Revision as of 13:02, 11 February 2011

From Queen's University Mine Design Wiki

This article is about the selection of an access method in order to create an infrastructure for the transport of ore, workers, and equipment in "underground mining projects".

Mine access is the method of which both people and equipment are able to get to and from an underground orebody. The selection of a proper access method is crucial to the overall design of the mine, since it is a long term decision that requires extensive time and capital. The two most common methods of underground mine access to be compared are ramp and shaft access. A variety of parameters must be understood and examined before proper access can be determined and designed, these include: Depth of mining, orebody geometry, production rates and rock quality.

Types of Mine Access

Shaft Access

Figure 1 - Underground shaft layout^[1]

Shaft access utilizes a skip and hoisting system in order to transport workers and materials through a vertical passage located near the orebody. Generally this system is best suited for deeper mines that will require higher production rates. A general rule of thumb states that a mine that has the potential to go deeper than 500m or plans to have production rates over 400000t/year will be best suited to a shaft ^[2]. Shaft access can also be utilized through an internal shaft, or a winze. This type of access is most commonly utilized in the case of deep lenses that extend past the initial mine life. The top of the winze is located underground and usually offset from the initial mode of access in order to get access to deeper pockets of the orebody. The development of a winze can differ slightly from that of a conventional shaft since raise bore technology can be used to ream the internal shaft.

Development Requirements

Figure 1 shows a general layout of what it required in a shaft access and handling system. In most cases the task of shaft sinking is allocated to a contractor who can usually advance between 3 and 5 vertical meters per day ^[3]. As can also be seen from this diagram, an ore pass exists which utilizes gravity to transport material from each level down to the skip where it can be brought to surface. This is not a requirement if ramp haulage is to be used. Production shafts are usually driven using traditional drilling and blasting methods with the use of a shaft mucker that is lowered down the shaft to pull out blasted materials.

Shafts are usually located along the centroid of the orebody, offset on the footwall side. It should be located at a large enough distance away to ensure that once the orebody is mined out the integrity of the shaft is not compromised. However, a large distance away from the orebody means higher operating costs from extensive materials handling; so a balance must be considered.

Costing

Costs for shaft development are heavily dependent on the rock type that is being worked with as well as the desired shape of the shaft. These prices are detailed from Cost Mine services in table 1 and 2 below. As well included in table 3 is the costing structure for an internal raise bored winze.

Table 1 - Circular Shaft Costs^[4]

Table 2 - Rectangular Shaft Costs^[4]

Table 3 - Raisebore Shaft Costs^[4]

Advantages of Using a Shaft

• Higher production capabilities

• More versatile with depth

• Smaller excavation; requires less support

Disadvantages of Using a Shaft

• Much longer to development

• Higher upfront capital

Ramp Access

Figure 2 - Underground ramp layout^[5]

Ramp access in a mine consists of driving a down-sloped heading with horizontal curves in order to transport mobile rubber-tired equipment from surface to the orebody, as shown in Figure 3 below. Ramp access is attractive for shallow orebodies, particularly where a decline portal can be sited within an existing open pit ^[2]. Ramps can be driven at grades ranging up to 20%, but are usually restricted to no more than 16% if they are to be used for truck transport of stope muck. ^[3] The slope of a ramp is determined by the capacity of vehicles to negotiate declines, with 15% grades being common, with 12% in curves and 10% at level intersections ^[1] The economic depth limit to which a single ramp access mine can extend is governed by haulage costs, equipment selection, productivities, and ventilation considerations. ^[3]

Development Requirements

In order to drive an access ramp, conventional drill and blast methods are usually employed (drill,blast, scale, muck and bolt), with average advance rates being is as follows: 0.3-0.5 m/man shift for an inexperienced crew; 0.7-0.8 m/man shift for competent crews; and 1.0-1.25 m/man shift for exceptionally productive crews. ^[1] In designing a ramp, one must first consider the capabilities of the equipment that will be travelling on the ramps. While one must consider equipment capabilities for the decline grade, as noted above, physical dimensions of the equipment as well as ventilation requirements need to be considered. It is common for the required turning radius for equipment to be 15m to 30m, but will vary based on what is selected. ^[6] It is also important to consider ventilation requirements for the equipment travelling on the ramp. A good rule of thumb is that 0.0633 m3/s of volumetric airflow will be required per kilowatt of operating or installed diesel equipment. ^[1] Also, air velocities in haulage and travelway ramps should be between 0.5 m/s and 3 m/s ^[1] to ensure proper air circulation without creating difficult working conditions. As such, it is common for ventilation requirements to have a significant influence on the face area of access ramps.

