Underhand cut and fill

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Underhand cut and fill, also referred to as undercut and fill, is a variation of the cut and fill mining method. It was originally developed by Inco Ltd. in order to address the abnormal ground conditions found in Sudbury, Ontario, Canada[1]. This method is
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Longitudinal and section views
used mostly to facilitate pillar recovery but is sometimes used as the primary stoping method. Historically, square-set mining has been the most efficient approach to pillar recovery but is quite costly in noncompetent rock. The need for a safer and more cost-efficient method of pillar recovery in poor ground conditions led to the development of underhand cut and fill. Underhand cut and fill offers reduced costs, improved safety, and a more accurate mining cycle estimation in comparison to square-set mining[1]. The ore is mined in a top-down manner by removing a cut of ore and then constructing timber stringers along the length of the cut. The timber stringers are used as support for a mat, created by laying logs and wire mesh. The cut is then filled. As suggested by the name of the method, mining continues beneath the filled mat. In the image to the right, the mats in place are visible at the bottom of each cut[1].


Contents

Geological Setting

Underhand cut and fill is a versatile mining method; it can be used in orebodies of varying widths and dips, from vertically dipping to flat lying[2]. It is most useful in ore where the wall rock or vein is unconsolidated and cannot be safely mined using traditional cut and fill[2]. Underhand cut and fill has been particularly successful in rockburst-prone areas[3].

Design Considerations

Many factors must be considered when mining under an engineered back, as is the case with underhand cut and fill. A high factor of safety is necessary since work occurs directly underneath the fill.

Loading

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Variation in vertical loading resulting from different analytical approaches

The design load is a key factor in estimating the strength required for the sill pillar. Overestimating the load leads to unnecessary expenditures in an already expensive method, while underestimating may lead to failures during mining. Determining vertical loading is a complex and inexact process because there are many influential factors that cannot be directly measured[4]. There are many methods of calculating this, which can cause some variation in the final load estimation. A graphical example of the variability is shown to the right[5]. The ratio between vertical and horizontal stresses, ‘k’, is extremely important in this process. k is an effective way of communicating the effect of arching and its associated allowable load transfer[4]. In the initial fill placement, the coefficient of earth pressure is at rest but this changes with each subsequent placement of fill. With each fill placement, the existing fill shifts and compacts, which results in a transfer of the load to the mats and posts. During this activity, the shear resistance follows a similar path; the resistance is low at the time of initial fill placement but increases with each subsequent fill[4].

The vertical stresses for cemented rock fill can be calulated using the equation:[4]

Ucf eq.png







Although this equation was developed for cemented rock fill, it can also apply to paste fill since any differences would be accounted for in the equation variables.

Failure

During backfill design, it is important to understand the failure mechanisms that may occur. The failure modes are most influenced by the stability of the backfill. The stability is affected by stope geometry, seismic activity, loading conditions, rock quality, and support. Typically, unconfined compressive strength is used to estimate the stability and potential failure modes. Failure modes that can occur in underhand cut and fill are shown in the image below[6]


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Potential failure mode models

Access

Since underhand cut and fill is a top-down mining method, the amount of preproduction development is limited. Preproduction work includes the development of raises, either bored or driven[2]. The stopes are accessed through winzes that advance with mining[2].

Ventilation

Ventilation in underhand cut and fill is treated similar to that of a drift heading. This method requires auxiliary ventilation because there is only a single access point and no positive airflow. Fans are installed at the top of the manway with plastic ventilation tubing to carry air into the face. There is also a plastic air line used to ventilate after blasts and between shifts. 406 mm compressed air fans with silencers are most commonly used but electric fans are used where possible[1].

