Equipment selection

The equipment selection process for underground mine design has an incredibly wide scope, as it incorporates the considerations for both hard and soft rock mining, with each respective variation possessing specific requirements, and the related implementation of both mobile and stationary equipment. This article primarily focuses on the initial selection process for hard rock mining with regards to mobile equipment.

Introduction

The determination of the initial mobile equipment fleet is an important step in the overall underground mine design process, and is typically based on the production targets, mining methods and stope dimensions. Not only does the mining equipment employed in the operation impact the overall mine productivity, ventilation requirements, and maintenance considerations of the entire mining operation, but also the overall project economics. The initial capital cost calculations, annual operating costs, equipment replacement considerations, and other financial implications play a significant role in establishing an accurate forecast of the net-present-value of the mining operation. Through the collection and analysis of pertinent data, the use of relevant performance measures and related studies, mine planners can identify the most cost-effective opportunities maximize productivity and the span of the equipment life, as well as reduce the overall cost.

As there is inherent variability and difficulty in acquiring precise data to arrive at a conclusion, various ranges and rules-of-thumb have been developed to provide a standardized process for calculations to be derived from, and various applications are summarized in this article. In the past, it has been a pervasive belief that bigger equipment yields better results; however, that is not always the case. Furthermore, the development of cross-sections, ramp gradients, the electric power network, and other factors are based on an initial selection of the mobile fleet. The selection of equipment for mining operations is not a well-defined process, which is primarily attributed to the inherent variability that exists in mining operations, as no two mines are alike. This article aims to provide guidance in the initial mobile equipment selection process by presenting background information, associated calculations, and case studies, in addition to identifying gaps that currently exist in the data.

Preliminary Selection Considerations

Equipment selection for a hard-rock mine typically begins with or soon after the planning parameters of the stoping operation is completed; therefore, equipment used for stoping should be suitably sized for the stope dimensions and the characteristics of the orebody. However, it is also impractical to commence mine planning and scheduling without a working concept of the relative type and size of equipment that may be employed. All underground deposits require a method of accessing the orebody; thus, multiple factors should be known, such as daily production rate and the equipment selected for development and production, with the associated sizing, utilization, and costing data to assist in determining the best layout.

Most modern underground mines typically operate with fleets of mobile production equipment that include, but are not limited to: load-haul-dump machines (LHD), haul trucks, drill jumbos, long-hole drills, rock bolting machines, and other ancillary equipment. All pieces of equipment should be compared on the basis of their capital costs, operating costs, specifications, performance, availability, and service life before equipment purchasing or leasing is to be considered.

Equipment Matching

It is imperative to ensure that individual pieces of equipment are also considered in terms of a system instead of an independent machine; in other words, each machine must be viewed as a part of an interactive development or production system. Ensuring that an equipment fleet is not only compatible, but can also interact effectively and efficiently, is integral to optimizing the productivity of the entire process. For example, a LHD must be selected in terms of the size of the trucks it will be loading the material into. This equipment matching is to ensure that the bucket of the LHD can reach the height of the box on the trucks and efficiently load it in 3 - 5 passes. If the size of the truck and LHD are not well matched, it may lead to hindrances to the overall production schedule.

The same concept applies to electric-powered machines as they should all operate from the same electric supply voltage. The fleet and individual equipment sizes and capacities are often constrained by work space limitation; therefore, integrating proper machine configuration can help achieve optimal safety, reliability, and maintainability. Equipment matching is an important step in the overall decision-making process for determining the mobile fleet that is required. Assistance can also be provided by the manufacturer and/or supplier of the equipment, as they can help ascertain how well-matched the equipment will be.

Figure 1: An example of a manufacturer's recommendation for equipment fleet selection for optimal mobile equipment interaction.

Equipment Selection Factors

There are a range of factors that must be taken into consideration pertaining to each piece of mobile equipment:

• Purpose and objective of each equipment
• Different types of each equipment
• Size and capacity
• Operating cycle time
• Turning radius / working radius
• Number of pieces required for an operation
• Health and safety considerations

Load-Haul-Dump Machines

Load-Haul-Dump (LHD) vehicles, also referred to as scooptrams, loaders, and muckers, are the fundamental mobile component of an underground mining operation. They are four-wheeled, centre-articulated, bi-directional, rubber-tired machines that can be diesel or electric powered and are designed to fit under restricted heights and narrow areas in an underground mine. The centre-articulation feature allows for the machine to have high maneuverability and a small turning radius to adhere to the designated stope sizing constraints. Furthermore, LHD’s are versatile as their small turning radius and bi-directional features enables for switchbacks to be employed. They are typically selected to complement the size of the fragmented material in a mining environment.

