Hard-rock room and pillar

The Robert M. Buchan Department of Mining

Queen's University

Created: March, 2012

Note: This article focuses primarily on hard-rock room and pillar mining. Complimentary to this article, is a case study on post room and pillar mining which can be accessed here.

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Room and pillar is an underground mining method that has applications to a wide variety of hard-rock deposits worldwide. It is commonly classified as an open-stoping method, meaning that development involves mining out underground cavities while leaving the surrounding un-mined waste or ore as primary support. Room and pillar is differentiated from other open-stoping methods, in that the support rock typically extends from hangingwall to footwall in the form of pillars. Pillars are usually round or rectangular and are completely surrounded by open excavations called ‘rooms’. The mining method often warrants the use of secondary support such as rockbolts, reinforcement rods, and shotcrete but this does not preclude its classification as an open-stoping method. There are four main variations of room and pillar: classic, post, step, and steep room and pillar.

Background

Room and pillar is one of the oldest existing mining methods, dating back over 1000 years. In its early use, detailed stope planning was very uncommon and mine operators would typically follow apparent high grade areas, leaving pillars only when necessary to stabilize openings. In early 20th century America, when this lack of planning still existed, room and pillar mining was referred to as ‘gophering’ because of the random and asymmetrical pillars that resulted from development. Today's mining industry is more systematic and most current room and pillar mines go through rigorous planning prior to development. As will be discussed more thoroughly in the planning section, early development can have a profound effect on the late stages of mining and project economics; especially in multipass mining systems.

Room and pillar has a rich history in soft-rock mining and is commonly associated with coal, potash, uranium, and other industrial materials. Although, it is not uncommon to see room and pillar used in hard-rock metal mines for such commodities as lead, zinc, and copper. A few noteworthy areas in North america that have utilized hard-rock room and pillar mining are Missouri, Tennessee, Nanisivik(Baffin Island), Subdury (Ontario), and Elliott Lake(Ontario). The latter three of the aforementioned areas, all of which are located in Canada, employed post-pillar mining.

Deposits that are exploited with room and pillar are usually sedimentary-bedded deposits because of their characteristic properties of being flat with large horizontal extent are ideal for room and pillar mining. Although, room and pillar is very versatile and can be applied to non-conforming deposits using different variations or in conjunction with other mining methods.
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Geological Requirements

Ideal conditions for room and pillar mining are regular, flat orebodies with large horizontal extent and competent ore and waste rock, however these are by no means limiting criteria. Room and pillar is considered one of the most versatile mining methods. To illustrate its versatility, room and pillar has been used in deposits with rock strengths ranging from 30 to 350 MPa, depths ranging from 15 to 900m, and inclines ranging from 0 to 55 degrees. That being the said, the profitability of room and pillar is very sensitive to the aforementioned parameters and it is important to understand the size, shape, incline, and grade continuity of a deposit before determining its suitability for room and pillar mining.

Strength

There are no strict guidelines for minimum ore and waste rock strength but ultimately, for room and pillar to be viable, the orebody should be competent enough to support itself without significant ground support. Therefore, any deposit that could support an open excavation without major ground support could be feasibly mined with some form of room and pillar. However, there inevitably becomes a point in some deposits where the necessary pillar size to support the mine becomes so large that room and pillar stops being the best economic option.

Pillar size and shape are designed based off strength and stress estimates from geotechnical information and are designed to be as small as possible to maximize recovery while ensuring a safe working environment. Some pillars must be able to provide support long enough to complete stope extraction while other pillars, located in more critical areas, must be designed to provide support for the duration of the mine life. In appropriate rock conditions, pillars can be designed to fail gradually under close monitoring but this is very dependent on stress conditions. At very large depths below surface, in hard-rock conditions, some pillars are able to absorb massive amounts of energy before deformation and fail violently. (see rock bursts)

Geometry

The geometry (size, shape, thickness) of the deposit are crucial in determining the suitability of room and pillar. Room and pillar can be feasibility utilized on a number of different types of deposits, however it is usually is not recommended for use in steep deposits (>55 degrees), where material is able to flow by gravity. This is especially the case in very thin (rooms become too small for mechanization) or very thick deposits (pillars become too high to support the open stopes). There are some case when realtively steeply mined orebodies with large vertical extent can be mined (post-pillar) but orebody inclination normally does not exceed 55 degrees. Room and pillar also becomes difficult in deposits with large vertical extent. Extremely large pillars cause problems with deterioration, and the roof becomes hard to monitor and maintain.

