Underground Stoping Methods - Cagatay Jay Guley

The selection of an appropriate stoping method is arguably the most critical strategic decision in the development and operation of an underground mine. This choice profoundly influences the project’s entire lifecycle, dictating capital investment, operational costs, production rates, ore recovery, dilution, and, most importantly, the safety of the workforce and the long-term environmental legacy of the operation. The decision is a complex, multi-faceted engineering challenge that must balance the unique geological and geotechnical characteristics of an orebody with demanding economic, safety, and environmental constraints.

The fundamental factors that govern the selection process are rooted in the physical environment of the deposit and the economic value of the commodity. These primary criteria include:

Geological and Geotechnical Properties: The geometry of the orebody—its shape, thickness, dip, and continuity—is the primary determinant of a method’s physical applicability. Equally important is the geomechanical character of the rock mass, specifically the strength and competency of both the ore and the surrounding host rock (often referred to as country rock). These properties determine whether an excavation will be self-supporting, require artificial support, or can be induced to cave in a controlled manner.
Economic Factors: The grade of the ore, required production rates, and the relative balance between capital expenditure (CAPEX) and operating expenditure (OPEX) are key economic drivers. High-grade, narrow-vein deposits can sustain expensive, selective methods, whereas large, low-grade deposits demand low-cost, high-tonnage bulk mining methods to be viable.
Safety and Environmental Considerations: The inherent risks to personnel, the potential for surface subsidence, and the management of waste rock and tailings are non-negotiable aspects of modern mine design that significantly influence method selection.

To provide a structured and logical analysis, this report classifies the principal stoping methods into three fundamental categories. This classification is based directly on the geomechanical response of the rock mass to excavation and the degree of artificial support required to maintain stability.

Unsupported Methods: These methods are employed in competent rock masses that are largely self-supporting. They rely on the inherent strength of the rock, often using systematically placed pillars of unmined ore as the primary means of ground support.
Supported Methods: These methods are necessary when the ore or host rock is not strong enough to remain stable on its own. They involve the systematic introduction of artificial support—most commonly engineered backfill—which becomes an integral part of the mining cycle.
Caving Methods: In contrast to the first two classes, caving methods are designed for weak or highly fractured rock masses. They operate by intentionally inducing and managing the controlled, large-scale gravitational collapse of the orebody and/or the overlying host rock.

Chapter 1: Unsupported Stoping Methods

Unsupported methods are applicable where the rock mass possesses sufficient inherent strength to remain stable with minimal or no artificial support. The primary means of ground control is the strategic leaving of ore pillars, creating a fundamental trade-off between maximising ore recovery and ensuring mine stability. These methods are characterised by their reliance on the natural structural integrity of the rock.

1.1 Room and Pillar Mining

Principles of Operation

Room and Pillar mining is one of the oldest and most widely utilised underground mining methods. The system involves excavating ore in a network of “rooms,” while leaving behind “pillars” of intact ore to support the roof, or overburden. The layout typically resembles a grid or checkerboard pattern when viewed in plan. The dimensions of the rooms and the size and spacing of the pillars are critical design parameters, engineered to ensure the stability of the mine workings. Depending on these dimensions, the amount of ore extracted from a given panel can range from 40% to as high as 70% during the initial, or “advance,” phase of mining.

Geological and Geotechnical Applicability

This method is ideally suited to tabular, or blanket-like, ore deposits that are horizontal or have a gentle dip. It is the predominant method for extracting bedded deposits such as coal, salt, potash, limestone, gypsum, and certain types of uranium and iron ores. A critical prerequisite for the successful application of Room and Pillar mining is the competency of the rock mass. Both the ore that forms the pillars and the immediate roof and floor strata must be reasonably strong and stable to prevent pillar failure and roof collapse.

Development and Production Cycle

The mining cycle begins with the development of a series of parallel entries that serve as main haulage and ventilation routes. From these entries, production panels are developed by driving intersecting rooms and crosscuts at right angles, which systematically forms the pillar grid. The process is highly mechanised and allows for multiple active working faces, contributing to high productivity. In some operations, a secondary phase of mining known as “retreat mining” is employed. During this phase, the pillars themselves are systematically extracted, starting from the back of a panel and working towards the main entries. This practice, also called pillar robbing, can significantly increase the overall ore recovery but intentionally causes the roof to collapse into the mined-out areas, increasing the risk of surface subsidence and large-scale instability if not managed correctly.

Key Equipment

Room and Pillar mining is highly amenable to mechanisation. The typical equipment fleet includes continuous miners (especially in coal and softer minerals), which cut and load the material in a single operation, and conventional drill-and-blast equipment, such as jumbo drills, for harder rock. Loading and hauling are performed by Load-Haul-Dump (LHD) vehicles, shuttle cars, and haul trucks. Roof bolters are also essential for providing localised support to the immediate roof strata by “pinning” weaker layers together to form a more competent beam.

Advantages

Simplicity and Flexibility: The method is operationally straightforward and can be adapted to variations in the orebody Market fluctuations can be accommodated by adjusting the number of active production faces.
High Productivity and Mechanisation: The regular layout and large openings are well-suited for large, efficient, and highly mechanised equipment, leading to high productivity at moderate mining costs.
Good Ventilation: The network of interconnected rooms provides multiple pathways for ventilation, which is crucial for safety.
Reduced Surface Subsidence: When pillars are left in place permanently, the risk of surface subsidence is significantly lower compared to caving methods.

