###### Abstract  
A methodology for determining overbreaks in hard rock tunnel construction using the drill-and-blast technique is presented in this paper. The methodology was developed for and applied to crystalline medium- to thick-bedded limestone, but it can be used in any jointed hard rock. Overbreaks are inevitable in hard rock tunnelling in a low-confinement environment (shallow tunnels up to several hundred meters deep) as a result of wedge failures along unfavourably oriented discontinuities caused by blasting. It is widely accepted in engineering practise that overbreaks will be inevitable even if smooth contour blasting is applied. If not controlled, overbreaks can result in extreme financial and time costs; and determining, predicting, and mitigating them is the key to successful tunnel construction in hard rock. Technological overbreaks, which are caused by the inappropriate use of drill-and-blast excavation, are not easily distinguished from the inevitable overbreaks dictated by the geological conditions with which they interfere and overlap. A methodology was developed with the aim of distinguishing the two causes of overbreaks, which can be applied in any phase of tunnel construction for evaluation or mitigation. The analysis of key inputs, including geological face mapping, shear strength tests along discontinuities of the rock mass, and their spatial orientation relative to tunnel advancement and survey overbreak measurements, is presented in this paper. Due to the stochastic and statistical nature of the problem, a probabilistic concept was also applied as part of the method so that the probability of failure around unprotected tunnel sections could be determined. The so-called stability criterion is introduced to distinguish between stable and unstable sections in terms of the probabilistic safety factor. The quantification of overbreaks, including the threshold value distinguishing technological from geological overbreaks, is proposed. The application of the methodology, demonstrated on an 8.1 km long section of a 12 km long pressure tunnel in hard rock, is presented in the paper. hard rock; drill-and-blast; geological overbreak; technological overbreak; stability criterion; discontinuity orientation; tunnel construction +
Footnote †: journal:  
increase the consumption of shotcrete as well as the consumption of cast-in-place concrete needed for the secondary lining, both at substantial cost. In engineering practise, the problem of overbreaks has been addressed mainly from a technological point of view, with an aim to evaluate blasting quality and improve efficiency. Typically, a smooth contour blasting technique [2] is used to reduce the damage to the rock mass surrounding the underground excavation. This technique involves drilling a number of closely spaced boreholes along the final excavation contour; low-charge density-decoupled charges are installed in the boreholes. The detonation of the contour charges is triggered simultaneously after the detonation of the main charges located in the blast holes at the face of the excavation, with the aim of obtaining as smooth a contour as possible. Sometimes presplitting blasting, in which the peripheral holes are detonated prior to the main boreholes, is used for the same purpose, but it typically has lower efficiency [3]. With some exceptions [4], geological conditions, which are a key cause of overbreaks, are seldom evaluated in the context of blasting efficiency [5]. The key question during mining and tunnelling excavations is whether overbreaks were caused by inappropriate blasting practice or unfavourable rock mass characteristics. However, the response to this question differs in the mining and civil engineering industries. In mining, the functional role of a secondary tunnel lining is rarely needed, and the focus is on critically evaluating the technological factors influencing blast damage to achieve better fragmentation and, thus, more efficient access to mineral resources [6].