Roof Support Evolution at Sigma Colliery

Transcription

1 Roof Support Evolution at Sigma Colliery F Hunter Manager Underground Operations Sigma Colliery INTRODUCTION Sigma Colliery is situated in the Sasolburg area South of the Vaal River in the Free State. Mining activities started up in 1952 and the official closing function for the mine was held in April 1999, although the physical coal production from the underground section is planned to cease at the end of February Up to the present date, 183 million tons have been produced at this Colliery. The mine was started as a highly mechanised underground Colliery doing conventional board and pillar mining. In the 1960 s experimentation was done with long wall mining in order to increase the percentage extraction of reserves. In the 1970 s a modern longwall was introduced into the mine and this unit produced coal for ±15 years before it was phased out. In the 1980 s pillar extraction and rib pillar extraction methods were introduced into the mine and these methods were evolutionised to suit the mining conditions. There are three mineable coal seams at Sigma and they are known as the 2A (lower seam) 2B (middle seam) and 3B (top seam). The average seam thickness of each of the seams is ± 4m. The parting between the 3B -and 2B seams is ±10m and the pasting between the 2B - and 2A seams is ±1m. The mining depth varies from 40m in North Eastern underground boundary to 160m in the Southern Boundary of the underground workings. 179

2 180

3 SITUATION Roof support was inadequate The mine is divided by a syncline having its axis North South and this divides the coal reserves East and West. Mining started in the Eastern reserves and progressed toward the Western area in the later years. In the East, the basement rock underlying the coal is lava and the seam is relatively flat rolling and dipping 2-3 degrees. In the West the basement rock is dolomite and pot holes occur and faulting in excess of 3m is common and the seam rolls and dips 4-10 degrees. Production rates in continuous miner sections dropped by 40 percent in the Western area when compared to the Eastern reserves. The roof reacted in an uncharacteristic manner. Numerous unplanned roof falls were occurring in the back areas, as well as in the production sections. One production shift per month was lost due do roof falls on main belts. Post roof support roof failure Areas would show signs of roof failure within hours, shifts, weeks and up to six months after they had been supported with roof bolts. Production levels were on the decline Production was hampered seriously due to the unplanned roof falls. Re-support costs were excalating At one stage Sigma Colliery was spending an average of a R per month on roof trusses and long anchors. Three back area personnel for each production man on the coal face This situation arose due to the numerous roof failures, and people had to be increased in the back area in order to assist with roof 181

4 repairs/maintenance. TASK Investigated the following: Alternative support strategies A general team was incorporated with the continuous miner development team to install cable trusses concurrent to the roof bolt support. Alternative cutting sequences and cutting depths The cutting standard was a 14m double lift from the last line of roof bolts. The final roadway width was 5.6m. Single lifts 3.3m wide and a cutting depth of 5-8 m were experimented with. Alternative panel layouts Staggered inter sections were tried, pillar dimensions were altered from squate pillars to rectangular. Pillar centrers were changed from 18m centres and 24m centers to 24m by 36m and 24m by 48m rectangular centers. Different orientations were also experimented with. Traditionally the main developments ran North South with the result that splits were East West. Experiments were done to let the main developments run North West, South East and let the splits run South West, North East. East West roadways normally showed the most deformation, guttering and the major roof falls occurred in this orientation. Alternative production equipment Experiments were conducted with onboard bolters on a narrow head continuous miner and double pass cutting was done. An Australian bolter miner was rented for experimental purposes. Finally and ABM20 Voest Alphine Miner was purchased. The changes were made from the various machines in an effort to improve the productivity in the dynamic conditions in the Western 182

