TIKESHWAR MAHTODY. Director of Mines Safety,

Bilaspur Region 07898033693, 08982493361

tikeshwarmahto@yahoo.co.in

HIGHWALL MINING – A NEW APPROACH IN INDIA

: - A Case study

Worldwide, there are two main High wall mining Technology being in operation in the business of High wall Mining. In India, both technologies are in operation by the different Agencies. There details are as follows.

Add Car High Mining system: It`s details are covered in this case study. It is basically an US Company marketed by ICG ADDCAR System LLC, No.1, HWM Drive, Ashland, KY 41102, USA, http://www.addcarsystem.com. In India it is operated by a Hyderabad based company named Advanced Mining Technologies Pvt. Ltd., Plot No.668, Road No. 33, Jubilee Hills, Hyderabad- 500 033, India, Tel: +91-4023548990, Fax: +91-4023608565, Email: hari@amt.net.in . This Technology is highly productive and can be achieved upto 1MT/Year. Only constraint with this technology is that, it is best suitable for competent roof. Because Addcar conveyors are open and if, roof collapses it can damage the conveying system.

Auger High Wall Mining system: It is also an US based company``BUCYRUS Mining``. In India it is operated by CBL (Coopram Bagdoria Limited). Presently, it is being operated in Sohagpur Area of SECL, a sub-sidiary of CIL. It is being operated on the name of Sharda High wall mining. It is less productive in comparison to Add Car Highwall Mining system, but it is safer. Because, it`s augers( conveying system) are armoured and covered. It can be applied in any condition. Presently, this company in India is headed by Sri B.N. Pan, a retired C&MD of SECL, whose contact No. +91-9830024733

INTRODUCTION

Highwall mining is a method of mining, originated from auger mining that involves the use of a remote controlled mining machine, which is driven into a coal seam to extract the coal. This method is often used to access coal left behind from previous mining operations or when difficult geological conditions restrict the use of other mining methods. Coal is extracted from the base of a cliff (a highwall) using horizontal or inclined drilling to create holes in the coal seam left in barriers of highwall of opencast mine. Another primary difference in a highwall mining operation is that it is carried out by remote control at the surface, where an operator located in a cabin uses a television camera to monitor and control the progress of the continuous miner machine. There are two main types of highwall mining: continuous highwall mining and auger mining.  Continuous highwall mining involves the use of a continuous mining machine that creates rectangular openings into the coal seam of a highwall. Coal is hauled to the surface using a conveyor system. In Auger mining, coal is extracted using an auger mining machine. This machine operates like a drill, with a cutter head rotating into the coal seam and creating circular holes to access the coal. The extracted coal is returned to the surface using the auger machine and a conveyor system. Auger has limitation of penetrating into the seam. It can go upto 60m, but Continuous mining machine can go upto 600m. Production capacity of continuous mining machine is about 1MT per annum.  

 

History

Historically, when an open-pit or open cut mine was excavated, a highwall remained after excavation and was abandoned or covered up. This eventually provided an incentive for coal miners to devise a means of extracting coal from an existing highwall, as much labour had already gone into excavation and exposing an accessible coal seam. The primary goal for establishing a highwall mining system was therefore to increase the production life of an open-pit mine. Initial attempts to mine high walls were met with very little success until the development of coal auger mining in US in the 1940s. In this respect, highwall mining has largely been viewed as an adaptation of auger mining. The problem with deploying just augers to mine a highwall was their restriction to penetrate coal seams to a limited depth. They were also unable to easily navigate over the rolls and dips of coal seams because of the rigid structuring of auger flights.

Eventually, the continuous miners used in the underground mining of coal were developed and outfitted to also recover coal from surface high walls. This spurned the development of new highwall mining machine systems such as the Terex SHM, the Highwall Hog, and the Addcar System developed in 1990. As on now, more than 60 highwall miners are in operation in US, and they may account for about 4% of total U.S. coal production. This technology is also very popular in Australia. More than 13 panels have been extracted till date in Australia. And now SCCL, the Indian Mining Giant witnessed the successful implementation of first Highwall Mining in Asia continent.

Machinery used for Highwall mining at RGOCP-II of SCCL are supplied by Addcar Mining company of US based.

The key components of the system comprises A continuous miner A launch vehicle Conveyor cars (Addcars or screw conveyors) A stacker Rubber tyred loaders Dozer

1. The Continuous Miner The continuous miner is a Joy 12 CM12, instructed to underground specifications apart from a shortened and fixed tail. Additionally cameras have been fitted to provide the remote operator with instantaneous pictures of the face, machine instrumentation and methane detectors.