Costing

Factors affecting the capital cost of driving a ramp include operating & maintenance cost of excavation equipment, consumables (bits, steels, explosives, ground support material, etc.)and manpower ^[1]. A summary table of standard costs per linear metre of advance is show below.

Operating costs for a ramp accessed mining operation will vary greatly between mining operations, but contributing costs include: operating cost of trucks (fuel, oil, tires, etc.) $/hour of operation, maintenance cost of trucks, $ / hour of operation, ventilation, grading and labour costs.

Table 4 - Ramping Costs^[4]

Advantages of Ramp Access

• Quicker advance rates in development stage allows faster access to ore

• Generally lower capital cost to develop

• Flexible production based on equipment selection

Disadvantages of Ramp Access

• Less productive than shaft access

• High operating costs as depth increases

• May require extensive ground support in poor rock conditions

Decision Analysis

Due to the higher capital costs associated with shaft access, the question of mine access decisions is often asked in terms of at what depth is ramp access no longer the most economical option. In the past, 350m was a highly quoted number for the depth at which shaft access is superior, but with improvements in diesel truck technology over the years, this figure may no longer be universally valid ^[2]. In fact, McCarthy and Livingston determined the feasibility limits of ramp access to be around 800m and 1.2millon tons per annum (Mtpa), set by truck performance and ventilation. Other important conclusions include that the optimum changeover depth from decline haulage to shaft hoisting becomes shallower as the mine life increases, as well as also becoming shallower as required productivities increase ^[2].

Due to the non-universality of access selection it is important that steps are taken to optimize the decision. After all, access can be the determining factor in a mines production rate, and thereby the mining method. The result then trickles down creating differences in mining and milling costs due to economies of scale and other factors. Moreover, equipment expenditures and working capital requirements can also vary widely between the options. It is then necessary to build a case for each option, in an attempt to optimize the decision from a financial basis; creating the highest possible return for investors. When building a case for each of the options it is important to create a defensible cash flow analysis for each. These cash flows should reflect the reality of operating conditions specific to the access method selected. It should go beyond back-of-the-envelope type calculations and demonstrate due diligence in researching, and substantiating estimations and projections.

The two flow charts shown below outline relevant factors in deciding between a ramp or shaft access method:

Figure 3 - Mine Access Flowchart^[1]

Other Factors

"""Shaft""" • Mining and Milling Constraints • Method of Shaft Construction • Cross Sectional Area • Shaft Lining • Hoisting System • Transport of Men and Equipment • Transport of Ore • Headframe Construction • Crushing Station • Shaft Lining • Collar • Loading Pockets • Level Stations • Level Interval

"""Ramp""" • Mining and Milling Constraints • Method of Shaft Construction • Cross Sectional Area • Grade • Ramp Lining & Ground Support • Transport Equipment (Men & Ore) • Portal • Level Interval & Layout

References

1. ↑ ^{1.0 1.1 1.2 1.3 1.4 1.5 1.6} DeSouza, E. (2010). Mine Access. 8-48. Kingston, Ontario, Canada Cite error: Invalid <ref> tag; name "DeSouza" defined multiple times with different content Cite error: Invalid <ref> tag; name "DeSouza" defined multiple times with different content Cite error: Invalid <ref> tag; name "DeSouza" defined multiple times with different content Cite error: Invalid <ref> tag; name "DeSouza" defined multiple times with different content
2. ↑ ^{2.0 2.1 2.2 2.3} McCarthy, P. L. (1993). Shaft or Decline? An Economic Comparison
3. ↑ ^{3.0 3.1 3.2} McIsaac, G. (2008). Mine 244 Underground Mining Course Notes. Course Notes . Kingston, Ontario, Canada: Queen's University
4. ↑ ^{4.0 4.1 4.2 4.3} InfoMine USA, Inc. (2009). Mine Cost Service. Spokane Valley: Jennifer B. Leinart
5. ↑ Oriel Resources. (n.d.). VOSKHOD MINE DESIGN & DEVELOPMENT. Retrieved February 09, 2011, from Oriel Resources Web site: http://www.orielresources.com/voskhoddesign.asp
6. ↑ M Brazil, D. L. (n.d.). Optimisation in the design of underground mine access. Uncertainty and Risk Management in Orebody Modelling and Strategic Mine Planning . Melbourne, Australia: University of Melbourne