Uwe Wyink, Hugo Bras, and Precious Jere, Master Builders Solutions, consider why new demands and new technology in hard rock mining mean that cementitious grouts are under more pressure to perform than ever.
T
he most commonly used grout for cable bolts in hard rock mines is ordinary portland cement (OPC). It is tried and tested, well understood by those working with it, and widely available. However, some mines are turning their backs on tradition to work with pre-formulated grouts that behave differently. This change is partly driven by a switch to automated bolting and grouting machines, which perform more efficiently with engineered grouting systems. Productivity gains and a reduction in failures and rework mean that the increase in material costs for pre-formulated grouts is more than offset by savings made elsewhere.
Why bolts fail
The role of a rock bolt is to help take loads from the ground which are no longer supported by the rock that has been removed in the mining process. They are not needed everywhere; it depends on the type of rock and on the stresses and strains in the strata. Geologists and engineers set parameters which determine the type of rock support that is needed at different points in the mine. Grout is installed to fill the gap between the bolt and the rock, as well as any fissures that run from the bolt hole. It is a vital part of the system because it transfers the load from the rock to the bolt. If the gap around the bolt is not properly filled for any reason, the loading on the bolt will not be uniform, which could mean that an overloaded section of bolt will fail at some point. One problem with OPC grouts is that they shrink as they cure, creating space between bolt and rock. This can also allow water to run through the bolt holes, increasing the pace of corrosion of the rock bolt and bringing more water into the mine, weakening the installed ground support.
global mining review // May 2022
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