Prediction of blast-induced overbreak from uncontrolled burn-cut blasting in tunnels driven through medium rock class

Drilling and blasting is the predominant rock excavation technique in driving horizontal tunnels. This often results in large overbreak. One of the prime reasons for overbreak is the unacceptable levels of ground vibration generated in blasting. From the literature survey and practical experience, it was found that threshold levels of PPV for overbreak depends on rock properties, namely, rock strengths, P-wave velocity, specific gravity, Poisson's ratio and rockmass parameters. Determination of threshold level of peak particle velocity (PPV) is crucial for controlling blast-induced overbreak and can be approximated by extrapolating the vibration predictor established from near-field vibration monitoring. This paper reports the experiments carried out in five horizontal tunnels for monitoring near-field ground vibration using accelerometer-based-seismograph planted in roof/sidewalls to establish ground vibration predictors. Blast-induced overbreak for each blast has been measured using a telescopic profiler. The threshold levels of PPV for overbreak have been estimated from the extrapolated vibration predictors to the overbreak zone and also using near field approximation technique. The estimated threshold levels of PPV for overbreak ranged between 590 and 1050 mm/s in extrapolation model and 410-890 mm/s in near-field HP model. Apart from these, a relationship between the percentage overbreak and rock/rockmass, charge and blast design parameters has been established through multivariate regression analysis of the data pertaining to five investigating sites. The result has been validated for four blasts within reasonable accuracy. (C) 2011 Elsevier Ltd. All rights reserved.