Most hard-rock room and pillar mines are large (2,000 to 7,000 tpd) but there are some very small zinc mines in Illinois-Wisconsin that use room and pillar. One common consistency between these small and large deposits are regular dimensions. When thickness is constantly changing, it is difficult to use room and pillar. Pillars of different sizes pose rock mechanics issues and make stress conditions very difficult to simulate during planning. Although, there are some obscure cases when room and pillar has been used in orebodies with both small and large veins. In Tennessee, there are some mines which follow very narrow veins and then open up into large collapsed dome structures, where stoping must expand both horizontally and vertically to fully exploit the areas.

Continuity

Continuity is also an important geological aspect for room and pillar evaluation. Pillars are usually designed in regular patterns with consistent dimensions to one, make development manageable, two, make simulation of stress conditions easy, and three, keep haulage roads straight to make mechanization efficient. If the ore grade is continuous, regular pillar layouts are easy to justify but complications arise with inconsistencies in ore grade. When it is difficult to predict the grade throughout the deposit, it is difficult to produce an economically optimal pillar layout.
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Geotechnical

Tributary loading diagram. .

To design pillars to a size where they are able to stabilize an opening without excessive ground support, a detailed geotechnical study should be carried out, documenting roof and ore strength, geological structures, joints and fractures (rock mass classification), and in-situ stress measurements. With these parameters, rock-mechanics engineers can anticipate pillar behaviour in different configurations and design them accordingly. The following table shows common rules of thumb for safety factor in pillar design.

Pillar design factors

Safety Factor	Pillar type
2	Pillar located in development headings.
1.1-1.3	Panel pillars after retreat mining.
1.0	Pillars which are planned to fail.

The stress on a particular pillar can be calculated using the tributary load equations. Tributary loading is the phenomenon where stress accumulates towards structural members (pillars). The image on the right shows a diagram, explaining tributary loading area.

With a regular pillar layout, pillar stress can be calculated in following way.

Pillar Orientation

Vibernum Trend pillar classification system. .

From the geotechnical study, it is also important to determine the orientation of in-situ stresses. Pillars should be orientated in the direction of the maximum stress. This is especially of concern in deep hard-rock mines with tremendous horizontal stresses.

Pillar Rating System

To evaluate the condition of existing pillars, mines often develop a pillar rating system in hard rock mines. As an example, the mines of Vibernum Trend use a rating system from 1 to 6 to monitor pillar condition. A rating of 1, indicates no signs of stress and a rating of 6, indicates complete failure. The figure on the right shows images representing the different pillar conditions. The maintenance policy at Vibernum Trend requires pillars reaching 3 on the rating scale to be reinforced to prevent further deterioration.

An important part of pillar maintenance involves monitoring convergence. Convergence is good predictor of pillar failure and can help engineers respond before convergence accelerates uncontrollably.
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Planning Considerations

The feasibility of room and pillar is subject to a detailed planning process that must aim to maximize net present value (NPV) while maintaining a safe working environment. The process does not have a unique methodology, varying quite significantly based on orebody characteristics. The fundamental key to creating a room and pillar mine plan is understanding the relationship between all the different economic factors. The most important factors being safety, recovery, ground support, efficiency, and legal requirements.

Pillar Size

One of the steps of the planning process is understanding the trade-offs between recovery and ground support and ultimately finding a pillar size that is economically optimal. As pillar size decreases, reserves increase but so does the need for ground support. The point at which the added value of decreasing the size of pillars equals the cost increase for the necessary additional ground support, is the point where optimum pillar size has been reached. In reality, orebodies are not homogeneous throughout, therefore this analysis must be done on each section of the mine and in some cases, each individual pillar.

