Ground control continues to be a crucial issue in Indian coal mines .....What we learnt?

* B. Ramesh Kumar Director(Planning & Projects) ** MD. Suresh Kumar, Addl Mgr (Mech.Cell)

1. INTRODUCTION

Ground control is still a crucial issue in Indian coal mines affecting production and safety. The technique of ground stabilization is a continuous process of technological development. It needs to be upgraded on par with the increasing scale of mechanisation and the depth of mining. There are numerous research works being taken up continuously all around the world to serve the mining industry in its changing scenario. Ground control is in advanced stage by reinforcing the roof & sides in terms of active support and secondary supports in the few coal producing countries. India, being third coal producer in the world, is in the mid way in achieving this.Effective roof reinforcement is an essential requirement in the mechanised faces. An improper supporting needs secondary supports which again involves men exposure, additional material requirement and additional time for supporting. Secondary supports are always uneconomical and increases risk factor of men involvement. Hence, supporting freshly exposed roof is one of the major requirements to improve safety and productivity in underground mines. The fall of roof and sides is the predominant cause of accidents in complied accidents statistics in Indian underground coal mines. Though some ‘advanced supports’ which can be set in minutes and can develop roof reinforcement in seconds are being practiced in our underground mines, the tale of accidents due to roof and side falls continues to be major cause of accidents for past several years.

2. NEED OF AN HOUR

The increased coal demands and accelerated growth of our country’s economy put immense pressure on coal mining sector to produce bulk coal from underground mines at ‘Zero accident rate’. Heavy mechanization and introduction of new technologies have made this possible. Both CIL and SCCL are in the verge of eradicating manual mining. Perhaps, the manual mining has been replaced by SDLs and LHDs in the most of the mines. Thick seams are being successfully extracted with Blasting Gallery technique in SCCL. The Continuous Miners have been introduced in the mines Anjana Hill, Chirimiri (SECL), Tansi (WCL), Jhanjra (ECL) and Surfi (ECL) of CIL and VK-7, GDK-11A mines of SCCL which are proving its potentiality. The success of Continuous Miners has brought solution for bulk production and extraction of coal standing on pillars. The first ever high productive and highly sophisticated Longwall technology is under implementation stage in Adriyala Longwall Project of SCCL. Though the scale of mechanisation that CIL and SCCL are implementing presently is definitely a solution for reducing the accidents, particularly those are due to ground fall. Concurrently, the present depth of operation has crossed 400m and the new projects are being planned upto the depth of 600m. At this depth, the coal deposits required to be extracted under difficult geo-mining conditions. The advanced mechanisation needs wide and high galleries in the coal faces, loading points, junctions and in the other operational areas. The movement of trackless mining equipment demands roof support by tensile bolting in the least possible time to allow free passage of equipment in the production areas.

Despite the fact, that roof bolting is being used as primary support in our mines, the ground control is still a major issues and main cause of accidents in Indian coal mines. Hence, there is a necessity for a scientific introspection of the existing support system and the adoption of world class reinforcement techniques for roof and sides.

3. BEST PRACTICES

3.1. Replacing cement with resin

3.1.1. Resin grout and its superiority

The load bearing of the cement grouted bolts starts only after its setting time i.e. 30min to 2hours. Still such time the miners are exposed to unsupported areas for long durations. It is proved that the cement grout failed in watery and weathered strata. It is also observed that the full column grouted bolts using cement capsules left with a column of 40-70cm void which gives scope for roof dilation. Whereas resin grout is proved its supremacy over cement and is being widely used all around the world. Resin grout is more effective and efficient because of its active support nature, utilizing the rock to support itself by applying its internal reinforcing stresses. The casted resin grout having compressive strength of more than 80MPa, could generate a bond strength of 25-30T within few seconds when proper consumables are used and effective pre-tension is given. Perhaps, resin provides opportunity for pre-tension at higher axial load in a short span of time. A fully grouted bolt resists both vertical and lateral stress from strata. The casted resin in the wet holes completely seals and excludes air, thus reducing corrosion of the bolt assembly and weathering of rock.Further, the resin grout is best suited for weak, geologically disturbed, fractured and watery strata. The obvious advantages of resin capsules in different strata conditions, demands the use of resins irrespective of the strata conditions. Presently, resin grout is being used for strong and sand stone roof using pneumatic bolters. But the prime requirement is immediate supporting of coal roof in development faces as a ‘green roof support’. Hence, there is an emerging requirement to replace cement grout with resin grout in all our underground mines.