SpecificationsLength of machine: 11 mWidth of the boom: 3.5 mWeight : 72 TGround pressure: 1.95 kg per square cmHeight of extraction: 1.52 to 4.0 m

Electrical Details:Cutter motors: 2 x 220 HP (1.1 KV)Gathering head motors: 2 x 60 HP (1.1 KV)Scrubber fan motor 1 x 35 HP (1.1 KV)

Pump motor 1 x 70 HP (1.1 KV)Traction motors: 2 x 35 HP DC motors

2.The Launch VehicleThe launch vehicle is the main platform from which the continuous miner and conveyor cars enter the coal seam. A central “ belly belt” receives the coal from miner and conveyor cars as added, and transports it to a stacker. Additionally the launch vehicle houses the power centre and operator control centre, plus hydraulic system to provide thrust force for mining advance and retreat and machine positioning. Specifications

Length : 29 mWidth : 9 mHeight : 8.5 mWeight : 300 TMotors: 2 x 200 HP, 1.1 KV ( operates hydraulic pumps)Belly belt motor 75 HP , 1.1 KVGround pressure: 1.95 kg per square cm

2. Conveyor Cars

Wheel mounted conveyor cars, known as Addcars are added as the continuous miner advances under the highwall. It is also known as screw conveyors.

Specifications Length: 12.5 m

Width : 1.4 mMotor : 40 HPWeight: 12 T

3. StackerThe stacker receives the coal from the launch vehicle belly belt and can either discharge into trucks directly or stockpile coal to a height of 5.0 meter. SpecificationsMotor: 75 HP, 480 V

Production: On and average 3000 tones per day and 1 MTPA can be achieved with this machine.

4. LoaderLoader is used for loading and unloading of conveyor cars.

5. Dozer

It is required for leveling and compacting the platform used for the highwall mining.

Safety Features of Equipment:

Miner:

HORTA (Honeywell Ore Recovery and Tunneling Aid) ---

It provides real time horizontal guidance in conjunction with miner steering system. The position of the miner head (heading, pitch, and roll) is continuously monitored using the (HORTA) system. The HORTA system was originally developed for the military in USA, and uses three-axis, ring-laser gyroscopes and three-axis accelerometers. A monitor in the launch vehicle shows the position of the miner relative to the planned heading, allowing the operator to steer the miner and make adjustments in cutting height to keep in seam. This ability is important to the overall safety of Highwall mining, as the highwall miner holes can be mined to designed specifications, ensuring that the web pillars are the correct size to carry overburden loads. Stability of Highwall and web pillars depend upon the accuracy & correct interpretation of HORTA by the operator sitting in the operator’s cabin of H/Wall machine. Deviation of web holes from the projected direction can lead to major disaster. There are many cases of Highwall collapse due to failure of web pillars in U.S. One case of Highwall slope failure is shown in figure below.

Figure 8-Site of massive web pillar collapse resulting in highwall slope failure. Photograph was taken from adjacent spoil pile. Highwall is about 150 It high. A llO-ton coal haulage truck is buried in rockfall debris.

Inflammable gas Detector----- Miner has fitted with CH4 detector. Automatic power cut off system when inflammable gas is 1.25% or more. Inclinometer—It is fitted into Launch Vehicle and used for setting suitable

inclination (50 to 70 from horizontal) to miner and conveyor cars before entering into inclined seam. If the seam is steeply inclined, the platform is made sloping (50 to 70 ) towards the Highwall toe, so that the total inclination to the miner matches with the seam inclination.

Gamma sensors—

It is used for vertical guidance to keep the miner within the seam to eliminate dilution. The system uses gamma detectors at the miner head/bottom gathering arm pan to avoid cutting into the roof or floor. This allows the system to attain high penetrations from the highwall and minimizes out-of-seam dilution.

All electrical apparatus are Flame Proof. Interlock with water supply---

Miner stops working when water supply to the cutting head is not available.

Launch Vehicle:

Rugged and robust canopy Pull chord Emergency buttons All electrical apparatus are FLP Metal halide lamps top down Cat walk with fencings and doors

Add Car:

All electrical apparatus are FLP

Stacker: Pull chord Emergency button

LIMITATIONS:

Miner penetration capacity is limited by seam gradient with a maximum of 16 deg. It can penetrate to a length of about 600 m to 300 with flat to 16 deg gradients, respectively.

It is not suitable for soft roof and soft floor. Roof should be strong enough to bear the strata load till the completion of entire length of web cutting and retreating of machine.

Geological disturbances, like fold, faults etc. in the path of cutter creates problem in further advancement.

Ground Control Analysis of Highwall Mining

Ground control analysis is the very critical pre-mining activity for High wall mining. This technology is new for India, but very popular in USA and Australia. Till now USA has completed around 20 High wall mining projects with few cases of highwall collapses due to webs failure.

The approaches to web and barrier pillar design involved three basic steps:

Application of empirical pillar design formulas, Back-analysis of available information from past augering operations, and

Numerical modeling analysis to confirm design performance and test its robustness.

When designing a high wall mining, the mining engineer must specify

1) Web pillar width, 2) number of web pillars between barrier pillars and 3) barrier pillar width. The other design parameters determined by the high wall miner are hole width (or web cut), the mining height and the overburden depth. In addition, the mine planner must estimate the pillar strength, the applied stress on pillars and the pillar stability factor.

Design of Web pillar

Strength of Web Pillar

Numerous empirical formulas are available to predict coal pillar strength, but Mark- Bieniawski formula for rectangular pillar is best suitable Web and Barrier pillar design.