5 Sigma coal field reserves. Establish roof failure mechanism Strata Control Technology, Roof Support Consultants from Australia were appointed to establish the roof failure mechanism and to make support recommendations. A roof rating was done in accordance with borehole information. The American roof rating system was applied to the geological bore hole cores of the immediate roof strata to identify potential weak roof areas pro actively. ACTION Local experts were consulted to identify what additional recommendations could be taken to prevent the roof falls. Established a project section An experimental section was established where new techniques and ideas were tried out before full scale implementation was adopted through out the mine. Built a proto type continuous miner with on on-board roof bolters Sigma modified a 120H2 Jeffrey continuous miner, with satellite roof bolters attached to both sides of the machine. This attempt was not very successful but the principle was proven. Sigma rented an Australian bolter miner, the 12CM30 continuous miner. Unfortunately the cutting head of this machine was not very reliable, but Sigma managed to improve roof conditions where roads were developed with this continuous miner. Sigma visited Australia. On this visit two big differences were noticed between what Sigma were trying to do and what the Australians had already accomplished. The first difference Sigma noticed was that the roof bolt consumables of Australia were very robust and user friendly. The discipline regarding roof control was excellent and their safety statistics were an example to be strived for. The second difference was the ventilation system that they were using. They were really effective with the way they ventilated their headings as that allowed people to work productively in a dust free atmosphere. 183

6 A Joy HM9 continuous miner was modified and equipped with locally made roof bolters which were built in Sigma s work shop. The principal of ventilating, cutting and bolting was successfully executed in one heading. This new mining sequence allowed Sigma to mine areas, which were previously considered as unmineable. Appointed Australian roof support consultants Strata Control Technology roof support consultants were appointed to identify the true causes of the roof failure of Sigma. Previously Sigma was catering only for vertical stresses. The Australians taught Sigma to reinforce the roof for horizontal stresses as well. Motivated the of purchase of a Voest ABM20 continuous miner. A Voest ABM20 continuous miner was purchased by Sigma Colliery and this machine addressed the roof support problem very successfully. Australian roof support consumables Australian roof support consumables were purchased for trial purposes with the implementation of the Voest ABM20 continuous miner. The roof support consultants highlighted the superior quality and user friendliness of the Australian consumables. A 200 percent improvement on pull out resistance was achieved with the Australian system when compared with the local system. RESULT For the first time Sigma was able to manage the roof. The unplanned roof falls reduced dramatically. The roof support strategy was introduced throughout the mine. The changes were implemented systematically. A major reduction in roof fall accidents was experienced. The fatal accident statistics graph is indicative of results achieved. All mining teams were trained and coached to adjust support density in accordance with prevailing roof conditions. 184

7 FIGURE 1 Modified Jeffrey 120 H2 equipped with satellite roofbolter FIGURE 2 Australian 12 C30 Continuous Miner 185

8 FIGURE 3 Modified Joy HM 9 with onboard roofbolters 186 FIGURE 4 Voest ABM20 Continuous Miner

9 187 Basic Strata Control Facts which were emphasized by Strata Control Technology

10 188

11 189

12 190

13 191 Sigma was using long anchors before improving the bolting horizon and roofs were failing. Since improving the bolts and bolting more intensively the necessity for long anchors has disappeared.

14 At one stage Sigma was installing roof trusses and mine poles as well. Presently with the improved roof bolt installations, the roadway dimensions need for these trusses and mine poles has disappeared. 192

15 Practice strata model for training employees from operator level to managerial levels. Roof support is in balance when loaded with a vertical load. 193

16 194 Roof support is not in balance when loaded horizontally

17 CONCLUSION To create stable roof conditions in bad ground the following issues must be addressed in combination. Narrow down road widths. Cut short lifts. The lift must be to the optimum distance such that the roof integrity is maintained Install roofbolts in the shortest possible time after cutting (consider onboard roof bolters/double boom roofbolters) Be flexible with the installation of roofbolts. Alter the number of roofbolts per row and the distance between rows of roofbolts in accordance with prevailing conditions. Ensure that full column resin roofbolts are full of resin from the top of the bolt to the collar. Ensure the minimum annulus to create the maximum pull out resistance ensure that the roofbolt has the optimum deformation profile. The crimp nut or nib nut for spinning the bolt through theresin must be user friendly and 100 percent effective and have zero defects. Optimum drill bit profile for drilling productivity/optimum drill hole profile formation. Install additional roof bolts in the failure zone developing after the initial roof bolt support has been installed. 195