3.1.2. Understanding the science of resin application

The active supports used for roof reinforcement are bolting, shotcrete, chemical grouting and active pre-set props. These supports have replaced passive supports such as cog, props and steel beams. Despite the passive nature, the advantages of passive supports are that the health of the supports and the strata movement can be observed visibly and timely. Hence it is easier for miners to take timely remedy such as erection of additional supports and replacing the older supports. Whereas the bolting with resin is applied interior to the rock mass and in the most of the cases the health of bolting and the quality of grouting are not readily known. Therefore, more scientific approach is required in selecting, designing and executing the resin bolting application. 3.1.2.1. Annular Space Annular space is an important parameter in resin bolting system. Annular space is difference between radius of drill hole and roof bolt. Larger the hole diameter, poor the confinement of the resin, reduction in the bond strength which reduces the load transfer capability of resin bond. Further, the larger holes can result in poor resin mixing, a greater likelihood of “finger-gloving,”

(Fabjanczyk and Tarrant, 1992). The smaller the annulus, the more efficient is the mixing of the resin as well as more effective is the shredding of the plastic casing of cartridge. Further the smaller the thickness of the resin, the stiffer the bond between the bolt and the rock. An extensive laboratory and field tests have been conducted all over the world in this regard and found the annulus of 3-5mm is best suited for resin application (Fairhurst and Singh, 1974; Karabin and Debevic, 1976; Ulrich et al., 1989). It is observed that the combination of 27mm holes, 20mm bolts and 24mm resin capsules gives best results in both coal and stone roof. Any further reduction in annulus, below 3mm may create negative effect on resin. It is also observed in the mines that, 22mm dia bolts in 27mm holes sometimes result in over-heating of resin as the annulus is only 2.5mm. Such overheating accelerates premature setting of resin which prevents bolts insertion to its full length.

3.1.2.2. Roof boltsThe resin grout provides compressive strength of 80 MPa, bond strength of more than 25T and shear strength of more than 15T which is higher than the physico-mechanical properties of steel, currently used in mines. Therefore the steel bolt fails earlier to resin. Hence roof bolts shall be of thermo-mechanically treated (TMT) rebar manufactured from MS Grade Fe-500, Fe-600 or above (DGMS circular No.3 of 2010 & No.11 of 2009). The resin grout between roof bolts and the strata generates only mechanical bond and resistance to the vertical, horizontal and shear forces. 14Strength of this mechanical bond merely depends on mechanical strength of the bolt and the ‘Grip factor’. The ‘Ribs’ in the bolts play major role to ensure grip factor and surface contact. Grip factor depends on height, angle and spacing of ribs.

3.1.2.3. Length of BoltLength of the roof bolt must be 1.8 m to ensure better bolt to bolt interaction. It is be borne in mind that a roof bolt assembly is a component in beam building. Two or more such component build beam. Hence the stiffness created by one bolt in its surrounding strata should have interaction with other one. So the height of the bolt and the spacing between two adjacent bolts are crucial factors. Bolt height of 1.8m at 1m grid pattern always creates better interaction and stiffness in the entire beam of the gallery span. In addition, the local strata condition also determines the length of the bolts. In case of presence of weak strata, clay in the roof within 1.8m, the length of the bolts shall be increased to 2.4m

3.1.2.4. Nuts and Thread The thread and the nut must be of equal strength so that it should not slip at the yield strength of the bolts. The thread prepared by machine cut reduces its strength. Hence the thread on the roof bolt shall be M20 cold rolled thread conforming to IS 4218. (Part- 2 & 3). The nut shall be of hexagonal shape of thickness not less than 30mm conforming to IS 1363 (Part-3) and the thread shall conform to tolerance class of 7H. (DGMs circular No.11 of 2009)

3.1.2.5. Bearing plate ( Dome washer plate) Dome washer is flat bearing plate domed at the center. It has a compatible central hole of required size and an angled side to accommodate conical seat and nut. The action of pre-tension simultaneously induces stiffness in the bolt and keeps the dome washer tight against the roof by deforming it. Thus it develops a proactive tensile stress around the collar of the bolt. The collar failure is the one, frequently occurs in the cement grouted bolt as the system generally fails to

develop stiffness in the bolt column and it get transferred to collar of the bolt. Further a dome washer takes the shape of the roof profile on pretension and gets full contact to the roof profile in gradient.

3.1.2.6. Torque nut/Conical seatThe torque nut is designed to ensure that it does not rotate relative to the bolt during the bolt installation. It is placed between dome washer & nut and gets tightened along with nut. The curvature portion of the nut ensures full contact to the dome washer when bolt is installed in angle or in gradients. When toque is applied to the nut, the conical seat transfers the load in all the directions equally in the contact area. Torque nuts shall be of forged steel and shall not deform 1.2 times of yield load of the bolt. There were numerous field studies conducted and laboratory testing was done in the countries like USA, South Africa and Australia to understand the efficient procedure of resin bolting system and correct combination its consumables. The best practices of resin application all around the world is reproduced here in the table No.1:

Bolt -steel Ribbed rebar, upto 600MPa Tensile strength

USA, South Africa (Dr J.N. van der Merwe)

Ribbed rebar, upto400-500 MPa tensile strength

Australia

Bolt dia 20mm & 22mm USA, Australia (PC Hagan, UNSW, Sydney)S/Africa (Dr J.N. van der Merwe)

Hole dia 25mm, 27mm,29mm USA, Australia (PC Hagan, UNSW, Sydney)S/ Africa (Dr J.N. van der Merwe)

Annulus 3mm USA (Roof bolting and Place change practice in US coal mines by William S McIntyre Jr, Chief Design Engineer, JH Fletcher and Co, USA)