Disadvantages and Practical Challenges

Limited Ore Recovery: The primary drawback is that a substantial portion of the ore reserve, typically between 30% and 60%, is left behind as pillars to ensure stability. This represents a significant sterilisation of the resource unless retreat mining is feasible.
Risk of Cascading Pillar Failure (CPF): The most severe hazard associated with this method is the potential for a catastrophic, progressive collapse. The failure of a single, overloaded pillar can transfer its load to adjacent pillars, which may then also fail, initiating a chain reaction that can propagate rapidly across a large area of the mine. Such events, also known as pillar runs or squeezes, can occur with little warning and often induce a powerful and lethal airblast from the sudden displacement of air. The risk of CPF is particularly acute in mines with high extraction ratios (over 60%) and slender pillars (width-to-height ratios less than 3).
Design Complexity: The apparent operational simplicity of the method masks a deep geomechanical complexity. The design of stable pillars and rooms is not an empirical exercise; it requires rigorous engineering analysis of the stresses and rock mass properties. Irregular or inconsistent pillar and room sizes can lead to uneven stress distribution, compromising ground stability and complicating ventilation. This creates a “simplicity trap,” where the ease of initial implementation can lead to designs that are not robust against long-term pillar degradation, creating a significant latent hazard that may only manifest years or decades later.

Ore Recovery and Dilution

Ore recovery during the advance phase typically ranges from 40% to 70%. If pillar extraction is successfully conducted on retreat, overall recovery can exceed 85%. Dilution is generally very low, often around 5%, as the ore boundaries are typically well-defined and the host rock is competent.

1.2 Shrinkage Stoping

Principles of Operation

Shrinkage stoping is a vertical, overhand mining method where the ore is mined in horizontal, upward-advancing slices. A defining characteristic is that the majority of the broken ore (muck) is intentionally left in the excavated area, known as the stope. This accumulated muck serves two primary purposes: it provides temporary support to the stope walls (the hanging wall and footwall), and it acts as a working platform for miners to access the next slice of ore at the back (roof) of the stope. When rock is blasted, it increases in volume due to the creation of voids, a phenomenon known as “swell.” This swell, typically around 30-40%, means that a portion of the broken ore must be drawn off from chutes at the bottom of the stope after each blast. This drawing-off process, or “shrinking” of the muck pile, creates sufficient headroom for the miners and equipment to work on the next cycle. The bulk of the ore, however, remains in the stope until the entire block has been mined from bottom to top, at which point the remaining ore is drawn down and emptied.

Geological and Geotechnical Applicability

This method is best suited for steeply dipping ore bodies (typically with a dip greater than 50°) that have relatively narrow widths (from 1 metre up to 30 metres) and regular, well-defined boundaries. The geomechanical conditions are demanding: both the ore and the surrounding host rock must be strong and competent. Strong walls are necessary to prevent excessive sloughing and dilution, while strong ore is needed to prevent packing or cementing of the broken muck pile, which could impede its flow during the final drawdown. Furthermore, the ore should not be susceptible to oxidation or spontaneous combustion, as it may remain in the stope for many months or even years, and such chemical reactions can create processing problems or severe safety hazards like fires and toxic gases.

Advantages

Simplicity and Low Capital Cost: The method is conceptually simple and does not require a large initial investment in specialised equipment.
Ground Support: The broken ore provides effective, albeit temporary, support to the stope walls, reducing the need for extensive artificial support like rock bolts or timber and helping to minimise dilution from wall sloughing.

Disadvantages and Practical Challenges

Poor Safety: The working conditions are inherently hazardous. Miners work on an unstable surface of broken rock, directly beneath a freshly blasted roof. This level of exposure is considered unacceptable by modern mine safety standards.
Low Productivity and Mechanisation: Shrinkage stoping is highly labour-intensive and offers very limited potential for mechanisation. The uneven and shifting working surface makes it difficult to use large, efficient equipment. Consequently, productivity is low, typically in the range of 3 to 10 tonnes per employee-shift.
Tied-Up Capital: A major economic disadvantage is that approximately 60% of the mined ore remains inaccessible in the stope until mining is complete. This ties up a significant amount of capital and delays revenue generation, negatively impacting the project’s cash flow.
Ore Flow and Quality Issues: There is a significant risk of the broken ore packing tightly or cementing itself due to oxidation, creating “hang-ups” that prevent it from flowing to the drawpoints. Resolving these blockages is a dangerous and costly process.

Ore Recovery and Dilution

Under ideal geological conditions, shrinkage stoping can achieve high ore recovery, typically between 85% and 95%. However, dilution from wall sloughing during the final drawdown can be significant, ranging from a moderate 10% to as high as 25% or more.

While now largely obsolete in its original form, shrinkage stoping serves as a crucial case study in the evolution of mining methods. Its fundamental principles have been adapted and refined, but its inherent safety flaws and productivity limitations represent a technological and safety dead-end. The core concept of upward slicing was retained, but its implementation had to be radically altered through technological advancements, such as long-hole drilling, to eliminate the key safety and productivity bottlenecks. This evolutionary pressure directly led to the development of safer and more productive bulk stoping methods like Vertical Crater Retreat (VCR), which removes miners from the stope entirely by conducting all drilling and blasting from separate, stable development drifts.

