Development of Predictive Models for controlling Blast Induced Overbreak in Tunnels

Abstract

Drilling and blasting continues to be the predominant rock excavation technique in driving horizontal openings and tunnels for underground construction purpose. Faster drivages, attempted these days for reducing the long gestation periods of projects, have often resulted in large overbreak. This is due to the unacceptable levels of ground vibration to which the rock is subjected. Factors contributing to this include longer pulls; burn cuts, higher explosive per hole and per delay etc. Blast-induced rock damage (BIRD) assessment based on far-field peak particle velocity (PPV) measurement, when extrapolated near the face, has often resulted in suggesting higher PPV threshold levels. Apart from this the geological and structural features play a dominant role in masking the intensity of blast waves. Therefore, near-filed monitoring using accelerometers has been attempted in one of the metal mines to study the blast damage in faces excavated by burn cut. Present seismographs available have limited range (2540mm/s) and are not suitable for near-field monitoring. This paper reports the investigations carried out for measuring acceleration, PPV and overbreak in a development heading for arriving at a suitable method of predicting blast-induced rock damage. The analysis of both acceleration and PPV measured against overbreak has revealed that rock damage is found to depend more on acceleration than PPV.