Access drifts

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A drift (or access) is an excavated tunnel built for the purpose of providing a pathway for the transit of equipment, services or ore between different locations in an underground mine.

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Installing Rock Bolts and Screen at Rio Tinto Diavik, 2014, Lac de Gras, NT, Canada. [1]

Contents

Drift Types

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Isometric view showing haulage drifts and sill drift access.

Ore Access Drift Types

The types of ore access drifts may vary depending on the mining method, however nearly all underground mining methods can categorize their ore access drifts into the following:

  • Sill (ore) Drifts
  • Haulage Drifts
  • Cross-cut Drifts
  • Exploratory Drift

Sill Drifts

Sill drifts (known as ore access drifts) act as the egress to the orebody in an underground mining operation and typically have the smallest cross-sections. They are usually secondary developments and act as temporary access points to an active area in the orebody that is being mined. Sill drifts usually become a part of the stope excavations and are backfilled with the stope or are left alone after ore extraction.The amount of ore access drift entries into the orebody are dependent on the chosen mining method. For example, longhole stoping methods typically require more ore access drifts than cut-and-fill mining methods because of the logistical nature of ore extraction. [2]

Haulage Drifts

Haulage drifts are the main access routes on a level. Access to sill drifts, level entry points (ramp, manways) and capital infrastructure access points (ventilation raises, rock passes, service bays) are through the main haulage drift. They are typically driven parallel to the strike of an orebody (dependent on mining method and orebody shape) and see the heaviest traffic in a mining operation out of all areas on a level. [3]

A rule of thumb for the placement of these drifts is at least 15 m away from ore in good ground, or one stope length, whichever is greater. This provides both an allowance for stress shadowing and scoop parking.[4]

Cross-cut Drifts

Cross-cut drifts provide access to sill drifts from the main haulage drift on a level. If sill drifts are developed parallel to the orebody strike on a single level, cross-cuts are made between the drifts to allow for greater ore recovery when sill pillars are left behind. [5]

Exploratory Drifts

To complete exploratory drilling programs in the rock surrounding the defined orebody, exploration drifts may be developed underground to gain access to appropriate drilling locations. The purpose of this could be to increase reserve confidence in areas of low confidence defined from original drilling programs or to start new drilling programs in an undefined rock mass. Exploration drift development is typically budgeted and planned for in mine planning and scheduling.  

Design Considerations

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Surveying drift progression at De Beers Snap Lake. [6]

Geometry

Drift geometry is affected by:

  • The purpose of the excavation
  • The service requirements
  • Rock characteristics, ground conditions in the area
  • Intensity and nature of mining activity near the excavation
  • Intended life of use
  • Equipment clearance requirements, including whether or not the haulage route allows for one or two-way traffic

High-traffic areas, or areas with significant auxiliary service infrastructure, may need to be larger to allow for additional clearance. The selected width and height must allow for the largest perceived piece of equipment used in the area.

Ground control requirements in the area must also be assessed. These can influence the shape (arching) and orientation of the excavation relative to the principal in-field stresses. Another important acknowledgement is the impact of ground conditions on support consumables such as rockbolts, shotcrete, and the advance rate of the drift.

Other types of ore access developments, such as drilling horizons, raisebore installations, and drawpoints, will need extra room for consumables such as drill rods. Longhole drills and raisebores are often allotted an additional 1 or 1.6 m respectively.

Drawpoints will often require slashing to install remote mucking stands, provide additional void or a wider brow for facilitating mucking and line-of-sight prior to bringing a stope on-line.

Example Excavation Sizes

From the author's experience, typical longhole mining access drifts are 5 x 5 m and can accommodate a 10yrd LHD. Headings that will house longhole drills, or a raisebore, may be up to 6 x 6 m . [7]

Turns

Turning development drifts is accomplished by blasting rounds with increasingly shorter holes towards the corner in the new direction. When the turn is accomplished, the face is squared off and development proceeds as normal.

In designing turns, there are two main considerations:

  • the ability of the development miners and equipment to develop a turn to design
  • the space required for mobile equipment to maneuver the turn

Example Turning Space Requirements

Diamond drills often require more space than LHDs to maneuver a turn and limit the minimum radius of the drift curvature.

Example Drilling Space Requirements

For example, An Atlas Copco E2 C operator will require 7 m of clearance [8] to line up to a hole on a drill pattern. This limits the depth of a single round that can be achieved near the corners or at an angle to the face, when turning or drilling off slashes,remucks, etc.