LHD’s differ from front-end loaders due to their narrow breadth and ability to maintain a low-profile, and they can productively carry a load in their bucket for up to 200 m. They tend to devote more time travelling than front-end loaders, which results in increased wear on the tires and incurs additional operating costs for repair and/or capital expenditure for replacement. The haulage routes in an underground mine tend to be uneven and/or wet, which cause the tires to experience increased wear. Therefore, as tires can account for 10 - 20 % of the total operation cost due to the frequency of tire re-treading or replacement every 75 - 1,000 hours, it is essential to try and maintain a smooth, level, and dry working surface for optimal mobile equipment performance. Sufficient drainage and road maintenance can assist in mitigating extensive tire damage and, subsequently, reduce operating costs. LHD’s will not demonstrate optimal productivity in poor haulage conditions, tight loading areas, or when moving poorly blasted material.

Note: Tires can be fitted with chains for increased traction and decreased wear; however, the application of chains can be dangerous in many situations, and are not a common application in underground environments.

Sizing

The size of an LHD should:

• Suitably fit in the mine layout while meeting the regulatory sizing requirements
• Be compatible with existing equipment for efficient interaction
• Satisfy the production requirements of the mine plan.

Figure 2: An example of Caterpillar design specifications for a range of LHD sizes.

LHD productivity also relies on the size and density of the fragmented muck produced; therefore, the capacity should be selected to match the expected muck size and distances to be traveled, horizontally and/or vertically. Most mines typically incorporate a fleet of LHD’s with varying bucket capacities to meet their specific production targets. For example, a mine may employ two LHD’s; one for stoping and one for development. Therefore, the determination of the size of an LHD in a mine plan is a function of the stope dimension and production requirements for the operation.

Typical bucket capacities range from 3 - 11.6 m³, with special machines designed to adhere to narrow-vein operations that have a capacity of 0.38 m³, which are commonly referred to as “micro-scoops”. Respective payloads can also range from 1 tonne to 25 tonnes. An example of Caterpillar LHD’s and their respective features are shown in Figure 2.

Most mines purchase the largest LHD their mining plan and budget can accommodate in favour of higher production rates. For example, a LHD with a capacity of 6.0 m³ may cost 30 % more than a LHD with a bucket capacity of 3.5 m³; however, it can be up to 60 % more productive in regards to tonnes per hour of material moved. The equipment manufacturer/supplier’s manual should provide detailed output curves to allowed justified decisions to be made based on the company’s production targets.

When considering the use of larger equipment in a mining operation, a comparison analysis or a critical approach should be adopted before any purchases are made to ensure the large equipment will be economical with respect to the size of drift and ramp dimensions. If the larger equipment requires excessive excavation activities and ventilation requirements to accommodate the extra size, it may not be a feasible option. The method in which to determine the optimum size for a LHD is not internationally standardized, nor is it well-defined. This ambiguity is primarily attributed to the inherent variability in a mining environment, which results in each mine having different targets, requirements, and constraints to adhere to. Therefore, it is important to identify this when conducting your mine design planning process.

Diesel vs. Electric

Diesel LHD’s have been the customary selection over electric LHD’s over the past few decades, as they are more versatile and have quicker tramming capabilities. However, with the influx of technological advances and enhanced safety features of electric LHD’s, they are becoming a more common choice.

Diesel

Figure 3: Diesel-powered caterpillar LHD.

Diesel LHD’s in underground hard rock mines are equipped with a diesel exhaust treatment device that employs the use of water and catalytic exhaust scrubbers to complete the combustion of hazardous gases that is typically accomplished at an efficiency of approximately 90 %. However, since the gases do not experience full combustion, diesel particulate matter (DPM) is produced. DPM is composed of minute solid particles that are the result of incomplete combustion and/or impurities that may exist in the fuel, and may consist of various hazardous substances, some of which are carcinogenic. The aforementioned catalytic scrubber is ineffective at removing the particulates. Therefore, this issue has become a primary focal-point for the application of electric over diesel LHD’s within a confined working environment.

LHD’s that employ the use of diesel also require a proper ventilation system to counter the excessive exhaust fumes that are generated in a confined working space. There are various ventilation regulations and occupational health and safety standards that have been established to ensure a healthy working environment. A common rule of thumb is that a minimum air velocity of 0.5 m/s (100 ft/minute) must exist in haulage-ways and travel-ways with diesel equipment, as well as 0.047 m³/s per brake horsepower of operating or installed diesel equipment.

Electric

Figure 4: Atlas Copco's EST1030 electric-powered scooptram.

Electric powered LHD’s are an effective alternative to diesel LHD’s in a variety of operations and/or specific locations within a mine. An electric LHD does not emit any emissions and are much quieter relative to their diesel counterparts. Atlas Copco has issued its’ support for electric-powered LHD’s with the release of the Scooptram EST1030 that offers lower noise levels while reducing energy and heat emissions by 40 % versus equivalent diesel machines in conventional operations.

They also have the capability to be highly productive in operations such as block caving, whereby the ore is transported from a series of drawpoints to a fixed location. As mining operations descend deeper into the mine, heat and ventilation requirements become increasingly more problematic and the costs increase; therefore, the implementation of electric LHD’s may be economical in these situations.

