FUTURE AND POTENTIAL IN-STOPE ROCK SUPPORT SYSTEMS IN CONVENTIONAL NARROW VEIN STOPING OPERATIONS TO REDUCE SAFETY RISKS AND ENHANCE PRODUCTIVITY I T Tshabalala, Sales Manager, Minova Rsa P H Ferreira Pr. Eng (Mining), Managing Director, Minova Rsa ABSTRACT In typical conventional narrow vein stoping operations, the use of temporary support components to offer some support to workers, whilst the permanent and generally more elaborate support system gets installed or constructed, still results in injuries to people. The authors wishes to explore the probability and practicality of making use of new generation support products to make the support operations safer as well as enhance productivity in the stope face. This paper will examine the use of Autorock Drills as a means to drill roof support holes and install appropriate support components into these holes, as well as the application of Thin Spray Liners, commonly called TSL s as an additional means of creating a safer operating environment. The installation of netting as a means of roof protection and other support aspects such as cemented roof-bolts will also be explored. These proposed items are envisaged to improve the safety of our people and reduce production down time and increase production and productivity. 1. INTRODUCTION Rock support is a phenomenon that has been practiced in the mining industry ever since mining started in South Africa around 1886 with somewhat promising results. It (rock support) delineates the application of a reactive force at the face of the excavation (Windsor et al 1993) and it includes techniques such as timber support, steel support, fill support, props etc. Whilst its objectives were clearly understood right from the beginning, the mining industry has to date never actually managed to eliminate rock fall related accidents and incidents. Having said that, it is befitting to acknowledge and realize a noticeable dent in safety statistics that recent techniques, methodologies and innovations have achieved in this regard. One such methodology has been the promulgation of the Mine Health and Safety Act in 1996, which introduced an element of accountability and responsibility on behalf of both mine employers and employees alike with regard to taking reasonably practicable measures to safeguard themselves individually and collectively. The recent spate of mine accidents and fatalities, especially in 2007 where 221 employees lost their lives and the mines lost millions in revenue due to the resultant workplace Page 1
stoppages and mine closures, has forced both the technology developers and implementers to come under one roof in pursuit of a common solution. One such solution is in-stope roof-bolting, a support regime that is designed to hold and keep rock layers clamped together and reduce the level of rock fall incidents. Whilst this technology has actually been introduced more than half a century ago, some mines never really appreciated this technology and as such became reluctant to even try it, partly because of their nature to resist change (something mine workers have became synonymous with) and partly because of the inadequacy of the available technology at the time. Mines that appreciated and introduced roof-bolting in the stopes achieved a phenomenal decline in rock fall incidents and as such reported improved safety performance in the stopes. However, the tendency since inception of this practice had been to use the same face drill machine (usually compressed air operated) to drill the roof support holes. This meant that the holes were therefore obliquely drilled, thus not yielding the designed hole length, especially in low stope widths (in the order of 80 120 cm). The introduction of new support technologies in the stopes, which included using Autorock Drills for drilling roof bolt holes at 90º to the hanging-wall and then using the same drill for spinning the roof bolt to mix resin where the latter is used, has given the mining industry a sense of hope and optimism towards realizing the 2013 targets and milestones. This is because of the resultant full column high strength bonding that takes place between rock and steel, provided by resin. The above-mentioned technology may be augmented, in badly fractured ground conditions by the use of thin-spray liners (TSL s) to consolidate the rock conditions and therefore reduce rock falls. Good papers and presentations have been produced on both these technologies (Autorock Drills and TSL s), based on their successful implementation at other local mines. 2. ROOF-BOLTING TECHNOLOGY In-stope roof-bolting is an effective rock reinforcement and consolidation system that has been available for strata control in the local metalliferous mines for about half a century now. Whilst the technology is currently used by a few shallow mine (up to 1000m deep), it has the potential to be used in hard rock stopes industry wide, inclusive of deep level stopes. Typical bolting techniques used include reinforcing elements such as anchors, bolts, glass fibre bolts, and tendons, with the bonding medium being cement grout or resin, or friction mechanical bolts. The installation procedure may include pre-and-post tensioning, grouted and un-grouted reinforcement, coupled and uncoupled reinforcement etc. The rationale behind these reinforcement regimes is: Rock support Strata reinforcement Rock anchoring and Spot bolting Page 2