1.3 Sublevel Open Stoping (SLOS)

Principles of Operation

Sublevel Open Stoping (SLOS) is a high-production, bulk mining method used for large, steeply dipping orebodies. The fundamental principle is the creation of large, underground voids or “stopes” that are left open (unsupported) during extraction. The orebody is divided vertically by a series of horizontal tunnels known as sublevels. From these sublevels, long blast holes are drilled into the ore, typically in a radial fan or parallel ring pattern. The ore is blasted in large sections or slices, allowing the broken rock to fall by gravity to the bottom of the stope. From there, it is collected at designated drawpoints and mucked out using mobile equipment. Unlike shrinkage stoping, the stope is kept empty throughout the production phase.

Geological and Geotechnical Applicability

The primary requirement for SLOS is a high degree of rock mass competency. Both the orebody and the surrounding host rock must be strong to ensure that the large open stopes remain stable without collapsing. The method is best suited for orebodies that are steeply dipping, have a large vertical extent, and possess regular and well-defined boundaries.

Variations

The evolution of SLOS is a clear demonstration of how drilling technology can fundamentally reshape a mining method.

Big Hole Stoping: This modern variant was made possible by the development of highly accurate and powerful Down-The-Hole (DTH) drills. These drills can create long, precise holes (often over 100 metres) with minimal deviation. This accuracy allows for a significant increase in the vertical spacing between sublevels compared to traditional ring drilling. By reducing the number of sublevels required to mine a given block of ore, this method drastically cuts down on expensive and time-consuming development work.
Vertical Crater Retreat (VCR) Mining: VCR is a specialised form of open stoping that employs a different blasting philosophy. Instead of blasting into a free face from the side, VCR blasts upwards into a horizontal free face (the stope floor). Large-diameter holes are drilled downwards from an “overcut” drift at the top of the stope to an “undercut” drift at the bottom. The holes are then charged with explosives in short, concentrated sections at the bottom. This “crater charge” is designed to blast a horizontal slice of ore downwards into the undercut. The process is repeated, with the charge being “retreated” vertically up the blast hole for each successive slice. This method is considered safer and more productive than conventional SLOS because all drilling and charging activities are conducted from stable development drifts, completely isolated from the open stope.

Key Equipment

The equipment fleet for SLOS is highly mechanised and includes production drill rigs (such as ring drills and DTH drills), jumbo drills for development, LHDs for mucking ore from the drawpoints, and mine trucks for haulage.

Advantages

High Productivity and Mechanisation: SLOS is a highly productive, bulk mining method that is fully mechanised, allowing for high tonnages at a moderate cost.
Safety: Personnel are not required to enter the open stope during production, which significantly improves safety compared to methods like shrinkage stoping.
Efficiency: The VCR variation is particularly efficient, offering high production rates at lower costs due to its simplified blasting and development cycle.

Disadvantages and Practical Challenges

Potential for High Dilution: The stability of the large, exposed stope walls is critical. Any failure or sloughing of the hanging wall or footwall rock can lead to significant unplanned dilution, which lowers the grade of the ore sent to the mill.
Drilling Accuracy: The success of the method is heavily dependent on the accuracy of the long production blast holes. Inaccurate drilling can lead to poor rock fragmentation (creating oversized boulders that block drawpoints), damage to the final stope walls (increasing dilution), and inefficient use of explosives.
Pillar Recovery: Like Room and Pillar mining, SLOS often requires that substantial pillars of ore (crown pillars at the top, sill pillars at the bottom, and rib pillars between adjacent stopes) be left in place to ensure regional mine stability. This ore is of considerable value, but its recovery is a major engineering challenge, as attempting to mine the pillars can trigger a widespread collapse.
Backfilling Requirements: The massive voids created by open stoping can pose a long-term stability risk to the mine. To mitigate this risk or to enable the recovery of adjacent pillars, stopes often need to be backfilled after they are mined out. This backfilling process is a major operation in itself, adding significant cost and complexity to the mining cycle.

Ore Recovery and Dilution

Typical ore recovery for SLOS is in the range of 85%, with dilution around 15%. For the VCR method, recovery can be higher, around 90%, with dilution at approximately 10%.

Chapter 2: Supported Stoping Methods

Supported stoping methods are employed when the rock mass is not competent enough to be self-supporting. These methods are defined by the requirement for substantial, engineered artificial support to maintain the stability of the excavation. The support, typically in the form of backfill, is not merely an auxiliary measure but an integral and cyclical component of the production process itself.

2.1 Cut-and-Fill Stoping

Principles of Operation

Cut-and-Fill is a highly selective mining method where ore is extracted in horizontal slices, or “cuts”. After each slice is mined, the resulting void is completely backfilled with material such as waste rock, hydraulic sandfill (tailings), or, most commonly, a cemented paste fill. This backfill serves two critical functions: it provides comprehensive support to the stope walls, preventing collapse, and it creates a stable working platform from which to mine the next slice of ore. The process is cyclical: cut, muck, fill, repeat.