One significant issue with electric-powered LHD’s is cable control; each unit carries a cable drum and relies on a trailing electric cable to provide power to operate the machine. Therefore, they are typically tethered to a fixed location during production and, consequently, do not possess comparable versatility to that of the diesel-powered LHD’s. Furthermore, the cables can experience increased wear from high tension when wrapped around corners or being stretched, as well as the potential of being run over, which would incur additional maintenance and operating costs.

Cycle Time

The manufacturer/suppliers manual for the selected LHD will provide the required information on the machines particular speed range, turning radius, and other design specifications needed for making a justified, economical decision. However, from a planning perspective, it is imperative to determine the overall cycle time of a LHD to determine its’ productivity, and, consequently, the stope life cycle to analyze the return on investment for an individual or fleet of LHD’s.

There are three main elements in determining the total cycle time of a typical LHD:

1. Loading time: this depends on the density of the material in the muckpile along with the condition of the muckpile, the horsepower of the LHD, and the underfoot conditions.
2. Travel time: the time it takes to haul a load from the loading point to the dumping point travelling at average speed, then return back to the loading point with an empty bucket.
3. Dump time: this should include any additional time for waiting on trucks, and is also a function of the area that the load is being dumped into; for example, when dumping in a truck, the operator should ensure that the load is evenly distributed in the box of the truck, which may result in slightly longer dumping times. However, when dumping into a chute or pass, there is no requirement for attention to placement or shape of the load.

The optimum cycle time for typical diesel powered LHD machines has been recorded as between 28 to 42 seconds, with an average of 35 seconds. Average speed is a function of the condition of the haulage-way, clearance to tunnel walls, traffic, and the grade of the road.

LHD cycle times are calculated to ensure that the LHDs will meet the production requirements of both daily ore production and development requirements. The stope life cycle will vary depending on the LHD production (i.e. mucking rate) achieved for a given stope. An example of the calculations required to determine the total cycle time of an LHD loading a haul truck, its' mucking rate, and the number of units required to sustain the target production, and the design specifications are outlined in Table 1.

Table 1: Cycle Time Calculation Parameters

Bucket Capacity	Fill Factor	Muck Swell Factor	Muck Density	LHD Availability	Stope Utilization	Load Time	Travel Time (Loaded)	Dump Time	Travel Time (Unloaded)	Delays (Traffic, Waiting)
8 yd3	90 %	30 %	3.39 m3/t	80 %	80 %	30 seconds	200 m at 6 km/h	30 seconds	200 m at 6 km/h	0 seconds

Bucket Volume	Tonnes Per Bucket
Travel Time	Total Cycle Time

⇒ Therefore, assuming that the mining operation is a continuous, 24-hour cycle, the LHD mucking rate can be determined to allow for a calculation of annual production:

Trips per Day	LHD Mucking Rate

⇒ Furthermore, if an effective hours of operation per year is determined to be 4,500 (effective hours per shift x the number of shifts per year), the annual LHD production can be calculated and number of LHD’s required for the operation determined:

Annual LHD Production

⇒ Using a production target of 1.5 million tonnes/year of ore brought to surface:

Number of LHD's

Note: The maximum economical tramming distance for an 8.0 yd3 LHD it is 800 feet and 800 tons per shift. Furthermore, LHD mucking rates can change depending on the location of the stope and also whether the stope is a primary, secondary, etc., stope.

Case Study

Figure 5: Atlas Copco's ST14 automation system.

The implementation of automated LHD’s has become an increasingly prevalent trend in underground mines, and offers many benefits over manually operated LHD’s. Replacing manually operated loaders with remotely automated loaders has had increasingly positive results with respect to safety and productivity. This trend has led to increased safety and productivity in underground mining operations, while also lowering operating costs (specifically labour costs).

At BHP Billiton’s Olympic Dam mine in South Australia, the implementation of LHD automation has produced an additional 2.4 hours per shift of productivity. Furthermore, poor ground conditions that can result in highly unstable zones present a significant safety hazard to the safety of an operator.

A case study was conducted on Codelco’s El Teniente copper mine in Chile, where they have implemented the Sandvik AutoMine automated loading and hauling system. The AutoMine system was found to increase operating hours by over 3 hours per shift, and eliminate the amount of lost time accidents over longer intervals. Although some challenges were encountered in fully integrating the AutoMine system within the overall mining system, as the machines had to operate in exclusive zones with carefully monitored access, the study had demonstrated success. At El Teniente’s 24 hour, 7 days per week operation, only 4 tele-remote operators are required to complete the work of 16 conventional LHD operator on a three-shift rotation in absolute safety.

Although automated LHD's may be attractive due to their elimination of manual operators and the related labour costs, only 50 - 60 % of underground mining employees are operators of equipment. The reduction of operating costs in labour by using automation is offset by the requirement for highly specialized maintenance and technical support to operate the software. Furthermore, the capital cost and infrastructure requirements of installing a remote system with the necessary components throughout an entire underground mine will require an extensive initial comparison analysis to determine if it would be a feasible option to incorporate into the operation.

