Underground Mining Methods

Two Main Methods
- Room and Pillar

• Mostly with continuous miners

- Longwall – Develop longwall panels with room and pillar using continuous miners

- About 10% of underground production

• still comes from drilling and blasting

- Total underground output = 421mt (1997 data)

FIRST, MUST ACCESS THE MINE

- Drift (Adit)

• Seam outcrops, access from ground level

- Slope

• Drive incline in rock at up to 16 degrees
• Allows belt haulage

- Shaft

• Use: elevators/skips, for: people/coal
• Use shaft if >1500 feet, economics dictate

LIKE A CITY, OR LARGE BUILDING, SERVICES MUST BE PROVIDED

• Transport people (rail, rubber tired) - Transport supplies (materials / maintenance)
• Transport product (coal)
• Support roof
• Provide electrical power
• Provide fresh air (& suppress dust)
• Provide fresh water
• Get rid of waste water
• Dispose of trash

ROOM And PILLAR

• Mine “streets & avenues” (entries and crosscuts)
• Leave pillars to support roof (may mine later)

– Designed by formula

• Plan view-looks like city with “greenbelts”

– “Greenbelts” are large barrier pillars left to separate work areas

• Use continuous miner

MINE PLAN

• Main entries (7-9 openings)
• Submains (5-7 openings)
• Panels (panel entries, butt entries)
• Rooms (at times)
• Openings limited to 20-ft width

– Openings serve as air ducts and travelways – Return air is isolated from fresh air, two escapeways must be provided from face

• Longwall panels are solid coal blocks, usually 1000 ft by 10,000 ft, accessed by “gate” roads

ALL SERVICES EXIST TO SUPPORT MINING AT FACE
Continuous miner

- rips coal, using tungsten carbide bits

- miner mines at 4-25 t/m and conveys coal into shuttle cars

• Shuttle cars are electric (cable) “trucks” which haul for up to 600 feet or so (usual = 300-400 feet)

– Haul to feeder-breaker which acts as surge bin/crusher and feed coal onto belt

– Hold 3-25 tons/load, depending on seam thicknesss and amount of rock mined

FEEDER-BREAKER FEEDS COAL ONTO BELT CONVEYORS

• Conveyors transport coal to surface or into skips for shaft access

– Usual sizes - 42” to 72”

– Speeds - 500 - 800 fpm

• Longwall requires largest conveyors

– 54”-60” usual from face

ROOF BOLTS INSTALLED BY ROOF BOLTING MACHINE

• Roof supported by inserting reinforcing rods
• No one may work under unsupported roof – Cut depths limited to position of shuttle car operator (35’ to 40’ with remote control miner)
• When miner place changes, bolter moves in

– Bolt 3-6 min/row or 0.75-1.50 min/ft

– Use two bolter operators, twin-boom bolter

• A few operations attach bolters to miners, bolt as they advance

ROOF SUPPORT

• Insert bolts into the roof on regular pattern (3’-8’ length, usually)

– 4’ x 4’ or 5’ x 5’ most common

• Either “glue” (resin) a re-bar bolt in, or
• Use expansion bolt anchors or � Glue in the anchor only

– Anchors allow pre-tensioning of bolts

ROOF BOLTS GENERALLY WORK WELL

• Form “reinforced” rock, strong beam
• Or, may “hang” weak rock from stronger overlying rock layer
• Roof fall fatalities are now at 8 -12 per year

– Half are in violation of the law, under non-bolted roof

– Roof fall fatalities exceeded 100 per year around 1970

VENTILATION

• Provides oxygen, dilutes methane & dust

– Methane explosive when at 5-15% concentration Most coninuous miners have dust scrubber

– Draw air into ducts at front of miner

– Efficiency up to 96-97%

• Air directed to working face with brattice cloth (plastic curtains)
• Alternatively, hang tubing & use fan to draw air to face
• Fresh air ventilates one face only, then it is “return” air

– Separate air streams with concrete block walls or “stoppings”

• Maximum allowable methane content is 1%
• Control major flow with adjustable doors in airways (“regulators”)
• 150 - 400 ft/shift usual, tonnage depends on seam thickness

– 500 - 2000 tons/shift (usual)

• New miners load at 10 - 25 tpm
• Most continuous miners load only 60-120 min/shift

– Load only 12

– 10-25% of shift time

LONGWALL

• More nearly continuous method
• Analogous to “deli meat slicer” (shearer)
• Shearer mounted on chain conveyor
• – Coal cut falls onto conveyor
• Width of face usually 850 - 1100 ft
• – Depth of slice is 30 - 42 inches
• Behind face supported for 20’ or so by steel supports
• - each 1.50 or 1.75 m wide
• – Each support holds up to 600-1200 tons
• Supports connected to conveyor – By pushing, lowering & pulling
• - can walk conveyor and selves forward
• Panels (solid block of coal)
• – Usually 850’ - 1100’ wide & 7500’ - 15,000’ long
• – Contain 1.5 - 4 mm tons per panel
• Shearers cut at 35 - 65 t/min (2000-4000 tph)
• Output per year = 2 - 6 mm tons
• 6,000 - 20,000 t/day (max = 40,000)
• Cut 200-500 min/day
• – 20% - 45% of time (???)
• Capital intensive
• – $30M for face equipment only
• – $50-80M additional for mine / processing
• Require large, regularly shaped reserve
• – 50M ton minimum
• – Prefer 100-200M tons
• Mine-specific design / limited ability to move to other reserves

CONTINUOUS MINER SUMMARY

• Capital for section is $3-5 million
• Flexible, can move readily to other reserves
• One longwall usually requires three continuous miners for development
• Annual output for miner section is 0.3 - 0.8 million tpy

ENVIRONMENTAL

Longwall strata caves behind supports

– Surface subsides to maximum of 50-70% of seam thickness

– “Tilt” area may damage structures, so must provide special support methods at the structures to minimize damage

– Subsidence trails face position by a few days to a week or two, about 95% occurs in a few weeks

LONGWALL SUBSIDENCE

• Ground water flow is altered
• Some wells lose flow, temporarily or permanently; a few gain
• May need to drill wells deeper
• Connection from near surface to mine is possible if depth to aquifer is less than 40 x seam thickness (240 ft for 6-ft seam)

SUMMARY

Longwall (45% of UG output from only 60 faces -- average of 3 million tpy each)

– High output, high capital

– Low operating cost, 70-80% (?) reserve recovery

– Low flexibility

Continuous Miners

– Medium output, low-medium capital

– Moderate operating cost, 40-60% reserve recovery

– High flexibility

• Can use underground methods in +100 ft of overburden (actual minimum depth depends on whether strip ratio favors surface mining)
• – Roof subject to surface cracks when shallower
• Use longwall in large, thick (mine 6-ft min.), regularly-shaped reserves
• – Only economic method if seam is >1500 ft deep
• Else, use continuous miner and room & pillar
• While best walls far exceed cm productivity, on average, tons per manhour are close