Remucks and Safety Bays

Remucks, safety bays, and passing bays should be included in drift designs. On bends, safety bays should be installed on the outer wall as a safety precaution, allowing equipment operators to have a better line of sight to personnel on foot. A 2 m wide x 2 m deep cut-out is adequate for a safety bay.

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Example of to-scale excavations to facilitate operations, maneuvering and turn-arounds. [9]

Orientation

The orientation of the drift in the mine plan should be considerate of the principal stress direction underground. Ideally, drifts should run parallel to this field. This is of extreme importance for life-of-mine infrastructure, such as main level accesses in order to protect their stability integrity. However, drifts should not run parallel to joint trends or foliated mineralization to avoid sloughage and unraveling. Excavations should also cater to local geotechnical domains as much as possible. For example, drift arching should complement jointing.

Incline

The incline and/or decline of the drift needs to recognize the maximum slope that can be handled by mobile equipment. Changes in slope should not be so sudden so that a loaded scoop has to raise its bucket above operator height to travel up slope.

All drifts should be slightly sloped towards sumps or drain holes, to prevent water accumulation. Ramp grade ranges between 13-15% and a drift grade of around 2% to a maximum of 17%. For more information on ramps, see Underground Ramp Design Parameters.

Determining Service Requirements for Drift Development

The services required in a drift depend on what the drift is planned for and are tailored to equipment needs. Ore access drifts provide access to a stope during its lifecycle of drilling, blasting, mucking and backfilling and have specific service requirements.

Ventilation

Ventilation is one of the main limiting factors when designing and scheduling ore access drifts. Use of equipment is constrained by ventilation requirements as regulated by the Ministry of Labour for health and safety concerns.

Example

This calculation is the ventilation requirement for an Atlas Copco Scooptram ST7, a typical LHD that operates in ore access drifts.

Scooptram ST7: Underground Loader (LHD) Source

\text{Required Ventilation} = \frac{ \left(0.06cm^3\right)} {kW} \times 144kW = 8.64cm^3/s [10] [11]

This equation states that in order to have a STF LHD operating in the crosscut, 8.64cm3/s of fresh air is required.


Ventilation planning typically uses a secondary ventilation system design to supply fresh air to ore access headings. These secondary systems usually involve attaching ventilation tubing to auxiliary fans that pull the required amount of fresh air to the face of the drift from the main ventilation system.

The design of ore access drifts and how they connect to other infrastructures should consider ventilation system design capabilities and limitations. Secondary ventilation system design takes in to consideration the length of the drift and how the drift connects to other openings, such as other drifts, rock passes and open stopes. The more openings in an ore access drift, the harder it is to control the flow of ventilation and hazards such as recirculated air or lengths of drift with no airflow may occur [12] . Additional ventilation challenges and design considerations are discussed in ventilation.

Equipment Selection

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Drift cross section showing profile and service infrastructure.][13]

Equipment such as jumbo drills for development, scissor decks for bolting, LHDs for mucking and backfilling and production drills can be expected to work in these drifts. Trucks are not usually designed to operate in ore access drifts due to size and ventilation constraints. The size of a planned ore access drift is a limiting factor in scheduling equipment. A smaller access will reduce the allowable size of an LHD able to operate. A smaller LHD will only be able to extract tonnage at a certain rate and must be considered when scheduling tonnage.

Air and Water Services

Most equipment used in drift development and stope production require the use of compressed air and water. Main pipelines for compressed air and water are hung at the back of the drift in a system to service the whole mine.

During development, access to these lines to operate equipment is required as the drifts advance. A system of pipelines and headers are established to provide access to the services as well as to mitigate the hazard of multiple hoses being laid out on the ground over long distances. As a rule of thumb, air and water headers are established every 50-100 feet from the pipelines to provide the miners access to attach their equipment [14]. Water sprays are also used in dust control to reduce health and safety hazards associated with breathing in dusts that are associated with the mine activity. For control of these substances, refer to the Regulation 822 of the Occupational Health and Safety Act.[15]

During stope production, compressed air and water is required to operate the production drills and hole cleaning equipment. The use of water sprays is also practiced for dust control measures when mucking a stope.

Power and Communication Services

Electric power lines are run from the main substation on surface down to substations (ESS) on every level and provide a certain amount of power. Drift development scheduling must take power constraints in to consideration as a level’s ESS will only be able to support a certain number of electric powered equipment. Power connections are run from the ESS along the walls to the face of an active heading, similar to compressed air and water lines. Caution needs to be taken when hanging these power connections around wet conditions as the equipment draws a lot of current and poses an electrocution hazard.

