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Sketch painting of miners stoping at the Burra Burra Mine, Burra, Australia, 1847.
Stoping with an air drill in an American iron mine in the 20th century (museum exhibit)

Stoping is the process of extracting the desired ore or other mineral from an underground mine, leaving behind an open space known as a stope.[1] Stoping is used when the country rock is sufficiently strong not to collapse into the stope, although in most cases artificial support is also provided.

The earliest forms of stoping were conducted with hand tools or by fire-setting; later gunpowder was introduced. From the 19th century onward, various other explosives, power-tools, and machines came into use. As mining progresses the stope is often backfilled with tailings, or when needed for strength, a mixture of tailings and cement. In old mines, stopes frequently collapse at a later time, leaving craters or flashes at the surface. They are an unexpected danger when records of underground mining have been lost with the passage of time.

Stoping is considered "productive work", and is contrasted with "deadwork", the work required merely to access the mineral deposit, such as sinking shafts and winzes, carving adits, tunnels, and levels, and establishing ventilation and transportation.[2]

Overview

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A stope can be created in a variety of ways. The specific method of stoping depends on a number of considerations, both technical and economical, based largely on the geology of the ore body being mined. These include the incline of the deposit (whether it is flat, tilted or vertical), the width of the deposit, the grade of the ore, the hardness and strength of the surrounding rock, and the cost of materials for supports.[3]

It is common to dig shafts vertically downwards to reach the ore body and then drive horizontal levels through it. Stoping then takes place from these levels.

When the ore body is more or less horizontal, various forms of room and pillar stoping, cut and fill,[4] or longwall mining can take place. In steeply-dipping ore bodies, such as lodes of tin, the stopes become long narrow near-vertical spaces, which, if one reaches the surface is known as a gunnis or goffen.[1] A common method of mining such vertical ore bodies is stull stoping, see below.

Open-stope systems

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A large stope converted into a chapel in a salt mine in Poland – now open to tourists

Open stoping is generally divided into two basic forms based on direction: overhand and underhand stoping, which refer to the removal of ore from above or below the level, respectively. It is also possible to combine the two in a single operation.

Underhand stoping

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Underhand stoping, also known as horizontal-cut underhand or underbreaking stoping, is the working of an ore deposit from the top downwards. Like shrinkage stoping, underhand stoping is most suitable for steeply dipping ore bodies.[5] Because of the mechanical advantage it offers hand tools being struck downward (rather than upward, against gravity), this method was dominant prior to the invention of rock blasting and powered tools.[6]

Overhand stoping

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In overhand stoping, the deposit is worked from the bottom upward, the reverse of underhand stoping. With the advent of rock blasting and power drills, it became the predominant direction of stoping.[3]

Combined stoping

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In combined stoping, the deposit is simultaneously worked from the bottom upward and the top downward, combining the techniques of overhand and underhand stoping into a single approach.

Breast stoping

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Breast stoping is a method used in horizontal or near-horizontal ore bodies, where gravity is not usable to move the ore around.[7] Breast stoping lacks the characteristic "steps" of either underhand or overhand stoping, being mined in a singular cut. Room and pillar is a type of breast stoping.

Timbered-stope systems

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Stull stoping

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Stull stoping is a form of stoping used in hardrock mining that uses systematic or random timbering ("stulls") placed between the foot and hanging wall of the vein. The method requires that the hanging wall and often the footwall be of competent rock as the stulls provide the only artificial support. This type of stope has been used up to a depth of 3,500 feet (1,077 m) and at intervals up to 12 feet (3.7 m) wide.[8] The 1893 mining disaster at Dolcoath mine in Cornwall was caused by failure of the stulls holding up a huge weight of waste rock.[9]

Square-set stoping

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Square-set timbering was invented in the Comstock Lode, Virginia City, Nevada in the 1860s.

Square-set stoping is a historical method of stoping which relies on interlocking timbers set into place forming a grid, wedged tightly against the rock. As the mining progresses, generally upwards, new timber sets are added to fill the void. Ore is dropped through the sets freely or fed into chutes and either loaded into ore cars or mucked out from there.

Depending on rock conditions and other technical considerations, once the stope has reached its engineered height, it may be left open or backfilled for support. A common historical method of hydraulic fill involved dumping waste rock into a completed stope, then slurrying in a mixture of mill sand and water. The water drains away leaving the sand to lock the fill in place. Other methods of hydraulic fill using cement and mill sand, such as paste fill, are more contemporary methods of stoping and unlikely used in conjunction with the antiquated method of squareset timbering.

Shrinkage stoping

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A large stope in the Treadwell gold mine, Alaska 1908; an example of shrinkage stoping

Shrinkage stoping is most suitable for steeply dipping ore bodies (70°—90°). In shrinkage stoping, mining proceeds from the bottom upwards, in horizontal slices (similar to cut and fill mining), with the broken ore being left in place for miners to work from. Because blasted rock takes up a greater volume than in situ rock (due to swell factor), some of the blasted ore (approximately 40%) must be removed to provide working space for the next ore slice. Once the top of the stope is reached all the ore is removed from the stope. The stope may be backfilled or left empty, depending on the rock conditions.[10]

Long hole stoping

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Long hole stoping as the name suggests uses holes drilled by a production drill to a predetermined pattern as designed by a mining engineer.[11] Long hole stoping is a highly selective and productive method of mining and can cater for varying ore thicknesses and dips (0 – 90 degree). It differs from manual methods such as timbered and shrinkage as once the stope has begun blasting phase it cannot be accessed by personnel. For this reason the blasted rock is designed to fall into a supported drawpoint or removed with remote control LHD (load, haul, dump machine).

The biggest limitation with this method is the length of holes that can be accurately drilled by the production drill, larger diameter holes using in the hole hammer drills can be accurate to over 100 m in length while floating boom top hammer rigs are limited to ~30 m.

