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BORD AND PILLAR METHOD

APPLICABILITY OF BORD & PILLAR METHOD

• A seam thicker than 1.5 m.

• A seam free from stone or dirt bands. Stoneor dirt bands, if present in a seam, can beeasily disposed of for strip packing in long walladvancing method of mining.

• Seams at moderate depth.

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• Seams which are not gassy.

• Seams with strong roof and floor which canstand for long period after development stageis over.

• Coal of adequate crushing strength.

APPLICABILITY OF BORD & PILLAR METHOD

Classification of Bord and Pillar Mining System

• Develop the entire area into pillars and then extract thepillars starting from the boundary.

• Develop the area into panels and extract pillarssubsequently panel wise. This is called panel system ofmining.

• “Whole” followed by “broken” working in which themine is opened out by a few headings only andthereafter development and depillaring go onsimultaneously.

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DEVELOPMENT

• Two sets of galleries, one normally perpendicularto the other, are driven forming pillars betweenthem of size that currently depends on depth andsize (width) of the gallery.

• In the present scene of underground minedevelopment by Bord and Pillar system, mostlysquare pillars are being formed of size dictatedonly by depth and width of galleries under theCoal Mine Regulations.

BORD & PILLAR LAYOUT 

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DEVELOPMENT• A group of such pillars form what is known as a ‘panel’and one panel is separated from another panel byhaving solid coal barrier in between in the form oflong rectangular pillars.

• In the panel system of mining the coal seam is dividedinto a number of panels separated from one another bysolid barrier of coal.

• Size of panel depends on many factors, two of which arethe incubation period for the coal to be extracted and itsrate of extraction.

BORD & PILLAR LAYOUT

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BORD & PILLAR LAYOUT 

Main Elements Of Bord And Pillar Workings 

• Size of the Panel

• Size of the Barrier

• Size of Pillars

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Size of the Panel

• Depends on incubation period of the coalseam (generally varies between 6 to 12months for Indian coalfields).

• Depends on the rate of extraction (Theextraction rate from depillaring districts inIndian coalfield averages about 250‐300 tonsper day per panel).

• The no. of pillars varies from 12 to 30.

Size of the Barrier

• The width of the barrier depends on the load whichit has to carry and its strength. Greater the depthof working, wider is the barrier and also softer thecoal, the more, the width of the barrier.

• usually the same as is the width of the coal pillarswhich are enclosed within the panel.

• In deep mines the width of the barrier may becomequite large (up to 45 m) and so during extractionthey are thinned down consistent with safety.

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Size of PillarsThe size of the pillars is influenced by :

• Depth from the surface and percentage extraction in thefirst workings or development.

• Strength of the coal: Seams with weak coal require largepillars.

• The nature of the roof and floor.

• Geological Considerations: In the vicinity of faults, largepillars are required. Dip and presence of water alsoinfluences the decision as to the size of pillars.

• Time dependant strain

Size of Pillars

• In India, the dimensions of pillars andthe width and height of galleries areregulated by Regulation 99 of Coal MinesRegulation 1957.

• It is stipulated that the width of galleriesshall not exceed 4.8 m and the height of thegalleries shall not exceed 3 m.

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Dimension of pillars and galleries at different depths

Depth of the seam from the surface

Where the width of galleries does not exceed

3m 3.6m 4.2m 4.8m

The distance between centers of adjacent pillars shall not be less than(in m)

Not exceeding 60 m 12 15 18 19.5

Between 60-90 m 13.5 16.5 19.5 21

Between 90-150 m 16.5 19.5 22.5 25.5

Between 150-240 m 22.5 25.5 30.5 34.5

Between 240-360 m 28.5 34 39.5 45

Exceeding 360 m 39 42 45 45

Percentage extraction in development at different depths

Depth of seam from surface

Where the width of galleries does not exceed

3 m 3.6 m 4.2 m 4.8 m

Not exceeding 60 m 43.7 42.2 41.2 43.17

Between 60-90 m 39.53 39.8 38.4 40.5

Between 90-150 m 33.06 33.5 33.8 34

Between 150-240 m 24.8 26.2 25.6 25.9

Between 240-360 m 9.95 19.7 20.1 20.2

Exceeding 360 m 14.8 16.4 17.8 19.0

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Advantages of Bord and Pillar Method 

Disadvantages of Bord and Pillar Method 

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Advantages of panel system• Risk of loss of coal through spontaneousheating is limited. In the event of fireoccurring, the panel can be isolated fromother parts of the workings. Similarly,explosions can be limited to the panel ofoccurrence.

• Crushing of pillars is avoided.

• Creep and thrust (crush) started in any partof mine are arrested in their course andisolated in their action.