In the pillar size optimization there are a few other dependent factors that must be considered. Two of these aspects are ventilation and equipment. As pillar size decreases, room size increases which changes ventilation requirements and equipment options. Larger rooms oblige bigger fans to achieve minimum air flow velocity but allow for much larger equipment which can improve efficiency and operating costs. However, these factors are not just dependent on pillar size but also on pillar layout.

Pillar Layout

Regular pillar layouts improve mechanization efficiency and decrease ventilation requirements, however depending on the distribution of economic and deleterious materials, irregular layouts can sometimes be justified. Regular pillar layouts create more efficient roads for mechanized equipment to travel on. Optimal mechanization planning works to minimize haulage grades, and keep haul roads straight as possible and in excellent conditions, while avoiding the need for abrupt turns. Regular pillar layouts also create less resistance to air flow which decreases flow requirements. There is also the added complication of pillar recovery in this analysis(click here to skip to pillar recovery section).

Overview

In the resulting plan, all federal and local legal requirements must be met. This may include a minimum pillar safety factor, minimum flow/velocity air requirements, and maximum pillar height. Government authorities should be contacted prior to development.

Room and pillar planning is usually fairly straightforward in deposits where grade and thickness is consistent throughout, as is typical of coal and potash deposits. Planning, however, becomes considerably more complicated in hard-rock metal mines where there can be considerable deviation in the shape, grade, and strength of the orebody. These plans must then be flexible to change but this is often difficult in open-stoping mines where pillars are dependent on one another for support.
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Multipass Mining

In room and pillar mining, engineers have a choice of whether to take the whole orebody in one slice or in multiple slices. The need for multiple slices can arise when the orebody is very thick (see post pillar) and the pillars cannot support the full height of the deposit. Multipass mining is also used in mines where there is uncertainty in stress conditions and the engineer decides to take a more cautious first-pass and determine from there what the best course of action would be. In hard-rock mines, it is often to difficult to verify the exact thickness of a deposit because of poor continuity. When this is the case, it is difficult to decide between single and multi-pass room and pillar and it is typically recommended to begin on the projected top most slice to make it easy for the back to be reached.

Variations and Development

Development for room and pillar mining methods is generally quite small. The bulk of the development takes place within the ore body, therefore a mine is producing ore as development is carried out. Depending on the geometry of the ore body, haulage drifts and access ramps may need to be developed in the footwall and truck loading bays may need to be developed into waste rock if the thickness of the ore does not allow for the necessary height to load trucks.

There are four main variations of room and pillar mining which are differentiated by their applicability to different shaped orebodies. Classic room and pillar is applied to flat-bedded deposits, post-pillar is used for inclined orebodies with large vertical extent, and step and steep room and pillar are used in inclined orebodies with limited thickness. A useful video showing classic room and pillar development can be seen here.

Classic

Development in classic room and pillar.

This is the most generic room and pillar mining, applied to flat regular orebodies, usually with substantial thickness. Development consists of making large open stopes with trackless equipment operating on the floor. This method usually has high utilization and efficiency and planning is usually relatively simple.

Development

When starting ore production, it is normal practice to drive a drift in the ore that will allow the mine to open about four or five rooms off of the initial drift. Since this initial drift will serve as the main haulage way, it is important to be cognizant of road grade and make an effort to keep them as flat and straight as possible. The next step in production is to mark off the pillars that will be left in place, as identified by the mine planner. At this point, the rooms can be driven off laterally from the main drift. If the mineralized region extends beyond the length of the main haulage drift, care should be taken to ensure that the drift continues straight and pillars are not intersecting at any point. In many cases, “doglegs” will occur where the haulage drift is forced to weave around these pillars which increases transportation times and results in more expensive haulage. The image on the right shows typical classic room and pillar development.

Post

For a comprehensive post pillar case study click here

Development in post pillar mining.

