Pressure to provide a workplace free from injuries and fatalities has added to the worldwide market pressure to improve profits and margins in the mining industry.
A rockfall study (Potvin et al 2001) conducted in 26 Australian underground metal mines has shown that over 90 per cent of rockfall injuries involved rocks smaller than 1t.
Furthermore, studies have shown that most injuries in underground space have occurred within a few meters of the active face. This suggests that the current approaches in controlling the small pieces of exposed rocks near the active face where workers are consistently exposed can be improved.
The approaches used range from using no surface support to the installation of mesh (steel and fibre) and in poor ground conditions the installation of shotcrete.
While applying the current methodology near the face would likely reduce the risks of rockfall injuries, it could negatively impact the operations with regards to cost and mining cycle times.
Thin sprayed liners (TSLs) can address the above mentioned problem.
The installation of surface support using remote and rapid spraying techniques has the potential to minimise interference with the mining cycle and reduce costs.
TSLs can be applied essentially on or at the face to keep the small key blocks in place and reduce the potential of gravity induced fallouts of small pieces of rock.
The products discussed in this paper were all developed by Minova RSA, the intent being to compare the characteristics of each type of product against one another, rather than against those of competitive products. Minova RSA for the purpose of this investigation has chosen to evaluate their products against the following set of criteria:
? Characteristics of various TSLs
? Ease of application
? Product Quality Performance
? Spray ability and coverage
? Comparison to shotcrete
? Scale of interference and effect on the mining cycle
TSL characteristics
The Capcem KT range of TSLs provides a protective coating to rock, concrete or coal surfaces which are susceptible to deterioration on exposure to mine atmosphere, or as a TSL for support.
These products are all compressed air propelled onto the rock surface. Development of a full liquid TSL that does not require compressed air for propulsions onto the rock surface is in an advanced stage of development.
Critical requirements of TSL’s have been identified as follows:
? Adequate pot life when mixing i.e. more than 30 minutes
? Factory pre-mixed products for consistent performance of product
? Fast development of strength for rapid performance after two hours of application i.e. compressive, tensile and shear strengths
? Excellent adhesion i.e. bonding to the substrate
? Visibility of the coating (white or light in colour)
The support performance of TSLs is largely determined by the substrates onto which it is attached to and can greatly assist in maintaining the initial integrity of the rock mass under general loading conditions.
To ensure effective surface support design, it is important to fully understand the required reinforcing capabilities of the skin support.
In friable ground conditions the first function of a TSL is to prevent the unravelling and loosening up of fragments, thus maintaining the rock mass integrity. If unravelling is not prevented, the support resistance within the rock mass will gradually decrease and the demand on the surface support will inversely increase.
Being in intimate contact with the rock surface, the coating action of TSL bridges joints, effectively penetrates fractures and bonds the rock mass together, thus restricting inter block movement.
Figure 1 illustrates block theory with the interaction between various support components in a support system, represented as a reinforced beam loaded in an orthogonal direction.
Classification of TSL characteristics
Laboratory interpretation of TSL technical characteristics could be as detailed below but should be read in conjunction with the geotechnical requirements for the specific loading conditions that may be expected and may not all be equally important or entirely applicable.
In selecting the most suitable TSL, cognisance must be taken of the specific requirements and intended application of the TSL. For example, in a static environment the requirement may be for a TSL with a high uni-axial compressive strength where tensile strength capabilities are less critical.
Underground Application
Minova RSA TSLs are being extensively used at present in platinum and chrome mines with relatively limited application in other hard rock and diamond mines.
The Capcem KT range is used in most applications with specialised Tekflex applications in vertical settler dams and raise bore holes for sealing against water leakage and in excavations requiring flexibility. The application’s success stems from the ease of mixing and spraying, and ultimately the cleaning of equipment after use. All this leads to quick and cost effective spraying cycles and will determine the success of the TSL application. Good surface preparation is also essential for a good bond.
Stress induced fracturing was observed in the rock mass surrounding the excavation and wedge type failure of rock occurred at the intersections of these and low angled joint sets, creating a very rough and uneven rock surface in the immediate hanging and sidewalls of the excavation. The condition of the rock mass around a tunnel is to a large degree controlled by the rock strength and the stresses it will be subjected to during its life cycle.
In high stress conditions, sound design and excavation techniques are required to reduce early fracturing damage and to prepare the rock walls for the installation of the support.
In these conditions, the installation of support at an early stage after exposure of the rock surface can significantly improve the prevailing ground conditions. It has become common practice in the deep level scenario to apply a thin layer of shotcrete as soon as possible after the blast to inhibit the adverse affect of stress fracturing and weathering, and to maintain safe working conditions till such time that secondary and more permanent support can be installed. However, it is often difficult to maintain the shotcrete within a reasonable distance from the advancing face, mainly due to logistical constraints and difficulty to fully integrate the shotcrete into the development cycle. TSLs have the potential to reduce accident levels and increase productivity, as the rapid spraying techniques involve minimised interference with the mining activity. In friable ground conditions the first function of a TSL is to inhibit the unraveling and loosening up of fragments, thus maintaining the rock mass integrity.
In relatively large excavations, tunnels and a mechanised production environment, the application of TSL is very well suited to a robotic arm spray system for fast and efficient applications.
If unraveling is not prevented, the support resistance within the rock mass will gradually decrease and the demand on the surface support will inversely increase. Being in intimate contact with the rock surface, the coating action of TSLs bridges joints, effectively penetrate fractures and bond the rock mass together, thus restricting inter block movement. More permanent structural support such as thick shotcrete can be applied on top of the TSLs with good bonding.