Variations

The direction of mining progression defines the primary variations of the method:

Overhand (Bottom-up) Cut-and-Fill: This is the most common approach. Mining begins at the bottom of a stope and progresses upwards in successive slices. The freshly placed backfill serves as the floor for the equipment and miners working on the next cut above.
Underhand (Top-down) Cut-and-Fill: In this less common variation, mining proceeds downwards from the top of the stope. Miners work underneath a previously placed, high-strength cemented backfill layer, which acts as an artificial roof or “sill mat”. This technique is particularly suited for mining in high-stress ground conditions or for extracting pillars between previously backfilled stopes, as it provides excellent overhead protection.

Geological and Geotechnical Applicability

Cut-and-Fill is the method of choice for steeply dipping orebodies that have irregular shapes and boundaries. Its key advantage is its applicability in poor ground conditions, where the ore and/or the host rock are weak and would not be stable with unsupported methods. Because of its high cost, it is typically reserved for high-grade, valuable mineral deposits, such as gold or silver veins, where its high degree of selectivity allows for the precise extraction of ore while leaving low-grade material and waste behind.

Key Equipment

The equipment fleet for Cut-and-Fill stoping includes jumbo drills for drilling the blast pattern, LHDs for mucking the broken ore and sometimes for spreading the backfill, and a specialised backfill plant and distribution system. This system can involve pumps, pipelines, and boreholes or raises to transport the fill material from the surface to the underground stopes.

Advantages

High Selectivity and Low Dilution: The method is extremely selective, allowing miners to follow irregular ore boundaries precisely. This results in very low dilution and high ore recovery, maximising the extraction of valuable minerals.
Excellent Ground Support and Flexibility: It provides excellent ground control, making it safe for use in weak rock masses. The method is highly adaptable and can be adjusted to mine complex or scattered mineralisation.
Pillar Recovery: It is an effective method for safely recovering high-grade pillars left behind by other mining methods, as the surrounding voids can be filled to provide support before the pillar is extracted.
Waste Disposal: It provides a productive use for mill tailings, which are used to create the backfill, thereby reducing the surface footprint required for tailings storage.

Disadvantages and Practical Challenges

High Operating Cost: Cut-and-Fill is one of the most expensive underground mining methods. The complex, multi-step production cycle is inherently less productive than bulk methods, and the cost of the backfill—particularly the cement binder, which can account for up to 75% of the fill cost—is substantial.
Low Productivity: The cyclical nature of the process (mine, then fill, then wait for the fill to cure) results in a lower overall production rate compared to continuous or bulk mining methods.
Complex Backfill Engineering: The backfill is not simply waste material; it is an engineered product that must meet specific strength and stiffness criteria to be effective. The design, mixing, transport, and placement of backfill is a complex material science and logistics challenge. Failures of the backfill or the barricades used to contain it can have catastrophic consequences.
Potential for Subsidence: Despite the use of backfill, large-scale, multi-stage Cut-and-Fill operations can still induce surface subsidence over time as the entire backfilled mass consolidates and deforms under pressure.

The application of Cut-and-Fill mining effectively transforms a part of the mining operation into a sophisticated civil engineering and material science project. The economic viability of the method is as much a function of the cost and quality of the backfill as it is the efficiency of ore excavation. Consequently, a significant focus of engineering effort in Cut-and-Fill mines is on optimising the backfill system—for instance, by developing high-density paste fills that use less cement and water while providing superior strength. This directly links mining economics with advanced waste management and material science, turning a waste product (tailings) into a critical structural component.

Ore Recovery and Dilution

Cut-and-Fill stoping is renowned for its excellent performance in terms of recovery and dilution. Ore recovery is typically high, around 85% or better, while dilution is kept to a minimum, often as low as 5%. Under ideal conditions, it can approach 100% recovery with near-zero dilution, which is why it is justified for high-value orebodies.

Chapter 3: Caving Methods

Caving methods represent a fundamentally different approach to underground mining. Instead of fighting against ground pressure and striving to maintain stable openings, these methods are designed to harness the force of gravity and induce the controlled failure of the rock mass. They are high-tonnage, bulk mining methods that are only applicable in specific geomechanical environments where the orebody or the overlying host rock is amenable to caving.

3.1 Longwall Mining

Principles of Operation

Longwall mining is a highly productive and highly mechanised underground system designed for the complete extraction of tabular deposits, predominantly coal seams. The method involves mining a large rectangular block or “panel” of coal, which can be up to 400 metres wide and several kilometres long. A sophisticated, integrated system of machinery operates along the long face of the panel. A cutting machine, typically a shearer with two rotating drums, traverses the coal face, cutting a slice of coal that falls onto an armoured face conveyor (AFC) running parallel to the face. Immediately behind the conveyor, a line of powerful hydraulic roof supports, known as shields or chocks, protects the machinery and personnel by holding up the immediate roof. As the shearer cuts the coal, the entire system—shields, conveyor, and shearer—advances forward into the seam. The roof behind the shields is then allowed to collapse in a controlled and predictable manner into the void, or “goaf”.

Geological and Geotechnical Applicability

Longwall mining is best suited for extensive, continuous, and relatively uniform tabular seams that are flat or gently dipping, such as those found in many of the world’s major coal basins. The geology must be consistent and predictable, as the inflexible nature of the longwall equipment system cannot easily accommodate significant faults, seam thinning, or other geological anomalies.

