Although pumpable resins for rock bolting and cable bolting have been introduced only recently, they are already making a big impact. One mine operator reports that it is cutting months from its development schedule, allowing production to start earlier.
As mines operate in deeper levels, with higher rock stresses affecting the stability of the strata, rock reinforcement becomes more challenging. Pumpable resin, with rock or cable bolts, offers significant benefits over traditional bolt fixing methods, saving time and reducing the risks of bolt failures during their expected lifetime.
Deep Mining Installation Difficulties
Traditional solutions for long-term rock reinforcement, such as standing support, sprayed concrete and mechanical or cementitious grouted rock bolts sometimes have drawbacks for deep mining operations. They can be difficult to install, especially in poor ground. This leads to quality concerns and longer installation times, creating bottlenecks in mining operations.
Rock bolts grouted with polyester cartridges have become popular, because they offer a faster installation time than traditional methods, but in unstable ground this system has its challenges too. Because the cartridge must go into the borehole first, before the bolt is rotated in to mix the resin, looser ground causes problems.
If a borehole falls in as the drill bit is extracted, the cartridge cannot be inserted. If the borehole is bigger than intended due to overbreak, or even due to a worn drill bit, there won’t be enough resin in the cartridge to fill the borehole. For instance, an increase in diameter of just 2mm on a 32mm bolt hole requires 15% more grout to fill the annulus.
Bolt Rotation Speed
The speed at which the bolt is rotated into the hole is also important when using a cartridge system. Too little rotation and the two components in the cartridge won’t be sufficiently mixed; too much and it will be overmixed.
Bolts that are not fully encapsulated by grout become weak points in the system. When the rock mass moves – which often happens due to other mining activities – load on the bolt will be unevenly distributed, increasing the risk of failures. Poor encapsulation could also lead to corrosion due to groundwater reaching the bolt.
Another downside of cartridges is that they are sensitive to temperature. If they are stored or transported at too high a temperature, their reactivity and performance are affected. If the temperature is too low during installation, the resin will be more difficult to mix.
The Benefits Of Pumpable Resins
As more and more mines experienced the problems outlined above, the need to develop pumpable resins became greater. The idea of pumpable resins for rock bolting had been around for many years, but chemical technology has only recently developed sufficiently to allow the idea to become viable.
The goal in developing them was to combine the effectiveness of pumpable cementitious grouts with a resin that would cure much faster. Cementitious grouts must cure for between 12 and 24 hours before it is safe to re-enter an area, causing severe schedule limitations. Resins can go off in a few seconds to minutes only.
For the chemists who developed systems such as the MasterRoc RBA 38X series of pumpable resins, the goal was to create a fast-setting resin that was also thixotropic to allow overhead installation without additional borehole sealing. A considerable challenge – and one that not every system has overcome – is how to ensure that the resins are stable and perform uniformly at different temperatures.
Cable Bolts & Hollow Bolts
Pumpable resins can be used with cable bolts, as well as with hollow bolts or self-drilling anchors. Open times can be adjusted to suit the application and the resins can be adjusted to work at a range of temperatures from 5°C upwards.
Faster Installation Cycles
The speed at which the use of these new resins is spreading demonstrates how effective they are. Following extensive field trials on surface and underground, the MasterRoc RBA 38X series was deployed commercially for the first time in 2019. Today the resin is being used in major mine development projects.
In less stable ground, the immediate benefits of the resin system are apparent: installation times for cartridges can take 10 or 15 minutes, whereas a self-drilling bolt combined with pumpable resin takes 2.5 to 3 minutes. Although cycle times in good ground are similar for either polyester cartridges or pumpable resins, some mines are finding that it makes sense to use pumpable resins throughout because there is no need to switch between systems if the rock quality falls.
There are longer-term benefits of using pumpable resin too: it penetrates cracks and fracture zones around the bolt, consolidating the ground and increasing its stability. In fact, the polyurea silicate resins used in the MasterRoc RBA 38X series were developed from those used in rock consolidation. Larger-than-planned borehole diameters do not create potential problems with this system as the resin can be pumped until the hole is completely full.
Lower Lifetime Costs
Given that the cost to refurbish a tunnel is three-to-five times the cost to build it originally, improved rock stability and fewer bolt failures over the life of the facility mean lower lifetime costs for the mine.
Adapting Equipment For Pumpable Resins
Switching to pumpable resins requires new or modified equipment. Rigs with built-in systems for pumpable resin have come onto the market in the past 12 months. For some existing equipment field trials have demonstrated that retrofit kits work well, at a fraction of the cost of a new machine.
As with any new operation, mining operatives will require advice and training as they start to work with pumpable resins. However, systems such as MasterRoc RBA 38X have been designed to be easy to use, and reports that workers prefer to work with the pumpable resin rather than with the cartridges confirm this.
As well as being easier to work with, pumpable resin is safer too, because there is no need to approach the rock face to install a cartridge. Everything can be done by the boom of the rig, with the operator positioned behind the boom in the cabin of the machine. In an environment where mine operators are looking for more automation to improve safety, this system offers another considerable benefit.
Uwe Wyink is global technical manager at BASF