2.1 Underground Mining Method Classification & Selection
Key Takeaways
- Underground mining methods are categorized based on ground support requirements into unsupported (room-and-pillar, sublevel stoping, shrinkage stoping), supported (cut-and-fill, square-set, stope-and-pillar), and caving methods (block caving, sublevel caving, longwall).
- The Nicholas Method Selection Methodology quantitatively ranks mining method suitability by evaluating deposit geometry (shape, thickness, dip, depth) and rock mass quality parameters (RMR/RQD) for the orebody, hanging wall, and footwall.
- Block or panel caving can achieve low unit cost and high production in sufficiently large deposits whose stress, fragmentation, draw, subsidence, and infrastructure conditions support controlled caving; Padcal is an important Philippine caving example.
- Cut-and-fill and square-set methods can provide strong selectivity and ground control in narrow, irregular, high-value deposits, but performance depends on support, fill, sequencing, dilution, access, and local rock behavior.
- Sublevel open stoping generally requires competent ore and walls, suitable geometry, drilling access, and controlled spans; Philippine applications must be verified from the mine's actual design and operating period.
2.1 Underground Mining Method Classification & Selection
Quick Answer: Underground mining methods are systematically classified into unsupported, supported, and caving categories based on the structural self-supporting capacity of the rock mass. Selection of the optimal mining method depends on deposit geometry (shape, dip, thickness, depth) and geotechnical characterization evaluated through quantitative systems such as the Nicholas Method Selection Approach. In the Philippines, underground mining spans a spectrum from massive low-cost block caving at Philex Padcal to highly selective cut-and-fill stoping at Benguet Corporation's narrow gold veins.
Underground Mining Method Classification
Underground mining techniques are designed to extract mineral deposits beneath the Earth's surface where stripping ratios or environmental constraints render surface mining unviable. The fundamental classification framework, established by Peele and refined by Hartman and Mutmansky, categorizes underground methods according to the degree of structural support required to keep stopes stable during extraction:
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Unsupported Methods: Excavations rely primarily on the natural strength of the orebody and surrounding host rock to maintain stability without artificial support or backfill during active mining. Pillars of unmined ore may be left temporarily or permanently.
- Room-and-Pillar Mining: Used in flat-lying or low-dip (<20°) tabular deposits. Openings (rooms) are driven in a regular grid, leaving pillars of ore to support the overburden. Common in coal, salt, potash, and bedded metallic deposits.
- Sublevel Stoping (Open Stoping): Applied in steep (>45° to 50°), tabular or vein orebodies with strong ore and strong wall rocks. Sublevel drifts are driven within the orebody, and vertical rings of blast holes are drilled using longhole rigs to break ore into an open stope cavity.
- Shrinkage Stoping: Used in steeply dipping, narrow vein deposits with competent walls. Broken ore is accumulated inside the stope as a working floor for miners drilling overhead; only ~35% of broken ore is drawn during mining to accommodate swell, with the remainder pulled upon complete stope extraction.
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Supported Methods: Require systematic artificial support (such as backfill, timber framing, or structural steel) to maintain stope wall integrity and prevent catastrophic convergence or surface subsidence.
- Cut-and-Fill Stoping: Ore is extracted in horizontal slices from the bottom upward. After each slice is mined and mucked, backfill (waste rock, hydraulic tailings, or paste) is introduced to provide a working platform for the next lift and stabilize stope walls.
- Square-Set Stoping: Extremely high-cost, highly selective historic method where timber frames (sets) are erected to support very weak, erratic, high-grade ore and wall rocks.
- Stope-and-Pillar Mining: A variation of room-and-pillar for thicker, irregular tabular deposits where pillars are arranged irregularly in low-grade zones to support stope spans.
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Caving Methods: Controlled collapse of the orebody or overlying wall rock is purposefully induced to break and transport rock mass via gravity.
- Block Caving: Applied to massive, low-grade, steeply dipping or blocky orebodies with weak-to-moderate rock mass ratings. An undercut horizon is excavated beneath the block, triggering continuous gravity-driven self-caving of the overlying orebody.
- Sublevel Caving: Mining progresses from top to bottom in sublevels. Ore is blasted in rings on each sublevel, and the overlying waste hanging wall caves into the void, following the drawn ore down.
- Longwall Mining: Continuous extraction system for thin, flat-lying seams using automated shearers and hydraulic powered self-advancing roof supports, allowing the immediate roof (gob) to cave behind the supports.
Quantitative Mining Method Selection: The Nicholas Approach
Selecting an underground mining method is an engineering decision balancing safety, geotechnical feasibility, ore recovery, dilution, and unit production cost. Nicholas (1981) developed a standardized quantitative ranking system that assigns numerical scores to candidate mining methods based on physical deposit geometry and rock mass rating (RMR) attributes.
1. Deposit Geometry Criteria
- Deposit Shape: Tabular, massive, or vein/narrow-vein.
- Deposit Thickness: Narrow (<3 m), intermediate (3–10 m), thick (10–100 m), or massive (>100 m).
