Ore and waste passes
An ore pass refers to an inclined passage used for the transfer of material in underground mine workings. Ore passes are designed to utilize the gravitation potential between levels in order to minimize haulage distances and facilitate a more convenient material handling system. The term ore pass is a misnomer since all mine material including both waste and ore are commonly transported using this method.
- 1 Types of System
- 2 Location
- 3 Orientation and Shape
- 4 Length and Dimensioning
- 5 Finger Raises
- 6 Development Methods
- 7 Support
- 8 Material Control
- 9 Ore Pass Monitoring Methods and Modelling
- 10 References
Types of System
Two operational scenarios have emerged in the use of ore passes; flow-through and full ore pass systems. Each system has advantages and disadvantages and the selection should be based on the specific needs of the operation. In both cases, the ore muck is dumped into the ore pass and eventually ends up at the draw point. Following this, the muck is typically transferred via a secondary transport method such as a rail-car, conveyor or LHD to a shaft or underground stockpile.
Full Ore Pass System
This system attempts to keep a certain level of material within the ore pass at all times. Although there is an increased risk of hang-ups associated with this system, the level of ore pass degradation and seismicity near ore pass walls is decreased.
The full ore pass system greatly reduces ore pass degradation because there are fewer high energy impacts with ore pass walls. When material is dumped into the ore pass, it is not given the chance to gain enough momentum to cause serious damage. In most cases, the material does not have a significant impact at all. Keeping the ore pass full also allows for the stability and shape of the ore pass to be maintained. 
In the case where there are long times between drawing events, full systems will experience hang-ups more often than flow-through systems. The settling of fines to the bottom of the pass and material oxidation will also increase the occurrence of hang-ups. If the particle distribution in a full system contains many large boulders, the occurrence of hang-ups will increase. Many operations which attempt to operate with a full ore system have trouble maintaining material levels due to the need to continually feed the mill. The drawbacks to the full ore pass system can be mitigated by minimizing time between draw events.
In the flow-through ore pass system, muck is dumped in the ore pass and flows all the way down to the draw point. In Quebec, 25 of 31 ore passes surveyed operated using the flow-through method, although many operations which utilize this method have guidelines against its use. 
This method is usually employed when muck has high levels of fines which would result in more hang-ups, and when mill production may exceed underground ore production in full systems. Hang-ups are less likely to occur in the flow-through system because the material is allowed to flow most of the way down the pass without experiencing flow delays. The minimizing of settling of fines also attributes to fewer hang-ups. It may be advantageous to use this system if production constraints cannot keep the ore pass full or if there are drawbacks to keeping the ore pass full. In a large ore pass, the levels should be kept lower than where new muck will be dumped or the ore levels in the pass may be insufficient, there would also be the risk of hang-ups occurring at the intersection. 
Despite the prevention of hang-ups with the flow-through system, this method often results in degradation of the ore pass. As the material travels further along the pass, it gains momentum and the accumulation of impacts has the potential to cause severe structural damage. These problems may be exacerbated with insufficient support of the ore pass walls. The damage created will cause extra costs associated with further maintenance, redesign, monitoring and production delays.
Choosing the correct ore pass location is a major part of the ore pass design process. It can have significant impacts on the maintenance and longevity of final product. Ore and waste passes should be designed to intersect levels and sub levels in order to offer access to clear hang ups with ease.  Several rules of thumb exist to aid in the design procedure. The intervals between ore passes should be no more than 150m or 500ft, while waste passed should be no more than 230m or 750ft apart.  A primary factor to consider with regard to the placement of ore passes is the rock type and stress conditions in the area. Placement of the ore pass in an area with weak host rock may cause the premature end of the functional life of the ore pass in question by facilitating hangups.  Additionally, modern mining sequencing is designed to displace stress away from the mining face and towards the far field where an ore pass may be located. This needs to be taken into account to avoid potential damage or provide the appropriate support. 
Orientation and Shape
The selection of an inclination angle has a crucial impact on the effectiveness of an ore pass. Clearly, steeper angles tend to cause higher material flow and flow velocities; however, vertical ore passes will cause unnecessary damage to the ore pass walls. Therefore, the inclination of an orepass depends on other ore pass design criteria, the material properties and fragmentation as well as the mine layout. Steeper angles are recommended if the material contains a high percentage of fines. Inclination of ore passes has been found to vary between 45 degrees and 90 degrees. In Ontario, mines use an average inclination of roughly 80 degrees. 
It is recommended to develop ore passes against the dip of any persistent joint surfaces. Ore passes that are oriented more parallel to a major joint surface will tend to undergo more rapid degradation. This is most likely due to slabbing and toppling of the rock mass.
From a survey of Quebec mines, it was found that 105 of 153 ore passes were square shaped however, ore passes may also be rectangular or circular depending on how the ore pass is developed.  Circular ore passes are often a result of raise boring development methods. No evidence suggests that different shapes result in higher material flow or hang-up prevention although circular ore passes may be implemented in regions of high stress.
