Careful blasting is a controlled blasting technique that is often used to minimise theoverbreak and damage to the remaining rock mass. Although decoupled charges in blastholes arecommon, the influence of empty guideholes in directing crack propagation has been less investigated.This study presents small-scale experiments using magnetite mortar cylinders with two and four emptyguideholes around a central blasthole, charged with 0.28 kg/m3 of PETN with a 2.5 decoupling ratio. Thiswork investigates the influence of the different guidehole configurations on crack development throughultra high-speed (UHS) photography and digital image correlation (DIC). The results indicate that theguideholes significantly influence the trajectories of the cracks, attracting their development and thusreducing the development of unwanted cracks. These findings provide insight into blast-induced cracksand contribute to knowledge for designing blasts where precise control of the cracks is critical. 

The propagation of cracks is a critical factor in the efficiency, stability, and safety of the blast in mining and tunneling operations. Understanding and controlling blast−induced cracks helps achieving goals such as reducing overbreak and minimizing damage zones. Despite interest in minimizing blast damage, few experimental studies have examined the effects of empty holes (guideholes) on blast−induced cracks. This work explores the influences of different blast designs on crack propagation patterns using an ultra−high−speed camera (UHSC) and the digital image correlation (DIC) method in small−scale blast experiments. Four samples of rock−like material with a 2.5 decoupling ratio and empty guideholes were blasted with PETN cord (3.6 and 5 g/m). The configurations included: two samples with a central blasthole with two and four guideholes, a sample with two blastholes with an empty guidehole, and a sample with four blastholes and a central guidehole. The experimental setup consisted of the samples and a DIC system with one UHSC to capture in 2D how the detonation of the different blast designs affects the material behavior. The configuration had a spatial resolution of 0.6 mm per pixel and a time resolution of 4.83 μs. DIC analysis revealed the exact location of the areas of cracks in the strain development, and the development velocity of several points along the cracks was calculated using the displacement results. The findings indicate that the use of guideholes has a greater influence than the specific charge on the number and the type of propagated cracks. There was no evidence that the crack development velocity (up to 1354.6 m/s) was more influenced by the specific charge than the propagation paths or the blasthole configuration. Cracks were classified considering their final role (cut cracks or damaging cracks) and shown to have a direct relationship with the blast design. The present research highlights the importance of combining qualitative and DIC analysis, which overall enhanced the understanding of crack propagation patterns and serves to improve explosive design strategies and optimize underground excavation processes.

In Sweden, drilling and blasting is generally regarded as the predominant excavation method for tunnel construction. While many studies focus on the blast performance within the tunnel perimeter, few studies have examined the effects of the induced cracks behavior in these designs (where the charge is decoupled). This work aims to improve the understanding of crack initiation and development during contour blasting. The investigation consists of a small–scale blast test to study the crack development around a blasthole using the Digital Image Correlation (DIC) technique. The experimental setup involved a cylindrical rock–like sample with a central blasthole and a decoupled PETN cord (Pentaerythritol tetranitrate) explosive charge by 3.9. The data was collected with an ultra–highspeed camera (UHSC) and processed using image engineering. Based on the experiment, numerical modelling was carried out to compare and investigate the behavior of the cracks. The analysis approach included tracking crack development using the strain values for both experiment and numerical modelling. These results emphasize the importance of combining the strain analysis in different directions to find the crack development sequence. The experimental and numerical results show good consistency, providing valuable information on crack development behavior around a blasthole. 

In mining, tunneling, and other blasting applications, excessive rock fracturing can compromise stability and deteriorate environmental and economic outcomes. Predicting the extent of blast-induced cracks is critical to accurately assess structural damage and optimize blast design. Seven blast tests were performed on 150 × 300 mm cylinders with a central blasthole of 280 mm length. PETN cord (5 g/m) was used, and the decoupling ratios (f) of 2.5, 4.2, and 5.0 were achieved by varying the blasthole diameters. In this study, qualitative and quantitative aspects of the induced cracks (number, type, velocities, propagation patterns, etc.) were examined using ultra-high-speed (UHS) photography and 2D Digital Image Correlation (DIC) method. Numerical modeling with LS-DYNA was conducted to simulate the experiments, to expand the analysis from 2D to 3D. The experimental results showed that crack initiation, gas ejection, and damage level decreased when increasing the decoupling ratio; notably, no damage occurred when f = 5. Crack propagation velocity using the DIC results showed V_c = (839.40 ± 46.57) m/s. A model for V_c from the experimental results was established through curve fitting with a R² > 97%. The DIC analysis enabled the distinction of the fracture process zone and the crack tip location. The numerical modeling results provided a realistic estimation of the cracks and damage compared to the experiments. The experimental and numerical results show that increasing the decoupling ratio reduces qualitative and quantitatively the blast-induced cracks, demonstrating how blast-induced damage could be limited.

Blasting is widely used in civil and mining engineering projects, with the side effect of introducing damage to the remaining rock. The damage can be differentiated from the cracks in the remaining rock, which increases the concerns of safety and requirements for rock support. In situ study of crack development remains complicated and costly; therefore, small-scale blasting experiments are a viable alternative for a detailed investigation of the crack propagation behavior. To fill the gap, this study examined a small-scale blasting test by investigating the velocity of the cracks implementing the digital image correlation (DIC) technique and avoiding contact methods such as strain gauges. An ultra-high-speed camera (UHSC) was used to record the blasting test in a single blasthole rock-like sample with a PETN cord. The experimental design underwent calibration until achieving the configuration of the equipment while ensuring the safety distance. The developed experimental methodology was tested successfully capturing the crack behavior. The analysis outcomes showed that the raw UHSC data needed to be preprocessed to enhance the tracking of cracks with the DIC method. The findings of the DIC data analysis indicated a fluctuation in the propagation velocity along the cracks (889–1129 m/s), revealing that the proposed methodology positively contributes to the propagation behavior of using the DIC method to track the blast-induced cracks.