In-stope roof-bolting technique has been popularized since its inception by its proven successes in providing stable rock conditions in the stope, thus improving both the safety conditions and productivity at the same time. The technique can be used over a wide range of rock conditions, including, but not limited to hard rock conditions in shallow mines (where the rock is strong and stresses are low) and deep mines with relatively weak rock and usually fractured. To realize these results, certain conditions must be met first, and they include: Drilling and installing roof-bolts at 90º to the hanging-wall or strata Achieving full column fill with the grouting medium used Correct combination of hole diameter, capsule diameter and steel diameter (where polyester resin is used) Correct spin time for resin or using Lockset Spin to Stall resin There full potential of in-stope roof-bolting technology has not been fully explored and appreciated by the mining industry, and until such time that this happens not much improvement in safety will be realized and thus the industry will continue to maim and injure its players. However, the position taken in this paper is a somewhat optimistic one and it is believed that because of ever increasing labour costs associated with expensive timber support at depth, more and more mines will start to appreciate the benefits of this technology. This view is shared by Hunter (1963), who concludes his paper titled Rockbolting practice at Rhokana Corporation Ltd by insisting that the use of in-stope rockbolting using grouted or tensioned bolts will continue to increase in the future. It must be mentioned at this stage that the ever increasing costs of steel in recent times is considered to be a discouraging factor in the strides to expose and encourage all miners to use in-stope roof-bolting. In the light of this, alternative systems such as glass fibre bolts or GRP should be considered instead to mitigate this aspect. 3. CONVENTIONAL STOPE SUPPORT VERSUS ROOF-BOLTING Current conventional support systems in the stopes includes both temporary support in the form of timber sticks and mechanical props or jacks, as well as permanent support provided by various types of packs, pre-stressed profiled elongates, cluster sticks, RYHP (Rapid Yielding Hydraulic Props), backfill etc. Whilst conventional support has been in use in the mining industry for over a century to date, it continues to raise the following concerns: Two support cycles: first temporary and then permanent Persons are still first exposed to unsupported roof whilst installing temporary support Conventional support is bulky and adds to logistical problems Support medium must leave path for the scraper, and as such It cannot be placed right at the face Falls of ground is still a matter of serious concern Page 3
Other concerns raised by Van der Merwe et al (2001) pertaining to the current mining practices include the less than adequate support in the immediate face area particularly during cleaning and making safe operations, as well as the inadequate aerial coverage by support in the stope. These concerns are addressed in the proposed solution provided in this paper. 4. PROPOSED SOLUTION FOR THE FUTURE What is considered a potential solution to the current unacceptably high fatality rate (0.34 gold, 0.16 coal, 0.17 platinum and 0.18 for other) and high injury rates, calls for a strategic consideration of a number of alternative support regimes strategically chosen to augment current practices. The systems proposed include: 1) the installation of roof-bolts in the stope using suitable drilling equipment, whether these are mechanical, friction, cementitious or resin bonded bolts 2) using Thin Spray Liners (TSL s) in conjunction with option 1 above or 3) using a safety net instead of TSL, for personnel safety in the stope face area Cognizance should be taken of the fact that the installation of roof-bolts in the stope will still be done in conjunction with timber or cement packs along the gullies. To achieve desired results from this system, it is proposed that support holes be drilled with Autorock drill rigs (or any other appropriate rig) which have been designed primarily and uniquely for hanging-wall drilling. It is also proposed that resin should preferably be used as a bonding medium in the subsequent installation of roof-bolts, which ideally should be installed on the face prior to drilling of the production or shot holes, as seen below. Figure 1: On the face drilling and installation of roof bolts with Autorock drills in a deep gold mine Page 4