Summary

When creating an initial plan for selecting a fleet of mobile equipment that contains LHD’s for an underground, hard rock mining operation, there are various factors that need to be taken into consideration:

• The drift dimensions with respect to the maximum width allowable in an operation
• Loading method and bucket capacity determination
• Compatibility with truck size
• Diesel or electric LHD
• Density and properties of material that will be loaded
• Ability to meet required production targets
• Service and maintenance requirements
• Capital cost and operation cost
• Estimated utilization and availability
• Technological advances and automation
• Equipment life

Haul Trucks

Haul trucks are typically diesel-powered machines that transport material to and from surface, development stopes, headings, and draw points. There are three main types of trucks that are employed to adhere to different requirements: tractor-trailer units that are typically side-dumping and can be assembled as an underground road train, rigid-body rear-dump trucks, and articulated rear-dump trucks that are designed to have a low profile. An articulated, rear-dump truck is one of the most common trucks in an underground mining environment, as they are highly adaptable to rough terrain and typically possess a low profile.

Figure 6: Atlas Copco's articulated rear-dump haulage truck.

Sizing

Figure 7: Typical allowances for an underground mining haul truck.

The selection of a truck that will be travelling through development headings and ramps must fit within the dimension of the openings and adhere to various safety allowances. These allowances include:

• Clearances from the top of the truck’s load to the bottom of a ventilation duct or any service systems. Regulations require that a minimum clearance of 30 cm must exist between the highest point of the tallest piece of mobile equipment and the ramp’s roof or bottom of any duct, station, walkway, etc.,
• Sidewall clearance to allow for workers to walk past a stationary truck, or in the event of equipment malfunction, maintenance work can be conducted on the machine. It also minimizes impact damage.

Figure # depicts a typical design profile containing the various regulations applying to the cross-sectional dimensions of a drift.

The turning radius (refer to Figure 6 of [1] Underground Ramp Design] for trucks is a function of the size of the truck; however, a typical rule-of-thumb for initial planning is that trucks typically require a minimum turning radius over 20 m. The truck manufacturer should provide a comprehensive manual of each trucks design specifications, as well as detailed approaches to equipment system compatibility and recommendations to the buyer to make educated decisions for planning. Furthermore, options for leasing the haul trucks also exist, and may be a feasible alternative to consider.

Most mines aim to use the largest haul trucks that will fit into their operation to increase productivity and reduce operating costs. However, the use of bigger and more powerful haul trucks can induce ground control and stability problems, as well as higher costs related to the need for larger stope sizes. In addition, the operators experience increased risk due to reduced visibility, increased noise, and vibrations. For example, a case study was conducted to compare the overall safety of a 50-tonne haul truck against a 30-tonne haul truck. The study revealed that the 50-tonne truck resulted in visibility challenges for the operators, and they had difficulty spotting, and consequently, avoiding large rocks in the haulage-way or holes in the road. Thus, an increase in head and back injuries to the operator was observed. The 30-tonne truck did not cause the same issues.

Cycle Time

Similar to the method in which to determine the overall cycle time for LHD’s, the manufacturer’s manual will supply the information required to estimate the respective cycle time. In general, the following breakdown can be used to determine the overall cycle time:

1. Loading time: this includes the time waiting for an LHD to arrive; this factor is contingent on the distance from the muck pile to the loading point, as well as the loading capacity of the loader.
2. Travel time: calculated using the average speed (when loaded) and is a function of the distance of the loading point to the dump area, and the average speed (when empty) from the dump point back to the loading area. This factor depends on the haulage path conditions, traffic congestion, road and/or ramp grade, number of curves or bends that the truck will have to take, and any speed limits that exist in the mining environment.
3. Dump time: this also includes wait-times for other trucks to dump.
4. Exchange time: the elapsed time from when a loaded truck receives its’ final loading pass on shift until the next truck receives the first load of its' shift. This is a vital element to consider, as it completes the cycle time of the shift and can be a factor in increasing the number of cycles per shift. The best practices target for this parameter has been recorded as 42 seconds per cycle.

An example of the calculations required to determine the total cycle time of a haul truck being loaded by an LHD with the outlined design specifications and required parameters outlined in Table 2.

Table 2: Cycle Time Calculation Parameters

Truck Capacity	Fill Factor	Muck Density	Truck Availability	Stope Utilization	Load Time	Travel Time (Loaded)	Dump Time	Travel Time (Unloaded)	Delays (Traffic, Waiting)
50 tonnes	90 %	3.39 m/t	80 %	80 %	2.5 minutes	3.5 km at 7 km/h	30 seconds	3.5 km at 9 km/h	60 seconds

Truck Volume	Total Cycle Time

Once the average cycle time has been determined for a particular haulage route, the tonnage of material moved, effective time per shift, and overall truck utilization can be factored in to establish the number of trucks required for that operation. For example, once the effective hours per year of a truck is established (effective hours per shift minus the allowance times for breaks and multiplied by the number of shifts), the annual truck production can be calculated.