The Ministry of Labour in Canada as well as many other places require personal communication devices to be held by each person underground (Section 16. Regulation 854 of the OHSA) [16] . The use of a leaky feeder system has provided the ability to use radios to communicate underground as well as up to surface.

Installing Services Considerations

When extending services as the development advances, appropriate sequencing must be used in order to provide the services to the heading when required but also to avoid damaging these services when blasting. As mentioned in Drift Development Constraints, the main drift on a level is typically developed first in order to access capital infrastructure required before production can begin. When cross cuts are planned to be driven off of the main drift, it is common practice to advance the cross cuts up to 12m before advancing the main drift past the cross cut[14] . Services must be extending in to both but if the cross cut face is too close to the main drift, rock thrown from the cross cut blast will damage services if they are in the blast radius. 12m is far enough in to the cross cut that blast damage will be minimized but access to services from the main drift is still possible. The static properties of air flow will pull air from the main drift in to the cross cut to ventilate it without the use of a secondary ventilation system.

Cost Components of Drift Development

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Example of a two-boom development jumbo drill, the Sandvik DT820, here pictured in Doğançay, Turkey. [17]

The cost components of drift development is fairly extensive, as a large variety of work must be completed in order for the drifts to be safe and usable. The cost to develop a drift is closely related to the size of the drift, the stability of the material, and the expected life and purpose of the excavation. Labour, equipment, consumables, and ground support are the principal components needed to develop a drift and are the main components.

Labour

Depending on the size of the drift planned for development the amount of labour needed will vary. For larger openings that require short drilling depths, it may be optimal to have fully mechanized two-boom drilling. Mechanized drilling can allow the drift development steps that follow to proceed quickly as there should not be any equipment-related delays for handling drill rod extensions. It should be noted that longer rounds can result in blasthole drilling inaccuracies, which can lead to increased drilling times, increased charging and blasting times, longer muck handling times (due to larger muck volumes), and larger roof spans to be supported. [18]

The alternative to using mechanized drilling would be the use of an air-leg drill and have the blastholes manually drilled. This situation could arise if the planned drift is too small to effectively fit a mechanized drill jumbo.

The dichotomy of mechanization and manual labour becomes once again apparent with regards to how any loose rock on the excavation wall is removed. Mechanized scaling provides the advantage of not exposing mine personnel to difficult and potentially dangerous working conditions. [18]

For more information regarding the Ontario Ministry of Labour's requirements regarding working at the face, this video is a good resource which also includes footage from underground heading inspections.

Equipment

The size of the excavation comes into effect with regards to equipment selection. Larger openings may allow for large pieces of equipment to be used in drift development however larger openings may also necessitate more equipment (i.e. work platforms). Other factors such as ventilation and available power must be considered and accounted for. The SME Mining Engineering Handbook 3rd ed. provides a table (Table 12.6-1) where the major work tasks and equipment needed for drift development are listed.

Consumables

Consumable materials that a mine will utilize during drift development are numerous and precautions should be put in place to ensure adequate supply and mitigate delays. The cost of these consumables is very much subject to the conditions of the mine and the size of the excavation. A small list has been created which includes some of the areas where materials may be expended during mine development.

Mine drift development uses:

  • Blasting Resources
    • bulk explosives (ANFO, emulsion)
  • Equipment (expendable parts)
    • drill bits
    • wear plate replacement
  • Construction Materials
    • steel (split-sets, reinforcement, rebar)
    • concrete (pillars, support)
  • Ground Support
    • mesh screen panels
    • shotcrete
    • rock bolts (split sets, plates)
    • resin

Ground Support

Ground support normally commences after the muck has been hauled away. The type of required ground support depends on the excavation size and the ground conditions. In poor conditions, it may be necessary to perform multiple ground support activities (initial and final) instead of a single activity at the end of each round. If roof stability becomes critical, initial ground support (i.e. shotcrete, mesh) can be installed immediate after scaling to stabilize the excavation before mucking out completely. Final ground support (i.e. bolting, shotcrete) can be installed after the muck has been completely removed and the exaction fully exposed for the next round. [18]

Example

Based on Kidd Mine in Timmins, Ontario, generally it can be assumed that a cost of $2,500 - $4,000/round (3m) is a realistic cost when planning drift development for a hard rock mine in northern Ontario.[7] Drift development costs reflective of fairly fractured and weathered ground around central Nevada would be in the range of $2,790-$3,100/m (2009 US$). [19]

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Typical Development Costs as per the Mining Engineering Handbook (SME, 2011).[19]

Scheduling Components of Drift Development for Ore Access

In the early development phase of a mine, ore tonnage targets are developed based on the calculated production rate; monthly ore tonnage targets are spread across the operating days per month to ensure that adequate tonnage is able to be sent to the mill every day. Typically, underground mines have production rates between 2000-4000 tonnes per day [20]. Daily ore tonnage sent from the mine to the mill comes from both stope production as well as from development drifts in ore. The amount of ore tonnage from development is planned for each month based on the drift development scheduling.

