Underground Ramp Design Parameters
The aim of this Wiki article will be to outline the factors to be analyzed in choosing each design parameter when considering a ramp system for an underground mine. Proper consideration of each design element will ensure ramps are built optimally to perform their desired function at the lowest possible cost.
There are many differing requirements for ramps in underground mining operations with varying degrees of importance to the mines output. In general they can be categorized as follows:
Surface production ramp:
A ramp that provides access to the mine from surface and is used to transport ore using haul trucks. In general these are used in more shallow underground mines as an alternative to shafts, however recently they have been widely used in high volume, moderate depth mines in Australia (1). see Shaft vs. Ramp Access
Ore handling ramp:
A ramp that does not go to surface but is still a fundamental part of production and ore handling. Generally used to move ore from sublevels to primary levels or to the nearest ore pass where it can then be transported to a loading pocket to be hauled to surface via skips.
A ramp that is needed solely to move equipment and personnel between levels and sub-levels as work progresses, rather than for production purposes.
Note: In some cases a ramp will perform some or all of the listed functions. In particular, a surface production ramp can perform all functions and an ore handling ramp can also act as a service ramp.
When considering a mine plant design, of which ramps are a vital component, it is important to consider how equipment and ore will be able to move throughout the mine efficiently. Some large mines with multiple orebodies may need more than one surface production ramp. Others may need several ore handling ramps to accommodate a shaft, or perhaps simply a single service ramp to occasionally move equipment. Each deposit has unique requirements. The type of ramp and its intended use will dictate all other ramp design parameters. A surface production ramp will need to be built to accommodate the desired production rate. This needs to be done while considering the ideal ratio of CAPEX toOPEX and an optimized NPV (or other project goals). Generally speaking, more spent on CAPEX will result in lower OPEX; larger, more efficient haul vehicles, optimized cycle times for reduced consumables and wages (see "Ramp Grade" section). On the other hand, service ramps need only to be designed to meet minimum regulatory requirements (See "Cross-Sectional Dimensions" section) and vehicle requirements.
Placement and Ramp Type
There are three main types of ramps: spiral ramps, switchback ramps and straight ramps. Each has advantages and disadvantages. Placement will also play a large role in determining what type of ramp is best suited to a project therefore these design parameters should be made in tandem.
Spiral ramps allow vehicles to travel at moderate speeds without having to slow to round corners, as in the case of switchback ramps, and can access multiple levels reasonably well with short cross-cuts. They also provide a compact footprint when land parcel size on surface is a design constraint. They can however be harder on equipment and operators who must be subjected to continuous centripetal forces.
Switchback ramps are ideal for accessing all levels and sublevels with minimal cross-cuts and should therefore be the primary choice for ore handling and service ramps. They do require vehicles to slow at each corner, which can substantially increase travel/ cycle times.
Straight ramps are rarely capable of accessing all the necessary levels of a mine and should therefore be reserved for surface production ramps. They allow vehicles to travel faster than other ramp types. Straight ramps will reach surface a substantial distance from the orebody and infrastructure planning should be planned accordingly. Consequently, larger land ownership requirements are also required. It should however be noted that in some cases these problems can be solved with a single well placed switchback.
Ramp placement is dictated by functional/ production requirements and is constrained by geotechnical considerations. Generally speaking, a ramp used for production should be placed centrally relative to the orebody to achieve minimal average travel times from stope to ramp. That said, geotechnical constraints such as dykes or faults can sometimes make this difficult or impossible. It is also preferable to place a ramp in the footwall rather than the hanging wall where tensile stress is less likely to be a significant factor (2). The geotechnical area of influence is also a key consideration, see Figure 1. That is, ramps must be placed so that the stress accumulations caused by mining excavations do not affect the structural integrity of the ramp and vice versa (2).
Figure 1: This diagram illustrates yielded rock mass surrounding excavations and their effects on one another. It demonstrates the effect the proximity of a ramp to other excavations has on its structural integrity. It should be noted that it shows yielded elements after the full stress redistribution cycle has occurred, a process which takes some time. For this reason it is not immediately problematic within stopes as it would be within permanent excavations.
Grade is a very important design parameter to consider. It has a substantial impact on both capital expenditure and operating cost. Generally speaking more capital spent will result in lower overall operating costs and vice versa. The typical gradient range for underground ramp systems is between 10% and 15.5% (1), although in some high volume mines with larger trucks (such as is often the case in Australia), they can be in a range more similar to open pit haul roads, 6% to 10% (3). It is important to identify your goals in terms of an ideal ratio of capital and operating cost when determining an ideal grade. This will depend on both the projects overall cash flow projections and the ultimate purpose of the ramp (see "Use" section).
Capital costs for ramps are based almost entirely on length with few fixed overhead costs, as is the case with shafts (which require a surface plant, headframe and hoist, and construction infrastructure set-up). Ramp costs are composed nearly entirely of items that increase linearly: development costs (drilling, blasting and mucking), support (rock bolts, mesh and shotcrete) and services (ventilation, lighting & electrical wire, etc.). Figure 2 demonstrates how length decreases as a function of grade. This length factor could also be considered a capital cost factor given length and cost are directly correlated.