Therefore, assuming that the mining operation is a continuous, 24-hour cycle, the haulage rate can be determined to allow for a calculation of annual production:

Trips per Day	Haulage Rate

Furthermore, if 4,500 effective hours of operation per shift is used again, the annual truck production can be calculated and the number of trucks’ required for the operation determined:

Annual Truck Production

Using a production target of 1.5 million tonnes/year of ore brought to surface:

Number of Trucks

Note: As the mine development descends deeper, the problems with delay time (i.e. congestion) are amplified; this is due to the increase in the number of trucks to meet production targets, and the equipment interference can also increase. Therefore, to use more accurate and representative cycle times for extensive planning purposes; a “congestion factor” should be applied. One approach to determining this factor would be to assume that an empty truck that is returning to the load area will wait in passing bays for loaded trucks to pass by in the opposite direction; therefore, the cycle time would have the additional time it takes for a loaded truck to travel half the average distance between passing bays multiplied by the number of trucks that are operating, minus the empty truck that is waiting. However, traffic congestion can be mitigated through the use of effective traffic management measures such as radio communication, well-spaced passing bays, and the use of traffic lights.

Case Study

Figure 8: 50-tonne truck productivity at depth.

McCarthy and Livingstone explored the relationship between truck productivity as a function of the vertical depth of a mine. Truck haulage production rates using ramping systems were determined to range from 38% to 89%. The results of the investigation into a fleet containing varying numbers of 50-tonne trucks operating at different production rates can be observed in Figure #:

It is apparent that even though a relatively optimistic utilization factor of 70% was used, a rapid decrease in truck productivity as the vertical depth of the mine increased. The rapid decline in productivity is exhibited as the vertical depth increases is much more prevalent when 9 trucks are used; therefore, increasing the number of trucks as you descend deeper into the mine will not entirely eliminate the production issues. Increasing the number of trucks can reduce efficiency, raise mine operating costs, and increase ventilation requirements.

Summary

In creating an initial plan for selecting a fleet of haul trucks for an underground, hard rock mining operation, there are various factors that need to be taken into consideration:

• Drift dimensions
• Ability to meet required production targets
• Trucks be loaded by an LHD or chute
• Compatibility of haul trucks with loading equipment
• Loaded from the rear or the side
• The use of ejector boxes or conventional rear-dump boxes
• Density of material that will be loaded
• Service and maintenance requirements
• Capital cost and operation cost
• Estimated utilization and availability
• Equipment life

Jumbo Drills

Jumbo drills are a diesel powered mobile carriage that typically support 1 to 3 drills. They are used for rock excavation and have an articulated transporter and an electrically powered hydraulic pump to control the boom(s) and rock drill(s). The size of the opening dictates the number of booms the jumbo will need to have. Single booms are used in small headings or narrow veins, and can cover face dimensions from 6 m² to 30 m². Jumbo drills with two booms can cover an area ranging from 8 m² to 100 m², which makes it the most common selection due to their versatility. Three boom jumbos are rarely employed due to their larger size and increased complexity of operation.

Figure 9: Atlas Copco Boomer 282 jumbo-drill with double-jumbo.

Jumbo drills must possess the ability to reach the limits of the largest excavation, which is usually determined by the production drills and trucks. The reach of a jumbo drill also sets a limit of cut and fill slice thickness or stope width. Furthermore, the jumbo drill must be able to reposition to ensure precise drilling and fragmentation to generate consistently broken material for reliable loader productivity.

Jumbo drills must also be considered on the basis of their respective face advance capabilities, which is a function of the number of faces they have access to and the number of rounds they are able to complete per shift. For example, a double-boom jumbo in a single development heading descending from surface can typically advance at a rate averaging from 45 m to 50 m per week, which is subject to change depending on the underfoot conditions, the variability in the rock, and the existence of faults.

Number of Jumbo Drills

To determine the number of jumbo drills required for an operation the following parameters should be considered:

• The number of faces available to a jumbo drill
• The type of drill (single, double, or triple)
• The face meters drilled per meter of advance
• The target distance for development for month are all parameters required.

For example, if the face meters drilled per meter of advance was recorded to be 60 m and the target development distance accomplished per month is 800 m in an operation with 5 faces available to the two-boom drill jumbo, the number of jumbos required would be 3.

Note: Jumbo drills are amongst the most difficult pieces of equipment to drive to surface once they have commenced their work underground; therefore, they will require a service station underground.

Production Drills

Production drills are employed to drill blast holes in a circular pattern around a drift. Production drills often control stope dimensions, and are most commonly used in block and sublevel caving, sublevel stoping, and vertical crater retreat (VCR) mining operations. The capabilities and limitations of production drills (i.e. the ability to drill a certain length while remaining sufficiently accurate, or reaching a certain height) can dictate the amount of drill deviation that will occur, which will affect dilution or poor fragmentation.The type of production drill utilized in a mining operation will depend on the drilling and blasting requirements, rock mass characteristics, and the stope design. There are two primary types of production drills: Longhole drills and down-the-hole (DTH), which are also referred to as in-the-hole (ITH) drills.