Bieniawski formula for square pillar design is;

SP = SI[ 0.64 + 0.36 W / h ] (1 Where: Sp=  strength of Square pillar 

SI = in situ coal strength W =  width of squar pillar, and h = mining height

Later Mark modified this formula to design the rectangular pillar, which is known as Mark- Bieniawski formula. The Mark-Bieniawski formula applies best for web pillars, which are very long, narrow rectangular pillars. By far, the most widely accepted formula for web pillars design in the United States today is the Mark-Bieniawski pillar design formula.

Mark- Bieniawski formula for rectangular pillar is; SP = SI [0.64 + 0.54 W / h – 0.18 W2/Lh] (2

Where: Sp= web or barrier pillar strength SI = in situ coal strength W = web or barrier pillar width, h = mining height, and L = length of the pillar.

t

Front view of Highwall mining face

In the case of high wall mining where the pillar length (miner penetration) is much greater than either the pillar height or width, the last term may be omitted, and Mark- Bieniawski formula (2) reduces to;

SP = SI [0.64 + 0.54 W / h]  (3

In situ coal strength is normally taken as 6.2 MPa (900 psi).

In India first high wall mining Technology was applied at RG OCP-II of SCCL and completed successfully with little problem of roof collapses. Ground control layout

HIGHWALL BENCHES

Web Cuts Web Pillars

was formulated by CIMFR, Dhanbad. Web pillars strength was designed by using the CIMFR`S formula, which is as follows.

Sp =0.27pc* h-0.36 + (H/250 +1)(We/h -1) Mpa………………………………….(5)

Where, Sp = strength of web pillar (in Mpa)

Pc = compressive strength of sample of 25mm cube (23.8 Mpa);

h = height of extraction (in meter);

H = depth of cover(in meter) , and

We = equivalent width of web pillar (in meter)

= 4A/P

= 2W1*W2/W1+W2 (for rectangular pillar )

= 2W1 (W1+W2 ≈ W2 ,for long narrow web pillar of high wall)

Where, W1 is the width of web pillar

Stress on web pillar

Once web pillar strength is determined, an estimate of pillar loading is required to calculate a safety factor. Pillar loading is estimated using tributary area load theory as follows:

LP= SV(W + WE)(L)/W*L

Or, LP= SV(W + WE)/W ----------------------------------------(6

Where, Lp = average vertical load on the pillar SV = in situ vertical stress W = web pillar width, WE = entry width (or web cut width) and L= length of web cut

The high wall mining equipment dictates the hole width, which varies from 2.7 to 3.6 m. In situ vertical stress depends on the overlying rock density and overburden depth. Vertical stress gradient is typically 0.025 MPa/m. Overburden depth may be taken as the average of maximum overburden depth and minimum overburden depth on a high wall mining web pillar. 75% weightage is given to the maximum

overburden depth and 25% weightage to the minimum overburden depth, which is as follows,

H = 0.75 * HMAX+ 0.25 * HMIN (7

Where: HMAX = maximum overburden depth, HMIN = minimum overburden depth, and H= average depth of cover

And In-situ vertical stress, SV = density of rock * overburden depth

Or, SV = 2.5 * H , t/m2 = 0.025 * H, Mpa

Hence, LP = SV(W + WE)/W or, LP = 0.025 H (W + WE)/W Mpa

Finally, the safety factor of web pillar is calculated as:

SF = SP/LP (9

For design purposes, the stability factor for web pillars typically ranges from 1.3 to 2.0. CIMFR designed the FOS of web pillar of high wall mining at RGOCP-II in the range of 1.5 to 2.0. FOS of 1.5 for top most seam (I-seam) , and 2.0 for lower seams (II, IIIA, III and IV- seams)

Design of barrier pillar

Strength of barrier pillar

Strength of barrier pillar can be calculated by using `Mark- Bieniawski formula or CIMFR formula.

Mark- Bieniawski formula is;

SBP = SI [0.64 + 0.54 WBP / h] (10

Where, SBP = strength of barrier pillar, SI = In-situ coal strength , WBP = width of barrier pillar and h= height of barrier pillar.

If the number of web pillars in a panel is selected as “N”, then the panel width is given by

WPN = N (WWP + WE) + WE (11

Where, WPN = width of panel , N = No. of web pillars in the panel, WWP = width of web pillar and WE = width of web cut (or width of cutting machine). A sub- panel may be created in the panel for more safety. Generally 10-20 web pillars are taken in a panel or sub-panel.

Stress on barrier pillar

Neglecting the stress carried by the web pillars (i.e. assuming that they have all failed), the average vertical stress on a barrier pillar using Tributary area method as in equation (6) is,

LBP= SV (WPN+ WBP) / WBP (12

Where, LBP= stress on barrier pillar, SV = In-situ vertical stress, WPN = width of panel or sub-panel and WBP = width of barrier pillar

Similarly, the stability factor for barrier pillars against strength failure is simply

SFBP = barrier pillar strength / barrier pillar stress Or, = SBP / LBP (13

Because the stress carried by web pillars within a panel is neglected, the stability factor for barrier pillars can be as low as 1

Barrier pillars with a W/h (width/height) ratio greater than 3 are superior for sound geomechanics reasons. So, assuming safety factor of barrier pillar, the barrier width can be calculated.