Key Equipment

The longwall system is a single, integrated “super-machine” composed of three main components:

The Shearer (or Plough): The cutting machine that extracts the coal from the face.
The Armoured Face Conveyor (AFC): A heavy-duty chain conveyor that transports the cut coal along the face to a gate road.
Powered Roof Supports (Shields): A line of hydraulic jacks that provide a safe, mobile canopy and control the collapse of the goaf.

Advantages

Very High Productivity: Longwall mining is one of the most productive underground mining methods, capable of continuous operation and very high extraction rates.
High Recovery Rate: Within the boundaries of the panel, ore recovery is very high, often exceeding 80-90%.
Enhanced Safety: Miners work within the protective canopy of the hydraulic shields, which significantly reduces their exposure to the primary underground hazards of roof falls.
High Degree of Automation: Modern longwall systems are highly automated, further increasing efficiency and removing personnel from the most hazardous areas of the mine.

Disadvantages and Practical Challenges

High Capital Investment: The specialised and integrated longwall equipment system requires an extremely high initial capital investment.
Inflexibility: Once a longwall operation is established, it is very inflexible. It cannot easily navigate unexpected geological disturbances, which can halt production for extended periods.
Surface Subsidence: This is the most significant environmental and social consequence of longwall mining. The total extraction of the coal seam leads to the inevitable and immediate collapse of the overlying strata, which manifests on the surface as a large, predictable subsidence trough. This can damage buildings, roads, pipelines, and other surface infrastructure, and can permanently alter surface and groundwater hydrology.

The operational model of longwall mining can be compared to a continuous manufacturing process rather than a traditional, cyclical mining operation. This industrialised efficiency, however, comes at the direct and unavoidable cost of severe surface impact. The method’s core operational strength—total extraction—is inextricably linked to its primary environmental drawback—subsidence. This creates a fundamental tension between maximising resource recovery underground and preserving land use, infrastructure, and ecosystems on the surface, making longwall mining a frequent focal point for regulatory scrutiny and community opposition.

Ore Recovery and Dilution

Ore recovery within the mined panel is excellent, typically around 80% or higher. Dilution is generally low, as the cutting height is precisely controlled to match the seam thickness.

3.2 Sublevel Caving (SLC)

Principles of Operation

Sublevel Caving is a mass mining method that proceeds in a top-down sequence. The orebody is divided into a series of sublevels at regular vertical intervals (typically 8-15 metres). On each sublevel, a network of parallel drifts is developed through the orebody. From these drifts, long blast holes are drilled in a fan-shaped pattern into the ore block above. The ore is blasted one ring or slice at a time. The broken ore flows by gravity into the production drift, where it is loaded by LHDs. As the ore is drawn out, the overlying waste rock (the hanging wall and previously caved material) is induced to cave and follow the production front downwards, filling the void created by the extracted ore.

Geological and Geotechnical Applicability

SLC is applied to large, steeply dipping orebodies (dip >60°). The geomechanical conditions are specific: the ore itself must be competent enough to maintain stable production drifts with minimal support, while the hanging wall rock must be weak and readily caveable to ensure that it follows the extraction front without creating large, unstable voids. This places SLC in a geomechanical niche between Sublevel Open Stoping (which requires a very strong hanging wall) and Block Caving (which requires a caveable orebody).

Key Equipment

The primary equipment used in SLC includes long-hole production drill rigs for blasting the ore rings and LHDs for mucking the broken ore from the production drifts (drawpoints).

Advantages

High Production Rate: SLC is a highly mechanised, bulk mining method capable of achieving high production rates.
Safety: All production activities (drilling, blasting, and loading) are conducted from within the relative safety of supported development drifts, away from the caving zone.
Applicability in Weaker Rock: It can be used in orebodies with weaker hanging walls that would not be suitable for open stoping methods.

Disadvantages and Practical Challenges

High Ore Loss and Dilution: This is the defining challenge and primary drawback of the SLC method. The process of drawing ore from beneath a column of caving waste rock makes the mixing of ore and waste inevitable. This results in significant dilution (waste rock reporting as ore) and ore loss (ore being trapped and left behind in the caved waste). Typical dilution rates are between 20% and 30%, while ore losses can range from 15% to 25%.
Complex Gravity Flow Mechanics: The economic success of an SLC operation depends critically on managing the draw of ore from the production drifts to control dilution. This requires a sophisticated understanding of the mechanics of gravity flow of granular materials, as well as meticulous planning and operational discipline. Over-drawing a ring will result in excessive dilution, while under-drawing will lead to high ore loss.
Surface Subsidence: As a caving method, SLC results in the progressive collapse of the rock mass, which ultimately manifests as large-scale subsidence on the surface.
Other Hazards: Potential hazards include airblasts if a large void forms and then collapses suddenly, and inrush of water or mud if the caved zone connects to surface water bodies or aquifers.

SLC is fundamentally a method of calculated compromise. It is employed in ground conditions that preclude other bulk methods. Its adoption represents an explicit economic decision to sacrifice ore grade (through high dilution) and a portion of the resource (through ore loss) in order to achieve the high production volumes necessary to make the operation profitable. The primary engineering challenge is not to eliminate dilution and loss, but to manage and control them to an economically optimal level.

