with the assumption that the rock mass displays linearly elastic behaviour. The tensile stress zones can then be mapped in order to locate areas that fulfil the failure criteria for a particular failure mode [3]. The preliminary sequence design may be subject to alterations to eliminate insupportable stress states while still satisfying geomechanical and operational requirements.

One of the main objectives when developing an extraction sequence is to assure recovery of the highest grade blocks in the deposit along with adhering to production targets, ore grade requirements, and operational constraints such as access points and equipment size [2]. Even under relatively low stress fields, an improperly designed extraction sequence can cause a substantial reduction of the ore reserves recovered. There are several parameters that can affect the way in which stopes are extracted. One of the most prevalent limiting factors is the maximum allowable void space that a rock mass can sustain without stope wall failure. These failures can be caused by major geological discontinuities and features, and high induced stresses. In general, the main influence on the extraction sequence is the particular orebody being analysed [1].


Contents

Massive Orebodies

Massive orebodies are generally extracted with the use of multiple stopes, which include primary, secondary and when required tertiary stopes, in combination with mass blasting techniques and cemented fill. The extraction sequence options available include temporary or permanent rib or transverse pillars, strike slots with continuous or discontinuous advance, and checkerboard sequence [2].

Temporary Longitudinal, Transverse and Crown Pillars

For extremely competent rock masses, an extraction option of mass blasting secondary stopes into adjacent primary stopes to create very large and stable openings is available. To create stability and increase the recovery rate, voids can be filled with consolidated or unconsolidated fill while installing transverse and longitudinal pillars separating each stope (Figures 1 and 2) [2]. The use of these pillars can cause a large quantity of ore to be left in them. Sublevel caving retreat may be used to accomplish complete recovery of the pillars if the orebody geology allows for it.

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Figure 1 - (a) Rib and transverse pillars at Mount Isa Mines and (b) transverse pillar at the Darlot Gold Mine.[1]
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Figure 2 - Temporary rib pillars within a top-down sequence at Mount Charlotte Mine, Kalgoorlie, Australia.[1]













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Figure 3 - Plan view of discontinuous transverse slot extraction sequence for a massive orebody.[1]

Continuous and Discontinuous Advance

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Figure 4 - Plan view showing a continuous, pillarless, stoping sequence.[1]

A 12-stope extraction sequence with a discontinuous strike slot approach is depicted in Figure 3. This method uses primary, secondary, and tertiary stopes designed with a stress shadowing and orebody abutment retreat to manage induced stresses with the assumption that the major principal stress is normal to the long axis of the deposit in question [1]. When the strike slot has been completed, the remainder of stopes are stress shadowed from the principal stress. Stress shadowing happens when multiple excavations fall along a major principal stress trajectory. The stresses may redistribute causing areas of high stress and providing relief in others as rock coincides with the shadow occurred due to the excavation [2]. At the early stages to provide more stress relief transverse pillars or discontinuous strike slots should not be used if possible as these geometries have a tendency to concentrate stress. Stopes in the corner of the excavation are extracted at the end so cemented fill is not needed [1].

For excavations involving high stress fields, transverse pillars or discontinuous strike slots as previously stated can concentrate stress which can form a continuous slot within an initial mining block to effectively stress shadow the remainder of stopes in this environment. This is depicted in Figure 4. For a continuous strike slot, the fill from the first stope must cure before any adjacent stopes are extracted which can slow production as only a single side of the fill mass is exposed at a time [1].


Checkerboard Sequence

A checkerboard extraction sequence depends heavily on the effectiveness of the mass blasting conducted and the development of stable fill masses which provide support with minimal dilution as multiple fill exposures occur. This sequencing pattern begins with primary stopes filled with consolidated backfill, followed by the extraction of the secondary, and if required tertiary, stopes with the pillars having various fill mass exposures. The stoping front moves in a longitudinal direction or is directed as a continuous retreat strategy depending on the in-situ stress and production requirements. An advantageous aspect of this type of sequencing pattern is that primary stopes are more stable as they must be filled to provide support to the residual crown pillars and stopes. The main disadvantage is that a large portion of the ore is left within the tertiary stopes of which the stope design can be complex as it is a function of existing development and infrastructure [2].

Steeply Deeping Orebodies

Steeply dipping tabular orebodies can be wide enough to validate multiple levels of stoping to increase flexibility and production, or they may be thin enough to only need vertical stopes for extraction with residual pillars.