2.5 to 4.5mm

3mm

Australia (Fabjanczyk and Tarrant, 1992)

Dr.PC.Hagan. UNSW2.5 to 3.8mm South African mines-A a survey by University of Van

PretoriaDr J.N. van der Merwe, South Africa

Pretension 70% of Tensile strength of rebar ( say 10T)

USA ( Ref: Roof bolting and Place change practice in US coal mines by William S McIntyre Jr, JH Fletcher)

Torque upto 260Nm with 500 KN thrust.(say-5T) Bolter RPM=250-500

South African mines-A a survey by University of Van Pretoria

Australian practiceNut Shear pin break out type @

80-120NmUSA, Australia, South Africa

Table No.1 Best practice of Resin bolting in various Countries.

Hence, it is necessary to promote our standards of rock bolting and reinforcing techniques on par with the international standards accompanied by the site specific studies to select the best suited consumables for our underground coal mines.

3.1.3. Understanding the correct bolting procedure

3.1.3.1. Roof supporting timeThe time taken for roof supporting after its exposure is an important factor to prevent roof dilation due to redistribution of forces. Therefore, *The Roof Response Time (RRT) or the Roof Standup Time (RST) of any working face must be well studied. It is learnt that of the stress following the excavation is immediate, but the failure process is time dependent. But it varies with geological, geotechnical and local geo-mining factors. It varies seam to seam and mine to mine. This is one of the critical measures for selection of bolting system and bolting machines. The available work force, bolting equipment and the bolting system shall be accordingly planned at unit level so that every meter of exposed roof in the mines shall be supported well before its dilation or bed separation.

*Roof Response time/ Roof stand up time: Time taken between roof exposure and first symptom of stress relief in the form of layering. Mark (1988 Australia): For the first time, he has drawn up relationship of stand up time and roof failure for tunnel. Bieniawski (1989): quantified this relationship between RMR and the roof stand up time. Molinda & Mark (1994 ASA) attempted to quantify roof response time with respect to CMRR for US mines. Dr. JN. Van der Merve (SA) found that redistribution of stress following the excavation is immediate, but the failure process is time dependent.

3.1.3.2. Full encapsulation 1When the bolts are partially encapsulated, the entire free length of the bolt is loaded by the sum of all loads coming from individual displacement zones. Therefore, the load from the adjacent strata exceeds the yield strength of the bolt and the bolt becomes soft (plastic mode). This significant elongation of the bolt may then allow roof deformation. The full encapsulation of bolts dramatically improves the support stiffness. Individual dilation horizons affect only the corresponding local section of the fully grouted bolt. This will minimise or delay the building up of loads exceeding the yield strength of the bolt. The more contact area along the bolt, the better the resistance offered by resin grout to the surrounding rock.

3.1.3.3. PretensionPre-tension is the direct application of an axial tensile stress into the roof bolt shortly after its setting. When an axial thrust or torque is given, a small pull load is applied in the bolt. With the effective resin anchorage, it generates stiffness in the bolt, grout and adjacent strata.

This stiffness in the bolt, grout and adjacent strata induces compressive and tensile stress around the adjacent rock beds.

It creates a stiff and stable beam by reinforcing different roof layers. The pre-tensioned bolts clamp the thinly laminated roof beds into a thick beam which is

more resistant to bending. A stronger beam can be built with the same bolt by applying larger installed load.

By increasing the frictional resistance along bedding planes, roof sag and deflection are minimised.

The lateral movement due to horizontal stress is also minimised. Pre-tensioned bolts are more effective when the immediate roof layers are weak. The pretension ensures the bearing plate is very tight against the immediate roof.

When the pretension is lost, the normal stress on the lamination interfaces is also lost and the layers are free to slide (Dr. JN. Van der Merve, 1998). To achieve a stiff and strong beam, a torque of nearly 150KN must be applied to tighten the bolt when resin grout is used. Besides, there are some practical difficulties in applying the required torque with the help of hand held pneumatic bolters. The inefficient air pressure or air leakage still affects the available torque to great extent. Hence, manual tension in addition to the bolter is required to be given immediately after setting time of the resin is elapsed. This is possible using torque wrenches to some extent. However higher capacity torque wrenches shall be purchased for this purpose. The heavy duty mechanised bolters are having inherent advantages that they could apply more than 200KN toque for pre-tensioning. The current trend around the world is to apply higher pretension load upto 400 KN.

3.1.3.4. ReinforcementLongwall gateroads, permanent galleries in coal roof or any other drivage which requires long term stability needs roof bolting accomplished with reinforcements like wire mesh and W-straps. The length of W-straps shall cover entire gallery span and the hole spacing pre-designed in the w-straps shall match with bolting pattern. The Strength provided by the wire mesh and w-straps can be attained by using pre-fabricated wire mesh.

