an induced blasting technique in coal miningvk-7 incline, kothagudem area, where the seam was...

AN INDUCED BLASTING TECHNIQUE IN COAL MINING

Dasari Appaji [email protected]

ABSTRACT

Induced caving by blasting during depillaring of

panels in underground coal mines has received

limited attention. This technique has become an

integral part of a mining operation known as the

blasting gallery (BG) method in India.

Systematical deep hole (16-30 m) blasting has

been successfully carried out from underground

split galleries in BG panels. A study of drilling

and blasting parameters, gas hazards, strata

behaviour and ground vibration was undertaken

as part of a research project for the Indian

Ministry of Coal and Mines

1. INTRODUCTION

Coal is the world’s most abundant and safe

fossil fuel. World coal reserves are estimated to

last almost 200 years from now. The

importance of coal as a primary source of

energy is significant in India. India has a long

history of commercial coal mining covering

nearly 220 years from 1774 in the Ranigunj

Coalfield.

Sector X plan XI Plan XII Plan

2006-07 2011-12 CAGR (%) of 2016-17

Actual Assessed Projected

Power Utilities 310.00 483.00 9.27% 750.00

Power Captive 31.50 57.06 9.50% 85.00

Cement 25.00 31.90 -- 50.00

Sponge Iron &

Others

50.00 90.64 12.41% 135.00

Total Non-coking 417.00 662.60 9.70% 1020.00

Coking - Steel 43.00 68.50 9.76% 105.00

Total 460.00 731.10 9.7% 1125.00

CAGR – Compounded Annual Growth Rate

JASC: Journal of Applied Science and Computations

Volume VI, Issue I, January/2019

ISSN NO: 1076-5131

Page No:393

Expediting the project approval process and

faster project commissioning

For faster execution of projects

after they are approved, MoC

grants ‘advance action plan’ for

larger projects.

Private mining

GoI has issued some of the coal

blocks to private companies for

captive mining.

Higher import

To enable higher imports, the

GoI lowered the effective

import duty on non-coking coal.

Overseas mining

CIL has formed a subsidiary,

Coal Videsh Limited (CVL) to

acquire coal blocks in a number

of countries including

Indonesia, South Africa and

Australia.

Enhancing producition from opencast mines

Opencast mines allow higher

mechanisation and can therefore

ensure faster step-up in

production.

Increasing the extent of outsourcing in various

mining operations

Outsourcing of mining

operations can lead to faster

project implementation as the

private parties can readily

procure machinery.

Creating an enabling policy environment for

private participation

The magnitude of coal shortage

is such that active participation

of private players is urgently

required to broad base the

country’s efforts at increasing

coal supply.

2. STRATEGY OF MINING IN SCCL

Exploitation of coal seams from u/g mines

became a major challenge for Indian Coal

Mining Industry. Ever increasing demand year

by year as coal is the main fossil fuel used by

power sector. SCCL has initiated several steps

like re-organization of existing mines to

augment production. One example is

mechanized open cast mining with techno-

economic viability. Introduction of

mechanization for higher rate of production

such as long wall system and intermediate

technology etc., liquidation of long standing

good quality coal pillars. About 50 % of coal

reserves in India are in seams with thickness

more than 4.5 m which fall under the category

of thick seams. Exploitation of thick seams by

u/g method poses certain difficulties /

problems. Extraction of thick seams by

conventional hand section is neither productive

nor viable from conservation point of view.

Percentage of extraction by hand section in

thick seams is as low as 25 – 30 %. However,

some attempts have been made to extract thick

seams with sand stowing. Sand stowing for

working thick seams cannot be considered as

an option because the cost is prohibitive as sand

is becoming increasingly scarce commodity

along with timber. At the same time, coal

industry was in search of an economic method

for dealing with thick / standing developed

pillars. SCCL is of the opinion that blasting

gallery (BG) method is the appropriate method

for the extraction of thick seams up to a

thickness of 8 – 11 metres. Mining by BG



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method produces about 1000 T/day with 85 %

of extraction which has been in SCCL.

3. CARBONNAGE DE FRANCE (CDF)

SUGGESTED BLASTING GALLERY (BG)

FOR EXTRACTION OF THICK SEAMS

IN INDIA

The 1st BG Panel was started in the country at

East Katras colliery in Jharia Coal fields

(BCCL) in 1987. The method was not

successful at East Katras colliery where

overriding of pillars (strata control) occurred.

In Chora – 10 Pit colliery in Raniganji (ECCL)

in the year 1987, the method was partially

successful giving encouraging results.

However, it was discontinued due to

spontaneous heating. Expected production and

percentage of extraction could not be achieved

in both the above mines. SCCL introduced BG

method in collaboration with Carbonnage de

France at GDK-10 Incline in the year 1989. for

extraction of a coal seam with 11 mtrs.

Thickness. The method was very successful

resulting in 85 % of extraction with high

productivity. At the same mine, where the thick

seam was developed fully in top section and

partially in bottom section, BG was introduced.

