# Measuring the Reduction of the Confinement along the Evolution of a Burn Cut
## 2 Materials and Methods  
### Blast Configuration  
The blast test was performed as follows:  
* The opening cut was designed with a burn-cut configuration: a central charged hole, surrounded by four relief holes that were not charged. The rest of the cut evolved with two concentric squares. Ten holes in the opening cut were shot in total. * Each hole was drilled with a pull \(p=80\) cm. * Each hole was charged with the same charge: four emulsion cartridges 1\" \(\times\) 8\" (119 g each, for a total hole charge q = 476 g), primed with a fire cap and ignited using a fuse. * Each hole was charged and shot independently. * After each detonation, ventilation was allowed, then pictures of the whole face and zoomed-in pictures of the cavity created were taken. Figure 1 shows the configuration of the blast face as drilled: 21 charged holes and 4 dummy holes are visible. During the test, only the first ten holes (whose sequence is reported in Figure 1) were measured. Such holes represent the opening cut and were the objective of the research. The other 11 production and contour holes were charged and shot separately in a single blast due to operational constraints. Figure 2 shows the evolution of the cavity formed by the holes of the opening cut, high resolution images of the cavities are reported in Appendix A. One can observe the following mechanism:  
1. Delay \"0\": Detonation of the central hole of the burn cut. Due to the high confinement, the rock was fragmented but not ejected. A clean fracture was visible between the central hole and the dummy hole to its right. * Delay 1: The detonation of this hole created the first visible cavity, which was a prism that joined the detonated hole with the dummy holes at the extremes of the burn cut. The angle of breakage of the prism was about 40 degrees. * Delay 2: The created that prism that was ejected was wider and it joined the detonated hole with the extremes of the previous cavity. Confinement was reduced and the angle of the prism was about 60 degrees. * Delays 3-9: The cavity was well defined and regularly increased in size for each hole. The angles of breakage of the prisms tended toward 90 degrees. A larger and larger free surface was available at each detonation, and confinement continued to decrease. Although irregular, the cavities formed followed the mechanism of the ejection of prisms, at whose extremities lay the detonated hole and the extremes of the previously created free surface. The surface of each cavity created (corresponding to the surface of the prism ejected) was measured using software image analysis (Plot digitizer v. 2.6.8--Free Software Foundation, Inc., 59 Temple Place--Suite 330, Boston, MA 02111-1307, USA) via pictures with a scale metric reference. Results are shown in Figure 3, which plots the evolution of the surface of the cavity formed by each hole along the experiment. Figure 1: Drilling mesh on the blast face. Numbers indicate the firing sequence (“z” indicating zero). Light green: charged holes, orange: dummy holes. The metric scale is visible. Figure 2: Evolution of the opening cut cavity. Numbers indicate the sequence number of the last hole that was detonated. The first picture shows the result of the first hole, where the rock was fragmented but not ejected. Ejection of the rock broken by the first hole happened at the detonation of the second hole. Figure 3 shows the evolution of the surface of the cavity formed by each hole along the experiment. Such a section was the one measured on the cross-face of the tunnel, and when multiplied by the drill length, it gives the volume of the cavity. The dashed line indicates the total surface of the cavity after the detonation of each hole. The difference between the surface cavity measured after the detonation of hole \(n-1\) and the cavity measured after hole \(n\) gave the surface of the actual prism broken by hole \(n\).