Slot – initial void

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Holes drilled underground are generally drilled perpendicular, in a radial pattern around the drive. For the blastholes to successfully extract the ore material they must be able to fire into a void in front. A slot is required in every stope to provide the initial void. The slot is often the most difficult, costly and highest risk component of mining a stope. Depending on the shape, height and other factors, different methods to create a slot can be used such as:

  • Raise bore, a circular shaft mined bottom up using mechanical rollers to achieve shaft profile. This method works well in larger stopes however requires both access to top and bottom of stoping block. Raise bores work most effectively between 45 and 90 deg.
  • Longhole raise, a pattern of tightly spaced blastholes and reamers (empty holes with no charge), similar to a burn cut in a development round. Can be done as downhole and fired in multiple lifts (15m rise in 3 lifts of 5 m to minimise chance of blast failing) or as uphole in one single firing. This method works well for shorter raises between 45 and 90 deg, however it is prone to freezing and may require remedial drilling to extract slot to full height.
  • Airleg raise, using an airleg (jackleg) machine to develop a sub vertical raise into the stoping block. This method has the advantage of giving geological and geotechnical teams further analysis of the stoping block prior to mining.
  • Boxhole boring, similar to raise boring but less productive as broken material is extracted from the same location as the drill, is used to bore vertically with no top level access required. (Hamza, 2016)

References

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Stoping

View on Grokipedia
from Grokipedia
Stoping is the process of extracting from a designated underground area known as a stope, where the orebody is removed in a controlled sequence to create an open excavation while maintaining the stability of the surrounding rock mass. This method is fundamental to underground operations, particularly for steeply dipping or irregularly shaped orebodies in competent rock formations, and it contrasts with by requiring careful management of ground support to prevent collapses. Stoping encompasses a variety of techniques tailored to the , characteristics, and economic factors of the deposit, with unsupported methods like open stoping and sublevel stoping suitable for strong, stable where minimal artificial support is needed, allowing for rapid extraction of immediately after blasting. In contrast, supported methods such as cut-and-fill stoping involve mining in horizontal slices followed by backfilling with waste or to provide immediate support, making them ideal for weaker ground conditions or high-value ores like and that require selective recovery. Shrinkage stoping, another common variant, utilizes a portion of the broken itself as a temporary platform and wall support during extraction, typically applied to vertical or near-vertical orebodies where the broken material's exceeds 45 degrees. Key considerations in stoping include for ground control, technologies using explosives like nitrate-fuel oil, and measures to mitigate risks such as falls of ground or gas outbursts, with modern advancements enabling higher productivity in deep or narrow veins. These methods are widely used for commodities including metals (e.g., , , lead-zinc) and , influencing mine design, costs, and environmental impacts like surface .

Introduction

Definition and Fundamentals

Stoping is the process of excavating in underground mines through a series of horizontal, vertical, or inclined workings, typically in veins, irregular ore bodies, or flat deposits, resulting in the creation of open spaces known as stopes. This method focuses on the extraction phase, distinct from development activities such as drifts, crosscuts, or raises, which prepare access to the orebody. Stoping allows for selective recovery of , particularly in vein or disseminated deposits where high-grade mineralization requires precise underground targeting. Key terminology in stoping includes the stope, which is the excavated void left after removal; the sill, referring to the horizontal floor or ledge at the base of the stope; , denoting the uppermost portion; the face, the active working surface where advances; and the back, the overhead rock surface or of the stope. These terms describe the geometric and operational elements essential for managing the excavation space and ensuring worker safety during operations. The basic process of stoping involves sequential steps: blast holes into the face, charging the holes with explosives, blasting to fragment the rock, mucking to remove the broken , and scaling to dislodge loose material from walls, , and faces for stability. Blasting breaks the ore into manageable pieces, while mucking transports it to points, and scaling prevents rockfalls that could endanger personnel. Unlike , which removes overlying earth to access near-surface deposits via open pits or strips, stoping operates entirely underground through tunnels and stopes, emphasizing rock support and selectivity for deeper, higher-grade in confined geometries.

Importance in Underground Mining

Stoping serves as the primary method for ore extraction in underground metal mines, particularly in hard-rock operations for and base metals. This approach is essential for accessing deep-seated or irregularly shaped deposits that cannot be reached economically through , allowing operations to extend to depths exceeding 2,000 meters while minimizing surface disruption. By creating open spaces known as stopes through , blasting, and removal of ore, stoping facilitates the targeted recovery of valuable minerals in environments where surface methods are impractical due to thickness or environmental constraints. One of the key advantages of stoping is its high selectivity, enabling miners to extract high-grade while minimizing dilution from surrounding waste rock, which typically results in recovery rates of 78-85% in suitable orebodies. This precision reduces costs and environmental impact by limiting the volume of low-grade sent to mills, making it particularly effective for narrow, steeply dipping veins common in precious and deposits. In contrast, methods like room-and-pillar offer lower selectivity due to the need for structural pillars that leave significant behind, while block involves bulk extraction with higher dilution rates, often exceeding 20%, as it relies on natural cave-in without precise targeting. Economically, stoping contributes significantly to global production of commodities such as , , and , where it supports efficient integration with underground hoisting systems and surface facilities to form a complete extraction cycle. For instance, sublevel stoping variants are widely adopted in copper-nickel operations for their in massive orebodies, driving substantial output in major mining regions like and . This method's ability to balance extraction rates with grade control enhances overall mine viability compared to less selective bulk methods in comparable deposits.

