Peripheral blasting in the driving of the hydraulic tunnels

“Instructions for Peripheral Blasting in Driving of Large Section Hydraulic Structures” was published in March of 1968. It sets forth in appropriate detail the technique of peripheral blasting along with substantiated recommendations for all principal instances of its application. In the meantime, some new details have come up in the continued development of this technique; their consideration is quite important in the correct working procedure. Details discussed in this article belong to this group: the effect of the type of bracing on over age in blasting cut-off timing for the lower bench, etc. Thus, this article complements the Instructions, to some extent, as a means of correct approach to some problems of peripheral blasting.

Conclusions

• 1. In driving tunnels and other undergound projects by the single section technique or with an upper lead section, peripheral blasting may prevent overage if the work is done without bracing or with an anchor bracing.

• 2. Peripheral blasting should be used, depending on the strength of rocks: with a closer spacing of charges in stable rocks; with a preliminary cut-off in the unstable rocks.

• 3. In working the lower bench following the driving of the lead section—always prior to concreting—it is necessary to blast the preliminary cut-off by placing the charges in boreholes as in the open-cut.

• 4. In driving the proximate parallel cuts, only the preliminary cut-off will protect the barrier from the effects of peripheral blasting.