Ore Access Development Scheduling

Long term drift development scheduling is based on stope sequencing from the life of mine schedule. From the life of mine schedule, stopes that are scheduled to begin production in the upcoming months require development access on both the drilling and mucking horizons. It is essential to schedule adequate time to create the stope design (not completed yet, should we try to link it anyways?), drill the stope and prepare the stope for blasting and mucking in order to begin stope production as scheduled. In short term planning, the work required to complete drift development for ore access is broken down in to a three month rolling plan and monitored. Three month rolling plans look at level plans to determine how much development is required to access a stope. From these plans, the length of a drift can be seen and, based on the updated drift surveys, the remainder of the drift to excavate can be calculated [14].

To view the blasting sequence for a development round, click here.

Advancement Calculations

To determine how long it will take to complete a drift depends on how many rounds (drift cycles) are able to be taken each shift. Drift size depends on equipment requirements. For instance, a standard 5m x 5m drift that can accommodate a two-boom jumbo (Atlast Copco Boomer 282) [21]. The jumbo can drill a round of 3m - 3.5m in drift length. A drift cycle, including drilling, blasting, mucking, scaling and installing ground support typically takes one shift to complete. Assuming a day and night shift are both working in the same drift heading, up to 7m can be advanced in a 24-hr period. Based on this rate, or whichever rate of advance is planned for, the time it will take can be determined[22]. For example, a drift is planned to be 30m long and requires 30m of development. A drift round is 3m in length and a round can be taken each shift. Therefore, to complete 30m while advancing 3m a shaft, it will take 10 shifts to complete. If a day and night shift is working to develop this drift, it will take 5 days to complete. If only one shift is working to develop this drift it will take 10 days to complete.

Development Constraints

One of the main constraints when scheduling drift development is manpower and equipment resource allocation [23]. Based on the production rate, a certain number of stopes will be required to produce the necessary tonnage to meet monthly and daily targets. To meet these production targets, stope access must be completed on time. The headings that need to be advanced to provide stope access dictate the number of active heading required. Depending on how many crews and required equipment are able to be allocated to these ore access headings, the advance rate of a drift may be limited. Crews and equipment must also be scheduled for capital development headings. In most cases, capital development on a level such as refuge stations, electrical sub stations, ventilation pass and rock pass accesses need to be completed before production can begin. Therefore, when planning for a production rate and scheduling stope production, crew allocation and equipment selection must be able to meet scheduling needs. Otherwise, a smaller production rate should be considered. Ventilation requirements and constraints must also be considered when scheduling equipment and have been discussed in Determining Service Requirements of Drifts.

Other Challenges

Drift Intersections and Breakthroughs

Drift intersections require revised support standards for their larger excavated span. For more information, refer to Ground support.

Example of Mitigating Challenging Ground Conditions

At Xstrata Bruinswick Mine , high stress levels lead to increased seismic activity as mine life and depth progressed. This influenced the severity of failures in drift intersections. Mining engineers decided to design their intersections to be offset from each other as much as possible, or in Ts to mitigate high stress concentrations in abutments. Offset drift intersections reduces the excavated span, creating a more stable structure. [24]

Dog Legs

A popular alternative to 90 degree intersections is the practice of "dog-legging" cross-cuts off existing ore access drifts at 45 degree angles to start the subsequent ore access drift. The biggest advantage to this practice is saving on the drift meters associated with developing exclusive drifts to each panel. Note that the narrow 45 degree abutment often requires additional ground support efforts. "Dog-legs" are often developed after primary stope extraction has begun in order to mitigate stress distribution and yielding.

Drifting through Backfill

Occasionally a mine plan may require development through or underneath fill to access ore later in the mine life, and to reduce dependency on leaving sills containing ore. A consideration in this situation is the increased time resource needed for supporting these excavations and realizing the unpleasant situations that could arise when working with backfill. Also in the case of pastefill, planning must have the foresight to pour a higher-than-normal binder content. Some times of fill are inadvisable for this approach due to poor cohesion between fill components and the likelihood of fill failure.