Historical Development

Origins in Early Mining

The origins of stoping trace back to ancient civilizations, where miners extracted from using rudimentary hand tools in underground workings. In and , techniques involved sinking vertical shafts and driving horizontal galleries along ore bodies, with workers employing iron picks, hammers, gads, wedges, and crowbars to chip away at , creating small stopes up to 200 meters deep. These methods relied on manual labor for extraction, often supplemented by fire-setting to fracture rock by heating it and with water. A notable example is the Laurion silver mines in , operational around 500 BCE, where miners used mallets and point chisels to systematically exploit mineralized veins in and , developing underhand stoping in galleries and shafts reaching 100 meters deep. During the Middle Ages, stoping advanced in European metal mines through the widespread adoption of fire-setting, a technique that heated rock faces with wood fires to induce cracking before manual removal. This method was particularly effective in environments of , allowing miners to excavate galleries and stopes along veins without advanced tools, though it produced characteristic smooth, rounded walls free of marks. Fire-setting remained a staple in underground metal until the introduction of explosives, enabling deeper extraction in regions like and Britain while requiring careful management of ventilation and smoke. The marked a pivotal shift with the adoption of black powder explosives, which facilitated the creation of larger stopes in deep vein mining. In Cornish tin mines, blasting, refined from earlier 17th-century innovations, allowed miners to deepen shafts and expand stopes significantly, boosting production to meet industrial demand. Similarly, at the in , discovered in 1859, black powder was immediately employed to drill and blast bodies, enabling rapid underground development in wet, unstable ground. Early stoping practices were fraught with challenges, including extreme from manual drilling and rock removal, often performed by teams in confined, poorly lit spaces. Frequent roof collapses posed a constant hazard due to the lack of systematic ground support, exacerbated by the removal of pillars and the instability of surrounding clay-rich material, leading to numerous fatalities in mines like the Comstock.

Evolution of Techniques

The early marked a significant transition in stoping techniques, driven by advancements in technologies that enabled more efficient excavation in challenging underground environments. Steam-powered drills, invented in 1871 by Simon Ingersoll, revolutionized rock breaking by replacing manual labor with percussion tools powered by steam engines, allowing for faster and deeper penetration in hard rock formations. Pneumatic drills followed in the late 1880s. Concurrently, the introduction of electric blasting caps around the improved detonation precision and safety over traditional black powder fuses, facilitating controlled blasting in timber-supported stopes. These innovations supported the use of square-set timbering, invented in 1860 for Nevada's mines, where modular timber frames filled with waste rock stabilized wide, irregularly shaped ore bodies previously prone to collapse. Post-World War II mechanization further advanced stoping by integrating powered equipment and backfill systems, enhancing in vertical and inclined bodies. Shrinkage stoping, which relies on broken ore for temporary support, saw refined applications in the mid-20th century as mechanized loading and reduced manual handling, though supported methods like cut-and-fill began to dominate due to better adaptability to . In Canadian mines during the 1940s, hydraulic fill techniques emerged as a key development for cut-and-fill stoping, using slurried piped into stopes to provide rapid, stable backfill and minimize dilution, marking an early adoption in North American metal mining. By the late 20th century, innovations in drilling patterns optimized large-scale extraction, particularly in massive orebodies. Ring drilling for longhole stoping, popularized in the 1970s, employed circular arrays of parallel holes blasted in sequence to create large, unsupported voids, improving ore recovery and reducing drilling time compared to earlier benching methods. Simultaneously, sublevel caving gained prominence in Swedish iron mines, with the Kiruna operation transitioning to this method in the 1950s, where sequential undercutting and blasting induced controlled caving of the overlying rock to extract ore with minimal support needs. In the , has transformed stoping operations by enhancing safety and efficiency through remote and autonomous systems. Sandvik's AutoMine suite, introduced in the early 2000s, enabled remote-controlled loaders for tasks like mucking in stopes, with initial implementations at mines such as El Teniente in in 2004, allowing operators to manage equipment from surface stations and reducing exposure to hazardous underground conditions. In the and 2020s, expanded significantly, with El Teniente achieving over 35% automated operations as of 2024, aiming for 50-60% by 2025 through AI-driven and remote operations to further enhance safety and productivity.

Basic Principles

Orebody Suitability and Stope Design

Stoping methods are most applicable to orebodies exhibiting tabular, , or disseminated geometries, where the deposit's shape allows for the creation of self-supporting excavations. These methods perform best on orebodies with thicknesses ranging from 2 to 30 meters, as narrower may require supported techniques to prevent excessive dilution, while thicker deposits enable larger, more efficient open stopes. Dip angles exceeding 45° are particularly advantageous, as steeper inclinations facilitate the gravity-assisted draw of broken , reducing handling costs and improving muck flow in methods like shrinkage or sublevel open stoping. Rock mechanics play a pivotal role in determining stoping suitability, with the strength and stability of the hanging wall and footwall being primary considerations. Competent hanging walls and footwalls, characterized by high uniaxial (typically >100 MPa), minimize the risk of collapse or excessive deformation into the stope, thereby controlling unplanned dilution from wall failure. The Rock Mass Rating (RMR) system, developed by Bieniawski, provides a quantitative assessment of these properties, incorporating factors such as rock quality designation, discontinuity spacing, and conditions; RMR values greater than 60 generally indicate fair to good rock mass quality suitable for unsupported or minimally supported open stoping, while lower ratings necessitate design adjustments or alternative methods. Stope design parameters are tailored to the orebody's and rock properties to optimize extraction while ensuring and . Typical stope heights range from 10 to 50 meters, allowing for multi-level blasting sequences, and widths from 5 to 20 meters, which balance recovery against dilution risks; for instance, narrower widths (5-10 m) suit thin veins to limit exposure, whereas wider spans (15-20 m) are feasible in stronger rock masses. Extraction sequences, such as primary-secondary stope sequencing or top-down progression, are planned to minimize dilution by sequencing blasts to maintain stability and prevent premature draw of waste rock, targeting loss below 5% through precise timing of adjacent stope . Geotechnical modeling tools are essential for validating these designs by simulating distributions and potential instabilities. Software like RS2, a 2D finite element analysis program, enables detailed stress analysis around proposed stope geometries, incorporating rock mass properties and excavation sequences to predict displacements and , thereby refining dimensions to avoid over-excavation or excessive support needs.