Subsequently, SCCL has introduced BG in

other mines also, namely :

i. GDK-8, where bottom section was

developed by bord and pillar method along the

floor of the seam.

ii. VK-7 Incline, Kothagudem area,

where the seam was developed in two sections.

During the development of BG panel, the

galleries were driven in staggered manner.

iii. GDK – 11 A Inc., RG-I Area and

later at No. 21 Area, YD Area in the year 2006.

ADVANTAGES OF BLASTING GALLERY

METHOD

• Full thickness of the seam can be

extracted in a single lift.

• Higher percentage of extraction i.e. 75 –

85.

• Capital investment is nominal when

compared to longwall project.

• Easy to train the man power and easy

maintenance of the equipment.

• Extraction is carried under fully

supported roof i.e. with remote

controlled LHDs.

• Safety of the workmen can be fully

ensured.

• This method can be also be adopted in

virgin/developed seams.

• Most of the equipment and spares are

indigenously available.

• Loss of production is minimum while

shifting the equipment.

• This method is highly flexible as several

units are in operation in a district

• Even if one of the units is under break

down, production from the district will

continue to come.

• The time required for preparation of

panel in relation to the total life of the

panel is less than other mechanised

methods.

LIMITATIONS OF THE METHOD

• This method is not suitable for gassy

mines and seams with degree-I gassiness

are most preferable.

• The method is suitable only for gradient

more than 1 in 5 to allow easy

movement of tyre mounted LHDs and

crawler mounted electro- hydraulic

jumbo drills.

• Though the percentage of extraction is

around 75-85, still coal left in the goaf is

likely to create spontaneous heating.

4. THE MANNER OF EXTRACTION

Each pillar shall be split into two equal parts

by level split of width not more than 4.2 m.



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and height not more than 3m. along bottom

section. The splitting of pillars shall be

restricted to one pillar from the pillar under

extraction. The long hole blasting shall not

be practiced at any place where two free

faces are not provided. In order to create

free faces in this operation / beginning of

extraction in BG Method. Before practicing

long holes blasting (ring holes) the operation

of drilling and blasting are carried out in

stages to a height of full thickness to expose

roof with increasing angle and length of

short holes in bye of the galleries i.e., called

“Potato blasting”. The full thickness of the

seam is extracted by blasting a ring of shot

holes with about 33 shot holes.

Potato Blasting

The shot holes are drilled in ring pattern and

sloping at an angle of about 300 to 400 from

the vertical towards the goaf. The spacing

between consecutive rings at shot holes shall

be 1.5 m. The shot holes are drilled in a ring

spaced 1.5m. apart by means of JUMBO

drill from the level rooms in such a way that

they cover half the pillar on rise side and

half the pillar on the dip side. Extraction in

level galleries shall be from in bye to out

bye forming a diagonal line at an angle of

about 600 to the level. A curtain of

thickness of coal not less than 1.5m shall be

left between two adjacent rooms after

blasting of rings. However, it was observed

that this curtain provides protection during

remote operation of the LHD in the goaf.

Blasting of Ring holes

After drilling is completed, shot holes are

charged with the explosive approved by

DGMS. The total number of shot holes in a ring

are 33. The cartridges of explosives are

distributed over the whole length of shot holes

by spacers tied together by a detonating fuse

called RING CORD which are initiated by No:

6 electric detonator. About 0.5 – 0.6 in length

of all holes are stemmed with clay at the end.

On completion of charging and stemming of all

holes, the circuit is connected in series. Before

blasting, the last installed roof support at the

goaf line is removed and next support line is

reinforced with extra support.

Specification of Explosives :

I-Stage

Explosive : Uniring (IEL)

(a) Weight of explosive : 200 g

(b) Diameter of explosive : 32

mm

(c) Length of explosive cartridge : 200 m

(d) II-Stage

(e) b) Explosive

(f) Explosive type

: Belgex Coal (R),

Bharat

Explosives Limited

(g) Weight of explosive

: 185 g



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(h) Diameter of explosive

: 32 mm

(i) Length of explosive cartridge

: 200 mm

(j) Density

: 1.18 – 1.19 g/cc

(k) VOD

: 2826 M/S

(l) Air – gap in unconfined condn

: 5 cm pass.

Composition of explosive

Nitroglycerine : 12 + 0.5%

Nitro-Cellulose : 0.2 + 0.5 %

Ammonium Nitrate : 46 + 3.0 %

Cellulose (Woodmeal) : 1.5 + 0.5 %

Salt : 37 +

2.5%

Sodium Nitrate : 2 %

Others : 6.0 + 0.5 %

5. SUPPORT SYSTEM IN BG METHOD

The main junction girders are secured tightly

by clamps suspended by steel bolts grouted in

the roof. The goaf edges and the entries of the

central dip / rise galleries of panel immediately

out bye of the goaf edges shall be kept

supported by row cogs set an interval of 0.25

m. All roof bars set on hydraulic props shall be

braced suitably. Adequate setting load of not

less 6 T shall be provided. The level split and

dip/rise galleries shall be kept supported by a

set of ISMB girders of 150 mm x 150 mm

mounted on 40 T hydraulic props at either end.