Ore Recovery and Dilution

Ore recovery is moderate, typically in the range of 85-90%, but this comes at the cost of high dilution, which is generally between 20% and 30%.

3.3 Block Caving

Principles of Operation

Block Caving is a large-scale, low-cost, mass mining method that uses gravity to do most of the work. The method involves defining a massive block of ore, which can be hundreds of metres in each dimension, and completely removing its support from underneath via a horizontal “undercut”. Deprived of its support, the ore block begins to collapse under its own weight. The immense internal stresses cause the rock mass to fracture and break apart, a process known as induced seismicity. This broken ore then flows by gravity into a series of pre-constructed funnels, or “drawbells,” located at the base of the block. The ore is then extracted from “drawpoints” on a production level situated beneath the drawbells. As ore is drawn from the bottom, the cave propagates upwards through the entire orebody.

Geological and Geotechnical Applicability

Block Caving is applicable to massive, steeply dipping orebodies that have a very large footprint in both horizontal and vertical dimensions. The geomechanical properties of the rock mass are paramount. The orebody must be amenable to caving; that is, it must possess a network of fractures and be under sufficient stress to break and fragment into manageable sizes once undercut. At the same time, the rock mass designated for the long-life production and haulage levels beneath the cave must be strong and competent enough to withstand the high stresses induced by the caving process above. The method is the primary choice for exploiting large, low-grade deposits, such as copper porphyries, that are too deep to be mined by open pit methods.

Key Equipment

While development requires standard equipment like jumbo drills, the primary “equipment” in Block Caving is the engineered rock mass itself. The production level relies heavily on LHDs to extract ore from the drawpoints. Due to the predictable and repetitive nature of this task, these LHD fleets are often fully automated, operating without human intervention. The high tonnages produced may also justify the installation of underground crushers and extensive conveyor belt systems for ore transport.

Advantages

Lowest Operating Cost: Block Caving has the lowest operating cost per tonne of any underground mining method, approaching the economics of large open-pit mines. This is because gravity performs the primary energy-intensive tasks of ore fragmentation and transport.
Very High Production Rates: It is the only underground method capable of achieving production rates comparable to surface mining, with some operations exceeding 100,000 tonnes per day.
High Automation and Safety: The separation of the production level from the active caving zone makes the method exceptionally well-suited to automation, which enhances both efficiency and safety by removing personnel from the most hazardous areas.

Disadvantages and Practical Challenges

Extremely High Capital Cost and Long Lead Time: The method requires a massive upfront investment in the extensive development of the undercut and extraction levels. This development can take many years to complete before a single tonne of ore is produced, resulting in a very long pre-production period and significant financial risk.
High Risk and Inflexibility: Block Caving is an “all or nothing” method. The success of the entire multi-billion-pound enterprise hinges on the accuracy of the geotechnical predictions of how the rock mass will behave. If the orebody fails to cave as expected (“cave stalling”) or if it breaks into excessively large blocks (“poor fragmentation”), the entire reserve can be sterilised, leading to catastrophic financial failure. Once initiated, the cave is largely uncontrollable.
Major Environmental Impact: The method causes extensive, large-scale surface subsidence that is unavoidable. This can create vast craters on the surface, permanently altering the landscape. It can also severely disrupt regional surface and groundwater systems, creating a significant long-term risk of water contamination from acid mine drainage as water percolates through the fractured rock mass.
Operational Hazards: Key operational risks include powerful and destructive airblasts, which can occur if a stable void forms within the cave and then collapses catastrophically, and the blockage of drawpoints by oversized boulders (“hang-ups”), which are hazardous to clear.

Block Caving represents a paradigm shift in mining philosophy, moving beyond simple excavation to a form of “manufacturing with rock.” The primary engineering challenge is not the mechanical breaking and moving of rock, but the accurate prediction and management of the geomechanical behaviour of the orebody to induce it to effectively “self-mine.” The mine’s success is therefore less dependent on the skill of the operational miners and more on the predictive power of the geotechnical models. The mine itself becomes the primary piece of equipment, and the dominant risk is systemic and predictive, rather than operational.

Ore Recovery and Dilution

If the cave behaves as planned, recovery can be very high, often around 95%, with dilution typically managed to around 15%.

Chapter 4: A Direct Comparative Analysis of Stoping Methods

The preceding chapters have detailed the operational principles, applicability, and characteristics of the principal underground stoping methods. This chapter synthesises that information to provide a direct, multi-parameter comparison, illuminating the critical trade-offs that drive the selection process in mine design.

4.1 Summary Comparison Table

The following table provides a consolidated, at-a-glance summary of the key attributes of each mining method. This allows for a rapid assessment of their relative strengths, weaknesses, and suitability for different geological and economic conditions.