Longhole Drills

Longhole drills are versatile and possess the capability to drill large diameter holes, typically 5 to 127 mm in diameter, over substantial lengths with sufficient precision and accuracy. The implementation of longhole drills can increase the spacing required between sublevels, which can effectively reduce the amount of development waste produced. Longhole drills deliver energy using percussion from a piston over the length of a steel string to the bit; energy can be wasted if the length of the steel string was dramatically increased. Subsequently, more torque would have to be applied to ensure sufficient energy levels were maintained, which could result in greater incidence of hole deviation and higher maintenance costs.

Figure 10: Atlas Copco longhole drilling rig.

Down-the-Hole Drills

Down-the-hole (DTH) drills employ the use of a hammer located directly behind the bit, which provides percussion rather than the piston over the length of a spring. This procedure results in a more efficient transmission of the power to the bit, and less hole deviation. The accuracy of a DTH drill establishes an upper limit on stope height. DTH drills typically range in diameter from 95 - 178 mm.

Production drills are employed in narrow-vein mining as well as larger operations, as they can eliminate the need for human intervention while the stope blastholes are being drilled; thus, increasing the safety factor in the working environment.

An overview of the various mining methods with a relative breakdown corresponding to the drilling equipment required for each method, along with the associated output parameters, is displayed in Figure #:

Figure 11: Breakdown of mining methods and applicable drilling equipment with typical output and other pertinent parameters.

Rock-Bolting Machines

A rock-bolting machine resembles a jumbo drill and may employ the same carrier; however, they are typically composed of a four-wheel-drive, articulated, scissor-lift platform whereby the rock bolting is conducted using a handheld rock drill. Development jumbo drills can also be used to install rock bolts and mesh; in these cases, the boom must have the capability to turn 90⁰ to the axis of the carrier, and a split-feed boom may be required to extend the boom to full length. This capability adds significant complexity and costs to maintenance, and the additional weight may result in bolting inaccuracy.

The size of a rock-bolting machine should guarantee suitable proximity to the development or production heading. One prevalent issue with selecting a rock-bolter is that they tend to be too small to suit the heading size and subsequently are unable to reach all of the designated areas in the heading. Rock-bolting machines may be slower and less mobile than jumbo-drills; however, they can be more productive and incur less maintenance costs.

Other Ancillary Equipment

There are other pieces of ancillary equipment that can be implemented in underground, hard rock mines independent from the most common pieces of mobile equipment listed above. Other pieces of equipment that may be considered include, but are not limited to:

• Scalers: suitable for headings that are between 4 - 6 m high.
• Shotcrete machines: these machines should be mobile, have an effective boom length, and have remote-control capability to remove human involvement in the unsupported areas.
• Front-end loaders: they are excellent for mucking but are extremely poor when required for haulage; therefore, the use of LHD’s are generally selected over front-end loaders.
• The use of rail equipment; although still employed in some mining operations, this technology is rapidly diminishing due to the meticulous and detailed planning required for location, costing, manufacturing, cycle times, gradients, safety, sizing, and ventilation requirements.

Furthermore, soft rock mining incorporates various pieces of mobile equipment such as continuous miners. However, further analysis is beyond the scope of this article.

Economics

The selection of the mobile equipment fleet for an underground mining operation is a vital part in determining the overall project net present value (NPV). As the underground mining industry is extremely capital-intensive, an analysis needs to be conducted on equipment fleet before any investments are made to ensure that the justification for purchase is quantified in advance. A minimum rate of return (MARR) or the weighted average cost of capital (WACC) are typically the primary hurdle rates that new mining companies strive to exceed to allow for a favourable rate of return and positive NPV to be achieved.

Conducting a life-cycle cost analysis is a recommended method for evaluating, comparing, and selecting equipment. This procedure examines the cost associated with a piece of equipment over its entire life, which includes the initial purchase, the production cycle, and disposal. This analysis allows for a determination of the best equipment selection, as the machine with the lowest life-cycle costs will be the best investment overall. For example, if an LHD was purchased at a relatively low price, but yields low productivity and incurs frequent maintenance stops over its productive life, it can become the much more expensive option.

Estimating Operating and Capital Costs

The estimation of capital and operating costs are part of the economic evaluation of a proposed project’s feasibility, and provides a basis for developing an authorization for expenditure (AFE). Accurate estimates of capital and operating costs are completed after detailed knowledge has been obtained regarding the mining operations target productivity, the quantity of material residing in the deposit, and the labour requirements. Underground mobile mine equipment and the corresponding maintenance, scheduling, and replacement requirements have a significant impact on initial capital investment, operating costs, and overall average annual cost.

Capital Costs

The capital expenditures (CAPEX) of a mining operation are any costs that are incurred over a one-time purchase, and encompass any already-produced assets such as equipment and infrastructure. Capital cost estimates for mobile equipment are typically provided by the manufacturers, who supply suggested list prices for pieces of mobile equipment. There are a variety of different CAPEX terms that apply to pre-production and production stages that must be considered when creating a financial study (i.e. fixed capital, sustaining capital, closure capital, and working capital). However, with respect to underground mobile equipment, the initial purchase falls under the category of fixed capital, with equipment replacement parts and the overall replacement of a piece of machinery being classified as working capital and sustaining capital, respectively.