Post pillar is a lesser known room and pillar variation that is applied to orebodies dipping from 20-55 degrees with large vertical extent. The ore is mined in a series of horizontal slices. Mining development progresses upward from the bottom horizon and the rooms are backfilled with backfill used on the next pass as the working floor (essentially a modified cut and fill). The pillars are left in the same location at each subsequent horizontal level as production moves upward. It is possible for production to move downwards but this is very uncommon as the backfill would need to be very strong, with the backfill becoming the back rather than the floor of the next subsequent horizontal stope.

Development

For more information about mining with backfill click here

The development into the ore body for the post-pillar mining method is similar to classic room and pillar on the first pass. A main haulage drift is driven into the ore body which will provide enough working faces for mining operations to be carried out efficiently. The main difference is that a hydraulic backfill delivery system would have to be installed and bulk heads would have to be built after each slice of ore is taken. In this version of room and pillar, it is important to have multiple headings available to mine so that the backfill has time to consolidate without interrupting ore production. Mining is usually conducted in an overhand style where the fill acts as the working floor but in some cases, an underhand method is used. Mining can continue with classic room and pillar with horizontal blasting on every level, or rooms can be blasted vertically starting on the second pass. In vertical blasting, the backfill is poured to fill the stope high enough for the drills to operate on the next pass. The disadvantage of this method is that mines may have to purchase two types of drills since horizontal drilling is required in the first pass and every subsequent pass would then use vertical drilling. The image on the right shows typical post pillar development.

Step

Development in step room and pillar.

Step room and pillar is found in orebodies dipping from 15 to 30 degrees. Orebody thickness is typically quite small, ranging from 2 to 5m. It is essentially an adaptation of ‘classic’ room and pillar with the orebody being developed in a series of horizontal ‘steps’. Haulage ramps are specially design diagonally against the dip of the orebody at shallow enough slopes to utilize trackless equipment. Mining advances downward along the step room angle with each step having a relatively flat production floor.

Development

The main development feature in step room and pillar is the development of an inclined haulage way within the ore body. The stopes are driven off this haulage way in an opposite direction so that the slope of the stope and the haulage way is appropriate for trackless equipment. Once an initial stope has been developed, the subsequent stopes can be slashed out sideways down the dip, following the slope of the floor. The process of taking adjacent stopes is repeated until a back reaches its maximum span. At this point, pillars are left in place for support. The image on the right shows typical step room and pillar development.

Steep

Development in steep room and pillar.

Steep room and pillar, similar to step room and pillar, is found in orebodies dipping from 15 to 30 degrees with limited thickness. It differs in that the working floor matches the orebody inclination. Because of this, trackless equipment is not able to be used and mining is considerably less efficient. This mining method is very seldom used in practice, especially in modern mine systems where significant improvements have been made to mechanized equipment.

Development

This version of room and pillar is less mechanized and requires the use of jacklegs for drilling and slushers for mucking since the slope is too steep for mobile equipment. The first step in development is to drive a horizontal transportation drift across the strike of the orebody. There are cut-outs in this drift that allow the slushers to accumulate the ore in specific locations where it can then be loaded and taken to surface. The pillar shape is usually rounded so that the slusher can efficiently muck the ore out of the stope without being caught up on a square corner or leave excess ore in front or behind a pillar. The image on the right shows typical steep room and pillar development.
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Use with other methods

Room and pillar is a versatile mining method and can be applied to a diversity of deposits. Often times, variations of room and pillar can be used alongside or in conjunction with other mining methods. In non-homogeneous deposits, room and pillar can be used in flat, thick portions of the orebody meanwhile other methods are being used to access steep, narrow portions. Also, in the case of post pillar mining, room and pillar can be used with cut and fill in an integrated system. Below are a few examples of room and pillar being used with other methods.

Room and pillar with sublevel longhole stoping

For main article on sublevel longhole stoping click here.

In very thick deposits, sometimes it is best to mine the room and pillar with two levels. Otherwise, development becomes difficult with large room heights. To do this, mines can opt to use sublevel longhole stoping to drill vertically between levels, leaving behind pillars as the primary structural support. This system was used at the Denison Mines at Elliot Lake.