Length and Dimensioning
Ore pass length is often governed by the company’s budget for capital development. To minimize capital, ore passes will be driven in short sections. However to maintain material flow and required mine production levels, more ore pass developments will be necessary. Inherently, longer ore passes have a greater probability of intersecting poor ground conditions making them more susceptible to degradation. Ore passes have been found to span anywhere from 10m to 300m in length. The average ore pass length in Ontario is 108m. 
Ore pass dimensions are typically measured based the the cross sectional dimension based on the shape of opening. In rectangular ore passes, the cross sectional dimensions refers to the smallest side length while in circular ore passes the cross sectional dimension refers to the diameter. The cross sectional dimension of an ore pass is a function of material flow, stress and practicality. A larger particle size would certainly warrant a larger ore pass however, only to the extent at which it is safe and practical to do so.
|Ratio of ore pass dimension (D) to particle dimension (d)||Relative frequency of interlocking|
|D/d>5||Very Low; almost certain flow|
|5>D/d>3||Often; flow uncertain|
Very high; slmost certain no flow
Table 1 shows the relationship between particle size, ore pass dimension and the material flow as well as relative hang-up frequency to serve as a guideline to ore pass design.
Finger raises are steeply sloping openings permitting caved ore to flow down raises through grizzlies to chutes on the main haulage level. Typically, they are used in conjunction with other raises to form a system of finger raises that branch together to the same delivery point.
Finger raises are used to collect rock from the draw-points and funnel it to the grizzlies. They are used again from the grizzly level to the chutes for loading into trucks or train cars. In this mining method the finger raises are arranged as branches of a tree, to gather ore from a large area and channel it eventually to a haulage level. 
Finger raises are also used to funnel material into larger rock passes. The finger raise typically in smaller in cross-sectional area then the rock pass it feeds. It is not unusual to have several finger raises leading to one rock pass.
Ore passes are traditionally developed using one of 2 methods. First is mechanical development and typically involves using a raise borer. The second is using drill and blast methods. These could include Alimak, conventional raising and drop raising. 
In Quebec, ore passes were developed 63% of the time by Alimak raise, 29% by conventional raise, 5% by drop raise and 3% by raise bore. The dominance of Alimak raises is attributed to the reasonable safety of the method, the fact that only one access is required as opposed to raise boring which both bottom and top access is required, and the tradition and expertise of local miners. 
In Ontario however, ore passes were developed only 39% by Alimak, 30% by raise bore and the rest by other raising methods. The rationale behind the higher usage of raise boring is to limit ground disturbance during excavation. Further review suggests that Ontario mines are going away from raise boring and are beginning to implement more Alimak raises due to the safety and ground support benefits. 
To ensure an adequate level of safety and operational availability, ore passes must be properly supported. Many support systems are available for ore passes including: resign-grouted rebar and resin-grouted short cable bolts. In addition to the primary reinforcement systems in place, many ore pass designs require additional support such as steel or shotcrete liners. The design of rock support is heavily dependent on the structural geology in the area of the ore pass and weaknesses such as faults, joints and folds should be avoided.
Primary Rock SupportAs stated previously, resin-grouted rebar is very common in ore pass reinforcement, especially amongst Quebec mines. The emergence of other types of rock supports such as resin-grouted short cable bolts, cement grouted cables, fiberglass rebar has also begun. The selection of rock support is often dependent on the method of excavation for the ore pass. The alimak raising method is typically found in conjunction with resin-grouted rebar although as time has passed, the use of resin-grouted short cable bolts has increased. Figure 1 shows a distribution of rock support used in ore pass support.
The design of rock support should be dependent on the following information from drill log data :
- Per Cent Recovery
- Rock Quality Designation (RQD)
- Angle between joints and core axis
- Type of joint surface
- Amount of joint opening, if any
- Type of joint infilling, if any
- Locations of disked core
- Locations of drill water gains or losses
L ≥ 6.0+0.004W2
Bolt spacing can further be estimated using the following rules, where L is the length of the bolt in ft. 
- Excellent rock – spot bolting only
- Good rock – (1/2 to 3/4) x L
- Fair rock – (3/8 to 1/2) x L
- Poor rock – (1/4 to 3/8) x L
- Very poor rock ≤ 1/4 x L
If there is significant jointing of the rock, weld mesh should be installed to prevent the bolts from spalling.
Concrete, shotcrete and steel liners may be used in an ore pass if the quality of rock is very poor. The decision to use a liner is typically made based on the level of wear expected to be experienced by the ore pass walls, usually for very soft/weak rock. The types of liners used in Quebec mines can are shown in Figure 2. The most used system is the abrasion resistant shotcrete.
The flow of material in an ore pass is usually controlled by chutes, feeders, chains, grizzlies and crash gates. These mechanisms control the size and amount of ore to enter the LHD or rail-car.
