Rock bolts are a vital element of ground support in longwall mining, providing structural support to the roof and ribs of main and tail gates. They are also a common source of problems since they can be difficult to install in poorer ground and sometimes fail in operation.
At the very least, such issues slow down the gates’ development rate. At worst, they can lead to roof or tunnel collapse. In some countries, roof falls remain a common cause of injuries and fatalities.
However, it is rarely the rock bolt itself that is at fault. The problem comes from poor encapsulation: the grout or resin used to fix the bolt into the strata is not filling all the space between the bolt and the ground or it has not been properly mixed.
A new form of pumpable resin has been developed which overcomes this problem, ensuring total encapsulation for every bolt. Using resin in this form brings other advantages too: the installation process is safer; it can improve the quality of poor ground, reducing the need for maintenance and lowering the risk of falls or collapses and it speeds up gate development rates, even more so in poor ground.
The resin, which is both pumpable and thixotropic, is already demonstrating significant benefits for tunnel development in hard rock mines. For coal mines, where the ground, by definition, is lower quality, the potential for improvement is far greater.
In introducing this new system to coal mines, the goal is to reduce the technology gap between the processes and equipment used to extract coal and that used to construct the access gates. While the last 10 - 15 years have seen significant advances made in equipment and automation at the longwall and in the transport of coal underground, the same cannot exactly be said for the access tunnels. This means that the development of the gates often becomes the bottleneck in the whole coal production process.
As mines go deeper, production rates for tunnel development decrease, using standard rock bolting technology. At greater depths there are increased stresses in the strata, making installation of rock bolts more challenging and increasing the likelihood of failure in poorly installed bolts.
A significant problem
The most widely used primary rock support method in underground coal mining today is rebar anchored by resin from polyester cartridges. Research and anecdotal evidence from quality control professionals suggests that a significant proportion of rock bolts installed in this way will not be fully encapsulated.
The installation process using polyester cartridges involves drilling a hole into the ground that penetrates through the weaker strata into stronger ones above, inserting the cartridge and then rotating a bolt into the cartridge. The rotation of the bolt ruptures the cartridge, mixing two components - the resin and a catalyst - which are separated by a membrane, so that they react.
A rock bolt’s purpose is to limit movement of the ground; if the strata around the bolt moves, it resists the movement through shear action in the resin around the bolt. The goal is for the resin to be perfectly mixed, completely filling the gap between bolt and ground so that loads are distributed uniformly. However, there are several reasons why this may not happen.
If the bolt is rotated too little when inserting it into the cartridge, the resin will not be correctly mixed. This means that the bolt is surrounded by a mixture of reacted and unreacted resin. If it is rotated too much, it can destroy the partially cured resin. Sometimes the plastic membrane inside the cartridge remains intact, wrapped around the bolt so that the grout is not connected to the bolt, a situation referred to as ‘finger gloving’.
The polyester cartridge system can also fail if the drilled hole is larger than intended, due to loose strata or even a worn drill bit. Just a few millimetres make a significant difference: an increase in dia. of 2 mm on a 32 mm dia. bolt hole requires 15% more grout to fill the annulus. Since there is a fixed amount of grout in a cartridge, the hole will not be completely filled.
If the diameter of the drilled hole is significantly bigger than that of the cartridge, the bolt may even slide along one side of the cartridge, only partially mixing the resin or failing to penetrate the plastic sleeve at all.
With poor ground, there are more potential problems. Not only could there be overbreak of the drill hole, but material may crumble into the hole, preventing the cartridge from being pushed in up to the end. If the hole cannot be cleared, re-drilling and flushing is required. This is highly time consuming and success is not always a given. If re-drilling does not work, a new hole needs to be drilled, with a high chance of the same issue. Clearly, this slows down the cycle time and construction progress severely.
Even though it may appear that rock bolts have been successfully installed, problems may emerge days, weeks, months or even years later. Once the ground starts moving, as the coal deposits are mined, the loading on the bolt will not be even if it is not fully encapsulated. That could mean that the bolt will fail, leading to increased safety risks and the need for costly repair. In addition to the potential human cost of such failures, there is a large capital cost too: repairing a tunnel is five times more expensive than constructing in it the first place.
Rock bolts failures due to corrosion are a widely reported problem, which is currently the topic of several research projects in Australia and China. Uneven encapsulation allows aggressive ground water to come into contact with exposed parts of the bolt, leading to corrosion and possibly failure.
This article was originally published in the July/August issue of World Coal. View the full issue here and register to receive your FREE trial of the magazine here.
Read the article online at: https://www.worldcoal.com/special-reports/09072020/bolt-down-on-quality/
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