Ground Support and Stability

In underground stoping operations, ground support systems are essential to maintain the integrity of excavations, preventing collapses and ensuring worker safety. Natural support relies on the inherent strength of the rock mass, where competent rock formations form self-supporting arches or spans that can sustain the load without additional intervention. This approach is viable in stable, massive types with minimal jointing, allowing spans up to several meters depending on the orebody's geological characteristics. Artificial support systems are employed when natural stability is insufficient, particularly in weaker or fractured ground. Common elements include rock bolts for reinforcement, wire to prevent spalling, and for surface protection, often applied in combination to distribute loads effectively. Selection of these systems is guided by factors such as excavation span, rock quality designation (RQD), and stress conditions; for instance, in spans exceeding 5 meters with RQD below 50, systematic bolting with is typically required to enhance load-bearing capacity. Stability assessment in stoping involves continuous monitoring to detect deformation early and verify support efficacy. Instruments such as multi-point extensometers measure rock displacement along multiple depths, while convergence meters track relative movement between and or walls. These tools enable collection, with stability criteria often requiring a greater than 1.5 for unsupported spans to ensure long-term integrity. A prevalent challenge in deep stoping operations, particularly below 1000 meters, is rock bursts—sudden, violent failures of overstressed rock that pose severe risks to personnel and equipment. These events arise from high in-situ stresses exceeding 40 MPa in brittle rock masses, leading to seismic release. Mitigation strategies include destressing blasts, which involve controlled explosions to redistribute stress and create fracture zones that absorb , thereby reducing burst potential in subsequent excavations. Ground support designs must also integrate with ventilation systems to facilitate safe , especially after blasting when toxic gases like nitrogen oxides and are generated. Open-pattern installations of bolts and , rather than dense linings, minimize obstruction, allowing auxiliary fans to efficiently dilute and exhaust contaminants from the stope. This coordination ensures re-entry times are shortened while maintaining air quality standards, such as diluting fumes to below 5 ppm NO₂.

Open Stope Methods

Underhand Stoping

Underhand stoping is a top-down extraction technique in underground mining where is removed in horizontal slices starting from the upper level of a stope and progressing downward, with miners standing on the broken that serves as a natural working platform. This method involves downward into the orebody from the platform, blasting the slice, and allowing the fragmented to fall by to lower levels or draws for support and removal. It is particularly suitable for steeply dipping orebodies with angles of 70° to 90°, where the ore's self-supporting nature helps maintain stability in the hanging wall. The process relies on conventional drill-and-blast cycles, using pneumatic or hydraulic drills for creating downward-directed blast holes, followed by loading and hauling with load-haul-dump (LHD) units to transport the broken from the stope to ore passes or levels. Stope heights typically range from 3 to 5 meters per slice, with the method allowing for selective extraction in narrow veins while minimizing exposure to unstable ground above. Key advantages include reduced ore dilution, often limited to 5-10% due to the controlled blasting and platform support that prevents excessive wall spalling, and enhanced safety for weak walls since the intact provides immediate overhead protection during operations. However, disadvantages encompass slower production advance rates of approximately 1-2 meters per month, attributed to the sequential slicing and manual handling in confined spaces, as well as higher labor requirements for and mucking in the accumulating ore pile. Historically, underhand stoping has been applied in narrow-vein gold mines, such as those in the Rand region of prior to the 2000s, where steep dips and competent allowed for efficient recovery without extensive artificial support. In contrast to overhand stoping, which advances upward for faster extraction in stronger ground, underhand methods prioritize stability in weaker conditions by working beneath the .

Overhand Stoping

Overhand stoping is an open stope mining method that extracts upward from the bottom, beginning at lower levels and advancing by blast holes upward or horizontally into the orebody, followed by blasting and mucking the broken material from the working level via platforms or drawpoints. This bottom-up approach leverages the natural support of the hanging wall, making it particularly suitable for competent rock masses in orebodies with flat to moderately dipping geometries, typically less than 20° to 35° dip, where the hanging wall provides sufficient stability without extensive support. The process typically involves initial development of a level, followed by raises or slot cuts to initiate the stope, with miners working from platforms or the broken pile to drill upward-angled or horizontal holes for blasting successive benches. Common variants include breast stoping, where horizontal slices are mined in a step-like progression across flat or gently dipping orebodies, and flat-back stoping, which maintains a level roof through systematic benching without pronounced steps, often employing timber square sets for support in wider veins. Advance rates in these sequences generally allow for steady progression, with stope heights developed in slices of 3 to 5 meters per cycle, enabling monthly advances of approximately 3 to 5 meters in height per face under manual or semi-mechanized conditions. Key advantages of overhand stoping include simplified mucking due to flow of ore to lower levels, reducing manual handling and enabling potential mechanization with loaders and trucks in larger stopes, as well as the ability to space haulage levels farther apart (up to 100 meters or more), which lowers overall development costs. Miners can also inspect and scale the stope back from a safer position below, minimizing exposure to falling rock, and the method self-drains water effectively in flatter deposits. However, disadvantages encompass challenges in upward or flat holes, which necessitate temporary platforms or stulls for access, potentially leading to poor ventilation if raises are spaced widely, and increased dilution from wall sloughing in less competent ground, where unstable walls mix waste rock with , often resulting in 15-20% unplanned dilution in open configurations without backfill. A notable historical application occurred in the Australian lead-zinc mines at during the early 1900s, where overhand stoping was employed to extract massive sulphide ores from wide veins between levels, using timber sets (approximately 7 feet by 7 feet) filled with waste rock for stability, starting from the footwall side and advancing upward in 12-foot-high cuts with ore chutes every 10 feet. This method proved effective for accessing rich silver-lead deposits but required substantial timber resources, costing around £1 10s per set, and was adapted to manage weak hanging walls through pillar support in alternate sections. Unlike underhand stoping, which prioritizes top-down extraction for weak roof conditions, overhand stoping at emphasized bottom-up efficiency in relatively stable, competent ground.