Advantages of panel system

• ‘Whole’ and ‘broken’ workings can be done at thesame time i.e., in one panel development and inanother panel depillaring can be done at the

same time.

• Ventilation is improved. Each panel can beprovided with its separate intake and return. Alsono. of air stoppings can be reduced.

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Advantages of panel system

• Control of subsidence is possible. By workingpanels of sub‐critical width, magnitude ofsubsidence can be reduced.

• By suitable design using yield pillar techniquespercentage extraction can be improved.

Disadvantages of panel system

• Considerable amount of coal is lost in barriers.Generally, in indian practice roughly 20% of coalis lost in the barriers.

• More number of air crossings are required forventilation purposes.

• Each panel must have its own independent coalcutting machine and haulage. Flitting of coalcutting machine from one panel to the otherpanel is not practicable.

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Pillar Working

Depillaring (Extraction of pillars)

Principles of Pillar extraction techniques• Roof exposure at one time should be minimal. In the Indian

coalfields, where caving is practiced, 60‐90 m2 exposure isnormally allowed. But in stowing districts the exposure may beincreases up to 90‐100 m2.

• The size of the panel should be such as depillaring can becompleted within the incubation period.

• The extraction line should be so arranged as to facilitate roofcontrol. In practice diagonal line, or step diagonal line of face iscommon. In special cases a steep diagonal line of face or evenstraight line of face has been selected.

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Depillaring (Extraction of pillars)

Principles of Pillar extraction techniques• Diagonal or step diagonal line of face provides protection as the

working places are supported by solid pillars and also when theroof caves, there is less risk of goaf flushing into the workingfaces. Diagonal line of extraction helps in the caving of the roof.

• The single lift extraction is limited to height of 4.8 m or less. Ifthe thickness of the seam is more than 4.8 m, the extraction isdone in multi‐lifts in conjunction with stowing. Seams up to 4.8m thick can be mined by caving in one pass.

• Whatever the method of extraction, the working area issystematically supported by cogs and props.

Diagonal Line of Extraction

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Step Diagonal Line of Extraction

Steep Diagonal Line of Extraction

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Straight line of extraction

Splitting of pillars• As laid down in the coal mines regulations 1957, During the

extraction of pillars, no splitting or reduction of pillars orheightening of galleries shall be affected for a distance greaterthan the length of two pillars ahead of the pillar that is beingextracted or reduced.

• Where pillar extraction is about to begin in a district suchsplitting or reduction of pillars or the heightening of galleriesshall be restricted to a maximum of four pillars.

• The width of the split‐galleries shall not exceed the widthprescribed for galleries.

• This is done to reduce the zone of stress concentration forensuring stability of the workings.

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Method of Splitting

Method of Splitting (diagonal)

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Method of Splitting (Step diagonal)

Method of Splitting (Steep diagonal)

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Layout for extraction of pillars in thin seam (1.5 m to 3 m)

Layout for extraction of pillars in coal seam (3 m to 4.8 m)

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SUPPORT SYSTEM (HAND SECT.)

SUPPORT SYSTEM (HAND SECT.)

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SUPPORT SYSTEM (LHD SECT.)

SUPPORT SYSTEM (SDL SECT.)

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SUPPORT SYSTEM (SDL SECT.)

UNDERGROUND MECHANISATION 

• Coal Cutting and Coal Ploughing machines

• Gathering arm loaders

• Shuttle cars

• SDLs

• LHDs

• Continuous miners

• Road Headers

• Conveyors

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SEMI‐MECHANISATION BY SDLs & LHDs

• In order to keep away with arduous labor of loadingcoal manually, to avoid exposure of persons at face andimprove productivity, efforts are made to introductionof intermediate technology and continuous miningsystems.

• As a result, in India SDLs & LHDs have taken footforward in intermediate technology.

• The Bord and Pillar method has been predominantlyadopted in India for the extraction of coal seams due toshallow depth and good roof conditions.

SEMI‐MECHANISATION BY SDLs & LHDs

• Drilling

• Blasting

• Supporting

• Coal loading (SDLs & LHDs)

Cycle of Operations:

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OPERATION OF LHDs and SDLs

• The method of development anddepillaring with LHDs/SDLs was similar toConventional development of formingrectangular or square pillars as perRegulation 99 of Coal Mines Regulations,1957 and subsequent extraction of pillarsby splitting and judiciously reducing it.

Method of working:

OPERATION OF LHDs and SDLs

Coal Preparation:

• Coal preparation for LHD/SDL was done byconventional drilling and blasting.

Transportation:

• The coal carried by LHD/SDL from the facewas discharged on to the pony beltconveyor, which was maintained as near tothe face as possible, even though LHD/SDLcope with about 100m lead.

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Load Haul Dumpers (LHDs)• an electro‐hydraulic powered underground mining

equipment.