With Autorock drill rigs, the following benefits are realizable (also see a paper on Autorock rigs for rock-bolting in narrow reef stopes by Johnson and O Connor) Support can be installed right up to the face in the beginning of the shift, thus ensuring worker safety throughout the shift The Autorock drill rig has two built-in clamping jacks acting as temporary support, each exerting a force of 100 kg on the hanging-wall Practically not requiring additional temporary support components to be installed, thus only one support cycle (permanent support immediately) Rock-bolts can be installed more rapidly than elongates or packs. Face area is left clear for people to maneuver without hindrance (scraper access always possible) Reduced timber requirements and related logistics Minimal or no fire hazard inherent with timber underground This increases safety as the workers spend less time under no support or temporary support; Roof bolts are a permanent support system and therefore ensures that work continues safely throughout the shift after their installation Much stiffer rock support mechanism is provided by rock bolts as compared to elongates or packs, even with pre-loading Figure 2: Roof bolts and pre-stressed mine poles installed in the gully (Platinum mine) The proposed support system is currently used successfully in the mining of the shaft pillar at a local deep level gold mine, with good safety results achieved to date. Since this introduction, the incidence of rock falls reduced dramatically in this work place, where the system is used in conjunction with pre-stressed mine poles as well as classified tailings back fill as the local support regime. The observation on safety improvement as a result of in-stope roof-bolting is also shared by Nel (2007). The polyester grouted resin steel bolts are installed 1m apart on dip, and spaced according to the current achieved face advance. Page 5
The pre-stressed profile sticks are also installed at 1m apart on both dip and strike orientations. The quality of installed support in this working place is good, as can be seen from the photos in figure 3. Figure 3: A combination of roof bolts and pre-stressed mine poles installed as face support at a local deep level gold mine 5. ROCK-BOLTING WITH RESIN In South Africa, roof-bolting with polyester resin capsules is mostly used at the various coal mines of the South African coal fields, as well as the platinum mines. This product is supplied as a two-in-one compartment capsule, comprising of polyester resin and a catalyst both covered and separated by a thin plastic membrane. The separating plastic is broken and destroyed by the spinning of the anchoring element (bolt), with the result that the two products are mixed together to form a very hard and strong bonding compound. The chemistry of the resin can be altered to give different gel and set times which allows for pre-tensioning by using two different speeds of resin in the same hole (The Minova guide to resin-grouted rock-bolts, p.50). With resin capsule products, it is important for users to understand the set time, spin time as well as the hold time, in order to fully appreciate and then apply resin capsules correctly to achieve desired results. A thorough delineation of these different times can be found in the Minova guide to resin-grouted rock bolts, p.51 53. 6. CEMENT BASED-GROUTING This consists typically of Portland A type cement with a number of additional aggregates and additives to achieve different and desired results. According to Sakkar (1997, p.105), the additives are designed to improve the mixing and flow characteristics of the grout, Page 6
prevent bleed from the curing cement, reduce curing time and increase the compressive strength of the cured grout. Minova RSA manufactures a wide range of pumped cement grouts as well as cement capsules, for various applications in both mining and civil industries (see figure 4 below). Both pumped cement grouts and cement capsules are relatively low in cost and also simple to use. Cementitious grout products are generally non-shrink and thixotropic for the appropriate application as well as non corrosive. Figure 4: Cement capsules and pumped cement grout (with pump) Resin Cement grout Advantages Disadvantages Advantages Disadvantages Quick installation Relatively high cost Lower cost of cement Longer installation time Quic setting time Short shelf-life High holding power Slow setting time Bolts with two resins with different setting time Very high holding power Good protection against bolt Ease of installation Resin vapours toxic to skin and eyes Sensitive to temperatures above 50ºC Setting time varies with temperature Resins are flammable Good protection from bolt corrosion with correct formulations More difficult installation in holes drilled upwards Less control over grout quality due to water : powder ratio sensitivity Table 1: Adapted from T.N. Singh, M.N. Bagade & S Jayanthu. Central Mining Research Institute, Dhanbad, India 7. COST CONSIDERATIONS AND OTHER TECHNICALITIES Resin capsules are generally perceived to be more costly compared to cement capsules, when one looks only at the price disparities on a per unit basis. However, the reality is not always true when looking at installed resin roof-bolt and installed cement rock bolt, because other factors such as bolt contribute significantly at the end. These cost issues are illustrated in the following table Page 7
