Application of the guideline for overbreak control in granitic rock masses in Korean tunnels

Overbreak, one of the most important factors in the evaluation of the results of a tunnel blasting, has been a challenging target for many tunnelling engineers as it is directly connected with construction cost. In Korea where the space produced by overbreak should be filled with shotcrete or the lining concrete, various efforts have been made to reduce overbreak. In connection with this, a guideline for overbreak control has been suggested by the author in a previous study based upon full scale field tests. The guideline provides the instructions as to how to draw a contour line, to drill blasting holes, and to charge explosives to reduce overbreak.In this study, field investigations and tests are carried out to check the feasibility of the suggested guideline for four tunnels in Korea. The average overbreak for the cases performed according to the guideline is compared with that for usual cases. The results from the comparison show that the guideline is generally working well and the overbreak is decreased by 10–77%, and it is analyzed that the overbreak reduction corresponds to approximately 65,000 US$ to 130,000 US$ in a support cost reduction.It is concluded that not only the use of a computerized drilling jumbo but also the carefully performed operations are important for the overbreak control.

► A guideline for overbreak control has been suggested by the author in a study.
► Field tests are carried out to check the feasibility of the suggested guideline.
► The overbreak when the guideline is applied is decreased by 10–77%.
► A computerized drilling jumbo has the effect to reduce overbreak by about 10%.
► The cost saving effect of the reduced overbreak is also analyzed.