18 196

19 No systematic support Mechanical bolts ROOF BOLTING EVOLUTION 16mm Resin bolts Point anchor 16mm Resin bolts Full column Two speed resin Additional support mine poles support in return airways Sheppard crooks Sandex of bolts 12.5m Spiral bar Roof support Cost saving drives that only considered the price of a roof bolt Experience proved that the above: efforts cost Sigma Colliery dearly in regard to safety and re-support costs Sigma roof truss Sigma cable truss 6m Long anchors 20mm Full column roof bolts with W-straps with 4mm washers 20mm Spherical seats roof bolts with 6mm washers with W-Straps 197

20 Sigma Roof Truss 198 Transition from roof truss to cable truss

21 Long Anchor Support Note the RHS long anchor slipped through the wedge Long Anchor were installed in areas where the roof had already failed. Too little done too late. 199

22 4mm Roof bolt Plates were used until the first nut pulled through the plate. The present Sigma Roof bolt standard is a 6mm roof plate with a spherical seat. 200

23 Improvements to Roof Botls 201

24 The change from crimp nuts to nib nuts resulted in a 30 percent improvement in roof bolt installations. 202 Wire mesh and pine head board support in friable roof conditions. Old standard.

25 Roof bolts and W-strap support in friable ground. New Standard. 203

26 Girder support installed for the recovery of a trapped continuous miner which was doing rib pillar extraction. Salvage equipment for pulling a trapped Continous Miner out of a Goaf. 204

27 FACTORS INDICATING HIGH HORIZONTAL STRESS Typically horizontal stress in most coalmining environments is equal to, or greater than the vertical stress. Normally horizontal stress has the most influence on roof and floor conditions. Roof falls out in a single lift cut-out-install support Roof remains intact when second lift is cut-out - install support Knife edge roof fragments Elliptical roof falls Falls in road ways between pillars as opposed to falls on intersections Roof test holes showing signs of horizontal strata movement At roof falls bolts indicating S-shaped deformations Load damage on roof bolts and plates far in excess of dead weight loading Not all roof bolts in a support line showing signs of stress damage Areas that will fail regardless of the support installed East West roof falls vs Stable roof conditions North South AREAS WHERE ELEVATED HORIZONTAL STRESS CAN BE EXPECTED Near streams and rivers. Located on surface In undulated coal seams - (ups and downs) 205

28 Roof falls out after single auger width is cut out. Roof remains intact when second lift is cut out. 206

29 Knife edge roof fragments 207

30 208 Roof falls out between pillars as opposed to falls on intersections. (This fall was 5m high and 120m long. It took 3 months to support and load out).

31 Roof test holes showing sings of horizontal strata movement Roof bolts showing S-shaped deformations. Load damage in excess of dead weight loading. (The way the roof bolt plate is deformed is indicative of more stress than the mass of 20-30cm of rock that broke out above the roof bolt plate). 209

32 210 Not all roof bolts in a support time showing signs of stress damage. The bolts outside the picture image showed no stress damage. This roof failure can occur within one shift, one week or months after being supported with roof bolts.

33 Not all roof bolts in a support time showing signs of stress damage. The bolts outside the picture image showed no stress damage. This roof failure can occur within one shift, one week or months after being supported with roof bolts. Areas that will fail regardless of the support installed. The straps were installed 1m apart, 6 bolts per strap, 3 x 6m long anchors were installed in every second strap. The support was stable for 6 months before the roof collapsed. 211

34 Brow of East/West roof fall looking West. Notice how the rock is crushed Southern brow of the same East/West fall. Notice how the rock is intact. 212

35 Coal basins/channels Seams thinning Near dykes and faults Near goaf edges LEARN TO FINGER PRINT ROOF CHARACTERISTICS The saying A stitch in time saves nine is very applicable to early detection and remedial action taken in areas indicating roof deterioration. Identify the maximum cut-in depth before roof failure takes place. Short double lift vs. Long single lift in bad (high stress) roof conditions. Telltale signs of gutter formation. Install bolts timeously in the failure zone to counteract gutter formation. Training needs Balance of support system Plate strength vs bolt strength Identity/Investigate specifications of support consumables. Roof plate thickness Diameter of roof bolt vs diameter of roof support drillhole Strength of resin Roadway design Width, height and life span Support requirements Understand roof failure mechanism 213