Characteristics End-anchor/ rock stud (full column cement): 16mm Equipment T-spanner/ requirements pneumatic wrench Resin / full column Hand inserted 0.9m inflatable type 25 Mpa pump, hoses & couplers 39mm x 0.9 Split set Pusher dolly Full column Cement Installation time 60 seconds 60 seconds 40 seconds 40 seconds 60 seconds Re-tensioning Often needed Not needed Not needed Not needed Not needed Full column support No 100% Appr. 87% Appr. 87% 100% Active support Yes Yes Yes Yes No Typical peak 10.5 tons 15.5 tons 10.7 tons Typically 5 tons Nil load (after 10 mins) on installation Installed reliability > 70% 100% > 90% > 90% < 50% Sensitive to undersize borehole dia. (e.g. bit wear) Not generally sensitive Damaged plate, Underground thread, bar if not resilience tensioned Shear Yieldability Appr. Cost/ bolt Complete R44.33 (incl. 10% wastage loss of shells) End anchoring & critical bond length Not sensitive Excellent Decreases Increases performance due to performance partial inflation Very good; valve damage unlikely Excellent Not sensitive Excellent Good 18 mm Excellent Good 18 mm Good 18 mm Good No, cable of appr. 650mm R44.40 (20mm X 0.9 bolt) Excellent Comments Mechanical bolt Excellent (1st world support system) R45.00 (R39 + R6 (pump), no bolt wastage No. cable of appr. 380 mm + 60 mm = 440mm or longer (partial) Friction bolt Table 2: Adapted from B.B Nel, R41.77 (no bolt wastage) No. cable of appr. 2000mm; poor Friction bolt R41.67 (with 16mm X 0.9 m bolt) No 8. THIN SPRAY LINERS (TSL s) Thin Sprayed Liners are used to provide a protective coating to rock, concrete or coal surfaces which are susceptible to deterioration on exposure to the atmosphere, and also for various support applications. The latter include pillar support, face support, raise bore linings, temporary support in ore passes and for concrete shaft lining repair. Products used for the TSL application are cement based, solvent free, non-toxic and non-flammable. They are supplied as a single component powder to which water is added, and the mixed material can be spray or trowel applied as seen in figure 5 below. It is proposed that an application such as this should be considered in the hanging-wall of the stope panel to augment roof bolting, before moving it into the panel upon achieving good results. TSL s have the potential to bond and hold key blocks competently together, resulting in a more stable hanging-wall condition, whilst providing excellent aerial coverage at the same time. Page 8
Figure 5: Typical application of TSL for surface protection and support Depending on the desired application and results, a choice can be made from a product with a fast setting time (45 55 minutes for the Capcem KT Fast) to that with a medium setting (2 3 hours for Capcem KT White), yielding phenomenal strength capabilities, such as 8.9 MPa Compressive and 0.7 Mpa Tensile strengths after 2 hours for Capcem KT Fast and 15.0 MPa Compressive and 1.0 Mpa Tensile strengths after 1 day for Capcem KT White respectively. The sealing properties of the TSL s also open opportunities for applications other than rock service support, as these could also be used to stop, reduce or slow down ground weathering, ground scaling and prevent water leakages. 9. SAFETY NET Safety net encapsulates the use of a rectangular safety net placed as close to the hangingwall of the stope face area as possible, to provide protection during cleaning and drilling operations. The net is used in conjunction with other stope support systems (props, tendons, RYHP, elongates etc). It provides extensive aerial coverage (up to 85% of the hanging-wall) and can hold up to 2 tons of rock, as proclaimed by Van der Merwe et al (2001, p.34). The net is installed and tensioned manually, by attaching the straps to the surrounding stope support system. Since it was introduced, the system has proved to be very effective in terms of providing personnel safety. A case in point cited as the success story by Human and Fernandes (2004, p.75) entails a fall or ground incident on 28/11/2003 at Impala Platinum s no.14 shaft, where a piece of rock with an estimated mass of 350kg fell and nearly injured a rock drill operator who was busy drilling at the time. The conclusion drawn by the mine s investigation team was that the safety strap had sufficiently reduced the momentum of the fall of ground, resulting in no serious injuries to the drill operator. Page 9