Numerical Modeling Analysis

The empirical method used for the web and barrier pillar design should be confirmed by mining experience in a wide variety of mining types and geological conditions by using Numerical modeling technique.

First experience of High wall mining in India at RGOCP-II of SCCL

Before 2005 no any Indian mining operators dared to think about this technology to

adopt in their mines. In 2005, SCCL tender was floated and at the end of year 2006,

the contract was awarded to a Hyderabad based company (Advance Mining

Technology). In tender 18 months time were given to the bidder to procure the

machinery and take permission from DGMS. The process included a visit of DGMS

and senior official’s of Labour Ministry to the equipment supplier (ADDCAR Highwll

System, USA) facility and mines in the US to witness and approve the concept and

its adaptability in Indian mines. After clearing all litigations, the High wall mining

equipment was transported to the project site (RG OCP-II) in July of 2010, but the

site preparation and equipment assembling took another 4 months and finally the

High wall mining was inaugurated on December 10, 2010.

Initially, High wall mining was proposed for four seams, viz. I, II , III and IV seams

respectively ( nomenclature from up to down ) having thicknesses 5.2- 6.0m, 3.1-

4m, 11 – 12.5m and 3.7- 4.1m respectively. There is clay parting of 1.0m in I and II

seams. III and IV seams are free from any contamination. Hence, major coal and

quality coal is blocked in III and IV seams. In the mine I and II seams have already

been extracted upto the boundary and open to air, but III and IV seams (bottom

seams) are partly extracted and backfilled and partly under extraction. Therefore,

High wall mining was not possible in III and IV seams. So, first high wall mining

witnessed II seam and then I seam. The Highwall mining block was generally

divided into Panel-A and Panel-B, the extraction cuts were planned in these panels

along apparent dip and full dip directions, respectively.

DGMS permitted the extraction of coal by High wall mining at RG OCP-II of SCCL by putting the following terms and conditions.

1. The extraction seams shall be in ascending order i.e. starting from No. IV seam and upwards.

2. The no. of galleries/entries, length and width of galleries, no. of web pillars, height of extraction, its dimensions etc., shall be made in accordance with conditions no. of Annexure I attached to this permission letter.

3. In this connection your special attention is invited to Regulation 117 of the CMR,1957 regarding precautions against fire to be taken for strict compliance.

4. Regulation 112(1)(c) read with this Directorate’s circular no.11 of 1959 regarding fencing of the surface area likely to subside.

5.0 Please note that, this permission is subject to the following conditions:

5.1 In the event of any change in the circumstances connected with this permission, which is likely to endanger the life of workmen employed in the mine or endanger the mine, the mining operations for which this permission has been granted shall be stopped forthwith and intimation thereof sent to this Directorate. The said mining operation shall not be resumed without express and fresh permission permission in writing.

5.2 If at any time any of the conditions subject to which this permission is granted is violated or not complied with this permission shall be deemed to have been revoked with immediate effect. The above permission may be amended or withdrawn at any time, if considered necessary in the interest of safety.

5.3 This permission is being issued specifically under the regulations mentioned above, and without prejudice to any other provisions of law including the Mines and Minerals (Regulation & Development) Act, 1957 and Mineral Concession Rules 1960, which may be or may become applicable at any time.

5.4 The above permission may be amended or withdrawn at any time, if considered necessary.

5.5 The Directorate shall be informed as soon as the mining operations are commenced in accordance with the above permission and intimation about completion of mining operations shall also b sent promptly and in any case not later than one month thereof.

Annexure- I

Conditions governing extraction of coal by High wall mining Technology at Ramagundem opencast II Mine of M/S SCCL.

1.0 Before commencement of extraction of coal:

1.1 Check survey and leveling of the area shall be made. If any discrepancy is noticed, the extraction shall not commenced and the Directorate shall be informed with a copy of correct plan/sections indicating the difference and extraction shall not be commenced except with the prior approval of the Directorate.

1.2 Adequate steps, including provision of suitable garland drains shall be taken to prevent accumulation of flow of water on surface above the panel.

1.3 Since the drivages are proposed to be made from floor of the quarry, the quarry bottom shall be kept free from water. Adequate pumping arrangement shall be provided to deal with water, if any.

2.0 The manner of extraction shall be as follows:

(i) The total identified area for high wall mining shall be divided into two blocks called A & B.

(ii) The extraction galleries/cuts shall be made in these locks along apparent dip in blocks A and along full dip in blocks B.

(iii) The width of the galleries proposed to be driven in all the panels/seams shall be kept at 3.5m(cutter width).

(iv) seams I and II shall be extracted in both blocks A and B.

(v) While extracting III and IV seams :

The galleries /web pillars in both the seams shall be columnised.

Long term stability of the pillars /wall shall be so designed as to ensure a factor of safety of more than 2 in IV seam and III seam.

Alternate long and short galleries as proposed shall be extracted due to wider variation in depth.

(vi) The galleries/cuts and the remnant web pillars shall be made to ensure long term stability of workings/slope etc. Seam I and Seam II shall be extracted in such a way that a factor of safety of more than 2 for Seam II and more than 1.5 for seam I shall be achieved.