Capital costs are also provided in terms of unit costs, which is the average capital cost per tonne of annual capacity for a piece of equipment. This measure allows for the estimation of capital costs of similar types of projects operating at different scales; however, it does not take into account economies of scale.

Equipment salvage values should also be included in the cash flow model, as those values provide a more accurate and much more manageable capital cost forecast. Salvage values are estimated on the basis of the proportion of useful life remaining for the equipment, and may vary depending on the extent the machine has been worn down, the quality of maintenance performed, and the demand for the equipment. A more comprehensive and detailed investigation is required to determine the operating and, subsequently, the annual costs to provide a reflective running cash flow spreadsheet.

The average annual cost (AAC) is a unit that tends to be a prevalent and common factor employed in the determination to select the most appropriate equipment in mine planning stages. It is the sum of the operating cost, interest, and depreciation of a piece of equipment. For example, if the estimated hourly operating cost of an LHD with a 4-year life span was $200 per hour, with the utilization of 5,550 hours per year, and an annual interest expense of $60,000 per year, the AAC can be calculated as:

Average Annual Cost

However, that equation does not factor in depreciation. Therefore, for a more representative and accurate estimate for AAC, companies tend to apply a capital tax factor (CTF) to adjust for tax effects on capital expenditures and salvage values.

Operating Costs

Figure 14: Typical operating costs of 40 and 50 tonne rear-dump trucks.

Operating expenditures (OPEX) encompass the various costs that are associated with production or maintain capital and can fluctuate over the life of a project; some examples include equipment operation, fuel, tire wear and replacement, hourly labour, ventilation requirements, and maintenance. The magnitude of these costs can be contingent on production output and their capital use, and are typically incurred at least once a year, if not multiple times throughout the year. A breakdown of operating costs for 40 and 50 tonne rear-dump trucks is displayed in Figure 14.

Maintenance requirements typically represent the largest fraction of the mine's controllable operating costs, accounting for 30% to 65% of the overall operating cost budget for a typical mining operation. However, new mobile equipment typically operates at a lower maintenance cost for the first 2 years or 10,000 hours. Therefore, the working cash flow sheet should reflect this consideration.

The unique and dynamic nature of a mining operation can make it challenging to establish consistent operating costs. Overall operating costs for hard rock mines can vary between US $5 to $100/tonne depending on the scale of operations and the mining method that is employed. A brief estimate of the applicable operating costs may also be calculated by obtaining from published, public annual reports of mining companies with a similar orebody. There are also resources such as The Mining Cost Service handbook that can assist in determining an reasonable forecast.

Note: A convenient method to estimate current operating costs for a particular piece of mobile equipment is to employ the monthly charges accumulated over a one-year period, and add the latest month and removing the last, or 13th, month.

Equipment Life

The equipment life cycle (useful life of equipment) is an important metric to consider as it establishes the estimated time a machine will have fulfilled its production and economic targets, and should be replaced. This factor may not apply to all pieces of equipment in a mining operation, as some will possess service lives greater than the mine life; however, most mobile equipment fleets have a finite equipment life. For example, LHD units, which typically have the shortest life span, will deteriorate in strength and efficiency after 5,000 hours of service if they are not well maintained.

Most haul trucks in a mining operation have a useful life of 20,000 hours or more depending on the preventative maintenance and management program and the implementation of newer technology and/or software. Haul trucks tend to have a longer equipment life than LHD’s, carry more tonnes per horsepower, incur lower maintenance costs, lower tire costs, and have a higher availability as they are used solely for hauling while LHD’s are used for both mucking and hauling. An alternative to extend the service life of a machine is to designate it to perform different duties; for example, a haulage truck can become a utility vehicle or production LHD’s become development LHD’s to alleviate the frequency of dedicated work.

It should be considered that error and uncertainty is inherent in most cost estimations due to the variability of the mining and geologic conditions, the current and future global economic trends, and the misapplication of estimating methods in determining costs.

Equipment Utilization and Availability

Figure 15: Rules of thumb for productivity.

The mining industry is extremely capital-intensive, especially when implementing bigger equipment with larger capacities to increase production rate. However, low commodity prices have forced companies to discover methods in which to decrease their overall unit costs by improving equipment productivity. One way to accomplish this is to utilize equipment more effectively and efficiently. An industry rule-of-thumb productivity breakdown for various activities is displayed in Figure #. Therefore, equipment utilization (EU) is an integral measurement that can not only evaluate mobile equipment performance, but also reduce production costs and improve the profitability of a mine.

Equipment Utilization

Equipment utilization (EU) is the proportion of total scheduled hours that a machine actually operates. EU can be expressed as the hours of service per year, the tonnes of ore handled per year, or, most commonly, the percentage of the available and/or production time that a machine is operating. An industry rule-of-thumb for new and/or well-maintained equipment is to assign an initial utilization value of 60 - 80 %. For pieces of equipment that will not be used on a regular basis (i.e. a two boom jumbo drill or scissor lifts), an EU value between 15 % - 40 % is acceptable. However, if the machine will be employed frequently (i.e. a longhole drill to conduct longhole stoping, haulage truck, or LHD), an EU value between 70 - 80 % would be applicable.