3.2. Mobile bolting rigs in underground replacing hand drilling

The introduction of pneumatic bolters in our underground mines for roof bolting has considerably reduced the human drudgery. But the utility of pneumatic bolters are restricted to the faces being worked with sand stone roof except in few in coal roof. The inherent drawbacks in the pneumatic bolters and the option for dry drilling refrain ourselves from using these in faces having coal, shale, weak and watery roof. Hence, the option for dry drilling with simple mechanism is to be explored. However, the mobile bolting rigs have to be introduced for long term requirement. It must suit to the working conditions of our mines. This also ensures the implementation of the recommendations of 10th conference of safety in mines. Further, it shall ensure proper drilling for roof bolting in all types of roof strata. Machines shall be capable of being operated from a distance or be provided with suitable canopy to protect the supporting personnel during drilling or bolting operations. It should give immediate reinforcement in poor roof and watery strata. It should also have ability of developing effective pretension to bolts in the order of 300-400KN.

4. FEW INNOVATIONS IN GROUND CONTROL TECHNIQUES

4.1. Cable bolting with pre-tensioned steel wire rope

Cable bolting practice has been started in Indian coal mines atleast two decades back and constantly improvised with different form. Earlier, old haulage ropes were grouted using cement grout. Then cement was replaced by quick setting cement capsules upto the roof horizon of 3-4m. Later, a special cable grouting system (Breather tube method) was introduced where cable

bolts were fixed to higher horizons upto 8-9m. Grouting was done by injecting liquid cement grout in the bore holes using compressed air pump along with air pipe. The entire column of cement was retained in the hole by wedging a wooden plug at the hole mouth. This system is believed to generate nearly 20-25T anchorage. Recent days, these cable bolts were replaced by pretensioned ‘Flexibolts’ and stranded bolts using resin grout or chemical grout. These cables are stiff enough for installation similar to solid bolts, flexible enough to allow installation of long lengths and could be pre-tensioned up to 5T using a nut on the threaded section of the strand. These cables are proved to exert a tensile load of nearly 50-60T to the surrounding strata. As the stratified coal/stone beams strong in compression and weak in tension, most roof collapses are caused by tensile failure of coal/rock. The cable bolting system decreases the tensile stress in the strata.Where there is a requirement of clamping of thicker roof beam which may be prone to dilation such as Blasting Gallery districts and Continuous Miner faces in pillar extraction districts, this cable bolting with resin anchor must be implemented in the days to come. (Technology news, US bureau of mines, US dept of the interior, No 433 dated Apl 1994)

4.2. GRP bolts/fiber bolts/Carbon fibre bolts for ribs

Traditionally, these products have always been priced well above that of steel bolts. The way that the steel prices are going, this type of support medium, used together with resin is becoming favorable. GRP bolts are totally non-corrodible and have strength comparable to steel bolts. They are cuttable as well as flexible so can be used in low stopes where longer bolts are needed.

4.3. Chemical anchor

The first commercially economic chemical bolts were manufactured from mild steel round bar (21.7mm diameter) and had a 150mm rolled M24 thread on each end. One end was threaded to fit a nut whereas the other end was threaded to provide deformations on the bar to improve the resin bond strength. The type of deformations achieved by threading was not very efficient. The relatively small pitch of the thread form (3mm ) generated only marginal radial forces required to wedge the resin between the bolt and the rock. The resistance, which prevented the bolt from being pulled out of the hole, relied mainly on the shear strength of the resin between the threads or on the rock interface. Since the shear strength of the resin is about 3 times less than its compressive strength the anchors could fail at a relatively low load. Subsequent extension of the anchor length by threading longer sections of the bar (up to 700mm) significantly improved its performance, but the real breakthrough was the introduction of the ribbed LHA (Left Hand Anchor) bar or “T-bar” (Norris & Yearby, 1980).The spacing of the ribs was 10 to 12 mm and their average height was 0.7 – 0.8 mm. The ribs move against the resin column as the bolt is loaded by strata movement and develop radial (wedging) forces which compresses the resin between the rock and the bolt ribs. This then resists the bolt/rock relative movement in a more efficient (stiffer) way when compared to the threaded bar. Further improvements in the anchorage stiffness have been achieved by the recent introduction of bolts (AXR and HPC) with significantly higher ribs when compared to the LHA bar. Their rib heights, which vary between 1.3 to 1.6 mm provide a 40 to 60% increase in the anchorage peak shear stress.

4.4. Truss support

These supports are very popular in USA mines in the recent days. Basically two bolts are fixed with resin grout at side corner 0.6 to 0.9 m from the edges at 45o angle into solid pillar. These angled bolts connected by horizontal members with turn buckles. On tensioning the horizontal beam using turn buckle, the bolts with bearing plates get tensioned with approximately 17800N force. These supports are relatively cheaper and very useful in weak strata. This may be useful for Longwall gate roads, rooms & half levels of BG panels and entries of Continuous Miner districts. The attachments like horizontal beam and turn buckles are re-usable and can be used in conjunction with long cable bolts too. ( Ref: Roof bolting and Place cnange practice in US coal mines by William S McIntyre Jr, Chief Design Engineer, JH Fletcher and Co, USA)