(vii) The galleries shall be driven along floor of in IV seam and along roof of III seam, so that a minimum solid parting of more than 5.0m is left between III and IV seams.

3.0 Keeping the conditions in view, the dimensions of galleries/web pillars etc. in different seams/sections shall be made and maintained as given below:

3.1 seam No. I, a factor of safety :>1.5

Seam Parameters Panel-A (in mtrs) Panel-B (in mtrs)

I Seam

Seam height 6.7 6.7

Gallery width 3.5 3.5

Extraction height 4.0 4.0

Web pillar width 4.8 5.5

Barrier pillar width 15.0 17.0

No. of galleries/entries 35 38

Panel Avg.width 320 375

Panel Avg. Length 107 135

Sub panel width 78.2 93.5

No. of web cuts 10 11

Cover 15-140 115-140

Factor of safety 1.5 1.5

3.2 Seam No. II, a factor of safety :>2.0

Seam Parameters Panel-A (in mtrs) Panel-B (in mtrs)

II Seam

Seam height 4.1 4.1

Gallery width 3.5 3.5

Extraction height 4.0 4.0

Web pillar width- L- 8.1 8.5

Web pillar width- S- 2.3 2.5

No. of galleries/entries L28, S28 L28, S28

Panel Avg. Width 315 325

Panel Avg. Length 145 145

Panel Avg. Length 35 40

Cover 60-170 140-170

Factor of safety 2.0 2.0

However, the stability analysis of the dimensions of the cut, web as well as barrier pillars shall be reassessed by another/International Scientific Agency having experience in design of cut & web pillars in High wall mining.

4. The high wall shall be monitored to detect any movement in any part of the standing slope using continuous scanning system to ensure any instability in advance any to protect any danger to persons at or near the High wall miner. Ground movements from surface shall be monitored to find out the web pillar failures, if any. The ground monitoring activities on surface and along slope of high wall shall be carried out in association with a Scientific Agency.

(i) The area shall be worked after thorough loose dressing. Reinforced wire mess shall be installed near the zone which may be vulnerable to rock fall.

(ii) A strong protective canopy shall be provided on the launch vehicle to ensure safety of work persons deployed on the vehicle.

(iii) A protective bund shall be provided at the working level to arrest the small boulders falling from the lower portion of the standing high wall slope.

(iv) A bund or g garland drain or a combination of the two shall be provided at the Highwall crest to avoid inrush of surface rain water into the pit. The water diverted along the garland drain/bund shall always be diverted away from the quarry.

(v) The existing tension cracks , if any shall be filled and sealed properly before the onset of the Monsoon. The drain should be properly graded to prompt quick water movement and minimize the chances of ponding.

(vi) High wall slopes shall be checked for loose rock and adequate precautions shall be taken for a distance of 15m on either side of the equipment under operation.

(vii) Persons shall not work or travel between the equipment and the high wall, where there is a likelihood of fall of rock or other injury.

(viii) A hand plan of 500:1 showing the sequence of extraction shall be prepared and given to all Supervisory staffs/Officers.

(ix) The supervisory staff and officers shall be adequately trained for the operation of high wall mining.

(x) Copy of the approved code of practice of high wall mining shall be given to all officers/supervisory staff and the same shall be implemented.

(xi) Stacker conveyor shall always be positioned at the rear end of the launch vehicle to receive coal from the belt conveyor and to transfer to a stock pile.

5.0 A proper navigation system shall be provided to constantly monitor the position of the continuous miner using HORTA (Honeywell Ore Recovery Tunneling Aid ) or similar equipment with facilities to give the coordinates of the cut/gallery. This system shall ensure guiding of continuous miner according the planned headings. This system shall also ensure horizon control of the miner, gradient, parting between the seams, web pillar left between the galleries etc.

5.1 A steering system shall be provided to steer the continuous miner while progressing and even crossing undulations. This system shall ensure that the rib thickness is not compromised or reduced.

5.2 Continuous miner proposed to be used shall be of approved type by this Directorate.

5.3 No workings shall be made or extended when the % of inflammable gas exceeds 1.25 in the face. Sensors shall be provided on the machine for shutting off power when the % of inflammable gas reaches 1.25%.

5.4 All electrical provided in addcars shall be flame proof and one such Addcar motor shall be subjected for test for FLP features in India.

6.0 After extraction of lower/bottom seam, the out bye areas shall be filled upto the level of the next higher seam to ensure complete packing of opening of the galleries to prevent heating. The galleries shall be filled with incombustible materials/debris within 72 hors of completion of drivages. The entire lengths of

cuts/Galleries shall also be filled with inert gases such as N2 or CO2 immediately before sealing off the mouth/entries by the incombustible materials/debris.

6.1 The width of the bench formed against high wall shall not be less than the maximum dimension of the launch vehicle plus 10m for movement of machinery, like front end loaders, dozers , graders, tractors with or without attachments.

6.2 The width shall be suitably increased in case of any need for safe operation and type of equipment in use and any other operational considerations.

6.3Bench formed by dump material shall be thoroughly consolidated prior to installation for safe positioning and operation of equipment.

7.0 Sequence of seam extraction. The sequence of extraction of coal shall be as follows:

7.1 Generally the extraction of seams shall be done starting from bottom in ascending order with opening designed for stability.