Method	Classification	Ideal Orebody Geometry (Dip, Shape, Thickness)	Required Rock Mass Strength (Ore & Host Rock)	Ore Recovery (Typical %)	Dilution (Typical %)	Relative Capital Cost	Relative Operating Cost	Productivity / Mechanisation	Selectivity
Room and Pillar	Unsupported	Flat to gentle dip (<20°), tabular, uniform thickness	Competent ore and host rock	40–70% (advance), >85% (retreat)	Low (~5%)	Low-Medium	Medium	High	Low
Shrinkage Stoping	Unsupported	Steep dip (>50°), narrow, regular veins	Strong ore and host rock	85–95%	Medium (10–25%)	Low	High	Low	Medium
Sublevel Open Stoping (SLOS)	Unsupported	Steep dip, large, regular shape	Strong ore, very strong host rock	~85%	Medium (~15%)	Medium	Medium	High	Low
Vertical Crater Retreat (VCR)	Unsupported	Steep dip, large, regular shape	Strong ore and host rock	~90%	Medium (~10%)	Medium	Medium-Low	High	Low
Cut-and-Fill	Supported	Steep dip, irregular shape, variable thickness	Weak ore and/or host rock	High (~85%+)	Very Low (~5%)	Medium	Very High	Low-Medium	Very High
Longwall Mining	Caving	Flat to gentle dip, tabular, very continuous	(Not primary constraint; must allow goaf collapse)	Very High (>80%)	Low	Very High	Low	Very High	Very Low
Sublevel Caving (SLC)	Caving	Steep dip (>60°), massive, large vertical extent	Competent ore, weak/caveable host rock	85–90%	High (20–30%)	High	Low-Medium	High	Low
Block Caving	Caving	Steep dip, massive, very large vertical & horizontal extent	Caveable ore, competent host rock for infrastructure	Very High (~95%)	Medium (~15%)	Very High	Very Low	Very High	Very Low

4.2 Analysis of Key Dichotomies

The selection of a mining method is rarely a simple choice but rather a complex optimisation process involving several fundamental trade-offs. The comparative table highlights these dichotomies, which are central to strategic mine planning.

Bulk vs. Selective Mining

This is perhaps the most fundamental distinction in mining methods. The choice is dictated almost entirely by the geometry and grade distribution of the orebody.

Selective Methods, such as Cut-and-Fill, are designed to precisely extract ore while leaving adjacent waste rock undisturbed. This is essential for high-grade, narrow, and geometrically complex deposits, like meandering gold veins, where mining even a small amount of barren waste would render the operation uneconomic. The high operating costs of these methods are justified by the high value of the cleanly extracted ore.
Bulk Methods, such as Block Caving, Sublevel Caving, and Longwall mining, are designed for high-volume production where selectivity is either impossible or unnecessary. These methods are applied to large, massive, or continuous deposits where the ore grade is relatively uniform. For a vast, low-grade copper porphyry deposit, for example, the concept of selectively mining higher-grade zones is physically impractical; the entire rock mass must be mined, making a bulk method like Block Caving the only viable option.

Capital vs. Operating Costs

The financial profile of a mining method is a critical factor in project feasibility.

High Capital/Low Operating Cost Methods: Longwall Mining and Block Caving fall into this category. They require enormous upfront investment in specialised equipment (Longwall) or extensive pre-production development (Block Caving). This high initial risk is offset by extremely low operating costs per tonne once production begins, driven by high productivity, automation, and the use of gravity. These methods are suitable for large, long-life mines with well-defined, extensive reserves that can guarantee a return on the massive initial investment.
Low Capital/High Operating Cost Methods: Methods like Shrinkage Stoping and, to a lesser extent, Cut-and-Fill, require less initial capital for equipment and development. However, their operating costs are much higher due to their labour-intensive nature and lower productivity. These methods may be more suitable for smaller deposits or operations with limited access to initial capital, where the high value of the ore can absorb the higher per-tonne operating costs.

The Recovery-Dilution Trade-off

Ore recovery (the percentage of the total ore reserve that is successfully extracted) and dilution (the amount of waste rock that is unavoidably mined along with the ore) are often inversely related. The optimal balance between these two metrics is a key driver of a mine’s economic performance.

High Recovery with High Dilution: Sublevel Caving is the classic example of this trade-off. The mechanics of the method, drawing ore from beneath caving waste, make a high degree of mixing unavoidable. To achieve high recovery (e.g., 90%), the operation must accept high dilution (e.g., 30%). The economic calculation is that the value of the extra ore recovered outweighs the cost of hoisting and processing the diluting waste.
High Recovery with Low Dilution: Cut-and-Fill stoping sits at the opposite end of the spectrum. By immediately backfilling voids and providing excellent wall support, it allows for the precise extraction of ore, leading to high recovery with very low dilution. This near-perfect extraction, however, comes at the price of very high operating costs.
The Economic Cut-off: Ultimately, the acceptable level of dilution is determined by the mine’s economic cut-off grade. Drawing of ore must cease when the grade of the mixed material falls below the point where it is profitable to process.

Chapter 5: Common Misconceptions and Practical Realities

The theoretical principles of mining methods often contrast with the complex and sometimes counter-intuitive realities encountered in practice. Several widely held misconceptions can lead to flawed decision-making and an underestimation of risk. This chapter addresses some of the most common of these “known wrongs.”

5.1 The Myth of Long-Term Stability (“If it hasn’t collapsed yet, it’s safe”)

A pervasive and dangerous misconception is that abandoned underground workings, particularly in Room and Pillar mines, that have remained stable for many decades are permanently safe. This belief often stems from a failure to appreciate the difference between human timescales and geological timescales.