Fig No.1 Truss support in underground

4.5. Foams and Resin injection

4.5.1. PUR ( Poly Urethane Resin ) (Gregory M. Molinda, Research Geologist National

Institute for Occupational Safety and Health Pittsburgh Research Laboratory Pittsburgh,

Pennsylvania USA)

Polyurethane injection for ground stabilization in coal mines was first developed by the German coal mine research organization Bergbau-Forschung GmbH in the early 1960’s (Stewart and Hesse, 1985). It became a standard stabilization method in Germany since its commercial introduction in 1971 (Knoblauch, 1994). With the introduction of the RokLok binder system in 1977, polyurethane stabilization, particularly in Longwall recovery, has become common in the U.S. (Stewart and Hesse, 1985). Nowadays Polyurethane injection in coal mines of USA is most commonly used in complex ground conditions including fractured rock, trunk roads, gate roads of Longwall face, face recovery chamber. But often it is applied as a last resort where conventional roof reinforcement and support has failed. It also has a low viscosity which allows it to penetrate cracks as small as 0.04 mm wide. It has engineered expansion properties (1:1 to 1:12) which also allow for penetration. It is both strong and plastic, preserving its’ integrity under load. In main purpose of polyurethane stabilization is to reinforce the fractured rock to the point where it can support its own weight and the weight of unconsolidated rock above it (26th

International Conference on ground control in mining, Strata management in New South Wales by Mr. Rob Thomas, Principal NSW and Australia). Mechanically, the polyurethane forms a beam with rock that has been separated along bedding or is broken into key blocks. It is the size and strength of this beam which enhances the stability of the roof. Once an expertise in the usage of resin is achieved, the utility of PUR injection can be explored for. The compressed air pump presently used for the breather tube cable bolting system can be used for pumping of PUR fluid. Option for replacing cement grout with PUR fluid in the breather cable bolting system also to be explored. Silpur PF 1K, Silpur US Foam, SilPur US Flex are fire resistant urethane-silicate resin available in the market.

Fig No.2 PUR application in underground

4.6. Flexible coating

Tekflex is a high tensile sealant specially designed to permanently stabilise the integrity of rock structures. It is widely used as a mesh replacement and it can able to accommodate the stresses associated with strata movement while providing a barrier to moisture degradation. It is a cement-based sprayable coating designed with highest flexibility, high tensile strength and excellent adhesive qualities.

4.7. Pumped grout support

Minova International is manufacturing both cementitious and resin based strata support grouts and foams. These grouts and foams are formulated to use a high water content, which leads to reduced material usage and reduced transport costs. Products include: Tekblend, Tekset, Durafoam and Tekfoam. These foams are also useful for cable bolting. Wilflex is two component phenolic foam which when pumped through suitable application pump forms rapidly reacting foam. Due to the quick reaction time and very high foaming factor, Wilflex can be used to rapidly fill cavities even in remote locations.

5. PREDICTION AND MONITORING OF STRATA BEHAVIOUR

Prediction of strata behaviour by physical observations like sounds, vibrations and roof to floor indicator props is almost impossible in the semi- mechanised and mechanised faces deploying LHDs and CMs. Hence, monitoring and prediction must depend on instrumentations rather than depending on human judgment. There are number of instruments available either imported or indigenous in the market for measuring

Roof convergence Multi point linear scale tell tales Rotary tell tales Auto warning tell tales and remote convergence indicators

Strata dilation Multi point bore hole extensometer upto 15m height from the working seam

Load on the supports Load cells for OC props in LW and BG faces, Load cells for roof bolts in developmental faces

6. QUALITY TEST

6.1. Anchorage Test and Short encapsulation Pull Test (SEPT)

Anchorage test is normal pull test is conducted in underground to test the bond strength of resin after its setting. 10% of bolts shall be tested for 10T load with the normal ‘Anchorage Test Machine’. Every such test shall be recorded and maintained at mine level.Whereas the Short Encapsulation Pull Test (SEPT) is the destructive test. Normally the resin grout provides anchorage of 80 MPa. Whereas the tensile strength of steel bolt is less than that of resin which means that the steel bolt fails earlier to resin. It is required to measure the anchorage properties of the resin system rather than the strength of the roof bolt. Hence a special bolt with a short encapsulation (international standard is 300mm) is installed in a well supported gallery maintained for the purpose. The bond strength is measured by applying pull load upto 210KN until the resin fails. This is an internationally recognised method of measuring the anchorage or bond properties of resin bolting system. This test must be conducted in our mines also. This test not only measures the performance of a roof bolt, resin, rock system but it also reveals out the strength of steel bolt, thread and nut. Hence it is required to set up ‘SEPT galleries’ in underground to conduct tests.

6.2. Quality checks

The quality checks shall be done at two levels, manufacturers end and at user end. The required test facilities shall be set up at mine level and at area level. The area laboratories can be so established with required facility to conduct quality checks for roof bolts and resin capsules. The facility required for quality control and checks for resin is given in the DGMS circular No.10 of 2009 and for quality control and checks for roof bolts is given in the DGMS circular 11 of 2009 and 3 of 2010. At every mine, proper set up for Short Encapsulation Pull Test (SEPT) must be established. The necessary equipment shall be purchased.