7.2 Superimposition of galleries and pillars shall be maintained in workings of all the seam.

8.0 Blasting operations shall not be carried out within 500m from the hog wall equipment. No person shall be admitted in to the galleries made in high wall.

9.0 Adequate instrumentation shall be provided to monitor noxious or inflammable gases at the machine face and a written record shall be maintained for the same.

9.1 In the gallery, coal cutting shall be done in conjunction with jet of water with adequate pressure and the generation of inflammable gas shall be continuously monitored, diluted or removed by air/inert gases.

9.2 Air born dust shall be suitably suppressed by adequate water spraying arrangements.

9.3 Water spraying arrangement/wet cutting shall be provided with the continuous miner to reduce coal dust concentration. A flow of 155-195 lit/min of water shall be supplied to the machine.

9.4 On continuous miner inter-locking arrangement in the form of flow switch with safety circuit shall be provided to trip the machine power centre stopping the equipment completely in the event of dropping of water flow rate in the miner at or below 160 lpm.

10.0 Adequate supply of water for all operations and dust suppression measures shall be provided at the site by portable water tankers and necessary head shall be created either by pumping or by sitting the tanks at an elevated platform to create the desired head.

10.1 A strong blast shield shall be provided on the front side of the launch vehicle to prevent propagation of fire/blast etc., if any from the gallery.

11.0 Supervision, control and Management:

11.1 The operation shall be carried out under the supervision and guidance of representatives of M/s Advance Mining Technologies, who would be operating this panel on contract basis complying with all statutory obligations.

11.2 Adequate no. of trained competent persons shall be provided for the operation and maintenance of high wall machinery and other associated equipment and they shall be duly authorized in writing by the Manager.

11.3 Extraction of coal by high wall mining technology shall be kept under overall supervision of one exclusive First Class Assistant Manager, who shall place and coordinate the work well in advance.

11.4 The entire operations in each shift of the high wall mining technology shall be placed under the charge of a 2nd Class Asst./Under Manager, who shall be assisted by an engineer having adequate experience for carrying the maintenance operations in each shift.

11.5 Overall maintenance, testing and repair operations shall be kept under the supervision of a senior engineer possessing Degree in Engineering under the overall incharge of Ist Class Asst. Manager. He shall also be provided with required facilities and adequate trained manpower for the purpose.

11.6 Routine condition monitoring and performance monitoring of the high wall mining face and associated equipments shall be kept under an Asst. Manager duly authorized for the purpose, who shall be provided with adequate number of competent persons and facilities for the purpose.

12.0 Standard Operating Procedures:

Code of Practice for high wall mining operations shall be framed and submitted to this Directorate for approval.

12.1 Standard Operating Procedures (SOP) for installation and maintenance of face machinery & equipment viz. Continuous Miner, Loader, Launch vehicle and associated power equipments shall be framed based on the hazard apprehended. A copy of the SOP shall also be submitted to the Directorate.

12.2 Suitable lock out system shall be provided and maintained at Continuous miner, in order to prevent self starting of the machinery inadvertently during repair or maintenance work.

12.3 A contingency plan for withdrawal of continuous miner safely in case of breakdown or stoppage de to fall or any other reason shall be formulated and submitted to the Directorate.

13.0 Safety Management Plan: A Safety management plan (SMP) on the manner of extraction, operation of

continuous miner, Launch vehicle and also for very principal activities during extraction is required to be prepared based on the Risk Assessment conducted at the mine and the suggestive control mechanism for each hazard/sub-hazard shall be documented, which shall also to be reviewed periodically for addition, deletion or modification as detailed in DGMS (Tech)(S&T) Circular No.13 of 2002.

14.0 Emergency plan: For dealing the situation, in the event of any abnormality Emergency plan shall be made and submitted to this Directorate. All persons working in the panel shall be appraised and trained in this regard.

M.M Sharma

Director General of Mines Safety

Extraction of II Seam by Highwall mining at RG OCP-II

II Seam has a maximum thickness of 4.1m that can be extracted in Panel-A and Panel-B. Alternate short and long cuts were designed in this seam for achieving maximum recovery. Highwall mining in Panel-A was designed along apparent dip, because mine barrier at this particular location was in dip direction and 4.1 m height of coal seam was not sufficient for the machine to drive in apparent dip. That is why proposal of Highwall mining in Panel-A was cancelled. And first Highwall mining started in Panel-B of II seam then in I seam.

Extraction in Panel-B of II- Seam:

First Coal extraction by Highwall mining at RG OC-II was started in Panel-B of II seam. Panel-B lies along dip-rise direction of the seam. The seam is having 0.75m clay parting lying 1.0m below the roof. Initially 0.5m of coal seam was left to keep intact the clay parting, but after some instances of roof collapses during web drivages 1.0m thickness of coal was left below clay for roof stability. After that no any cases of roof collapse was observed. In some holes, stone bands were observed while driving (in mid-length), which forced the machine to retreat back. Panel-B was designed for alternate long and short holes for better stability and maximum recovery of coal. The pattern is shown in next page.

