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Roof bolts: it's the initial reinforcement that counts.

Roof bolts: It's the initial reinforcement that counts

Research in Sweden has shown it is the initial roof reinforcement that is most important and disproved assumptions that grouted re-bar rock bolts are less prone to rusting.

The latest Swedish study into the durability and effectiveness of various rock bolts headed by Professor Hans Helfrich of the Swedish Rock Engineering Research Foundation, was conducted at Yxhult Mineral AB's Centralgruvan mine at North Kvarntorp. Here, systematic rock bolting has been employed in a regular room-and-pillar sandstone-mining operation for over 20 years. Load conditions in the horizontally layered sandstone are uniform and exact records of bolts installed have been maintained.

The results of the project suggest that: * Grouted bolts show more signs of corrosion than ungrouted bolts. * Only 50% of grouted bolts show satisfactory grouting despite using recommended grouting procedures. * Poor grouting quality reduces the effectiveness of the bolt. * Reduced bolting quality is not necessarily a safety risk where systematic rock bolting is practiced. * Degree of effectiveness reduces with time. * Corrosion resistance is not as important as thought. * And that roof areas reinforced with grouted re-bars 20 years ago show no signs of convergence in spite of corrosion attack and reduced bolt effectiveness due to poor grouting.

The project involved the testing of 52 bolts ranging from 5 to 20 years in age. The four types of bolt tested were cement-grouted re-bars, resin-grouted re-bars, ordinary expander bolts, and a test batch of Swellex bits installed seven years ago.

Among the most important conclusions drawn by Professor Helfrich is that it is the immediate effectiveness of a roof bolt that has the greatest influence on long-term rock stability. Deformation forces are greatest immediately after blasting and indications are that rock surrounding an underground excavation stabilizes rapidly if immediate reinforcement measures are adequate. This suggests that as forces are redistributed, demands on the support function of a bolting system reduce. Thus, while it is essential that the effectiveness of the bolting system be sufficient to overcome initial deformation, a gradual reduction in bolt effectiveness has little influence on long-term roof stability.

If this can be proved conclusively, it will cast new light on the demands traditionally placed on roof bolts. Periodic measurements at Centralgruvan show no serious signs of convergence.

Professor Helfrich's conclusions are seen to improve prospects for Atlas Copco's Swellex bolts since it is generally accepted that the two major advantages of the Swellex rock bolt are the speed and ease with which it is installed, and the immediate full-column support it provides. Furthermore, Atlas Copco is now taking positive anti-corrosion measures for its roof bolts. The new Coated Swellex bolt is dipped into a one component physical curing paint (Iso-Galvan, which is based on a bitumen-modified cycled rubber) to give the entire bolt a tough and flexible coating.

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Publication:E&MJ - Engineering & Mining Journal
Date:Sep 1, 1990
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