GATE Mining Engineering Mining Methods and Machinery Syllabus

What is highwall mining and when is it used?

Highwall mining recovers coal from the exposed highwall of a terminated opencast pit (or contour mine) by driving a series of parallel, unmanned entries into the seam using a remotely operated continuous miner with an auger/conveyor train. It is used to recover coal economically beyond the final highwall where the stripping ratio becomes uneconomic for further opencasting, without going fully underground.

In highwall mining, what is the role of "web pillars" and what limits penetration depth?

Web pillars are the narrow coal pillars left between adjacent highwall entries to support the overlying strata. Penetration depth into the seam is limited by the cutting/conveyor module reach, steering accuracy (guidance), ventilation/inertization needs, and the strength of the web pillars to prevent collapse; typical penetration is several hundred metres.

What are the two principal methods of underground coal mining?

(1) Bord and pillar (room and pillar) method — develops a network of roadways leaving pillars, which may later be extracted (depillaring); and (2) Longwall method — extracts a long continuous face in a single pass while the goaf caves behind a line of powered supports.

Describe the two phases of the bord and pillar method.

Phase 1 — Development: driving a grid of galleries (bords) and cross-cuts to form coal pillars, extracting about 25–40% of coal. Phase 2 — Depillaring (pillar extraction): systematically extracting the pillars on retreat, allowing the roof to cave into the goaf, recovering most of the remaining coal.

What is the difference between "bord" and "pillar" in the bord and pillar system?

A bord (board/room/gallery) is the excavated roadway or working place from which coal is extracted. A pillar is the block of solid coal left in situ between bords and cross-cuts to support the overlying strata during development.

State the formula for percentage extraction during bord and pillar development with square pillars.

For a square pillar of side $w$ (centre-to-centre spacing $w+B$, where $B$ = bord width): $$\text{Extraction} = \frac{(w+B)^{2} - w^{2}}{(w+B)^{2}} \times 100\%$$ Equivalently, recovery during development is the gallery area divided by the total panel area.

How is pillar strength estimated using the Obert–Duvall (squat pillar) formula?

$$\sigma_{p} = \sigma_{1}\left(0.778 + 0.222\,\frac{w}{h}\right)$$ where $\sigma_{p}$ = pillar strength, $\sigma_{1}$ = strength of a cubical specimen, $w$ = pillar width, and $h$ = pillar height. Strength increases with the width-to-height ($w/h$) ratio.

How is the average vertical stress on a pillar found using the tributary area theory?

$$\sigma_{p} = \gamma H \left(\frac{w+B}{w}\right)^{2}$$ (for square pillars), where $\gamma$ = overburden unit weight, $H$ = depth, $w$ = pillar width, and $B$ = bord width. The pillar carries the load of its own area plus the surrounding opening (tributary area).

Define the "factor of safety" for a coal pillar.

$$FoS = \frac{\text{Pillar strength}\;\sigma_{p}}{\text{Pillar stress}\;\sigma_{v}}$$ It is the ratio of the pillar's load-bearing strength to the actual induced vertical stress. For stable bord and pillar design (e.g. CMRI guidelines), an FoS of about 1.5–2.0 or more is typically required, depending on depth and whether the pillar is permanent.

How does "room and pillar" mining of metals differ from coal bord-and-pillar?

Both leave pillars to support the roof, but metal room-and-pillar is applied to flat-to-moderately-dipping tabular orebodies (often in stronger rock), pillars may be irregular and sometimes left permanently (unrecovered), openings are larger, and drilling-and-blasting with LHDs is common. Coal bord-and-pillar uses continuous miners/shuttle cars and usually targets later pillar extraction (depillaring).

What are the principal advantages of longwall mining over bord and pillar?

Higher percentage extraction (80–90%+), higher concentrated output and OMS, better roof control by powered supports with planned caving, improved safety, lower manpower, better gas/dust management, and suitability for greater depths. Disadvantages: high capital cost, inflexibility, geological-disturbance sensitivity, and rapid surface subsidence.

Differentiate advancing and retreating longwall layouts.

