# Investigating an Enhanced Contour Blasting Technique Considering Rock Mass Structural Properties
## 1 Introduction
The overbreak can be kept within acceptable design levels by using advanced drilling and blasting methods in combination with common blasting strategies aimed at controlling overbreak, which include the following: delay control in the perimeter and buffer rows; increasing the number of holes to compensate for the light charge concentration at the periphery; avoiding drilling too close to discontinuities; reducing perimeter hole spacing if a severe weak geological zone, i.e., a fault, crosses over; using customized explosives for the tunnel perimeter blasting that facilitates better control; using electronic detonators to  
Figure 1: (a) Illustration of blast-induced rock mass damage zones around an underground excavation; (b) overbreak and underbreak in underground excavations opened with drill-and-blast method. facilitate precise delay control; and opting for drilling precision, because even a slight angle difference in the drilling rig can change the direction of the produced blast impact, which can lead to overwhel [2; 10; 13]. The blasting of continuous stressed media in general, and contour blasting in particular, and the identification of its characteristics have been the subject of research by many scientists. Significant contributions to the study of rock breakage mechanisms, the formation and propagation of stress waves in the rock mass during the explosion, and the extent of the damage zone have been achieved. For instance, [PERSON] and [PERSON] [16], as pioneers in this field, in 1963 laid the foundations of modern blasting techniques and introduced smooth blasting, which has guided most existing contour blasting methods in tunneling. Two decades later, [PERSON] and [PERSON] [17] provided insights into the essence of the phenomena occurring in the blasting of contour charges via a full-scale investigation, since most existing works were not fully validated, and some appeared to be unsuitable for engineering purposes. They concluded that, in a system of contour charges, there was no need to incorporate any empty blast holes. Since then, many studies have been carried out to further elucidate the complex mechanism of fracturing in the excavation contour in different engineering conditions and to provide guidelines for the blast-induced overbreak using a wide range of methods, such as field experiments, numerical modeling, empirical models, and machine learning techniques. [PERSON] and [PERSON] [1] investigated the extent and causes of overbreak in tunnels during contour blasting; their results indicated that the in-situ stress condition, in addition to the rock mass properties, plays an important role in enhancing the magnitude of the overbreak. These results were also corroborated in other research [11; 18]. Examples of more recent studies include the introduction of the overbreak resistance factor; a proven empirical approach for predicting and managing the overbreak phenomenon in tunnels [10]; a comparison of the rock cracking and damage in tunnel contour blasting (smooth blasting and presplitting) under various initial stresses via experimental and numerical simulations [19]; the modeling of the rock fracture and fragmentation process induced by the controlled contour blasting [20]; a site investigation of the effect of the excavation sequence on contour blasting efficiency [21]; a phase-field formulation with a dynamic constitutive model capable of capturing the blasting-induced failure and breakage phenomena in rock masses [22]; and an excavation overbreak extent prediction using AI techniques [7; 23]. Despite the large number of studies that have been undertaken and the successes achieved in studies related to the blasting mechanism with explosives and its effect on rock masses, the current literature indicates different schools of thought, each with its own merits and limitations. Until now, there has been no single and widely accepted blasting design method for underground excavations.