Breast Stoping

Breast stoping is an open stope method employed for extracting from flat-lying or gently dipping orebodies, where the stope face advances horizontally across the in successive slices known as breasts. The process involves ore in horizontal layers between levels, with broken material loaded directly at the face using scrapers or chutes and transported via cars on tracks laid close to the working area; pillars or ribs of ore are often left between slices for temporary support, and the method can incorporate overhand or underhand techniques depending on ground conditions. Typical dimensions for breast stopes include slice heights of approximately 2 to 4 meters (6 to 13 feet) and widths of 3 to 6 meters (10 to 20 feet), though these vary with thickness and stability, allowing adaptation to narrow veins up to 20 feet wide while level intervals are spaced 30 to 45 meters (100 to 150 feet) apart. This method offers advantages such as high ore recovery rates exceeding 90% in massive, stable deposits due to direct face loading and minimal dilution, making it suitable for competent ground where little support is needed beyond occasional timbering. However, disadvantages include risks of pillar instability leading to in weaker ground, increased timber and track maintenance costs from close level spacing, and slower production rates in narrow veins owing to labor-intensive extraction and limited . Historically, breast stoping was commonly applied in the copper mines of the Keweenaw Peninsula from the 1850s through the 1920s, particularly at operations like Calumet & Hecla and , where it facilitated extraction of native copper from flat-dipping amygdaloid and conglomerate lodes to depths of up to 2,400 meters (8,000 feet).

Supported Stope Methods

Cut-and-Fill Stoping

Cut-and-fill stoping is a selective underground mining method that involves extracting in successive horizontal slices, typically 3-5 meters high, followed by backfilling the excavated void to provide a stable working platform for subsequent slices. The process begins at the bottom of the stope and progresses upward (overhand) or, less commonly, downward (underhand), with each slice drilled, blasted, and mucked out using short-hole equipment before the void is filled with waste material. This sequential approach allows for precise extraction in areas where geometry is irregular or the surrounding rock is moderately stable, minimizing exposure to unsupported ground. Backfill materials commonly used include hydraulic fills, consisting of a sand-water with 60-70% solids by weight, and paste fills made from cemented for enhanced strength. The backfill is compacted to achieve 80-90% to ensure , with hydraulic fills often requiring drainage systems to solidify within hours or days for safe access. Paste backfill, which incorporates additives, provides higher and is preferred in deeper operations to resist ground pressures. The method offers advantages such as minimal dilution, typically less than 5-15%, due to its high selectivity, and versatility for extracting in steeply dipping (35°-90°) or irregular shapes up to 40 meters wide. However, it incurs high backfill costs, representing 10-20% of total expenses, and slower advance rates of approximately 0.5-1 meter per day, making it labor-intensive compared to bulk methods. Cut-and-fill stoping is particularly applied in polymetallic deposits with steep dips, such as those at Kidd Mine in Canada, where overhand and underhand variants with paste backfill enable recovery of copper-zinc ores in challenging ground conditions.

Square-Set Stoping

Square-set stoping is a labor-intensive, timber-supported mining method designed for extracting ore from highly irregular orebodies in weak or swelling ground conditions where other techniques may fail. The process begins with the excavation of small rectangular blocks of ore, typically in horizontal or inclined slices, followed by the immediate installation of interlocking timber sets to form a rigid grid-like framework that supports the surrounding rock and prevents caving. Each square set consists of vertical posts, horizontal caps, and girts framed into cubical or rectangular units, usually measuring about 1.8 m × 1.8 m × 2.4 m, placed contiguously to create a continuous support structure spanning the stope. As mining advances, the open spaces between sets are lagged with wooden planks on the walls and back to seal against loose material, and the sets are often filled with waste rock for enhanced stability in heavy ground. The primary materials employed are durable timbers, such as sawn logs formed into square sections for posts, caps, and girts, which are precisely cut and fitted without nails to ensure tight interlocking. In historical applications like those in Butte, Montana, untreated timbers were initially used but later treated with preservatives like creosote to combat rapid deterioration in hot, humid underground environments. Sets are typically spaced 1 to 2 meters apart, depending on ground conditions, forming a dense lattice that accommodates irregular ore shapes and dips greater than 45 degrees. This method contrasts with simpler timber supports like those in stull stoping by relying on permanent, fully framed structures rather than temporary props. One key advantage of square-set stoping is its adaptability to structurally weak and walls, enabling safe extraction in conditions prone to while achieving high recovery rates, often exceeding 85% in suitable deposits. It allows for selective of high-grade in complex geometries, minimizing dilution from surrounding . However, the method is highly labor-intensive, demanding skilled carpenters for the precise assembly of thousands of sets per stope, which drives up operational costs significantly, especially in deeper mines where timber demands escalate. Additionally, the extensive use of wood introduces substantial hazards, particularly in sulfide-rich orebodies where can ignite the timbers, leading to dangerous underground s. Ventilation challenges and the need for constant manual handling further limit productivity to low rates, typically 0.5 to 2.5 tons per worker per shift. Historically, square-set stoping peaked in the early , most notably in the copper mines of , where it supported vast underground networks amid unstable , with stopes extending hundreds of feet and consuming enormous quantities of timber. By the mid-1900s, the method began to decline sharply due to the rise of mechanized alternatives like cut-and-fill stoping, which offered better efficiency and reduced reliance on manual timbering amid increasing labor costs and timber scarcity. Post-1950s, it was largely phased out in favor of more automated supported methods, though remnants persist in some high-value, low-mechanization contexts.