• used to collect the coal, haul upto the discharge point ofhopper/chain conveyor/belt conveyor located within 100 mfrom the coal face and to dump on it.

• 50 HP (811 Model) Motor and 100 HP (912E Model) Motor

• Three forward and three reverse speeds.

• The working pressure of hydraulic system is 2200 PSI anddischarge 16 GPM.

• The hydraulic tank capacity is 210 ltrs.

• Oil is cooled by a radiator and fan run by hydraulic motor.

• The power to the machine is fed by a trailing cable 16 m2handled by a cable reeler, which can accommodate 100 mlength.

EIMCO ELECON 811 LHD

Electric motor : 50 HP Buck Capacity : 1.5 cu.m (coal)

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EIMCO ELECON 912 E LHD

Electric motor : 100 HP Buck Capacity : 2.7 cu.m

LHD mechanization – where the gradient is 1 in 6 & flatter

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EIMCO ELECON 625 SDL

Electric motor : 500 HP Buck Capacity : 1.1 cu.m (coal)

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SDL mechanization – where the gradient is 1 in 4 & flatter

A typical method of depillaring by stooking and slicing

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A plan view of typical manner of pillar extraction (splitting and slicing) and placement of roof bolt supports during depillaring by continuous miner and shuttle car combination.

Modern continuous miner

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Continuous miner and shuttle car (roofbolter in the top left)

Air path around a working section

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Overview of a working room and pillar section

DRILLING OPERATION IN THE COAL FACE

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Permitted Explosives• Explosive to be used in coal seams must be safe for usein methane‐air mixtures or coal‐dust mixturesencountered in underground coal mines.

• To meet these requirements stipulated official tests arecarried out to assess the safety of the explosives underthese conditions. Only on passing these tests in anexplosive approved as a ‘permitted’ explosive.

• types of permitted explosives

– P‐1, P‐2 and P‐3 Type Permitted Explosives

Permitted Explosives• P‐1 Type Permitted Explosives: These explosives can be used for

drifting or ripping and simultaneous firing in an undercut /middlecut / overcut and in depillaring faces, in coal seams of firstdegree of gassiness.

• P‐3 Type Permitted Explosives: These are Equivalent‐to‐SheathedExplosives (which have replaced the earlier Group P‐2 SheathedPermitted Explosives) used for drifting or ripping andsimultaneous firing in an undercut / middlecut / overcut and indepillaring faces, in all coal seams.

• P‐5 Type Permitted Explosives: These are special type ofexplosives with a high degree of intrinsic safety against all types oflikely hazards in delay blasting in coal and specially designed forSolid Blasting in conjunction with non‐incendive copper shortdelay detonators.

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Permitted Explosives

• All Indian coal seams have been classified according to theirdegree of gassiness.

Degree of Gassiness of Coal seam

Classification of Gassiness Type of Permitted Explosives

I < 0.1% of gas in the general body of airand rate of emission of such gas is less than 1 cu. m/t of coal production

P1 / P3 / P5

II > 0.1% of gas in the general body of air and rate of emission of such gas is greater than but less than 10 cu. m/t of coal production

P3 / P5

III Rate of emission of the gas is greater than 10 cu. m/t pf coal production

P3 / P5

 

Permitted Explosives

• DGMS (India) Stipulations on Maximum Permissible Charge in aShot Hole

Types of Explosives Degree of gassiness / Type of Application

Max Permissible charge per

Shot hole (gms)

P1

P3

P5

P5

Degree 1 mines, Cut face, Depillaring Degree I, II & III mines, Cut face, Depillaring

Degree I-‘BOS’,

Degree II &III-‘BOS’

800

1000

1000

565  

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Detonators• In conjunction with Permitted explosives, only electric detonators

with copper tube are permitted for use in coal blasting. These areof two types (both of N0.6 strength).

• 1. Instantaneous Copper Electric Detonators (CED): These areInstantaneous Electric Detonators with a copper shell. P1 and P3Explosives can only be used in conjunction with CEDs.

• 2. Copper Non‐Incendive Short Delay Detonators (CDD):These are copper tube detonators, used for delay firing whensolid blasting in coal is done. These are used along with P5explosives. The nominal delay of 25ms between two successivedelay intervals (from 0 to6) is available to the maximum delayperiod of 150ms.

Principles of blasting in coal

• From the point of view of blasting, undergroundcoal mining operations could be divided into twocategories, namely,

– Development and Depillaring.

• There are two techniques employed for blasting in development galleries.

–Blasting on pre‐cut face. 

– Solid blasting of coal.

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Blasting on a pre‐cut face• In many mines coal‐cutting machines are used to provide an

additional free face for blasting. While, this may be under, over,middle or side cut, the most common practice in Indian mines is tohave undercut face. For blasting on cut faces the followingconditions must be complied with:

• The length of the shot hole should be at least 15cm (6 inches) lessthan the depth of the cut.