Support/Backfill

As an artificially supported method, the cemented sand should bind to the mined out walls in order to adequately support the weight of the fill. In addition, the pressure caused by mining underneath the mat causes a voussoir arch to develop above the mat, supporting the majority of the load[1]. The log mats support the remaining mass as well as prevent the fill from settling. This is important because excessive settling would cause the voussoir arch to break, resulting in the entire load being transferred to the mat. The creation of the voussoir arch is integral to the overall method. Since the voussoir arch supports the majority of the load in each level, there is no significant increase in weight as mining progresses to the lower levels[1]. The appropriate backfill characteristics depend on the source of the sand, the compressive strength of the cemented fill, and the geology of the mine. Some typical ranges are shown below.


Backfill and delivery system specifications[1]
Sand Delivery Rate 145 - 180 t/h
Pulp Density 65 - 70% solids
Ratio 15:1 - 30:1
Water Inflow 0.02 m3/s


Delivery

Backfill is delivered using a series of pipes. Pour points are located every 9 m, originating at the manway, and are maintained through the fill for use in lower cuts. They are constructed using a 101 mm thin-walled pipe starting in the top sill[1]. Each pipeline is connected to the main line by its own valve, allowing the sand to be more concisely directed to wherever it is needed. The pipes extend through the mat and then turn back using 90° elbows to extend back above the cut. This turn is created so that the pipes can be disconnected and extended down to the next cut even after filling. A 51 mm plank is fixed to logs above these pipes so that they are not dislodged during filling. A drainage system is installed through the fill fence to allow the sand to flow for a longer period of time before the fill crew must stop the flow and drain the water.

Method

On a newly mucked cut, the backfilling procedure begins with the construction of the mat. Before the mat can be placed, some preliminary preparations must be done. As an example scenario, a 3.65 m wide cut will be examined. In this scenario, two longitudinal stringers are constructed 2.7 m apart. These are constructed by setting 127 x 203 mm x 3.65 m timbers so that the end of one stringer is positioned at the midpoint of the other until the necessary length has been reached[1]. The timbers are held together using 25.4 mm wide steel straps. During the stringer installation process, effort is made to keep them on a flat grade. The stringers are also secured to rockbolts every 6 m and at the ends using 12.7 mm cabling. In some cases, they may be secured to the mat from the previous cut instead of rockbolts. The cables provide temporary support in the case of any posts being damaged during mining and prevent the stringers from collapsing during blasting. Two 3.65 m logs with a diameter of 203 – 254 mm are placed at every joint (every 1.8 m). These logs will become the posts during mining and so are not secured in any way at this point.

For the construction of the mat, 3.65 m logs with a diameter of 152 – 203 mm are placed across the stringers at 0.6 m centers[1]. 101 x 101 mm No. 9 wire screen is placed on top of the logs and up the walls. This screen helps prevent loss of fill during the mining below. An isometric view of a mat in place is shown below to clarify this process[7].

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Stope preparations prior to backfilling

Simultaneous to the construction of the mat, the surrounding work area is prepared for filling. The grizzly is removed and the orepass and manway are sealed off. Although there are multiple sealing methods, a commonly used method is as follows: The borehole is covered with a bulkhead constructed of 127 x 203 mm timbers, which is then covered with a plastic-coated synthetic fabric called Fabrene[1]. This fabric is sealed to the floor with thick mortar. Three stringers spaced at 1.8 m are used to create a manway over the orepass. A fill fence, constructed from timbers, screen, and Fabrene, encloses the manway and is supported by posts spaced 1.8 m apart. The manway is used to house a ladderway for access, a nipping compartment to move supplies, service tubing, and space for storage of miscellaneous equipment[1].

Before pouring the sand fill, downholes for the next cut must be drilled and the collars must be fitted with a 51 mm bit and 31.7 mm antistatic tubing[1]. The bit and tubing prevent sand from entering the drillholes during filling. An area is opened up around the orepass and the area is handmucked or cleared with a blowpipe. Once the cut is 3.65 m high, a slusher and grizzly are brought in and set up. The manway ladder and nipping compartment are extended so that they reach the top of the grizzly. As the cut is opened, the previously established stringer joints are supported with posts. Drilling on the face does not begin until all the stringer joints are supported.