References

  1. Rio Tinto (2014). Dave Brosha Photography. Diavik Diamond Mine. Retrieved from http://www.riotinto.com/media/photo-library-263.aspx
  2. A. Makuch, "Dynatec's Approach to Narrow Vein Longhole Stoping at Midas, Nevada," Dynatec, Midas, 2001.
  3. C. Jude, "Structural Analysis of a Mechanized LHD Trench Undercut Caving System," United States Department of the Interior, 1995.
  4. Stantec, Hard Rock Miner's Handbook. (2014). 5th ed. available online: http://www.stantec.com/content/dam/stantec/files/PDFAssets/2014/Hard%20Rock%20Miner's%20Handbook%20Edition%205_3.pdf
  5. P. Andrieux and B. Simser, "Ground-Stability-Based Mine Design Guidelines at the Bruinswick Mine," in Underground Mining Methods- Engineering Fundamentals and International Case Studies, 2001, p. 206.
  6. De Beers Canada (2014). Snap Lake Mine. Retrieved from https://www.canada.debeersgroup.com/Media-Centre/Image-Galleries/
  7. 7.0 7.1 A. Gauthier, Interviewee, [Interview]. 25 January 2015.
  8. Atlas Copco. Drilling Rig Technical Specifications. Available Online. http://www.dukraft.com/wp-content/uploads/2014/09/SPECS-Atlas-Copco-Rocket-Boomer-E2C.pdf
  9. SME. (2011). Mining Engineering Handbook.
  10. Atlas Copco, "Scooptram ST7: Underground loader (LHD) - Atlas Copco Estonia," January 2012. [Online]. Available: http://www.atlascopco.com/eeus/products/loading-and-haulage-equipment/3506190/1471533/. [Accessed 3 February 2015].
  11. Ministry of Labour Ontario, "Occupational Health and Safety Act - R.R.O. 1990, Reg. 854," Government of Ontario, Toronto, 2015.
  12. C. Pritchard, "Methods to improve efficiency of mine ventilation systems," National Institute for Occupational Safety and Health (NIOSH), Spokane, 2010.
  13. M. Morin, "5x5 Drift Profile with CAT AD45 Truck - MINE 448 2014/15 FW Underground Design," January 2015. [Online]. Available: https://courses.engineering.queensu.ca/d2l/le/content/36482/viewContent/161336/View. [Accessed 3 February 2015].
  14. 14.0 14.1 14.2 F. Borysenko, Interviewee, [Interview]. 3 February 2015.
  15. Ministry of Labour - Ontario, "Occupation Health and Safety Act - R.R.O. 1990, Reg. 833," 28 January 2015. [Online]. Available: http://www.e-laws.gov.on.ca/html/regs/english/elaws_regs_900833_e.htm. [Accessed 2 February 2015].
  16. Ministry of Labour Ontario, "Occupational Health and Safety Act - R.R.O. 1990, Reg. 854," Government of Ontario, Toronto, 2015.
  17. Sandvik (2014). News Archives. Retrieved from http://www.understandingunderground.sandvik.com/category/news/
  18. 18.0 18.1 18.2 J. Cline, "Construction of Underground Openings and Related Infrastructure," in Mining Engineeing Handbook, 3rd ed., P. Darling, Ed., Society for Mining, Metallurgy, and Exploration, Inc., 2011, pp. 1223-1237.
  19. 19.0 19.1 R. Bullock, "Mechanical Excavation Methods of Development," in Mining Engineering Handbook, 2011, pp. 1203-1221.
  20. G. McIsaac, "Strategic Design of an Underground Mine under Conditions of Metal Price Uncertainty," Queen's University, Kingston, 2008.
  21. Atlas Copco, "Boomer 282: Face drilling rig - Atlas Copco Canada," April 2013. [Online]. Available: http://www.atlascopco.ca/Images/Technical%20specification%20Boomer%20282_9851%202500%2001_tcm795-1533190.pdf. [Accessed 2 February 2015].
  22. The Northern Miner, "The Mining cycle," The Northern Miner, vol. 90, no. 17, 2004.
  23. J. G. Hazan, "The Implications of Planning and Scheduling by CPM," in Annual General Meeting by CPM, Toronto, 1965.
  24. P. Andrieux and B. Simser, "Ground-Stability-Based Mine Design Guidelines at the Bruinswick Mine," in Underground Mining Methods-Engineering Fundamentals and International Case Studies, 2001, p.206.
  25. McKinnon, S.(2014). Mine 469 Empirical Design of Open Stope Dimensions: Course Notes.
  26. MineWiki(2014). Louvicourt Case Study. Retrieved from https://www.minewiki.org/index.php/Sill_pillar_recovery_at_the_Louvicourt_mine
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