6.2.1. Quality Checks for Steel Bolts

The random sampling shall be collected that atleast 2 roof bolts out of 1000 pieces of roof bolts in a batch shall be thoroughly checked for

1. Its physical parameters such as length, diameter, height of rib, rib angle, thread length, bolt straightness and size of nut etc.,

2. Further the sample shall be subjected to the following test The roof bolt assembly is tested for tensile test and should not fail upto 20T load in case

for use with resin grout. The roof bolt assembly shall be subjected to accelerated weathering test in mine water

and acid water conditions for more than 20 days. Roof bolt assembly shall be subjected to field exposure and corrosion resistivity tests to

test the resistance to corrosion of the steel.The roof bolt shall pass through all the above tests. These tests must be conducted at Area Laboratories. The bolts received at the Central stores/ Area stores must initiate test process. As a second level of check, the batch of bolts received at mine sites, shall also be tested.

6.2.2. Quality Checks for Resin

As on date, the users have been left with little scope on quality check of resin capsules at user end except physical verification and anchorage tests/ or SEPT. The quality check for resin is predominantly of two types:1. Check on ‘Standards of manufacturing and composition of resin’ and testing its physical &

chemical properties at manufacturers end using specified and appropriate apparatus.2. Performance of resin at user end.

a. Testing Physical conditions of resin at mines/laboratories.b. Testing for its bond strength-Short encapsulation Pull Test (SEPT) and normal pull test

that is 10% of the installed bolts.

6.2.2.1. Tests required to be carried out at mines/LaboratoriesApart from the anchorage tests and SEPT, the following checks can be carried out at mines/ laboratories:1. Physical condition of the capsules

a. Proper crimping at ends to prevent leakageb. There shall not be hardening of material inside the capsulec. Capsule should be straight and shall not be too loose or flexibled. Fragileness test of double chamber tube for its damage or breakage.

2. Testing of gel and set time shall be done in the field by pull test.3. Thermal stability of resin shall be tested to be stable from temperature 50C to 450C.4. Some of the physico-mechanical properties of casted resin shall be measured

a. Compressive strengthb. Shear testc. Elastic modulus testd. Shrinkage test

6.2.2.2. Tests required to be carried out at manufacturers endThe following tests for both physical and chemical properties shall be carried out at manufacturers end using specified and appropriate apparatus by SCCL personnel..1. Physical conditions of resin capsules as above.2. Gel time and set time using gel time measuring apparatus.

3. Viscosity of the resin matrix using viscometer (suitable viscosity of fluid ensures proper mixing and grouting).

4. Acid value of resin matrix using PH meter (More value of free acid will lead to rusting of steel bolt).

5. Reaction temperature 6. Water resistance (casted resin should not absorb water which may lead to micro cracks in the

resin matrix) 7. Flammability test (Resin matrix should not catch fire when temperature raised above 1200C).8. Toxicity test.9. Self life of resin. The facilities required for quality control at manufacturing units are given below, so that the manufacturing units also to be visited by company representatives and shall make quality checks accordingly.

7. Monitoring

A team of officers headed by a General Manager at corporate level, area level and unit level in both the disciplines, mining and E&M shall be identified for effective monitoring of the implementation of this policy. Such monitoring committee shall,

ensure implementation of this policy in all the underground mines and departments monitor the bolting activities in the mines monitor the performance of different mechanised bolter including the ‘Health

Monitoring’ of bolters ensure proper spares and consumables available at area stores and mine site. ensure standard size and quality of bolting consumables are purchased, supplied to and

being used at mines co-ordinate with mines, regional laboratories, stores and manufacturers to carry out

different tests and quality checks on roof bolting consumables. ensure ‘SEPT’ galleries are set up in every mines with required test facilities The core committee at corporate level shall have regular interaction with DGMS and

manufacturers.

8. CONCLUSION

Roof bolting in SCCL is being practiced with handheld electric drill machines for drilling and quick setting cement capsules as the grout medium. Drilling in the hard roof was problematic and created drudgery on workmen. When the drills are used to drill holes in moist sandstone roof, the overload protections of the electric drill machines were frequently disturbed creating unsafe conditions. Face blasting was delayed due to break down of drill machines and production was badly affected. Besides the load bearing of the cement grouted bolts starts only after its setting time is elapsed. Until such time the miners are exposed to unsupported areas for long durations. It is proved that the cement grouted bolts in watery and weathered strata got failed. The resin grout maintains its superiority over cement due its, high, effective anchorage capacity in shortest time creating a safe environment in coal face. The resin enables pretension of bolts with high setting load. The anchorage provided by the resin bolts independent of strata type and address to the complex geo-mining conditions. The increased semi mechanisation demands roof support by resin bolting system to allow free passage of trackless mining equipment in the production areas

and to have faster drivage. Since the bolted mine roof can provide an unobstructed opening with minimum maintenance, production has increased, costs have decreased and ventilation has improved.Keeping the above concrete base, SCCL has purchased pneumatic roof bolters with compressors to address the above problems and also to fulfill the recommendations of 10th conference of safety in mines. These bolters were distributed to different mines depending on the requirement given by the mine authorities. But there are few lapses in application of resin bolting system which need to be reviewed at this juncture. Hence, it is need of hour to have a better understanding of resin bolting system and to have a corporate ‘Policy on bolting system’ at this changed scenario of mechanisation in underground mines for sustaining improvements.