A summary of the fall of ground test results by Human and Fernandes is tabulated in page 74 of their paper. According to Van der Merwe et al (2001, p.35) safety net offers the following advantages Low mass and simple installation and removal procedure High aerial coverage between support units Ideal for panes with friable hanging-wall conditions Net can be re-used Figure 6: Safety net used in the stope face area 10. SAFETY: 2013 SAFETY TARGETS & MILESTONES Fatalities at the local mines increased for the first time in six years last year, to reach a disappointing figure of 221 deaths. This record erased a promising downward safety trend which started in the mid-1990 s. It may be qualified to assume that the industry was doing everything right, hence the promising results. The question to be asked is now what was done incorrectly in 2007. Will the industry experience a better performance in 2008, especially after the presidential safety audit of all the mines? These are the questions that all the stake holders will be seeking answers to at the end of the year (2008). In an endeavor to reduce accidents and fatalities in South Africa such that the local industry compares to other international players in safety, the industry set for itself what it calls The 2013 Safety Targets and Milestones. According to these targets, specific, measurable, achievable, realistic and time bound (SMART) safety performance never achieved in the industry before has been set. The milestones set for safety performance are: Gold Sector: achieve safety performance levels equivalent to current international benchmarks for underground metalliferous mines, at the least, by 2013. Platinum, Coal and Other Sectors : achieve constant and continuous improvement equivalent to current international benchmarks, at the least, by 2013. Page 10
It is a known fact that falls of ground are a major cause of accidents and mine fatalities. The industry experts have also indicated time and again that these accidents can be prevented if the correct support choice is made and installed according to pre-determined standards and procedures. CONCLUSION The implementation of in-stope roof bolting has undoubtedly brought about significant improvement in personnel safety conditions in the underground stope face, the area that has for centuries been known and almost accepted as the high risk area. This improvement has been so noticeable that even the Minister for Minerals and Energy could not resist commenting on this issue in her department s budget vote for 2007/8 dated 30/05/2007, where she proclaimed that there has been a significant improvement on rock related fatal accidents over the past year subsequent to the implementation of preconditioning and instope roof-bolting. In-stope roof-bolting, coupled with the application of TSL s where necessary are proven support techniques for reinforcement of the roof in future, to reduce fall of ground incidents and improve personnel safety. Typical cost of drilling a 1.0m roof-bolt hole, excluding labour costs and logistical advantages nor safety improvement aspects, could be around R6.00 per hole (i.e. capital cost of machine and drill accessories). A resin capsule bonded steel bolt of 0.9 m length could cost around R36.00 (steel bolt and resin). This equates to a total cost per installed roof-bolt of about R33.00. Add to this the application of TSL s on the roof at around R60.00/ m² sprayed (excluding labour). Therefore, under special circumstances, the cost to install roof-bolts with TSL applied could be around R100.00/ m². It then qualifies to ask the question is ± R3000.00 per typical 30m blasted panel advanced at a rate of 1.0m per blast too much to pay to safeguard the lives of our people working on the face? Should this not be a consideration as an additional solution in stope panels with potential bad rock conditions? REFERENCES Human, J.L., Fernandes, L., 2004. Testing of temporary face support systems under rock fall conditions. Hunter, J.K., 1963. Rockbolting practice at Rhokana Corporation Limited. Symposium on rock mechanics and strata control in mines Johannesburg. Journal of the South African Institute of Mining and Metallurgy, 194-203 Nel, B.B., Implementation of in-stope roof bolting at Elandsrand Gold mine. Paper presented at the Association of Mine Manger s meeting, May 2005. Page 11
O Connor, D., Johnson, F. 2008. Autorock rigs for rock-bolting in narrow reef stopes Sarkar, S.K., 1997. Ground Control in Mining. Select papers Sonjica, B., 2007. Minerals and Energy Department Budget Vote (30/05/2007) The Minova Guide to Resin-Grouted Rock bolts Van der Merwe, J.N., Wojno, L., Toper, A.Z., 2001. Implementation of state-of-art mining knowledge and technologies in design and operation of a safe and efficient deep mine stope for 21 st Century. Safety in Mines Research Advisory Committee. AUTHOR I TSHABALALA, B-Tech Mining Eng. (UJ), Sales Manager, Minova RSA CO-AUTHOR P H FERREIRA Pr. Eng (Mining), Managing Director, Minova RSA Page 12