Stull Stoping

Stull stoping is a simple, timber-supported underground method designed for extracting from narrow to moderate-width, steeply dipping veins in relatively competent rock conditions. The process involves advancing the stope in overhand slices, where horizontal timber beams called stulls are installed at regular intervals to support the roof (back) and provide working platforms for . proceeds upward from a bottom drift or , with each slice typically 1.5 to 3 meters high, allowing broken to be drawn off below while maintaining stability through the timber framework. Timber design in stull stoping emphasizes practical, wedged installation for quick deployment. Stulls are round timbers, generally 10 to 25 cm in diameter, placed perpendicular to the walls and wedged tightly between the footwall and wall to span the stope width. They are installed every 2 to 3 vertically and horizontally as advances, often in a systematic to form a grid-like support; in wider spans exceeding 3 , vertical posts are added between stulls to enhance load distribution and prevent sagging. This setup also serves as platforms for miners using hand-held drills or jackhammers to bore overhead blast holes. The method's advantages include its economy for veins 2 to 5 meters wide, where minimal timber volume enables quick setup and low material costs, facilitating faster production rates than more elaborate systems. It is particularly suited to competent walls and footwalls that require only selective support, minimizing dilution and allowing selective recovery. However, stull stoping is restricted to ground conditions, as weaker rock demands fuller framing like square-set stoping; additionally, timber decay from moisture and ore contact necessitates periodic replacement, increasing maintenance demands over time. Historically, stull stoping found widespread application in vein during Colorado's silver boom in the late 1800s, where it supported extraction from narrow, pitching lodes in operations around Leadville and other districts, leveraging abundant local timber resources for efficient development.

Caving and Bulk Methods

Sublevel Caving

Sublevel caving is a mass mining technique employed for extracting large, vertically extensive orebodies with competent hanging wall rock that caves naturally as ore is removed. The method relies on flow for both ore extraction and waste rock caving, making it suitable for steeply dipping deposits greater than 15 meters in thickness. Development begins with the creation of haulage and ventilation levels at the base, followed by the establishment of multiple sublevels spaced 15 to 30 meters vertically apart to facilitate systematic top-down . The process involves driving parallel production drifts on each sublevel, typically spaced 20 to 30 meters apart, connected by crosscuts for access. From these drifts, longhole —often in fan or ring patterns with hole lengths up to 30 meters—is performed using mechanized rigs, followed by charging and blasting to fragment the . Blasting induces controlled of the overlying rock, creating a cavity that propagates upward. At the lowest sublevel, drawpoints are established for selective extraction of the broken , which flows under gravity into loading areas. Load-haul-dump (LHD) machines collect the muck from drawpoints and transfer it to ore passes or muckbays for to the main level below. Extraction proceeds sequentially from the top sublevel downward, with each level mined out before advancing to the next to ensure stable propagation. Draw control is critical to manage flow and minimize dilution, involving regulated loading rates at drawpoints to prevent excessive waste rock ingress from the caved . This top-down approach allows for ongoing monitoring of cave development and adjustment of blasting patterns to maintain fragmentation suitable for LHD handling, typically achieving ring blasts of 7,000 to 12,000 tonnes per ring. The method offers significant advantages, including low development requirements compared to supported stoping, high potential, and substantial rates exceeding 10,000 tonnes per day in large operations. Operating costs are relatively low for underground methods, driven by efficient use of and minimal ground support needs in the drifts. However, challenges include dilution levels of 20 to 30 percent due to waste rock mixing during draw, limiting recovery to 85 to 90 percent, and the irreversible nature of , which precludes re-entry into mined areas and requires precise geotechnical assessment to avoid uncontrolled . Sublevel caving has been successfully applied to massive deposits, notably at the in operated by since the 1950s, where it supported annual production of approximately 24 million tonnes through large-scale implementation with sublevel heights up to 28 meters as of the early 2020s. The technique's adaptability to deep, tabular orebodies has also made it prominent in and other mines worldwide, though ongoing assessments at sites like are evaluating adaptations for increasing depths and rock stresses as of 2023. emphasizing its role in resource-efficient extraction under favorable geological conditions.

Block Caving

Block caving is a bulk underground mining method that utilizes to extract large volumes of low-grade from massive deposits, typically those exceeding 100 million tonnes in reserves. The process begins with the creation of an undercut level beneath the entire ore block, which is usually 100 to 300 meters high, to induce natural fracturing and downward movement of the ore mass without the need for primary blasting in the production phase. Once the undercut is established—often through sequential blasting of narrow stopes to propagate the —draw columns are installed above extraction points to channel the caved ore. The ore then flows under to drawpoints on the production level, where it is mucked and transported to crushers using load-haul-dump (LHD) machines. This gravity-driven flow continues as the cave propagates upward, potentially reaching the surface over time. The layout of a block caving operation is designed for high-volume extraction and stability, featuring a grid of drawpoints typically on 15 to 20 meter centers, with pillars of similar dimensions left between them to support the extraction level during initial . In modern implementations, automated LHDs are employed at drawpoints to enhance and efficiency in handling fragmented , often in drifts oriented at right angles to the drawpoints for optimal flow control. Surface resulting from the caving process is monitored using (InSAR) technology, which provides wide-area deformation mapping to predict and manage ground movement. Block caving offers significant economic advantages, with operating costs as low as $3 to $5 per due to minimal requirements after undercutting, making it ideal for large-scale, low-grade deposits. However, it is capital-intensive and prone to challenges such as 20 to 40 percent dilution from waste rock intermixing, which can reduce grade, and lead times of 2 to 3 years for undercutting and setup before production commences. A prominent application is at the El Teniente mine in , where block caving has been employed since the mid-20th century and expanded in the through the New Mine Level project to access deeper reserves, with a designed capacity exceeding 130,000 tonnes of per day, though a in October 2025 has temporarily disrupted operations and reduced output.