Acknowledgements

The authors are very much thankful to the management of The Singareni Collieries Company Limited for permitting to publish this paper. The views expressed in the paper are of the authors and doesn’t necessarily of the organization to which they belong.

References

1. Evaluating development in roof bolting technology in Australian coal mining by DSI minova.

2. Minova guide to Resin grouted rock bolts 3. DGMS circulars4. BS Standard BS7861 Part1:1996, Strata reinforcement support system components used in

coal mines, Part 1. Specifications for rock bolting5. Developments in improving the standard of installation and bond strength of full column

resin roof bolts: Peter Altounyan, GM, RMT,UK, Alan Bugden, Manager, Goedehoop Colliery, Anglo Coal, SA, Donald O’Connor, Managing Director, Minova International, Johannesburg, SA ,Robin Berry, Senior Vice President, Anglo Coal Operations, SA

6. Developments in Long Tendon Systems Used In Coal Mining Reinforcement : Brian Clifford, Senior Geotechnical Engineer , Lorraine Kent Geotechnical Engineer ,Peter Altounyan GM, Dave Bigby Manager, Instrumentation and Research, RMT UK

7. Variation in load transfer of a fully encapsulated rock bolt : PC Hagan, UNSW, Sydney8. Performance of roof support under high Stress in a US coal mine : David Oyler and

Christopher Mark NIOSH Pittsburgh, PA, Winton Gale,SCT, NSW, Australia, Jinsheng Chen,RAG, PA

9. Control mechanism of a tensioned bolt system in the laminated roof with a large horizontal stress : Song Guo, asst VP, John C.Stankus, VP, Jenmar corporation, PA

10. Status of roof bolting research at UNSW, Australia11. Roof bolting and place change practices in coal mines, William S Mcintyre, Chief Design

Engineer, Jh Fletcher, Huntington, USA12. Higher performance in rock-bolting technology by use of immediate-bearing grouted bolts,

Dipl.-Ing. Jürgen Wiegard And Dipl.-Ing. E.A. Eigemann, minova carbotech GMBH13. Rock support in Southern African hard rock mines. Dennis Van Heerden Minova RSA14. Evaluating Development in Roof bolting Technology in Australian Coal Mining –DSI.

15. Field performance testing of fully grouted roof bolts, C. Mark, C. S. Compton, D. R. Dolinar, D. C. Oyler, Natl. Inst. For Occuptnl. Sfty. & Health, Pittsburgh, PA.

(ii) Geotechnical Hazard PlansWhilst Geotechnical Hazard Plans aim to capture a number of thepoints raised in the Coal Mine Health and Safety Regulationsnamely, the estimation and recording of geological conditions thatmay affect roadway stability, critical items to be addressed in a

Geotechnical Hazard Plan can be summarised as follows:

High Strength - High Tension Cable BoltsAn ACARP assessment of high strength, high tension cable bolts was carried out in a particularly weak area wheredevelopment was being slowed by excessive roof movement. The A heading (travel road) of Maingate 7 requiredcable bolting after every about 40 m of drivage, resulting in the machine being flitted to the belt road and “B”heading being driven just slightly behind A heading. The B heading didn’t require cable bolting which was assumedto be due to the stress relieve caused by A heading being driven first. This was further demonstrated when the 80tonne cables in A heading were finally tensioned to 40 tonnes and subsequently the B heading became unstableand required a full complement of cable bolts to regain stability. It was a result of this trial and the constantproblems maintaining stability on installation roads that 80 tonne cables with 40 tonne tension became the

standard support on installation roads.

PUR InjectionThe Crinum mine was one of the largest consumers of PUR in Australian coal mines for two years, exceeding 300 tboth years. This was primarily for longwall falls but also included installation roads and other outbye roads.Bolted Roof InjectionWhen injecting in roadways a procedure was setup which included the following:• Standing support (usually propsetters) was required to be installed in the roadway prior toinjection• Pump pressures were limited to 85 bar to reduce the risk of “jacking” the roof down.• Electronic roof to floor convergence monitoring was employed and roof movement limited to 8 mmin any one hole injected• No inexperienced operators were allowed under the unstable ground• The pump was located outside the unstable area• Low or no expansion PUR was used• The ability to pump more than 200 kg in a single hole not intersecting a coal seam was of greatbenefit• Roof extensometry was reestablished after the PUR had set and before the props were removedLongwall Roof Fall InjectionInjection on longwall roof falls was initially under the control of PUR contractors. However after initial experience,detailed PUR support and injection plans were developed by the geotechnical engineer including number, depths

and angles of holes, injection limitations, use of roof bolts, cable bolts, dowels or spiles and sequence of injection(Figure 17). Injection quantities were required to be submitted at the end of each shift and plotted on the support

plan for migration

Factors that may influence rockbolt anchorage performanceDesign Operation Quality Control(non-conformance with specifications)• spacing• length of rockbolt• rockbolt inclination• strength of rockbolt• resin characteristic• resin annulus thickness• roughness of borehole surface• straightness of borehole• length of borehole• spin and cure times• location and inclination of boreholes• rockbolt diameter – core and rib height• rockbolt straightness• length of rockbolt• surface corrosion• drill bit size• turnover of resin cartridges – physical environment