36 214 This tunnel was mined with the experimental Joy 12HM9 Continuous Miner with onboard roof bolters. The bolters could bolt within 3.6m of the face. The road width was 4.8m wide. Roof bolts were installed at 1.8m intervals. This roof fall was limited to 0.75m high by 3.6m long and 4.8m wide. In this same area when place change continuous mining methods were used and 14m long lifts were cut out from the last line of roof bolts. Roof falls occurred which were 3-5m high and 100m long.

37 This roadway was mined with the Joy 12HM9 continuous Miner with onboard roof bolters. The road was 4.8m wide and 2.9m high. This East West roadway was supported with local support consumables. The first set of support was W-straps with 6 x 1.8m, 20mm full column resin bolts per strap at 1m intervals. Guttering took place on both sides of the road and additional straps and 6bolts per strap were installed between each strap. Test holes were drilled for petro scope inspection and bed separation was identified in the order of 20mm at 1m, 1.5m and 1.7m into the roof. 6m Long anchors were then installed, 3 in a row every 3m of road way. 215

38 This is an East/West Roadway mined in the same area as the above mentioned road. The Voest ABM20 Continuous Miner was used together with the imported support consumables. The same bolt length was used i.e. 1.8m. Note the difference in mining conditions. The road was 4.9m wide and 2.9 high. 216

39 Training need of supervisors and operators top down. Repeat training and continually coach on the job, to create understanding. Difference in mining conditions North South vs East West. LESSONS LEARNT AT SIGMA Barrier pillar sizes. - in an old mine barrier pillars are an insurance for safe retreat extraction. Barrier pillars allow flexibility. Multi seam mining, the time span between mining top and bottom seams. - limit the time span between mining the top and then the bottom seam. The overlying seam becomes a water storage dam in time. The interburden parting can become incompetent due to water leaching into the strata rendering poor roof conditions when mining the bottom seam. Plan mine from day one to mine closure. A lack of long term vision is very costly oversight. Support specifications for main development, secondary development. A main development road well supported initially is an asset. Poorly supported main developments become liabilities. Finger print the reserve East West bore hole info. - if you understand the stresses causing roof failure you can optimise your support densities. At Sigma the majority of roof falls occur in East West roads. How applicable is the safety factor form formula - over estimating the coal strength of pillars becomes 217

40 a costly oversight when coal pillars age and stabilization of pillars is necessitated. - Width to height ratio critical - Always mine/develop with high safety factor Straight line stooping not 45 degrees have a continuous miner recovery/salvage equipment available when a machine is trapped load out the minimum without spiles. Don t fight the dolerite join forces with it. - leave strategic support pillars and utilize the strength of the dolerite to keep the overburden intact and thus limit vertical stresses on pillar remains. Panel width is critical for stability in total extraction panels. Stooping practices. - herring bone/robbing - snooks, size, placement - finger lines - speed - pillar size - panel width - dolerite - support pillars Diesel policy - timeous replacement of units - fleet management - infrastructure - sound maintenance practices - sound operational practices Don t leave vacant geologist posts empty. Get a drilling contractor to do horizontal drilling. The strata on your mine is a given factor, learn to manage it 218

41 Training all personnel and coaching them in strata control is the way to get team involvement in the prevention of roof falls. Unplanned roof falls are not a normal part of the daily happenings on an underground mine. - establish what reasons caused/contributed to the fall - adjust procedures accordingly to prevent similar roof falls. The strata control officer is an integral part of the training programme to create awareness of how to read roof conditions 19. Re-visit the basics regularly to keep the works ticking smoothly. 219

42 220

43 221 TYPICAL PILLAR ROBBBING LAYOUT

44 222