Scaling of the face and walls as well as removing loose sand from the mat must be completed before any drilling of the face may begin. The holes should be drilled at least 0.6 m from the above mat to prevent damage[1]. The top row of holes should be drilled flat[1]. To maintain stability, blasting is confined to a small area so that only one stringer joint is unsupported at any time.



Costs

The development required in underhand cut and fill mining is roughly the same regardless of tonnage. Information on the costs of underhand cut and fill mining is scarce. Estimated costs from 1977 can be seen below[8]. However, it should be noted that technological advancements have reduced mining costs; improvements in placing dense sandfill have reduced the labour requirements for underhand cut and fill[9].

Underhand cut and fill stoping costs in 1977

Potential Issues

Mat Failure

The mat is an integral part of the support in underhand cut and fill. As such, the stringers and posts are inspected and, if necessary, repaired every shift. Repair of the mat may include the installation of additional posts or, in extreme cases, additional sets[1]. Poor drilling and blasting is the most common cause of mat failures, but poor quality fill or excessive water saturation are also potential causes[1].

Run of Dry Fill

Run of dry fill in an underhand cut and fill stope can create voids in the mat above which may lead to mat failure. An excessive run of dry fill may result in the current stope being filled before mining has been completed[1]. If the run can be stopped and the resulting void is not too large, mining may continue. During the backfilling process, an additional 'breather' pipe is used to help fill the void[1]. To prevent the occurence of a run of dry fill, the height of a cut should be decreased when mining near noncompetent shrinkage or blasthole stopes or if a nearby fence is in poor condition[1].

Advantages and Disadvantages

Advantages

  • Adaptable to different orebody geometries
  • Selective
  • High recovery
  • Low Dilution
  • Low accident frequency
  • Requires minimal preproduction development

Disadvantages

  • High cost
  • Labour intensive
  • Low productivity
  • Requires consistent fill material

Case Study

For a case study on the underhand cut and fill mining method, please see Lucky Friday Mine.


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 W. Murray, “Undercut-and-Fill Mining: An Introduction,” in Techniques in Underground Mining. Littleton: SME, 1998, ch. 31, pp. 557 – 571.
  2. 2.0 2.1 2.2 2.3 W. A. Paroni, “Excavation Techniques,” in SME Mining Engineering Handbook, 2nd ed. Littleton: SME, 1992, ch. 19.2 , pp. 1749 – 1755.
  3. S.A Orr, “Hard-rock Mining: Method Selection Criteria,” in SME Mining Engineering Handbook, 2nd ed. Littleton: SME, 1992, ch. 21.1, pp. 1838 – 1842.
  4. 4.0 4.1 4.2 4.3 R. Pakalnis et al, “Design Spans – Underhand Cut and Fill Mining,” in CIM-AHM, Toronto, CAN, 2005, pp. 1 – 9.
  5. R. Pakalnis et al, “Estimation of Vertical Loading onto Sill Mat,” in CIM-AHM, Toronto, CAN, 2005, pp. 3.
  6. R. Pakalnis et al, “Limit Equilibrium Criteria Adapted from Mitchell, 1991,” in CIM-AHM, Toronto, CAN, 2005, pp. 4.
  7. S. Jain. (2011, October 23). Mining Methods [Online]. Available: http://www.slideshare.net/smhhs/mining-methods
  8. G.M. Pugh and D.G. Rasmussen. “Cost Calculations for Underhand Cut-and-Fill Mining,” in Techniques in Underground Mining. Littleton: SME, 1998, ch. 34, pp. 595 – 601.
  9. D.E. Nicholas, “Selection Variables,” in SME Mining Engineering Handbook, 2nd ed. Littleton: SME, 1992, ch. 23.1, pp. 2051 – 2057.


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