- Deposit Dip: Flat (<20°), intermediate (20°–55°), or steep (>55°).
- Depth below Surface: Influences in-situ stress magnitudes ($ \sigma_v = \gamma z $).
2. Geotechnical & Rock Mass Quality Criteria
Nicholas assigns numerical weights to three key geotechnical domains:
- Ore Zone Rock Mass: Measured via intact rock Uniaxial Compressive Strength (UCS), Rock Quality Designation (RQD), joint spacing, and shear strength.
- Hanging Wall (HW) Rock Mass: Determines stope span stability and caving behavior.
- Footwall (FW) Rock Mass: Crucial for drift stability, drawpoint integrity, and infrastructure development.
Each candidate mining method accumulates a total score. Preferred methods achieve positive rankings (+3 to +4), whereas structurally unsuitable methods receive negative scores (-49), ruling them out immediately.
Comparative Matrix of Underground Mining Methods
| Mining Method | Deposit Dip & Shape | Ore Strength | Wall Strength | Production Capacity | Relative Cost | Ore Recovery % | Dilution % |
|---|---|---|---|---|---|---|---|
| Room-and-Pillar | Flat (<20°), Tabular | Moderate–Strong | Moderate–Strong | High | Low–Moderate | 60%–80% | 10%–15% |
| Sublevel Stoping | Steep (>45°), Tabular/Vein | Strong | Strong | High | Low–Moderate | 75%–90% | 10%–20% |
| Shrinkage Stoping | Steep (>50°), Narrow Vein | Strong | Strong | Low | Moderate | 75%–85% | 15%–25% |
| Cut-and-Fill Stoping | Steep (>45°), Any Shape | Any Strength | Weak–Moderate | Moderate | High | 90%–100% | 5%–10% |
| Square-Set Stoping | Any Dip, Irregular | Very Weak | Very Weak | Very Low | Extremely High | 90%–100% | 5%–10% |
| Block Caving | Massive (>100 m), Steep | Weak–Moderate | Weak (HW Caves) | Extremely High | Extremely Low | 75%–85% | 15%–25% |
| Sublevel Caving | Steep (>60°), Massive/Vein | Moderate | Weak (HW Caves) | High | Moderate | 70%–85% | 15%–30% |
| Longwall Mining | Flat (<12°), Thin Tabular | Moderate | Weak (Gob Caves) | Extremely High | Low | 85%–95% | 5%–10% |
The comparative performance ranges above are broad teaching illustrations, not guaranteed recovery, dilution, capacity, or cost. Method selection requires local geometry, geotechnical evidence, design, schedule, and economics.
Underground Mining Applications in the Philippines
Philippine underground mining history reflects diverse geological environments, from massive island-arc porphyry copper deposits to epithermal narrow-vein precious metal systems.
1. Block Caving: Philex Mining Corporation (Padcal Mine, Benguet)
Philex's Padcal Mine in Tuba, Benguet is the premier Philippine example of block caving. Operating in a massive porphyry copper-gold deposit, Padcal has used gravity-driven block caving to achieve the high tonnage and low unit-cost characteristics associated with the method:
- Undercut & Extraction Levels: Undercut drifts are driven to fracture the base of the ore block. Drawbells funnel collapsed ore into extraction drifts.
- Haulage & Secondary Breaking: Load-Haul-Dump (LHD) units collect ore from drawpoints and transfer it through finger raises to underground grizzly levels for secondary breaking before rail/conveyor transport to the surface mill.
2. Cut-and-Fill & Square-Set Stoping: Benguet Corporation (Acupan/Balatoc Operations)
The historic Acupan Mine in Itogon, Benguet extracted high-grade epithermal gold quartz veins hosted in complex diorite-dacite breccias. Because the narrow veins are steeply dipping but structurally erratic with weak wall rock:
- Cut-and-Fill principle: Selective horizontal lifts and engineered backfill can control dilution and support irregular vein ground. Actual lift height, fill material, exposure time, sequence, and support must come from the mine plan and geotechnical design, not from one historical generic value.
- High selectivity minimizes waste dilution in high-grade quartz-calcite-gold structures.
3. Sublevel Stoping: Lepanto Consolidated Mining Co. (Victoria & Enargite Mines)
The Mankayan district contains structurally controlled copper-gold mineralization developed through changing underground mine phases. Method claims must be tied to the orebody, period, and operator source:
- Sublevel open-stoping principle: Where ore and walls are competent and geometry is suitable, longhole drilling between engineered sublevels can provide productive stoping. Interval, span, blasting, dilution, support, and sequencing are site-specific.
Which underground mining method relies on undercutting and gravity-driven cave propagation in a large orebody whose cavability, fragmentation, draw behavior, stress regime, and infrastructure support the method?
According to the Nicholas Method Selection approach, which geotechnical combination makes an orebody MOST suitable for Sublevel Stoping?
Among the choices, which method is generally the best fit for selective extraction of a steep, irregular, high-value vein where engineered backfill is needed to limit exposure and support the walls?