• The detonators in a round should be fired simultaneously,preferably in series.

• The maximum charge in hole should not exceed 800 gms for P1explosives and 1000 gms for P3 and P5 explosives.

Blasting on a pre‐cut faceThumb  rules for u/g coal blasting in Cut Faces :

1.   No. of holes For each 1 square meter area = 1 hole

2.   Depth of hole 15cm less than cut length

3.   Charge per hole 40% of hole length

4.   Stemming 60% of hole length

5.   Explosive P1 type (Solarcoal‐1) for degree 1 mines

P3 type (Solarcoal‐3) for degree 2/3 mines

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A typical round of hole used in blasting an under cut face

(Numbers shown sequence of firing rounds)

Depth of Hole - 1.5 MetersNo. of Holes - 8Charge / Holes- 300 gTotal yield of Coal - 14 tonesYield per kg - 6.00 tones

 

Solid blasting of coal (BOS)• Both in Longwall and development faces coal can be blasted

without giving an under cut by a coal cutting machine.

• The technique of blasting of the solid is simpler, more economicaland less hazardous than conventional method of breaking coalwith machine cut and blasting.

• Points remembered while doing Solid Blasting in coal:

• To create an initial cut, holes need to be drilled in inclined positionso as to provide direction and force for coal to move outwardly, tocreate free face for other holes. (Lever action theory)

• Only P5 Permitted Explosives (Solarcoal ‐ 5) and 'Non‐incendive' copper short delay detonators shall be used.

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Solid blasting of coal (BOS)Points remembered while doing Solid Blasting in coal:

• Maximum charge per hole is 1000 gms for Degree I mine and

565 gms for Degree II & Degree III.

• The estimated period of Delay between the first and last shotsshall not exceed 150 ms (100 ms for Degree III Mines).

• The estimated period of delay between any two consecutive delayshots shall not exceed 60ms.

• The maximum distance between the two adjacent shot holes ofdifferent delays shall not be less than 0.6m.

• Atleast 284 cu.m of air per minute shall be conducted up to everyface where solid blasting is being done.

• Test for inflammable gas should be made within the radius of 20mof the blast.

• More than 25 shots in one round should not be blasted.

Recommended Pattern for Seam Height 2.6 m Face Size 4.6 x 2.6 Total Holes = 15 Type of Holes

Number Depth of Holes

Explosive / hole Delay Number

Stemming

Cut Holes

6 1.7 m (5.6) 3 555 Z 60-65%

Other Holes

6 3

1.6 m (5') 1.6 m (5')

3 3

555 555

I II

60% 60%

Total Explosives per Round

Expected Pull Expected Coal (Insitu)

Powder Factor T/Kg.

Detonator Factor

8.32 Kg. 1.27 m (4') 19.74 M. T. 2.37 1.31 Remark 1. 2-4 holes to be increased if encountered with shale /stone band.

2. Full packaging with sand clay cartridges as stemming gives 10% better results.  

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Solid blasting of coal (BOS)• In development headings the technique of solid blasting is done

by creating initial opening either by

(a) Wedge Cut Pattern, or (b) Fan Cut Pattern

(a) Wedge Cut Pattern

• In a 'Wedge Cut', two or more holes are drilled so as to convergeat the rear by not less than 0.3m. This arrangement provides aconcentration of charge at the back.

• With this pattern, the advance is limited. For deep pulls burdenshould be reduced by providing steeply inclined hole at the centre,known as 'Stab Hole' or by giving another shallow wedge cutknown as 'Baby Cut'. This relieving hole should be fired prior tothe main wedge cut pattern.

Wedge Cut Pattern

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Delay Number Charge (Cartridge) Explosive (gm ) Explosives Charged (gm)

0 1 3 555 555 1 2 3 555 x 2 1110 2 2 3 555 x 2 1110 3 4 2 370 x 4 1480 4 4 2 370 x 4 1480 5 2 2 370 x 2 740

No. of Holes = 14+1stab hole = 15 (Total), size of gallery = 4.3m x 2.4m

Wedge Cut Pattern

Total Charge :- 6.5 kg, Av. Pull :- 1.35m to 1.45mBlasting Ratio (PF) :- Av. 2.6 t/kg Detonator Factor :- 1.1 to 1.2 t

Fan Cut Pattern

(b) Fan Cut Pattern

• In Fan Cut Pattern, the initial opening is createdby firing a hole drilled at an acute angle to theface. The inclination generally varies from 30 to60 degree. The inclination of subsequent hole isgradually reduced.

• This pattern suitable for medium hard tohard coal seams.

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Fan Cut Pattern