# Investigating an Enhanced Contour Blasting Technique Considering Rock Mass Structural Properties
## 1 Introduction  
With the growing demand for natural resources, an increasing number of ore deposits have been exploited in underground mines to supply the global market demand over the past few years, resulting in the construction of thousands of kilometers of excavations for mine development and ore extraction. At present, underground mining is marked by higher productivity as a result of advancements in mine mechanization technology. The efficiency of mining operations is fundamentally influenced by the speed and quality of the excavation and stopping processes. Although the use of mechanical excavation methods such as tunneling boring machines (TBMs) is increasing, the drill-and-blast method remains the primary excavation technique used for rock excavations and fragmentation in underground mines due to its low cost and high flexibility, despite its inherent destructive nature [1, 2]. During rock excavations, an inappropriate blasting design might cause overbreak or underbreak. The rock mass damage area surrounding the excavation comprises three zones: the overbreak zone (failed zone), the damaged zone, and the disturbed zone, as shown in Figure 0(a). Overbreak refers to an excavation void that exceeds the designed tunnel perimeter, whereas underbreak refers to an unremoved part inside the expected contour. The rock mass damage zone surrounding an underground opening consists of an overbreak zone (failed zone), a damaged zone, and a disturbed zone (refer to Figure 0(b)). Excavation overbreak not only decreases stability and safety but also raises construction costs to a certain degree. It is one of the factors reducing the advance rate of excavating, leading to the formation of weakened zones with fall of ground, and ultimately to an increase in the costs of ground support and the hauling of excess rock mass [3; 4; 5; 6; 7; 8; 9]. Hence, controlling overbreak in underground excavations is critical for economic and safety reasons. In general, rock breakage by blasting is directly proportional to the exposed rock's stress level and pre-blasting conditions. Many studies have been undertaken to better understand rock damage and the overbreak phenomenon. In high-stress situations and adverse geological conditions, blasting disturbances can cause substantial ground control and overbreak problems. Perimeter control may be implemented to mitigate these undesired consequences, but its use frequently yields suboptimal outcomes. Factors that influence overbreak include rock mass properties (e.g., discontinuity characteristics and geological conditions), explosive parameters, and blasting design parameters (e.g., hole spacing, burden and diameter, and initiation systems) [1; 2; 6; 10]. In addition, tunnel geometry and orientation, the presence of nearby excavations, groundwater, and thermal, chemical, and biological effects all have a significant impact on the overbreak [11]. Although rock mass properties cannot be modified, understanding them allows for making informed decisions about selecting the most suitable explosive features and blasting variables to achieve the optimum results. The existing literature related to this topic suggests that in the mining industry, more than 90% of rock excavations are achieved using drilling and blasting methods, and considerable theoretical and empirical knowledge has been accumulated [12; 13]. In spite of this, the effective control of a blast effect in rock masses has not been fully investigated [14; 15]. Consequently, investigating the problem of controlling the blast effect and maintaining the overbreak in excavations within an acceptable limit is highly relevant. One way to achieve that is the improvement of the methods of explosion energy control, which rely on a sound understanding of the physical processes of rock breakage by blasting.