Prediction of overbreak depth in Ghalaje road tunnel using strength factor

It is well known that the overbreak caused by the blasting damage during tunnel excavation increases costs associated with filling the collapsed area with shotcrete and results in filing of a claim by the contractor. This paper outlines a new approach for prediction the overbreak depth during tunnel construction. Hence, firstly excavation damage zone (EDZ) are determined by average specific charge in each zone. Numerical modelling is used to simulate the EDZ around tunnel boundary and the overbreak depth are calculated by the rock strength factor. The predicted overbreak depth compared with observed field data from a case study. The results show that there exists an approximately up to 40% difference between the prediction and the observed volume of overbreak depth. Therefore, the method can be well used to predict the overbreak depth to estimate more precision of shotcrete and concrete volumes in tunnelling cost during design phase.