Question by Gerdeen (for Barton, Lien and Lunde) The authors are to be congratulated on a very interesting and potentially useful paper. Our work has been mainly with roof bolting and so my question deals with this method of support. Would you explain the difference between categories 10 and 11 in Table 8 where the quality indices for block size are the same, but where different types of bolting are recommended? It seems to me, you favor tensioned mechanical bolts over untensioned grouted bolts for larger spans. The question is why? Are there not cases where tensioning may cause additional damage? Does your analysis apply as well to flat horizontal roofs or only to arches? Reply by Barton The two support categories referred to in Mr. Gerdeen's comments were categories 10 and 11, and you can see from Fig. 3 that between these two categories there is no change in the rock quality. The rock quality ranges from 40-100 in each case. That's pretty good rock. Some people would say it's the best rock you ever get, but that's not our Scandinavian experience. So we've just got a difference in span and/or in the use of the excavation. You can say the EST (excavation support ratio) which describes the type of excavation makes the effective span for category l0 less than that for category 11. I am in full agreement with your point on the application of tension bolts as opposed to, grouted bolts. If this paper were expressing purely the opinion of we three authors in NGI, we would have, I think, in most cases have recommended just grouted bolts. We have analysed approximately 200 case records, or 200 usable case records, from which it appears that the general practice of some years ago, 5 years ago let's say for an average construction completion, was the use of tensioned bolts in these larger excavations. I think that when the rock quality is good, this is a very questionable practice, but when the rock quality is very poor, and really requires an active support pressure, then one may obviously favor tensioned bolts. However, I have so reservations ca this and I'd like to refer to the Roman arch in the following figure. It is not the usual type of rock mechanics we deal with but it is quite interesting. This is part of a famous Roman aqueduct in Spain. There are something like four levels going right across the valley, if I remember correctly. Now, let's suppose this were a continuous rock mass. If we consider the different effects of grouted untensioned bolts, and tensioned bolts in this arch, it's very easy to see that the tensioned bolt is going to destroy the arching effect. When you apply a tensile force in good rock, I believe that even though you are applying a support pressure, of course, you are also reducing the arching effect, or the ability of the rock to support itself.
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-, -. "General Discussion." Paper presented at the The 16th U.S Symposium on Rock Mechanics (USRMS), Minneapolis, Minnesota, September 1975.
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