In advancing longwall, the face moves away from the main roadways and the gate roads are formed and maintained behind the face in the goaf. In retreating longwall, the gate roads are driven to the panel boundary first and the face then retreats back toward the mains, leaving solid coal ahead and goaf behind. Retreating is now preferred for better strata control and pre-drainage.

What are the key components of a modern mechanized longwall face?

(1) Shearer (or plough) for cutting coal; (2) Armoured Face Conveyor (AFC) carrying coal along the face; (3) Powered roof supports (chock/shield supports) that advance hydraulically and control caving; and (4) Beam Stage Loader (BSL) and crusher transferring coal from the AFC to the gate (panel) belt conveyor.

What is the function of a powered (shield) support on a longwall face and the term "setting" and "yield" load?

Powered supports hold up the immediate roof at the face and advance themselves, controlling roof convergence and inducing caving in the goaf. Setting load is the initial preset force applied by the hydraulic legs against the roof; yield load is the maximum force at which the support's relief valve releases to prevent damage under increasing roof load.

What is a "goaf" (gob) and what is periodic/main roof weighting?

The goaf (gob) is the void left behind a retreating/advancing longwall face after the supports are withdrawn and the roof caves. As the main roof overhangs and then fails periodically, the increased load transmitted to the face supports is called periodic weighting (the first major collapse is the main fall / first weighting).

List the main thick-seam mining methods used in underground coal.

(1) Multi-slicing longwall (descending or ascending slices, often with caving or stowing); (2) Longwall Top Coal Caving (LTCC / caving of top coal behind supports); (3) Sublevel caving adapted for coal; (4) Blasting Gallery (BG) method; (5) Inclined slicing; and (6) Hydraulic/sand stowing of slices for surface protection.

Describe Longwall Top Coal Caving (LTCC).

In LTCC, the lower part of a thick seam is cut conventionally by the shearer along the face, while the upper top-coal section is allowed to cave under induced/natural fracturing and is drawn through a rear (back) opening between the rear canopy and the rear AFC of specially designed supports. It allows extraction of seams 4.5–20 m thick with a single face.

What is the Blasting Gallery (BG) method?

The Blasting Gallery method is a thick-seam (bord-and-pillar based) technique where a gallery is driven in the lower section of the seam, ring/fan holes are drilled into the overlying coal and blasted to fragment it, and the broken coal is loaded by LHDs. It enables high recovery from thick, steep, or disturbed seams unsuitable for longwall.

What is the broad classification of underground metal (stoping) methods by ground support?

(1) Unsupported (naturally supported) methods — e.g., open stoping, sublevel stoping, room and pillar; (2) Supported methods — e.g., cut and fill, shrinkage, square set, stull stoping (use artificial support/backfill); (3) Caved methods — e.g., block caving, sublevel caving, longwall (roof allowed to cave).

What is a "stope" and what are the associated terms "sublevel," "drawpoint," and "raise"?

A stope is the underground excavation (void) formed by extracting ore. A sublevel is an intermediate horizontal level between main levels for drilling/access. A drawpoint is the opening through which broken ore is drawn from the stope. A raise is a vertical or steeply inclined opening driven upward connecting levels (used for ore passes, ventilation, access).

Describe open stoping (and sublevel open stoping) and where it is applied.

Open stoping extracts ore from large open voids supported only by remnant pillars; in sublevel open stoping, long blast holes are drilled from sublevels and the broken ore is drawn from the bottom. It is applied to steeply dipping, regular orebodies with strong ore and strong walls so the open stope remains stable. Gives high productivity and low cost.

Describe the shrinkage stoping method and the meaning of its name.

In shrinkage stoping, ore is mined upward in horizontal slices and only about 30–40% of the broken ore is drawn off after each blast; the remaining broken ore is retained in the stope to provide a working platform for miners and temporary wall support. The name comes from the fact that broken ore occupies more volume than in situ, so the swelled excess is "shrunk" by drawing it. After the stope is complete, the retained ore is fully drawn.

Describe cut and fill stoping and its main advantage.