A VIEW OF HIGHWALL MINING MACHINE WORKING AT RG OCP-II OF SCCL

PLANE VIEW OF HIGHWALL AT OC-II

Panel-A

Panel-B

Active Quarry Edge

Surface Boundary Edge

Seam-II

Seam-I

Sectional View of Highwall panels

Pattern of Long and short web cuts Seam-II Characteristics

Empirical design parameters for Ground Control

Seam Parameters Panel-B (in mtrs)

Seam height 6.0

Extractable height 4.1

Extraction height 4.0

0.75m clay parting

1.0m coal left Against roof

Height of web cut

Panel-APanel-B

8.5m

Web Cuts Web Pillars

II Seam

Gallery width 3.5

No. of Long galleries/entries 28

No. of short galleries 28

Panel Avg. Length 325

Web width of Long holes(WL) 8.5

Web width of short holes (WS 2.5

Avg. length of Long holes (LL) 145

Max. length of short holes(LS) 40

% of extraction 36.6

Cover 140-170

Seam gradient 1 in 4.9

Factor of safety 2.0

Web pillars were designed by using CIMFR`s pillar design formula. Factor of safety was assumed 2.0 for long stability. Using CIMFR formula and data shown in above table, the graph between depth of cover and width of web pillar has been drawn here.

Web Pillar Width for 3.5m wide hole and FOS-2.0 using CIMFR formula

0.00

5.00

10.00

15.00

20.00

25.00

50 100 150 200 250 300 350 400 450 500 550 600 650

Depth of Cover(m)

Wid

th o

f W

eb P

illar

(m)

For 4m Gallery height

For 3m Galler height

Graph of web pillar vs. depth of Cover as per CIMFR formula

The comparative graph by using the above mentioned data and Mark- Bieniawski formula of pillar design has been drawn as shown below.

Web Pillar Width for 3.5m Wide Hole with stability factor 2.0 using Mark- Bieniawski formula

0.00

5.00

10.00

15.00

20.00

25.00

30.00

35.00

40.00

50 100 150 200 250 300 350 400 450 500 550 600 650

Depth of Cover (m)

Wid

th o

f W

eb

Pil

lar

(m)

FOR 4m Gallery height

FOR 3m Gallery height

Graph of web pillar vs. depth of Cover as per Mark-Bieniawski formula

Design of barrier pillar :

Barrier pillar should be left at regular intervals. While designing barrier pillars

to ensure arresting of cascading web pillar collapse, following assumptions were

considered.

1) Barrier pillars should be able to take additional load from the excavated

portion on both sides assuming complete collapse of the web pillars.

2) Wilson’s approach is used to find the abutment loading on to the barrier

pillars from the caved goaf. This approach is based on the theory that the full

cover pressure in the goaf occurs at 0.3xH distance from the edge of the

barrier pillar.

3) The barrier pillars should have a minimum safety factor of 2.0, when the web

pillars on one side of the barrier are fully crushed forming a caved zone; and

a minimum safety factor of 1.5 when both the sides are caved.

4) The spacing between the barrier pillar should be such that the width of each

sub-panel should not exceed the critical width resulting in the occurrence of

full cover pressure in the middle of the sub-panel, say WBP < 0.6 x H.

Re-design of web thickness in II Seam extraction from numerical modeling analysis by CIMFR.Earlier web thickness design was based on the theoretical vertical cover acting on the web pillars based on the final slope angle and seam inclination. However the

numerical modeling studies show that the vertical stress at the seam level may be considerably more than the cover pressure near the slope surface due to the induced stresses caused by heavy blasting of open pit. However the vertical stress equals the theoretical cover pressure after about 100-120m of whole length. At 40m short hole length the actual cover pressure may be 70% higher which may bring down the safety factors to about 1.2-1.4. At 85m hole length, there is still about 20% increase in the vertical stress as compared to the theoretical cover pressure. At 160m hole length, it is as per the theoretical cover pressure.

With this additional knowledge following points are strongly recommended:

1. For 85m long hole region, after considering hole deviation and 20% increase in vertical stress, the web pillar thickness required is 5.5m; and all the holes could be kept long, up to 85m length.

2. For region beyond the 100m face length, towards active workings of the mine, where the long hole length will be 160m, the web pillar thickness should be 8.5m; all the holes could be long (160m) and the short holes (40m) as per earlier design should not be driven.

3. The concept of an alternate short hole (40m) for improving recovery should not be practiced for Seam II from now on.

The above change will result in about 4% less recovery, but considering better stability and success of the first Highwall face, CIMFR strongly recommends the above extraction pattern and suggests to change the current pattern with immediate effect. Please do the needful to effect the above changes and may also discuss with the SCCL officers on this matter if needed.

Extraction of I -Seam :

After completion of extraction by Highwall mining in Panel- B of II- seam, overburden was filled upto Panel-B of I-seam to extract I-seam. Extraction in Panel- A of I- seam was done after extraction in Panel- B of I-seam and back filled upto Panel- A. The web and barrier pillars were designed by taking FOS= 1.5, considering the short term stability of web cuts. The web pillars in this seam are designed only for short-term stability; say a minimum safety factor of 1.5. All the extractions are to the full length possible. However, this requires leaving barrier pillars at regular intervals.