Room and pillar with modified shrinkage stoping

For main article on shrinkage stoping click here.

There are some mines that use modified shrinkage stoping after first-pass room and pillar mining. In low recovery room and pillar mines of sufficient thickness (>15m), "scram drifts and finger raises are developed and the ore is blasted with flat longholes, drilled from a raise with the ore going to what is termed a "modified shrinkage stope"".This modified system was used at the Ambrosia Lake uranium district.

Room and pillar with cut and fill

For main article on cut and fill click here.

Possibly the most used method used in conjuntion with room and pillar is cut and fill. This integrated system is used when backfill is necessitated for pillar structural support. The most common example of this is the multi-pass systems of post pillar mining where pillars would be too high to support the openings without backfill support. Room and pillar can also be used in very weak deposits where rooms are completely backfilled and pillars are extracted from sublevels beneath.
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Drilling and Blasting

Plan view of a stope showing advancement by swings and slabbing.

Ore production in room and pillar mining uses the same drilling and blasting techniques used in drifting. In this process of full face slicing, the drift dimensions are equal to the width and height of the stope. In cases where the ore thickness dictates that the stope must be mined in multiple benches, vertical drilling can take after the first slice has been taken.

The initial advance in room and pillar mining uses a cut pattern when there is only one free face open. The burn cut is the most common drilling pattern in metal mines. This refers to a group of holes that are parallel and centrally located at the face and are detonated on the first few delays. This cut provides relief to the remaining holes allowing them to break. This type of blasting is known as a swing when there is only one free face available. The next type of blasting used in room and pillar is known as slabbing. This type of round is used once a free face has been established so that there is a group of drill holes parallel to an open face. This free face allows the fragmentation of the rock to be the same as a swing with less explosives which leads to lower costs. It is important that the drilling and blasting engineers carefully monitor and plan out the cuts so that the number of slab rounds can be maximized. About half of the rock can be broken using slabbing. The image on the right shows a plan view, showing advancement by swings and slabbing.

Material Handling

In most room and pillar mining operations, ore is usually handled with rubber tire equipment. Due to the nature of this mining method , material handling equipment must be mobile and cannot be fixed in place since stopes are continually developing in different directions. Scoops are used to muck the ore and move it short distances but trucks are typically used the majority of transportation. Depending on the depth and size of the deposit, the mine may use shaft conveyance or ramp haulage. Ore movement from the stope to the shaft or ramp is mostly horizontal, so the practice of using ore passes has very limited applications.

Equipment

Drilling

Atlas Copco 2-boom jumbo.

Drilling in room and pillar is generally carried out with two or three boom jumbos equipped with hydraulic drills. Drill sizes vary by operation but it is usually preferential to use the largest jumbo that is feasible since drilling plays such a key role in the mining cycle. The image on the right shows a picture of a 2-boom jumbo. Drilling is continuously taking place at different faces so it is important that the drills are mobile so that they can move to different areas quickly and efficiently without blocking main haulage drifts. In some cases, crawler mounted long hole drills may be used for vertical drilling, although the vertical drills may provide higher production. Many mines find it more practical to just carry out horizontal drilling so that additional equipment does not need to be purchased.

Mucking and Material handling

Room and pillar mining is a highly mechanized mining method and makes use of trackless equipment. This results in a very mobile and flexible equipment fleet. Loading of the ore and waste is usually carried out with a load haul dump machine (LHD) which can then tram the ore economically for about 500 feet . LHDs range from 1 tonne to 25 tonne capacities . In cases where the ore needs to be moved over a longer distance, LHDs will load low profile underground trucks. These trucks have capacities of up to 60 tonnes and can economically transport ore over long distances. The use of slushers and dozers are another material handling alternative when rubber tire vehicles are not practical.
In addition to the main production equipment, bolting and scaling rigs are necessary for room and pillar mining. With this mining method comes a very large back that needs to be maintained since the entire stope is open to personnel. These pieces of equipment are an essential part to the mining cycle and to the safety of the workers.
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Pillar Recovery