Equipment utilization can also be calculated using the function below:

Equipment Utilization

Equipment utilization impacts the operating costs, which affect the overall production costs of a mine. For example, in a mine utilizing 5 LHD’s, each possessing a bucket capacity of 8 yd³ and an operating cost per hour of $90.50 with a utilization of 15 %, the total operating cost per day can be calculated to be:

Total Cost

Note: An EU value of 3,500 - 4,500 hours per year is typical for a captive unit of underground mobile equipment working 6 to 7 days per week and 3 shifts per day. LHD units at a shallow mine with ramp entry should have a utilization of 5,000 - 6,000 hours per year.

Equipment Availability

Equipment availability (EA) is described as “the ability of an item to perform a required function under given conditions at a given instant of time or over a given time interval, assuming that the required external resources are provided.” EA is a key element employed to determine the overall equipment utilization and meet production goals because production and maintenance are two factors that are inter-related. In other words, production cannot proceed without equipment, and equipment will not be available unless it is maintained properly. Equipment availability has many contributing factors, such as equipment reliability and maintainability, which can be monitored and employed in isolating the areas in which a piece of mobile equipment needs to improve to increase the productivity of the cycle.

Conducting an availability assessment can be an extremely useful tool for mine operators in effort to reduce operating and production costs. For example, an increase of availability in a piece of equipment by 1 % may result in a profit gain between 1.7 - 3.5 %.

Equipment availability is typically calculated using the equation below:

Equipment Availability

Equipment Reliability

Equipment availability and reliability are intricately related; however, availability measures the proportion of the total time that a piece of equipment is available and reliability measures the frequency in which the equipment fails or breaks down. Reliability is measured by the Mean Time Between Failures (MTBF), assuming a constant failure rate throughout a period of time, using the equation below:

Mean Time Between Failures

Note: It is possible to have a piece of equipment that is highly reliable, but that exhibits a low availability due to service requirements for long periods of time; conversely, a piece of equipment can be reasonably available, but break down quite frequently for short periods of time.

There are inconsistencies when calculating equipment availability in regards to the contributing values. For example, the measure of downtime hours is, in practice, described as the interval of time that commences once a mechanical failure renders a machine incapable of performing work, or it was taken out of service for maintenance reasons. However, some definitions classify end of 'downtime' as the moment a piece of equipment is no longer hindered by a failure or maintenance outage, and others consider it to be the moment the machine recommences required work. Therefore, an international standard for classifying the key terms in determining these measures of reliability, availability, and utilization should be determined to ensure consistent and precise calculations.

Evolution of Equipment

The selection of equipment for mining operations is not a well-defined, standardized process as no two mines are alike; inherent variability and complexity with regards to geology, location, company objectives, etc., make each design planning process unique. The selection of the underground mobile equipment fleet is an integral step in the overall underground mine design process with respect to production, costing, health and safety, and many other pertinent factors. Mobile equipment has improved and evolved significantly over the past century, and especially within the past few decades with regards to technological advances. Most mines are becoming highly mechanized, and are implementing automated pieces of mobile equipment to minimize the need for human interaction at the working face in potentially hazardous conditions.

Furthermore, as the pressure builds on mining companies to reduce costs and improve productivity, many turn to implementing bigger sizes of equipment; however, it is paramount to comprehend that most decisions regarding one piece of equipment will impact other selections either directly or indirectly. For example, the use of larger trucks can require the need for larger LHD’s, larger shafts, larger raises and drifts to be excavated and supported; larger drifts are more expensive to construct and maintain, especially at a depth. Therefore, optimizing the materials-handling process with regards to mobile equipment interaction is complex process with several inter-related components. The use of simulations for underground, hard rock mining design can be an incredibly useful tool in examining the productivity of haulage equipment, and isolating the areas in which have the greatest influence on productivity.

Current economic conditions, global competition, the inherent capital-intensive nature of the mining industry, environmental regulations, and enhanced health and safety protocols demand that mining operations employ many efficient and innovative methods to improve their productivity through optimizing their equipment utilization and effectiveness. Finally, it is an important concept to comprehend that while underground equipment should be selected to suit the mine, it is equally true that the mine should be designed to suit the equipment.

Further Reading

Should further information on mobile equipment selection for hard rock mining be required, it is advised that the following sources be used due to their quality and scope of material covered.

1. Darling, Peter. "SME Mining Engineering Handbook" (2011) Third Edition, Volume 2. Society for Mining , Metallurgy, and Exploration Inc. (SME)
2. De La Vergne, Jack. “Hard Rock Miner’s Handbook.” (2014) Edition 5. Stantec Consulting Ltd. http://www.stantec.com/content/dam/stantec/files/PDFAssets/2014/Hard%20Rock%20Miner%27s%20Handbook%20Edition%205_3.pdf