Top-Slice Stoping

Top-slice stoping, also known as top slicing, is a horizontal caving method employed in underground mining for extracting flat-lying or near-horizontal orebodies, particularly massive, thick-bedded deposits greater than 15 feet (approximately 4.5 meters) wide with weak ore and competent walls. The process begins at the top of the orebody, where horizontal or near-horizontal slices are mined in a retreating panel configuration, typically 10 to 20 feet (3 to 6 meters) thick, advancing systematically across the stope area. Ore is extracted using timber supports to create working rooms, and as each slice is completed, the supports are removed or blasted to induce controlled roof collapse, allowing the overburden to cave and fill the void behind the mining face. To facilitate ore handling, chutes are installed beneath the working level for gravity-fed transport to haulage points below, minimizing manual labor and enabling efficient drawdown. A key feature is the use of a timbered mat or platform—constructed from broken timbers, lagging, and waste—to separate the active slice from the caving overburden, preventing premature dilution while maintaining a safe working space under the mat as mining progresses downward slice by slice. This method is well-suited to flat-dipping orebodies where vertical caving methods like may be less applicable due to the deposit . Slices are developed in panels that retreat toward a central access drift, with the caved material providing natural backfill and support for overlying strata. The technique requires initial development of levels, raises for ventilation and , and timber bulkheads to control caving, ensuring the process remains systematic and contained. Applicable at depths from 150 to 2,500 feet, top-slice stoping relies on a friable capping that caves readily to form a tight fill, avoiding large wedging blocks that could disrupt operations. One primary advantage of top-slice stoping is its high recovery, approaching 95 to 100 percent in suitable conditions, as the sequential downward progression allows nearly complete extraction while handling irregular hanging wall and footwall contacts effectively. It provides robust timber support at the working face, making it ideal for soft or weak that cannot stand unsupported, and it minimizes waste dilution through the protective , typically keeping unplanned loss low. The method also facilitates pillar recovery in previously mined areas and is amenable to mechanization in accessible panels, promoting productivity in thick deposits. However, it demands abundant and inexpensive timber for extensive framing and matting, which can elevate costs if materials are scarce; it is non-selective, precluding sorting at the face; and the induced often leads to surface , limiting its use in areas sensitive to ground disturbance. Ventilation and dust control present ongoing challenges due to the confined spaces and falling rock during , with moderate dilution around 15 percent possible from wall sloughing despite the mat. Historically, top-slice gained prominence in the early for wide, soft deposits with unconsolidated , as detailed in U.S. Bureau of Mines publications from . It was applied in operations at depths of 150 to 2,500 feet, including soft hydrated deposits under friable caps, demonstrating its viability for massive orebodies during periods of intensive underground extraction in the United States.

Specialized Methods

Shrinkage Stoping

Shrinkage stoping is a semi-supported underground method designed for extracting from competent, steeply dipping narrow veins, typically with dips exceeding 55 degrees and widths of 1.2 to 4.5 meters. The process begins at the bottom of the stope and advances upward in horizontal slices, where blasting breaks the , and only a partial amount—approximately 30%—is mucked out to create working space while the remainder stays in place to form a self-supporting platform. This broken experiences a volume swell of 25-40% due to fragmentation, providing a stable floor for subsequent operations and temporary wall support until the stope reaches full height, at which point the accumulated is fully drawn down from the base. Blasting employs overhand ring drilling techniques, with rings of short holes (2-3 meters long) drilled into the face and fired to advance the slice by 2-3 meters per cycle, allowing miners to work safely above the swollen pile. This incremental approach ensures controlled fragmentation and minimizes immediate , though it relies heavily on the 's competence to prevent premature collapse during the buildup phase. The method offers several advantages, including operational simplicity, low dilution rates around 10%, and suitability for steep, irregular deposits where selective recovery is essential, achieving recoveries of 75-95% with minimal ground support needs. However, it presents notable disadvantages, such as hazardous conditions from working atop potentially shifting muck piles, variability in swell factors that can compromise the platform's evenness, and overall leading to slower production rates. Unlike cut-and-fill stoping, which uses artificial backfill for support, shrinkage stoping depends on the 's natural properties, limiting its use to stable ground. Shrinkage stoping found widespread application in narrow vein operations in prior to the 1990s, particularly at sites like Agnes Gold Mine, where it enabled high-grade extraction from shale reefs at depths up to 650 meters before economic pressures from rising costs, labor , and trends prompted a shift to bulk methods.

Longhole Stoping

Longhole stoping is a mechanized underground mining method designed for extracting from competent, steeply dipping orebodies, involving the of long parallel or fan-patterned holes followed by large-scale blasting to create expansive open stopes. The process begins with the creation of an initial void, or slot, typically 2-4 meters wide, using raise boring techniques to provide a free face for subsequent blasts. Production then employs specialized rigs to bore holes 20-60 meters in length, often in a fan configuration, with diameters ranging from 50-110 to target the orebody precisely. These holes are charged with bulk explosives such as and detonated to blast the entire stope volume, which can reach up to 10,000 cubic meters, allowing broken to drop for remote mucking without personnel entry during blasting. This method's efficiency stems from its reliance on automated for , enabling high productivity rates of 500-1000 tonnes per day per stope while maintaining relatively low unit costs through bulk extraction and minimal development requirements. However, it carries risks associated with large blasts, including ground vibrations that can propagate significant peak particle velocities exceeding 600 mm/s, potentially damaging adjacent workings or infrastructure. dilution typically ranges from 10-15%, arising from overbreak in the hanging wall, drill hole deviation, and blast-induced fracturing, which necessitates careful stope design and backfill to mitigate. Since the 1980s, longhole stoping has been widely adopted in nickel mining operations in the , , where it supports high-volume production from deep, competent sulfide orebodies with sublevel arrangements and paste backfill for stability.