As described previously in the planning (hyperlink) section, the mining engineer must acknowledge pillar extraction in the initial planning phase. Failure to do so can significantly affect the economic success of a mine, as pillar recovery can considerably increase reserves. Oftentimes, mines take too much ore during first-pass mining to a point where the mine cannot support itself, requiring extensive secondary support just to keep the mine in production. When pillars are left too small, convergence can accelerate uncontrollably requiring massive reinforcement or backfill. This reduces the opportunity of pillar extraction and noticeably increases costs. It is usually advisable to take a less aggressive first-pass, and then decide on the second pass how aggressive to be, depending on the stress conditions. This way, support costs are reduced while achieving the same, if not better recovery. There are four main pillar extraction methods depending on the variation of room and pillar being used and the orebody characteristics.

• High grade material can be slabbed off from pillars during a retreat of the mine.

• Pillars can be completely removed in a retreat.

• Select valuable pillars can be completely removed

• Massive backfill can be used to support the mine, while pillars are typically mine from a sublevel beneath.

Another pseudo pillar recovery method, which is rarely used in practice, is utilized in step room and pillar mining where the hangingwall is caved in moving along strike after first-pass mining .
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Advantages and Disadvantages

Maximize advantages , while eliminating disadvantages

Advantages

• Flexible – Can utilize multiple faces, and therefore be selective of production and grade. Especially advantageous with base metals that have cyclical price cycles.
• Highly Mechanized – Not very strenuous on the workforce. Allows for high efficiency and productivity.
• Easy Maintenance – Usually utilizes mobile, trackless equipment which is easy to transport in and out of maintenance areas. Equipment can also be transferred easily between levels.
• Low Operating Costs – Largely due the mechanization and productivity, operating costs are usually considerably lower than most underground mining methods.

Disadvantages

• Roof Maintenance – A large portion of the roof is exposed making monitoring and maintenance very time-consuming and costly. The roof can often require high lift equipment for appropriate inspection.
• High Capital Costs – Initial infrastructure and equipment fleet can be expensive, although typically cheaper in the long run with lower operating costs.
• Low Recovery – Room and Pillar can have one of the lowest recovery rates of any underground mining method. There must be significant reserves left behind to support the mine.
• Lack of flexibility in structural planning – It is difficult to make structural changes part way through production because most of the previously mined out rooms must be supported for the duration of the production life. The stress on a pillar is dependent on the location of other pillars and stress distributions can change drastically with changes in pillar location.

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Ventilation

For main article on ventilation click here.

Hard rock room and pillar mines are generally quite large. Rooms can be stoped out to 12 by 15 m in some cases, which requires significant air flow rate to meet minimum velocity requirement. The large air flow rate has effects on other structures such as stopping and ventilation doors, which must be built to withstand the enormous air pressure. Additionally, Room and pillar is highly mechanized which adds to the ventilation requirement. In Ontario, Canada, at minimum 0.06m^3/s of flowrate is required for each KW of power of the diesel equipment underground. Sometimes, when surface fans cannot handle the high flowrates, ventilation fans are often installed underground to meet ventilation requirements.
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Economics

Average operating costs from 6 large tonnage hard-rock room and pillar operations. .

Room and pillar historically has lower operating and capital cost in comparison to equivalent systems. The table represents average operating costs for hard-rock room and pillar based on 6 highly productive mines, with production rates ranging from 2000 to 7000 tonnes per day. Note that these mines are highly mechanized and have higher tonnes /shift than industry average. The mine prices are converting using inflation rates into 2008 US dollars.
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References

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External Links

1. http://en.wikipedia.org/wiki/Rock_burst[[1]]
2. http://www.youtube.com/watch?v=t2WoMaXizHs.flv[[2]]
3. http://en.wikipedia.org/wiki/Ambrosia_Lake
4. http://en.wikipedia.org/wiki/Rock_mass_classification
5. http://www.ritchiewiki.com/wiki/index.php/LHDs
6. http://bcgold.com/b/bulkhead

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Created by Group 5 2012: Chris Cameron, Lucas Dale, Kain Petterson, Scott Simpson, Courtney Squires