Cut and fill removes ore in successive horizontal (or inclined) slices, and after each slice the void is backfilled (with sand, tailings, or cemented fill) which serves as the working floor for the next cut and supports the walls. Advantages: high selectivity (good for irregular, high-grade orebodies), excellent ground/wall support, high recovery, and surface subsidence control.

What is square-set stoping and when is it used?

Square-set stoping supports the excavation with a continuous interlocking framework (sets) of timber (or steel) members forming cubical cells as ore is removed. It is a supported method used for very weak ore and weak walls in high-grade, irregular deposits where no other method maintains stability; it is labour-intensive, expensive, and now rare.

Describe block caving and its key requirement.

Block caving undercuts a large block of ore so that the ore caves under its own weight and stresses, fragmenting progressively as it is drawn from drawpoints below; subsidence reaches surface. Key requirement: the ore must cave and fragment readily (suitable caveability and a large, weak/jointed orebody). It gives the lowest cost and highest productivity of underground methods but with low selectivity and high capital/development cost.

Describe sublevel caving.

In sublevel caving, ore is drilled and blasted from a series of sublevel drifts; the broken ore is drawn while the overlying waste (hanging wall/cap rock) caves to fill the void. Extraction proceeds top-down. It suits large, steeply dipping orebodies, is highly mechanized and productive, but suffers higher dilution and ore loss because waste mixes with ore at drawpoints.

Compare supported, unsupported, and caved stoping methods on selectivity, cost, and dilution.

Unsupported (open/sublevel stoping): low cost, high productivity, low selectivity, low-moderate dilution — needs strong rock. Supported (cut-and-fill, square set): high selectivity and recovery, high cost, low dilution — for weak rock/high-grade ore. Caved (block/sublevel caving): lowest cost, highest productivity, lowest selectivity, highest dilution — for large weak orebodies, causes surface subsidence.

What is stope mechanization, and which machine is central to it?

Stope mechanization is the use of mobile, often trackless, diesel/electric equipment to perform drilling, loading and hauling in stopes. The central machine is the LHD (Load-Haul-Dump) unit, a low-profile articulated loader that scoops ore at the drawpoint, hauls it, and dumps it into ore passes or trucks. Jumbo drills, longhole production drills, and low-profile trucks complete the trackless fleet.

What is an LHD and how is its productivity estimated?

An LHD (Load-Haul-Dump) is a rubber-tyred, low-profile loader for trackless stope haulage. Its production per hour is $$Q = \frac{3600\,C\,F\,E}{t_{cycle}}$$ where $C$ = bucket capacity, $F$ = fill factor, $E$ = efficiency, and the cycle time $t_{cycle} = t_{load} + t_{haul} + t_{dump} + t_{return}$. Haul/return times depend on the one-way distance and tram speeds.

What are the components of an underground ore handling system from stope to surface?

Drawpoint/grizzly → ore pass (gravity flow) → chute/loading station → level haulage (LHD, rail cars, or conveyor) → underground primary crusher → ore bin/measuring pocket → skip hoisting in the shaft (or conveyor/decline trucking) → surface bin/stockpile. Grizzlies and crushers control fragment size; ore passes and bins provide buffer storage.

What is the difference between mechanical, hydraulic, and pneumatic power transmission in mines?

Mechanical power is transmitted by rotating shafts, belts, ropes, and gears (direct, efficient, limited distance). Hydraulic power transmits energy using pressurized (near-incompressible) fluid (oil/water emulsion) through pipes/hoses — high force, smooth control (e.g., powered supports). Pneumatic power transmits energy using compressed air — flexible, intrinsically safe in gassy mines, but lower efficiency due to air's compressibility and losses.

State the basic relations for power transmitted by hydraulic and pneumatic (fluid) systems.

Hydraulic power $$P = p \cdot Q$$ where $p$ = fluid pressure and $Q$ = volumetric flow rate; and force from a cylinder is $F = p \cdot A$ (A = piston area). For a fluid the continuity relation is $Q = A_{1}v_{1} = A_{2}v_{2}$. Mechanical shaft power is $P = T\omega = 2\pi N T$ (T = torque, N = rev/s), and electrical input is $P = \frac{P_{output}}{\eta}$ accounting for transmission efficiency $\eta$.