Primary and Secondary Stope Sequence

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Figure 5 - Primary and secondary stope extraction sequence for a shallowly plunging orebody. P1, primary stope cemented fill; S1, secondary stope unconsolidated fill. The numbers show the sequence of stope extractions.[1]

In order to maximize the extraction ratio, waste-rock should not be blasted along with the ore. Accordingly, optimized design incorporates precision blasting techniques and uses thin extractions that are parallel to one another along the strike of the orebody and that closely follow downward direction of the orebody dip. For this method, support pillars in between the stopes are often necessary during excavation to prevent collapse and to remain within legal geotechnical limits [2]. These pillars are usually also composed of ore and hence should be extracted at a later time once the currently empty stopes are filled with waste and/or tailings paste for reinforcement. Therefore, these support pillars are actually secondary stopes, which are simply excavated later in the sequence.

Keep in mind that this option of complete extraction and complete filling is only done when the ore body itself has enough value to merit the use of cemented fill. This value is often characterized by a high ore grade or relatively high market prices. It is often more economical when dealing with low grade orebodies to have stope extraction occur in conjunction with unconsolidated fill and permanent pillars. These permanent pillars are composed of sacrificed low grade ore, but are more valuable in the long run as support than extracted ore [1].

Mines which use this method and have this shape are often coal and lead based, due to the natural tendencies of such deposits to form tabular orebodies. For example, the Lead Mine in Mount Isa Mines in Queensland, Australia is a singular tabular steeply dipping lead deposit [2].

Multiple steeply dipping tabular orebodies can be managed similarly to singular ones. Primary and secondary stoping sequences can work as with high grade orebodies before. Similarly low grade orebodies may require permanent pillars as sacrificed ore due to the economic value of such a pillar. The main difference between multiple and singular orebodies is that the process can be mirrored in the 3-dimension as described in 2-dimensions for a singular orebody. This means that extraction will ideally be double if there are two parallel orebodies. If the geometry of the orebodies is not identical then some slight adjustments can be made easily enough, however the overall method remains more or less constant.

The advantages of this cut and fill method as mentioned is high flexibility and production in the early stages of primary stope excavation, as well as an overall cost cut by using unconsolidated fill in the stopes for post-excavation support. The downfall to this method is the risky stress redistributions from excavation which may cause damage to secondary pillars. This disadvantage can be mitigated by technical blasting, being careful not to undercut pillars, and also by mass blasting high stress areas within the block in question. Furthermore, multiple lifts within the primary and secondary stopes also helps to reduce stress by allowing minor deformation to occur between stages/lifts and hence lowering the accumulated stress [2].

Top-Down and Bottom-Up Bench Stope Sequence

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Figure 6 - Longitudinal view of a top-down extraction sequence, permanent pillars, and retreat to a central crosscut, no fill.[1]
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Figure 7 - Longitudinal view of a bottom-up extraction sequence, retreating to a central crosscut; pillars are optional.[1]

These two methods are executed within many stoping sequences, such as primary and secondary stope sequencing. They are a method of extraction and not so much a stoping sequence themselves. That is, they are the method used to get material out of a stope, not which way to set up a series of stopes.

A top-down extraction often requires permanent supplementary support such as rib pillars along the strike of the orebody between individual stopes. This helps to reduce dilution between the two adjacent stopes. Furthermore, a collection of sill pillars may be needed to assist overall stability and to isolate any unconsolidated fill from the upper stopes as the extraction moves downwards [1].

A bottom-up extraction requires backfill in order to build up a working level surface beneath machinery and personnel as the extraction moves upwards. Again, rib pillars along the strike of the orebody helps reduce dilution and minimizes the need for crown pillars [1].



Center-Out, Pillarless Sequence

Center-out, pillarless stope sequencing is a method used to eliminate the need for secondary stopes. By extracting small stopes at the center of the orebody and advancing outwards towards the surrounding host rock (Figure 8), a slow rate of convergence of the host rocks can be achieved[1]. This slow rate of convergence has been claimed to likely minimise the effects of local seismicity, and may also reduce the amount of stored seismic energy released during excavation[1].