Figure 2: This graph represents the length to rise ratio as a function of grade. It also demonstrates how capital costs increase as a function of grade.
The driving force in ramp operating costs are related to equipment: consumables, operator wages, ventilation requirements and equipment maintenance. As a result, it is optimal to have the least amount of equipment possible capable of meeting production targets. This can be achieved by reducing cycle times.
As grade decreases equipment is capable of travelling faster but must overcome a greater distance. It is therefore beneficial to determine cycle times at variable grades to find the optimal grade for a minimum cycle time, see Figure 3.
Figure 3:The following is an analysis using both Rimpull and Retarding curves for 25-ton and a 40-ton underground articulated trucks. (See "Appendix" for equipment specs and data.)
Increasing the size of haul vehicles in general reduces operating costs. There are fewer operators and larger vehicles are generally more efficient in terms of $/tonne moved. That said, production rate should be the driving force for vehicle sizing. The general consensus for the optimal number of operating vehicles on a ramp is in the range of 4-8 (1). The desired number of vehicles coupled with production rate will allow you to size haul vehicles. Every vehicle has unique performance characteristics therefore the optimal cycle times with regard to grade will vary with different vehicles (see Figure 3). In this particular case a 12% grade is optimal for both 25-ton and 40-ton rear loading trucks.
Increasing vehicle size also requires a larger cross section, which will have a substantial effect on capital costs (see the "Cross-Sectional Dimensions" section).
The cross-section of a ramp is regulated under Section 82 of the Occupational Health and Safety Act. It states that there must be at least 1.5 meters of lateral clearance between the maximum width of the largest piece of mobile equipment and both sides of the ramp or any obstructions the mobile equipment may pass. Vertically, the regulation is that there has to be a minimum of 30 centimeters clearance between the highest point of the tallest piece of mobile equipment and the ramp’s roof or the bottom of any obstructions the equipment may pass under .
These obstructions can include walkways, ventilation ducts, safety stations, etc. One must remember that these are minimum regulations and consideration must also be made to situations such as the height of the muck in a loaded haul truck. When considering height, it is also necessary to consider the equipment that will be used to develop the ramp. There must be clearance enough to allow mucking equipment to load the haul vehicles during the construction of the ramp.
Cross-sections may vary from 2.2 by 2.5 meters to 5.5 to 6.0 meters. Smaller ramps are allowed for rail-bound equipment, while a loaded heavy mine truck together with a ventilation duct requires approximately 25 meters squared of cross-sectional area . A visual example of this larger cross-sectional area can be found in Figure 4.
Figure 4: Illustrates typical vehicle specs combined with regulatory requirements to generate ramp cross-sectional dimensions.
The main considerations when determining the appropriate size of a ramp system intersection include: vehicle sight lines, vehicle turning radius requirements and grade at the intersection. To better illustrate the effects of these considerations, an analysis of a ramp system intersection is described below in Table 1. For the purpose of this analysis, the equipment specifications of a variety of sized CAT haulage trucks will be used.
Table 1':Summary of equipment specifications for the ramp system intersection analysis.
|Equipment||Clearance Width (m)||Turning Radius: Outside (m)||Turning Radius: Inside (m)|
These values were used to create an example of a ramp system intersection, the minimum clearance width for each truck type was used to design the ramp width, and the maximum turning radius for each truck type was used to design the corners at the intersection. This is shown in Figure 5. Typical manufacturer specifications are similar to the ones used in Table 1 are also shown in Figure 6.
Figure 5:Example of a ramp intersection with varying parameters based on truck size shown in Table 1.
Figure 6: demonstrates typical manufacturer specs relating to turning radius. Note that these are minimum dimensions for clearance and that additional width needs to be applied as detailed in the "Cross-Sectional Dimensions" section of this article. Further clearance may also need to be applied to acheive improved sightlines for safety purposes.
as shown in Figure 5, an increasing minimum truck clearance width yielded a larger maximum turning radius for the corners of the intersection, thus making the intersection larger. It should be noted that In addition to the turning radius requirements to accommodate equipment, the appropriate clearance needs to also be added. These clearance values are regulated and more details can be found in the cross- section portion of this article.
Sight lines should also be taken into account. Maximum vehicle speeds and their corresponding stopping distances should be correlated to sightlines to ensure safe vehicle interaction. All underground vehicles have operating lights which increases vehicle visibility beyond direct sightlines but this benefit should be viewed as a safety buffer.
Another important aspect of intersection parameters includes the ramp grade. Ideally, the ramp grade should be almost or near flat, particularly where the ramp intersects a level drift or cross-cut. This will greatly impact the two main considerations explained above. While it is not required by law to have a flat grade at intersections, for high traffic areas on the ramp system, it may be beneficial for safety purposes.
It should be noted that more substantial ground support is required at intersections than in the rest of the ramp system due to the fact that the various openings that converge at an intersection have an effect on one another and result in large stress accumulations. This is particularly pertinent at corners, where the more acute the angle and the smaller its radius, the larger the stress accumulation . Sharp corners should be avoided as they will inevitably fail over time potentially causing minor blockage and production delays.