###### Abstract
In civil engineering, the focus is on the quality of blasting but not on fragmentation. Instead, the efficiency of blasting is measured by the reduction of inevitable overbreaks, which must comply with the required pace of tunnel drift, so in this case, the rock mass characteristics are the most significant factor. An understanding of rock mass characteristics is necessary to determine the explosive properties and blast design parameters necessary to obtain optimum results. However, as emphasised by [PERSON] and [PERSON] [5], the heterogeneity of the rock mass material is in fact rarely considered during blast design. At the same time, it is commonly accepted that rock mass discontinuities have a controlling influence on the outcome of a blast. Most formulas in blasting are designed based on controllable parameters such as burden, spacing, bench height, hole diameter, stemming, docking, firing pattern, quantity of explosive, etc. However, according to [PERSON] et al. [7], many researchers have reported that uncontrollable parameters such as joints and bedding planes, and rock compressive and tensile strength also significantly affect blast performance. This implies that some overbreaks can be attributed to technological aspects of the blasting operation, which are referred to as technological overbreaks, whereas others are caused by unfavourable geological conditions, which are referred to as geological overbreaks. In terms of geological overbreaks, local geological conditions have a significant role, whereas the single most important consideration is the geological structure. The system of discontinuities, dips and strikes of bedding planes, and their relative position to the direction of the tunnel drift are decisive factors for the success of the blasting process and infrequent occurrence of overbreaks. In particular, it is commonly observed that the joint orientation and patterns can influence both fragmentation and overbreak in a blast [8]. The presence of joints affects the attenuation of the stress wave induced by blasting, which is minimal when the angle of occurrence is parallel or perpendicular to the face and increases to a maximum when the angle is between \(15^{\circ}\) and \(45^{\circ}\)[9]. Thus, if dominant joints are parallel to the face or inclined at an acute angle, there can be blocky fragmentation and severe endbreak. On the contrary, when the dominant joint direction is perpendicular to the face, no overbreak should occur when the fragmentation is poor, as block breakage can be expected. Other authors also reported that spatial alterations in joint angles will have a significant effect on the blast results [10; 11], as well as joint intensity [12]. The influence of the aperture of discontinuities on overbreaks was studied by [PERSON] et al. [13], who concluded that increasing joint surface separation severely decreases the quality of the final excavation profile as a result of increased cratering of joints. [PERSON] and [PERSON] [14] showed that a joint aperture as small as 3 mm acts as a free face, reflecting back the explosive energy and reducing the efficiency of blasting. The frequency of discontinuities in highly fractured rock mass is the critical factor in the incidence of overbreak; [PERSON] and [PERSON] [5] emphasised that a drilling pattern wider than the joint spacing for a frequency of 2-3 joint planes per spacing could have an adverse effect on perimeter control and cause large and consistent overbreaks. It should be stressed that geological overbreaks are more pronounced in hard rock tunnelling within a low-confinement environment (shallow tunnels up to several hundred meters deep) due to the incidence of wedge failures along unfavourably oriented discontinuities. In order to analyse the stability of the wedges, the parameters of the shear strength of the discontinuities need to be evaluated as part of the overbreak assessment.