The reality is that the stability of old mine workings cannot be assumed. The pillars left for support are subject to slow but continuous degradation through processes like weathering, groundwater action, and the redistribution of regional stresses. This can gradually reduce their load-bearing capacity over time. Catastrophic collapses of mines that were closed 50, 80, or even more than 100 years prior are well-documented. A study of subsidence incidents over the Pittsburgh Coal Seam found that 50% of incidents occurred above mines that had been closed for at least 50 years. Therefore, the absence of past failure is not a guarantee of future stability. Any plan to interact with or build over old workings requires a thorough, modern geotechnical assessment to evaluate the current state of the pillars and the risk of future collapse.

5.2 The Myth of the “Safe Depth” from Subsidence

Another common fallacy, particularly in relation to caving and total extraction methods, is the idea that if a mine is sufficiently deep, surface subsidence will not occur. This belief is based on the flawed premise that the rock overlying the mined-out void will break up and “bulk” (increase in volume) enough to choke the void and support the upper strata, preventing the effect from reaching the surface.

This concept is physically incorrect. For any mining method that involves total extraction of a deposit, such as Longwall or Block Caving, the volume of material removed from the ground must be accounted for by deformation of the overlying rock mass. While depth does influence the shape, extent, and timing of the surface subsidence trough, it does not prevent it. Indeed, significant subsidence is routinely observed above very deep mines. For example, subsidence equivalent to 90% of the mined seam thickness has been measured over longwall mines operating at depths of 2,000 feet. The engineering question for these methods is not if subsidence will occur, but rather how it will manifest, how to predict its extent, and how to manage its impact on the surface.

5.3 The Reality of Ore Dilution: More Than Just Waste Rock

In project planning, dilution is sometimes treated as a simple accounting adjustment—a matter of mining a bit of extra waste that lowers the average grade. This view dangerously underestimates its true economic impact.

Dilution is an economic poison that affects every stage of the mining and processing value chain. It not only reduces the head grade of the material sent to the mill, but it also actively increases costs throughout the system. Every tonne of diluting waste must be drilled, blasted (in some cases), loaded, hauled, hoisted, crushed, and ground—all energy-intensive processes—for zero economic return. In the processing plant, this barren material consumes valuable reagents and occupies capacity in flotation cells or leach tanks that could have been used for valuable ore. Consequently, controlling dilution is a primary driver of profitability and a critical consideration in the selection and design of a mining method. A method that appears cheaper on a “cost per tonne mined” basis may prove to be far more expensive on a “cost per unit of metal produced” basis if it incurs high dilution.

5.4 Mining as a “Low-Tech” Industry

A persistent external perception, and sometimes an internal cultural artefact, is that mining is an archaic, low-technology industry based on brute force. This image is decades out of date.

Modern underground mining is a technologically sophisticated and data-driven sector. The viability of methods like Block Caving is entirely dependent on advanced geotechnical modelling and complex numerical simulations (using software like FLAC3D and Elfen) to predict rock mass behaviour. Operations are increasingly reliant on automation and robotics, with tele-remote and fully autonomous LHDs and trucks now common in many mines, improving both safety and efficiency. Data analytics, AI, and real-time monitoring are used to optimise everything from blast fragmentation to equipment maintenance schedules. The industry is a major driver and adopter of advanced technology, from sophisticated sensors and communication networks to battery electric vehicles and virtual reality training systems. This technological underpinning is essential for mining deeper, lower-grade, and more complex orebodies safely and economically.

Addition: Future Trends in Underground Mining

The field of underground mining is in a state of continuous evolution, driven by the depletion of near-surface resources, technological advancements, and increasing economic and societal pressures. The selection and application of stoping methods in the future will be shaped by several key trends.

The Push Deeper: As easily accessible surface and near-surface orebodies are exhausted, the future of mining lies at greater depths. This trend inherently favours large-scale, highly mechanised, and cost-effective bulk mining methods. In particular, Block Caving and its variations are becoming increasingly important as the only methods capable of economically exploiting the deep, massive, low-grade deposits that will constitute a growing share of future metal supply.
Automation and Electrification: The drive towards automation and robotics will accelerate. The goal is to remove human operators from the most hazardous areas of the mine, leading to step-changes in safety and allowing for continuous, highly efficient production. Concurrently, the transition from diesel to Battery Electric Vehicles (BEVs) is reshaping underground logistics. Electrification reduces heat and eliminates diesel particulate emissions, which in turn lowers the substantial costs and energy consumption associated with mine ventilation systems.
Data-Driven Design and Operation: The selection of mining methods will move further away from empirical rules-of-thumb and towards rigorous, data-driven optimisation. The integration of advanced geotechnical simulation, real-time monitoring data from sensors embedded in the rock mass, and powerful analytical software will allow engineers to design and operate mines with greater precision. This will enable them to maximise resource recovery while proactively managing complex geotechnical risks.
The Social and Environmental Licence to Operate: Environmental performance and social acceptance are no longer secondary considerations but primary constraints on mine design. The environmental impacts of a chosen method, particularly surface subsidence from caving and the management of waste from all methods, will be a critical factor in securing permits and maintaining community support. This will increase the attractiveness of methods that minimise surface footprint and incorporate sustainable practices, such as the use of paste backfill, which turns mill tailings from a liability into a valuable structural component.

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