The first set of cross girders shall be erected

close to the goaf edges, but in no case less than

0.7 m. The distance between two such sets

along the gallery shall not exceed 1.00 m/1.5 m

depending on the roof conditions. The roof

between the bars shall be suitably lagged by

wooden sleepers. Entire roof is supported for a

distance of 40 m. from the face by 250 mm x

250 mm IBMS steel bars placed on 2 No. of 40

T capacity open circuit hydraulic props. The

interval between the bars being 1.00 m / 1.5 m.

All junctions are supported by 6 Nos of 5.5 m

long roof bars which are held in position by

means of 2 Nos of 5.5 m roof bars at either end

set over a cluster of 4 Nos of hydraulic props at

each corner. In between the roof bars, the roof

is further supported by 3 Nos of grouted

wooden/bamboo roof bolts. As a precaution

against the dislodging of hydraulic props

accidentally by moving machinery, the

hydraulic props are tied together to roof bars by

means of flexible steel wires. All junctions are

supported by 6 Nos of 5.5 m long roof bars

which are held in position by means of 2 Nos of

5.5 m roof bars at either end set over a cluster

of 4 Nos of hydraulic props at each corner. In

between the roof bars, the roof is further

supported by 3 Nos of grouted wooden/bamboo

roof bolts. As a precaution against the

dislodging of hydraulic props accidentally by

moving machinery, the hydraulic props are tied

together to roof bars by means of flexible steel

wires.

5.1 INDUCED BLASTING

When the method of extraction is to remove all

the coal or as much of coal practicable and to

allow the roof to cave in, the operations shall be

conducted in such a way as to leave as small an

area of uncollapsed roof as possible. Where

possible suitable means shall be adopted to

bring the goaf at regular intervals. In blasting

gallery method of extraction, induced blasting

has become an integral part of mining operation

during depillaring of developed coal pillars.

When the method of extraction is to remove all

the coal or as much of coal practicable and to

allow the roof to cave in, the operations shall be

conducted in such a way as to leave as small an

area of uncollapsed roof as possible. Where

possible suitable means shall be adopted to

bring the goaf at regular intervals. In blasting

gallery method of extraction, induced blasting

has become an integral part of mining operation

during depillaring of developed coal pillars.



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5.2 INDUCED BLASTING IN BG PANELS

In Blasting Gallery method of winning coal,

usually more area of goaf is exposed after

extraction of coal. If the roof does not cave by

its own weight, induced blasting is generally

carried out at regular interval when the roof

span is about 120 – 190 sq. mtrs. As per the

DGMS strictures, explosive charge per hole is

limited to 1.0 kg only. However, it is found

practically difficult to conduct successful

induced blasting with such limited charge.

5.3 CHARGING OF HOLES

Generally, 6m long plastic pipes, 36 mm and 38

mm inner and outer diameters respectively are

used for charging of holes. Where plastic pipes

are not available, charging arrangement is done

by cutting the blastic spacers and then making a

slot where explosives are inserted and tied

together with cello tape as practiced in Chora-

10 pit colliery of ECL. Approximately, 6.5-8.5

kg of explosive is charged in each ole

depending on the site condition and actual hole

length. Remaining portion of the hole is

stemmed with bamboo spacers. All the holes

are fired in one round with miscellaneous

detonators. Before firing five rows of girders

are reinforced with extra props. During blasting

all related measurements like gas survey,

vibration, convergence and strata loads are

monitored for safe and useful implementation

of design and charge loading parameters.

CONCLUSION

• Induced caving by blasting becomes

more important especially, in case of

winning of coal by BG method when the

overhanging of roof in the goaf area is

difficult to cave by it’s own weight.

• Though, the whole operation is non-

productive work, it ensures safety to the

persons and the u/g working

environment.

• Strata monitoring through measurements

of roof convergence, strata load and

ground vibrations during enduced

blasting are very essential for succesful

implementation of induce blasting.

REFERENCES

1. Banerjee S.P. Mining coal from deep

horizons, the technical, economical and

environmental Considerations, International

Mining Engineering Journal, Volume 45(2006):

pp. 24.

2. Jayanthu S. Strata behavior observations in

depillaring experimental panels vis - à- vis

applicability of convergence date for working of

goaf falls, International Symposium on

Advances in Mining Technology and

Management, (2005): pp. 337-341.

3. Ray A.K. Cavability assessment of roof rocks

to understand interaction of strata and support at

long wall face, Minetech, Volume 26(2005): pp.

16-28.

4. Satyanarayana I. Strata monitoring and

analysis of blasting gallery panels - a case study,

International Symposium on Advances in

Mining Technology and Management,

November–December (2005): pp. 203-217.

5. Venkatanarayana G. Measures to

control/delay spontaneous heating in BG

panels”, JMMF, Volume 52, No. 122 (2004): pp.

8-9.



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an induced blasting technique in coal miningvk-7 incline, kothagudem area, where the seam was...

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