(temperature, moisture content, etc) and duration in storage

IMPROVEMENTS IN SUPPORT DESIGN AND MINING PRACTICE Smartbolting A roof control strategy known as “Smartbolting” has been adopted by Goedehoop Colliery (Altounyan and Taljaard, 2000)[1] Smartbolting is a roof control strategy based on applying four basic principles:- 1:Understanding the roof failure mechanisms 2:Using a rockbolt system which provides effective support against these failure mechanisms 3:Designing the support pattern using in situ measurement 4:Monitoring the performance of the system Investigations at Goedehoop have confirmed that the principal roof failure mechanisms (lateral shearing due to horizontal stress) are similar to those in many other coalfields worldwide, and aspects of current world best practice, including the use of high strength stiff bolting systems and design based on measurement (Arthur et al 1998)[3], are therefore relevant. The most effective bolting system to resist shear failure is one with high bond strength and stiffness. Short encapsulation pull testing of the bolting system in use confirmed that the existing system had low bond strength and stiffness. An improved rockbolt system with the required performance, and features allowing rapid installation and installation quality and performance audit, has been developed and is now in use.

The use of measurement and monitoring systems depends on the practicality of installing and reading monitoring instrumentation in a room and pillar operation with place changing and extended cuts and special devices and procedure have been developed specifically for the Goedehoop situation. The main objectives of this roof control management system are: 1. To ensure the roof is appropriately supported at all times 2. To systematically detect changes in ground conditions using in situ deformation monitoring, visual observations, short encapsulation pull tests, routine risk assessments and audits 3. To optimise mining and support parameters in accordance with changing conditions based on 2 above. The operation of the Smartbolting roof control management system is described in more detail below:

Bolting System and Installation Procedure

A performance specification for a new bolt system has been drawn up based on performance criteria in particular the short encapsulation pull testing in rock in the laboratory and underground of the roofbolt system. A high bond strength and stiffness is specified to maximise resistance to roof movement and shear. Local suppliers have been able to meet the specification by changing the resin formulation and bar profile. The new standard 20mm bolt is installed in a hole drilled with a 25mm drill bit with a minimum of a 500mm long resin capsule. The properties of the resin

200 annulus results in efficient mixing and a minimum encapsulation length of around 0.9m. Typical test results with the new system are also shown in Figure 7. Other features of existing bolt systems which can lead to poor installation quality include the use of a crimped nut which gives inconsistent breakout and is liable to jam, a low strength cut thread and bolt plates which are stronger than the bolt. The nuts have been replaced with shear pin nuts, which are free running providing consistent breakout at a predetermined higher torque. The thread is rolled onto the bolt so that the threaded section of the bolt has a strength similar to that of the bar and a deforming plate is specified, which allows deformation and eventually pull through of the bolt at slightly less than its ultimate strength thereby providing a visual indication of high bolt load. These changes make installation easier and at the same time, breakout of the torque nut provides confirmation that the resin has been properly mixed and has set. The length of exposed thread after installation can be inspected to confirm nut breakout and drilling of the correct length holes. In a further development to make installation as straightforward as possible and to provide in built quality assurance, Goedehoop have pioneered the adoption of “spin to stall” installation. The setting time of the resin and the breakout torque of the nut have been selected to optimise this procedure. The rock bolt is spun through the resin as normal, but spinning is continued until shear pin breaks and the nut is tightened against the plate. There is therefore no spin or hold time to be adhered to and stalling of the drill motor indicates successful installation. The length of thread protruding provides a final check. The bond of this system is regularly checked by conducting underground short encapsulation pull tests.

All mining sections at Goedehoop are now using spin to stall installation and the quality of the system is audited for installation quality and post installation performance. Standard

operating procedures and checklists have been drawn up for each aspect of the system and comprehensive staff training undertaken to all concerned.

CGM (HRD)CorporateSCCL.

Sub: Submission of Technical Paper for "TMSMI-2011” -reg

Please find herewith the Technical Paper on Ground control issues is submitted for the Golden Jubilee International Conference on “Technological Challenges and Management Issues for Sustainability of Mining Industries (TMSMI-2011) during 4-6 August 2011 at Rourkela for scrutiny and onward submission to the Organisers.

Topic of Tech Paper : Ground control continues to be a crucial issue in Indian coal mines...What we learnt?

Authors : B. Ramesh Kumar CGM (CP&P) M. Raghavulu AGM (R&D) MD. Suresh Kumar, Addl Mgr (Mech. Cell)

CGM (CP&P)