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Figure 8 - Center-out, pillarless extraction sequence[1]

Center-out, pillarless extraction sequence

Implementation of this method of sequencing is challenging and can limit the productivity due to severe constraints on individual stope cycle times, especially with the addition of backfill[1]. A single stope must first be mined, filled and cured, all before the adjacent stopes can begin extraction. In large mining operations with many sublevels, challenges in development, scheduling and logistics can be encountered. In order to reduce this cycle time, rapidly curing and draining cemented backfill is used in all stopes[1]; this may result in an increase in operating costs. Cemented rockfill is often used, however tight backfill of the stope crowns is difficult to achieve and adding hydraulic fill to achieve a tight fill is expensive and time consuming. This results in the need for extensive rock reinforcement, and can cause issues with drilling and blasting due to the potential for large slabs parallel to the stope edges being released[1].


Primary and Secondary “1-5-9” (or "1-4-7") Stope Sequence

By still using the general triangular retreat shape of a pillarless sequence but instead using an arrangement of small lift primary and secondary stopes, a compromise can be made. This increases the productivity by making multiple primary stopes available for mining simultaneously. During this sequence however, stress redistributions in pendent pillars can cause damage and dilution if not properly controlled[1]. To prevent this from occurring, secondary stopes must be recovered as early as possible. A general rule is that no more than two sublevels are mined ahead of the pillar before recovering it, and both sides of the pillar cannot be mined simultaneously[1].

A variation of this method is to use a 1-5-9 stoping sequence. An example of this sequence as used for the George Fisher orebody, Australia, is demonstrated in Figure 9. Stopes 1-5-9 are extracted as two lift primaries and filled with consolidated fill. Following this stopes 3-7-11 are also extracted as two lift primaries and filled with consolidated backfill. Once the fill in the primary stopes has cured, a set of single lift stopes 2-6-10 are then extracted and filled with unconsolidated fill. The remaining set of single lift stopes 4-8-12 are then extracted and filled with unconsolidated fill[1]. This entire process is then repeated up-dip.

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Figure 9 - Longitudinal section view of the George Fisher conceptual orebody extraction[1]

A disadvantage of this method is the inefficient stope mucking characteristics[1]. By requiring a bottom up moving draw point following the vertical retreat of the stope, muck must be carried out in areas at the stope crowns that have previously been subjected to stress distribution and stope blasting. With the bottom up vertical retreat, each stope access becomes a drawpoint and thus requires significant reinforcement, along with all exposed backs[1]. Additional footwall development in waste may also be required on each sublevel to establish multiple points of access to each stope to allow for effective mucking.


Shallow Dipping Orebodies

Orebodies which are large and tabular with a dip angle that does not permit broken ore to flow with gravity can be extracted using uphole retreat panel stoping. The deposit is generally divided into panels which run parallel to the strike. A footwall extraction drive is developed in which drilling, blasting, and subsequent mucking is conducted in order to extract the stopes. To reinforce the hangingwall in the primary stopes the use of cablebolts or leaving permanent pillars within the secondary stopes is recommended [2].

This type of orebody can also be extracting using individual stopes with cablebolt support and backfill. The extraction of the stopes is done by making a trough undercut horizon in waste which will let ore flow into drawpoints. Downhole drilling is then performed from a number of hangingwall drives. This method of extraction involves added costs as there is a longer lead time [2].

References

  1. 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 1.11 1.12 1.13 1.14 1.15 1.16 1.17 1.18 1.19 1.20 1.21 1.22 1.23 1.24 1.25 1.26 1.27 Ernesto, Villaescusa. (April 2003). “Global Extraction Sequences in Sublevel Stoping.” Western Australian School of Mines. Retrieved February 01, 2015
  2. 2.00 2.01 2.02 2.03 2.04 2.05 2.06 2.07 2.08 2.09 2.10 2.11 Ernesto, Villaescusa. (April 2014). “Geotechnical Design for Sublevel Open Stoping.” CRC Press. Retrieved Jan 30, 2015.
  3. 3.0 3.1 3.2 Brady, H.G. Barry. (2006). “Rock Mechanics: For Underground Mining, Third Edition.” Springer. Retrieved February 01, 2015.

Further Reading

  • Alford, Christopher. Brazil, Marcus. Lee, L. David. (2007). “Optimisation in Underground Mining.” Vol. 99, pp. 561-577. Springer. Retrieved February 02, 2015.
  • “SME Mining Engineering Handbook, Third Edition.” Vol. 1, Chap. 6.5. Society for Mining, Metallurgy & Exploration. Retrieved Jan 29, 2015.
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