Applications and Considerations

Economic Factors

The economic viability of stoping methods in underground mining hinges on cost structures that vary significantly by technique, deposit , and operational scale. Typical operating costs for stoping range from $10 to $50 per of extracted, with lower-end figures associated with high-volume methods and higher costs for selective, labor-intensive approaches like cut-and-fill stoping. For instance, longhole stoping can achieve costs as low as $4.37–$18.55 per at capacities of 187–3,056 per day, while shrinkage stoping ranges from $12.40–$28.98 per at 33–1,320 per day. Cost breakdowns reveal labor as the dominant component, often accounting for 50–65% of total expenses due to activities like , blasting, and mucking; ground support contributes 3–17%, reflecting the need for in supported methods; and supplies (including explosives and consumables) make up the remainder, with typically comprising 20–30% of direct costs in mechanized operations. Mucking and add approximately 15% in methods requiring manual or semi-mechanized extraction, though this drops in caving techniques where natural collapse reduces handling needs. These proportions underscore why caving methods like block caving offer the lowest overall costs per , approaching open-pit levels, while supported stoping incurs higher expenses from ongoing support and dilution control. Note that these figures are based on 1991 USD; as of 2025, and increased may reduce labor shares to 40-60% in modern operations. Productivity metrics further influence , with advance rates and output determining mine life and capital recovery. Supported methods like square-set or cut-and-fill stoping exhibit slower advance rates, typically 5-10 meters per month per face, limited by installation of timber or backfill, yielding 3–38 tonnes per manshift. In contrast, methods such as sublevel or block achieve rates exceeding 10 meters per month and up to 90 tonnes per manshift, enabling higher throughput (e.g., 622–6,000 tonnes per day in shrinkage or blasthole stoping) and positively impacting (NPV) through accelerated cash flows in mine planning. These differences can significantly alter project NPV depending on production scheduling. Selection criteria for stoping methods prioritize ore value, depth, and grade to optimize returns. High value supports selective methods like shrinkage or longhole stoping for low-dilution recovery, particularly when grades exceed 5%, allowing premium pricing to offset higher unit costs. Depth plays a critical role, with deposits below 1,000 meters favoring methods like block or sublevel to manage stress and reduce support needs, while shallower orebodies suit supported techniques. Overall, method choice balances these factors against and geometry to ensure economic extraction. Economic modeling in stoping relies on (NPV) calculations to evaluate long-term profitability, using the formula: NPV=∑t=1nCash Flowt(1+r)t−Initial Capex\text{NPV} = \sum_{t=1}^{n} \frac{\text{Cash Flow}_t}{(1 + r)^t} - \text{Initial Capex} where rr is the discount rate (typically 8–12% in projects to account for and ), tt is time in years, and cash flows incorporate revenues minus operating and capital expenses. This approach guides mine planning by quantifying how method-specific productivities and costs affect overall project value, with often yielding higher NPVs in deep, massive deposits due to scale efficiencies.

Safety and Environmental Impacts

Stoping operations in underground mining present significant safety hazards, primarily from roof falls and exposure to blasting fumes. Roof falls account for approximately 18-40% of fatal incidents and injuries in underground mines, depending on the and method, often due to unstable rock masses in stoped areas. Blasting fumes, including toxic gases like nitrogen oxides and , pose immediate respiratory risks during and after explosive operations. Mitigation strategies include adherence to (MSHA) standards under 30 CFR Part 57, which mandate proper blasting procedures, ventilation to dilute fumes, and post-blast inspections to ensure clearance before re-entry. Additionally, proximity detection systems, required by MSHA for mobile equipment in underground coal mines since the 2015 final rule but increasingly adopted in metal/ stoping post-2010, use electromagnetic fields or to alert workers and halt machinery, reducing collision risks in confined stopes. Health risks in stoping are exacerbated by prolonged exposure to respirable silica dust generated during , blasting, and mucking, which can lead to , a progressive causing and impaired breathing. Silicosis affects miners through of crystalline silica particles smaller than 5 micrometers, with cumulative exposure increasing incidence rates in operations. Ventilation requirements under MSHA regulations stipulate minimum airflow volumes, such as 2000 cubic feet per minute (approximately 0.94 cubic meters per second) at working faces for certain operations, to dilute dust concentrations below permissible exposure limits of 50 micrograms per cubic meter over an 8-hour shift, though effective face velocities typically range from 0.5 to 2 meters per second for . , like powered air-purifying respirators, and wet suppression methods further reduce risks. Environmental impacts of stoping, particularly in caving variants like block and sublevel caving, include surface and water ingress into mined voids. can reach depths of up to 10 meters or more in large-scale block caving operations, leading to ground deformation, damage, and disruption over areas spanning hundreds of hectares. Water ingress occurs as aquifers drain into open stopes, potentially contaminating with and lowering local tables. Reclamation efforts employ backfill materials, such as cemented or waste rock, to stabilize voids and minimize , restoring ground integrity and preventing long-term ecological degradation. Modern regulations address these issues through international and national frameworks. The International Labour Organization's Convention 176 on Safety and Health in Mines, adopted in 1995 and effective from 1998, requires member states to implement policies for hazard identification, worker training, and emergency preparedness in all mining types, including stoping. Following high-profile tailings failures, such as the 2015 Mariana collapse (43.7 million cubic meters of , 19 deaths) and the 2019 Brumadinho disaster (12 million cubic meters, 270 deaths) in , global standards for tailings management have tightened, emphasizing dam stability assessments, real-time monitoring, and phased decommissioning to prevent similar environmental catastrophes in stoping-related waste storage.

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