Product description
Capcem KT White and Grey have very similar properties and are supplied as a single component powder (pre-mixed in the factory) to which water is added. The White cures to a brilliant white, and the Grey cures to a light grey. Capcem KT Fast is a product developed by Minova RSA with the intention to provide early aerial support coverage to underground excavations offering strength after two hours. KT Fast 2C and Tekflex consist of two packed components: a pre-mixed powder and a liquid polymer forming a kit.
The range forms a semi rigid support membrane that assists the rock to retain its initial integrity by reducing the adverse effects of scaling, spalling and weathering.
It is supplied in 25kg bags which produce on average 15 litres of product when mixed with 5.4 litres of water or instead, of polymer.
It can theoretically cover 3.4m2 if applied to a flat surface 5mm thick. However, due to the roughness factor of the underground rock, it is estimated that 2.5m2 can be covered by one 25kg bag or kit.
Application Methodology
Typical mixing arrangement
The contents of a number of 25kg bags of White, Grey, or Fast is mixed together with clean water as per recipe, for approximately four minutes and then pumped (electric pump or compressed air driven pump), through a 25mm hose to the spray nozzle some 15m away from where it is sprayed onto the hanging and sidewalls either by hand held or robotic arm arrangement. The spray tempo is in the region of 500 to 800 litres per hour for effective application.
With continuous mixing of the TSL whilst spraying, a surface area of some 45m2 can be covered in 30 minutes. Thus, a total cycle time of some 35 minutes measured from entrance into the tunnel to having moved out.
Minova RSA conducted numerous tests with TSLs to determine the relative performance of the products. Tests were done with TSLs only and also with a combination of TSLs with a relatively thin webbing being sprayed over. The results are illustrated.
It could be concluded that TSLs offer a practical solution for limited ground support. If used in conjunction with appropriate webbing and followed in extreme cases by a layer of shotcrete, it offers a superb practical and mechanized solution to ground support in a sacrificial or permanent tunnel or excavation environment with increased safety as very few people are involved. All applications from the application of webbing to TSL to shotcrete can be done mechanised, remotely and fast.
Spray Equipment
The ease of application of TSLs will ensure minimal interference with the development cycle and improve safety and productivity. The equipment required for TSLs are small when compared to conventional shotcrete equipment and can readily be moved between sites.
There are various types of TSL pumping and mixing equipment available, however the use of equipment with independent mixing and pumping arrangements is recommended to reduce the application cycle times.
Logistics
In the case of 25mm shotcrete, approximately 158 no. 25kg bags are required to cover an area of 45m2, which equates to around 4t of material. In the case of 5mm TSLs such as Capcem KT, 23 no 25kg bags or 0.5t of material is required to cover 45m2.
The use of TSLs can significantly reduce the demand on the already burdened logistical systems of the mine whilst maintaining safer mining conditions right at the advancing face.
A typical conventional development cycle might consist of the following:
? Cleaning operations
? Making safe
? Installation of temporary support (jacks and netting)
? Marking off support holes
? Drilling and installation of support
? Marking off of the round
? Drilling the roundt
? Charging up and blasting
? Removing temporary support
? Re-entry
Depending on the requirements, the application of surface support liners should ideally be integrated into the overall production cycle where it is most practical and causes the least interference with the development cycle. If it is required to apply the surface liner from the last line of permanent support up to the face, then ideally this activity should take place immediately after the installation of temporary support or even permanent support if a resin bolt installation is chosen.
In a bad ground undercut development end the support regime might call for the following support:
? A bolting pattern
? Shotcrete at 25mm thickness
? Steel mesh and or lace installation
? Shotcrete at 50mm thickness
With the new generation support available the following could be proposed:
? GRP or synthetic netting installation with a netting installing unit mounted to the face of the jumbo
? Some 5mm TSL on washed and made safe rock surface that is applied with a robotic arm applicator
? A GRP and resin capsule bolting pattern with a mechanised bolter, this could be adequate to safely allow critical and inline activities to continue at the face and then follow up with
? Some 50mm Shotcrete approximately 20m behind the advancing face
What the authors attempt to demonstrate is that a combination of TSLs, appropriate netting, GRP bolts and shotcrete will offer the same support resistance but with less material, faster development cycles, safer development methods and support products that can be broken up and destroyed when an undercut level gets blasted away and product goes through the crushers.
Allowance must be made in the production cycle of at least half an hour for the application of the TSL assuming the product and equipment is on site.
This excludes the time taken to set up the equipment and the cleaning afterwards. Furthermore, it could be advantageous to spray a thin layer of TSL over the previously sprayed surface of the previous blast to increase the TSL thickness to say 8mm and produce a both good and consistent cover and protection.
Conclusion
Rock related accidents are the major cause of injuries and fatalities in underground and most of these accidents occur near the active faces where workers spend most of their time. One of the major causes of instability is the lack of support and/or coverage between support units.
Support tendons alone do not provide sufficient rock reinforcement in friable ground conditions and hence the potential for the gravity induced fallout of small pieces of rock.
TSLs have the potential to benefit the works by offering improved productivity, profitability and safety.
From a manufacturing perspective it is very unlikely that all the characteristics that TSL could have can be included in one ‘super’ product. Proper selection and application of the correct TSL is very important to be able to improve ground conditions and reduce related injuries.
