# Measuring the Reduction of the Confinement along the Evolution of a Burn Cut
## 1 Introduction  
The main objective of this research was to document and analyze the evolution of a tunnel opening cut. The concept of the opening cut in a tunnel face is as old as the blasting science itself. One of its first definitions by [PERSON] and [PERSON] [1] reads: \"The first holes in the sequence aim at creating an opening towards which the rest of the rock is successively blasted. This opening, the cut, is the key that opens the rock.\" Cuts come in a wide range of schemes and conceptual breaking mechanisms. Nevertheless, it is common to adopt a general division into two main groups [2; 3; 4]:  
1. Parallel hole cut: cuts with holes parallel to the axis of the tunnel and uncharged relief (\"dummy\") holes.
2. Inclined hole cut: cuts with the hole inclined in some fashion with respect to the axis of the tunnel.  
The present work analyzed the effect of a \"burn cut\" [1; 5], which falls into the parallel holes cut category. [PERSON] and [PERSON] defined the burn cut as a parallel hole cut that \"operate[s] with empty holes of the same diameter as the charged ones.\" The burn cut specifically employed in the present study is defined by [PERSON] and [PERSON] [1] as a \"Gronlund cut\": a central charged hole surrounded by a square of dummy, empty holes that create the hollow prism for the expansion of the rock broken by the subsequent squares of the cut.  
The evolution of the opening consists of the increase in volume of the cut prism and the consequent reduction of confinement of the holes along the initiation sequence. To analyze such an evolution, this research focused on two aspects:1 Provide photographic documentation, in high definition, of the evolution of an opening cut.
2 Evaluate the reduction of confinement by measuring the energy transferred from the explosive charge to the rock mass.  
Regarding the first aspect, this is a rare opportunity. To document the hole-by-hole evolution of a blast via photographs, especially underground, one has to shoot one hole at a time as part of a heavily time-consuming operation. Such an activity can only be performed in a dedicated research facility, since no production operation would allow such a time-demanding task. In fact, the few pictures available in the literature of an evolving cut are present in out-of-date textbooks, such as [PERSON] and [PERSON], and are in black and white.  
Regarding the energy transferred to the rock mass: normally it is evaluated by considering the blast-generated vibration velocity. Many scientific works deal with this subject; in the last year alone, the subject has been covered by several authors [6; 7; 8; 9; 10; 11; 12; 13; 14]. Also, national and international regulations deal with the peak particle velocity of blast-generated ground vibrations (such as B56472-2:2008 in Great Britain [15], NZS/ISO 2631-2:1989 in New Zealand [16], DIN 4150-3:1999 in Germany [17], and USBM RI 8507 in the United States [18]).  
The present work intended to investigate the same energy phenomenon, but under a different point of view, focusing on the shock energy. Other authors have dealt with different ways of looking at blast-generated shockwaves [19; 20].  
We focus on the employment of a low-cost monitoring system. Particularly, we collected the vibrations by adopting low-cost shock sensors. The adoption of shock sensors was justified by the requirements to place the sensor very close to the blast face to minimize the impact of the effect of seismic propagation in heterogeneous materials. The vibrations induced by the blasting have been processed in both the time and frequency domains in order to estimate the most useful parameters and to establish relationships with the blasting performance. Information about the peak amplitude, duration of phenomena, and energy of the signal have been extracted.