Design parameters for I-seam are shown below.

Seam

Parameters Panel-A (in mtrs) Panel-B (in mtrs)

I Seam

Seam height 6.7 6.7

Gallery width 3.5 3.5

Extraction height 4.0 4.0

Web pillar width 4.8 5.5

Barrier pillar width 15.0 17.0

No. of galleries/entries 35 38

Panel Avg. width 320 375

Panel Avg. Length 107 135

Sub panel width 78.2 93.5

No. of web cuts 10 11

Cover 15-140 115-140

Factor of safety 1.5 1.5

Plan view of Pattern of web cuts in I-Seam with sub- panel barrier

Drawing depth of cover vs. width of web pillar as shown in fig. below.

Sp =0.27pc * h-0.36 + (H/250 +1)(We/h -1) Mpa

LP= SV(W + WE)/W where, SV = 0.025H , WE = 3.5m and H = Avg. depth of cover , FOS =1.5

Web Pillar width for 3.5m wide hole with stability factor of 1.5 using CIMFR formula

0.00

5.00

10.00

15.00

20.00

50 100 150 200 250 300 350 400 450 500 550 600 650

Depth of Cover(m)

Wid

th o

f W

eb P

illar

(m)

For 4m Gallery height

For 3m Gallery height

Graph of web pillar vs. depth of Cover as per CIMFR formula

Problems faced while operating Highwall mining at RGOCP-II

Lack of expertism in Highwall mining in India posed a problem in proper operation of machine.

Navigation system was not able to picturise clearly the parting being left against the roof.

Instance of roof fall for full length of hole while cutting the coal.

Highwall mining along apparent dip was not successful in Panel-A, due to face in dip- rise direction.

Back filling upto the seam level for shifting the machine in another seam caused delay in operation.

Encountering stone bands in the face.

Accumulation of water in the galleries due to intermittent delays in drivage when m/c is under breakdown.

Post Highwall mining activities

The entire lengths of cuts/galleries shall be filled with inert gases such as N2

or CO2 immediately before sealing off the mouth/entries. At RG OCP-II, atmospheric N2 was used, but in author’s view as N2 is lighter than air, so it is not suitable for dip galleries.

The galleries shall be filled with incombustible materials/debris within 72 hours of completion of drivages.

Complete back filling of area with O/B, so that chances of accumulation of rain water in the cuts/galleries will be less.

Suggestions for future Highwall Mining operations put by CIMFR

When the open pit touches its boundaries and being worked with benches, the Highwall operations may be planned in descending order. The working benches will provide platform for the extraction of the remaining portion of the seams.

The Highwall operations may go parallel with the open-pit final slope formation in descending order.

A suitable backfilling technology may be developed in order to improve the recovery and ground condition.

A suitable Highwall cutter may be used such that it takes a narrow cut while entering and widens the cut while retreating by thinning down the web pillar on both the sides. This can give better ground conditions and can also improve the recovery.

CONCLUSIONS

Highwall mining is an efficient and economic means of surface mining. HWM requires a thorough knowledge of the strength and physical properties of the immediate roof, coal, and immediate floor strata. Engineering design and surveying the orientation of each HWM cut is necessary to avoid ground control problems. Ground control is critical in HWM because a significant portion of the project cost (i.e. cost of HWM machine) lies beneath the hillside with virtually no easy access to resolve a roof fall or pillar squeeze. The analytical design equations provided in this paper present the base from which web pillar design should begin. However, as illustrated by the case histories, site conditions and characteristics of the coal seam and immediate roof strata frequently require deviation from the web pillar width calculated using the Mark/Bieniawski equation or CIMFR equation. In author’s view Mark-Bieniawski formula is more suitable for designing long web pillars of Highwall mining.

REFERENCES

1. Newman, David President ,Appalachian Mining and Engineering, Inc. Lexington, KY and Zipf, R. Karl Mining Engineer, NIOSH-Pittsburgh Research Laboratory Pittsburgh, P. Analysis of Highwall Mining Stability - The Effect of Multiple Seams and Prior Auger Mining on Design.

2. Shen, Baotang and E. Duncan Fama, Mary. Review of Highwall mining experience in Australia and a case study

3. Vandergrift, Tom. Associate Agapitos Associates, Inc.Golden, CO, USA and Arcia,Juan, Senior Mining Engineer Kennecott Energy, Colowyo Mine Meeker, CO, USA. Highwall Mining in a Multiple-Seam, Western United States Setting

Design and Performance. 24th International Conference on Ground Control in Mining Morgantown, West Virginia, USA, August 2-4, 2005.4. Mark, Christopher, PhD and E. Chase, Frank. Analysis of Retreat Mining Pillar

Stability (ARMPS). 5. Karl, R., JR, Zipf and Christopher Mark. Ground Control for Highwall Mining.

6. CIMFR, Extraction Strategy and Pillar Design for Highwall Mining at

RG OCP-II of SCCL.

7. Rao, Nageshwar, Gen. Manager, SRP, SCCL. Summary of Highwall Mining at OC-II, of SCCL.

TIKESHWAR MAHTO DY. MANAGER, RG OCP-II, SCCL