Grizzlies, Scalpers and Mantles
These material screening devices prevent oversize muck from entering the ore pass. The most common types of these screens are grizzlies and scalpers, although scalpers tend to experience greater amounts of oversize blocks wedging between bars, resulting in higher maintenance costs. In the use of scalpers, it is common for operating crews to push the oversize through the scalper, this practice usually leads to damaged bars and inadequate material screening. 
Grizzlies are the most commonly used screening mechanisms due to their ability to prevent oversize muck from entering the ore pass. Despite this, the sizing of the grizzly is critical. Mantles are also effective in material screening and require minimal rock breaking but the flow of large rocks in the ore pass will result in wall degradation.
Control chains are typically used at the draw point to restrict flow of material. This system utilizes very large chain links connecting to either the rock around the ore pass or a chute. The mass of the chains is used to prevent rock from flowing when the chains are down although a hydraulic cylinder is sometimes used to supplement the system. They may be used with or without a chute although they are not as effective without the use of a chute.  Chains are more effective than steel doors because they are not blocked by large rocks and they are less susceptible to damage once an ore pass has been emptied. 
Chutes are one form of material transfer to the LHD or rail system. Installed into the walls or perpendicular to the opening, they are opened and closed by an operator at the desired time. The size of the chute is generally the same as the ore pass and must allow for the maximum muck fragmentation to pass through. There are no empirical values for the height of a chute.
The chute is also used in conjunction with a control gate of some type such as:
- Undercut arc gates with control chains: good for controlling fines and blocks, will discharge if compressed air is shut off
- Underhand guillotine gates: good for fines but difficult to close in the case of large blocks, will discharge if compressed air is shut off
- Finger gates: very good for blocks but do not control fines.
- Bar and chain gates: very good for blocks but do not control fines, will likely discharge if compressed air is shut off
The undercut arc gates and control chains are typically seen as the most effective and the most used in Quebec ore pass systems.
Feeders are a type of chute less common in ore pass designs. They are designed similarly to chutes but are inclined with angles greater than the angle of repose for the muck. Used with control chains, the material is restrained by the chains and as flow is required, the chains are rotated, allowing the material to slide beneath. 
Hang-ups of broken rock hindering material flow may occur in even the most well designed ore passes. Hang-ups are mainly caused by either interlocking arches of oversize material or by the cohesion between materials. Interlocking arches tends to be more common as cohesive arches occur in zinc rich materials and other materials with cohesive properties.
Typically, mines will favor steep ore passes to limit hang-ups as well as prevent oversize material from entering the ore pass through the use of scalpers, grizzlies and mantles. However, even in a well-designed ore pass, hang-ups will likely occur.
There are two main types of hang-up release mechanisms; using explosives or using highly pressurized water streams (more common for cohesive arches). Methods using explosives make use of various delivery methods such as long poles, make-shift buggies or air propelled launchers to dislodge the blockage. 
Ore Pass Monitoring Methods and Modelling
Ore pass monitoring is important to the longevity of the pass. Several different aspects of ore passes need to be monitored. Often times ore passes are kept full or at a specified level in order to avoid impact loads, thus in these circumstances various methods exist to monitor these levels as outlined in Table 2. Additionally, monitoring systems can be used to ensure ore passes are not overfilled thus mitigating any damage to nearby equipment of affecting access to the pass.
Monitoring the wall quality and associated damage due to wear and tear is often done reactively to issues with hang-ups rather than proactively. Table 3 summarizes the monitoring systems used in Canadian mines.
Liners can be used to improve the strength of the ore passes in zones where ore passes are placed in very weak rock (see Liners and Support). These conditions often merit the use of monitoring systems be the inherent use of such supports, thus there are several monitoring systems that can be used to monitor the wear in liners as summarized in Table 4.
Ore pass monitoring can be extremely important from a safety standpoint in order to prevent failures or unwanted runs of ore that could potentially cause fatalities, injuries or damage to equipment or underground infrastructure. Monitoring programs should put emphasize on highly used ore passes that are in weaker rock mass conditions and special attention should be given to finger raises as they are often more narrow. 
Various modeling techniques exist in order to predict the flow of material within and ore pass as well as identifying the effects of dropping ore down the pass. Ore will deform when it is dropped down an ore pass due to the dynamic conditions of rock striking rock or rock striking the bottom of an ore pass. Additionally, once the ore is poured in an ore pass, the particles have different velocity profiles depending on the way they spread out and the inclination of the ore pass. The benefits to ore pass modeling manifest with both ease of use and safety. Ore pass modelling allows for the prevention and troubleshooting of hang-ups which can be extremely unsafe.
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- Optech. (2006). Underground Ore Pass Level Monitoring. Retrieved January 16, 2013, from Optech: http://www.optech.ca/pdf/Appnotes/21_undergrnd_ore_pass_level_mon.pdf
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