Delay time is an important factor in the quality of bench blasting. The development and application of electronic detonators make it possible to control the timing of detonation by a highly precise delay time. It is an easily achievable way to seek a better blast fragmentation by controlling the delay time. In order to investigate the influence of delay time on rock fragmentation, eight experiments on bench blasting models with double holes were carried out. The average weight of 4.59 g of pentaerythritol tetranitrate (PETN) was charged into each blast hole with a diameter of 10 mm. Delay times between the double holes were set as: 13.69, 27.36, 54.72, 60, 90, 120, 180, and 339.6 µs. During blast loading, the breakage processes of specimens were captured in detail by high-speed cameras. Full-field strains on the bench slope were analyzed by the three-dimensional digital image correlation (3D-DIC) technique. After each experiment, the coarse fragments were backfilled into the post-blast specimen to observe and record the bench slope’s final breakage pattern. The experimental results showed that the fragmentation of the bench slope transformed from horizontal crack dominance to vertical crack dominance as the delay time increased. In addition, post-blast fragments were collected and sieved, including fine materials. In the premise of approximate excavation, the optimum delay time was 180 µs. Compared to short delay times such as 27.36 µs, x₅₀ was improved by approximately 25% at the delay time of 180 µs. The results showed a significant difference and great improvement in fragmentation when the delay times were in the time range of no-shock-wave interaction compared to interactions. When determining the optimum delay time in multi-hole blasting, except for the stress wave interaction, factors such as crack propagation should also be considered. The results of experiments could contribute to references for relevant research.

Rock fragmentation by blasting influences ore recovery and the cost of downstream operations. The development of electronic detonators makes it possible to improve fragmentation by controlling the initiation timing in blasting projects, and the effect of the mechanism of delay timing on rock fragmentation should be studied. Fragmentation of granite bench specimens with different initiation timing was investigated in blast experiments. Conclusions are obtained by studying the surface strain field and post-blast specimens. A total of six blasting tests were carried out on granite bench specimens with four boreholes each having a diameter of 10 mm and a length of 450 mm. Each borehole used pentaerythritol tetranitrate (PETN) as the explosive charge, which was approximately 4.84 g with a charge diameter of 5.5 mm. Delay times between adjacent boreholes in the same row were set as 0, 50, 100, 150, 200, and 250 µs. The surface strain field of the bench specimen under blast loading was analyzed using three-dimensional digital image correlation (3D-DIC) techniques based on two cameras that captured high-speed images. Additionally, the post-blast specimen was also observed and recorded. Fragments of each bench specimen were carefully collected, weighed, and sieved with a set of sieves, including very fine particles. According to the 3D-DIC analysis for bench specimens, the propagation pattern of the main strain concentration zone transformed from horizontal to vertical with the increase in inter-hole delay. The maximum blast excavation weight was obtained by the bench specimen with an inter-hole delay of 100 µs, while the bench specimen with the longest inter-hole delay (250 µs) obtained the minimum blast excavation weight. By combining the results for blast excavation weight with the results from fragment size distribution analysis of all specimens, the optimal inter-hole delay was 200 µs. Compared to simultaneous detonation, the median size was decreased by about 14.5% for the inter-hole delay of 200 µs. The results of experiments show that delay time significantly influences rock fragmentation, but the stress wave superposition in short delays cannot improve rock fragmentation. For long delays, the blast-induced crack propagation time should be regarded as an influential factor when choosing the proper delay time. The experimental findings of this study could provide a better understanding of the effect of the mechanism of delay time on rock fragmentation.

Underground saturated jointed rock is prone to engineering geohazards under the combined effects of in situ stress and dynamic loading. A modified split Hopkinson pressure bar (SHPB) system was used to conduct dynamic loading tests on artificially fabricated saturated jointed rocks. The effects of joint matching coefficient (JMC) and confining pressure on the dynamic strength, deformation characteristics, energy evolution, and stress wave propagation of the specimens were investigated. The test results show that the dynamic compressive strength and stiffness of saturated jointed rocks increase with the increase in JMC, but the compressive strength is still lower than the typical dynamic strength range. Rock damage mainly occurs at the joint location, and the damage mode is dominated by tensile fracture. In terms of energy, the energy dissipation rate of the rock decreases with decreasing JMC and increasing confining pressure. The propagation of stress waves is mainly affected by the coupling of JMC and three-dimensional static stress, which is manifested as a transition from a rapidly changing phase to an unstable changing phase, a process accompanied by an energy distribution mechanism. These insights fill a gap in the mechanical response of saturated jointed rocks under complex loading conditions underground and help predict the risk of dynamic instability in underground engineering and mining operations.

The drilling and blasting method is commonly employed for the rapid demolition of outdated buildings by destroying key structural components and inducing progressive collapse. The residual bearing capacity of these components is governed by the deformation morphology of the longitudinal reinforcement, characterized by bending deflection and exposed height. This study develops and validates a finite element (FE) model of a reinforced concrete (RC) column subjected to demolition blasting. By varying concrete compressive strength, the yield strength of longitudinal reinforcement, the longitudinal reinforcement ratio, and the shear reinforcement ratio, 45 FE models are established to simulate the post-blast morphology of longitudinal reinforcement. Two databases are created: one containing 45 original simulation cases, and an augmented version with 225 cases generated through data augmentation. To predict bending deflection and the exposed height of longitudinal reinforcement, artificial neural network (ANN) and random forest (RF) models are optimized using the hunter–prey optimization (HPO) algorithm. Results show that the HPO-optimized RF model trained on the augmented database achieves the best performance, with MSE, MAE, and R² values of 0.004, 0.041, and 0.931 on the training set, and 0.007, 0.057, and 0.865 on the testing set, respectively. Sensitivity analysis reveals that the yield strength of longitudinal reinforcement has the most significant impact, while the shear reinforcement ratio has the least influence on both output variables. The partial dependence plot (PDP) analysis indicates that the ratio of shear reinforcement has the most significant impact on the deformation of longitudinal reinforcement.