The numerical analysis results from an improved design of large-scale dynamic test of rock support (Test 6) is presented in this paper. The improved field test was designed based on the results obtained from field tests and the numerical analysis of the earlier tests (Tests 1 – 5) conducted at LKAB Kiirunavaara mine. The performed numerical analysis investigates how the improvements including minimizing the expansion of blasting gases into the burden, avoiding the complete damage of the burden, and creating sub-planar waves were achieved under the improved design of the test. Furthermore, the response of supported and unsupported excavations as well as the complex interaction of stress waves and rock support was numerically studied. The numerical analysis comprised of two stages (i) the explosion stage modelled with the finite element code LS-DYNA and (ii) the wave propagation stage which was modelled using UDEC with the results from LS-DYNA as input. The accuracy of the developed models was investigated by comparison of the UDEC models results to the data obtained from the field test. The numerical analysis results confirmed that the improved designed burden has assisted in reducing the areas of tensile yielding in the burden and as a result, the gas expansion and complete damage of the burden was avoided. The simulation results showed that the used support system (Swellex Mn 24 and reinforced shotcrete) has effectively limited the displacement of the test wall and prevented ejection during the dynamic loading. The combined numerical technique has shown its advantage when simulating blasting as well as interaction between waves and opening and it can thus be used as a tool for evaluating rock support performance.

The numerical analysis of four dynamic large-scale field tests conducted at LKABKiirunavaara mine are presented in this paper. The aim was to numerically studythe behavior and response of the burden and the tested walls in field Tests 1, 2, 4and 5. For this purpose, two numerical methods were combined, i.e. the finiteelement code LS-DYNA and the distinct element code UDEC. The LS-DYNA wasused to calculate the blast load, and the UDEC was used to propagate the calculatedload in the model where the geological conditions of the test site and the installedrock support in the field tests were modelled. The model was calibrated bycomparing the velocity and displacement calculated on the surface of the opening,and the zones yielded in tension were used to study the failure mechanismdeveloped in the burden. The numerical models were able to mimic the behavior ofthe jointed rock mass and the rock support fairly well. It is concluded that thenumber of major joint sets was the main reason to the difference between the failuredevelopment in Tests 1 and 2 and Tests 4 and 5. The numerical analysis of Tests 1and 2 confirmed that the gas pressure in the vicinity of the test wall in those testswas minimum. In Tests 4 and 5, it was observed that, the generated fractures in theburden combined with the natural joint condition of the burden, increased thepossibility for blocks to rotate and move within the burden. The complete burdendamage in Tests 4 and 5 was concluded to be the be due to the ejection of rockblocks in the vicinity of the test wall upon the arrival of stress wave, and ejection ofthe remaining portion of the rock blocks in the burden by the gas expansion. 

Seven large scale dynamic tests of rock support were conducted at LKAB’s KiirunavaaraMine in order to develop a methodology for the design of dynamic load resistant rocksupports. The tests were performed to increase the understanding of the response of rocksupport installed on the walls and roof in a tunnel and subjected to strong dynamicloading. The tests were originally numerically simulated by using two numerical tools,LS-DYNA and UDEC, in a former PhD Project at Luleå University of Technology, LTU(Shirzadegan, 2020), to investigate the performance of the test set up, the response ofsupported and unsupported excavations during the field tests as well as the complexinteraction of stress waves and rock support. The comparison of the numerical results andthe results obtained from field tests showed that the combination of LS-DYNA and UDECcould satisfactorily simulate the field tests. This work is the continuation of the LTUproject with LS-DYNA and UDEC.Since the analyses were previously conducted with 2D models, while in reality thestructural geology, ground motion measurements, fracture investigations and supportmotion, as well as the deformation measurements, occur in a 3D space, in the presentproject three-dimensional analysis are carried out using 3DEC that can assist in studyingthe interaction of the dynamic load with rock joints, rock blocks, tunnel wall and installedrock support. In this project the analyses are carried out in two stages: (i) the explosionstage is modelled in 3D with the finite element code LS-DYNA and (ii) the wavepropagation stage is modelled in 3D with the program 3DEC, where the results from LS-DYNA are used as input.The numerical simulations in this project can be used to analyse the behaviour of rockmasses and rock support systems under dynamic loading conditions. These simulationsprovide valuable information about the response of the rock mass to dynamic loading andcan be used to optimize the design of rock support systems. The numerical results evaluatedifferent rock support performance (Swellex Mn24 bolt and D -bolt) under dynamicloading conditions.3DEC modelling in the present study shows that: a) numerically calculated velocitypatterns match well those observed in the field for Test No. 6, b) larger displacementsoccurred for certain rock blocks in the unsupported crosscut and, c) the patterns of themost displaced rock blocks in the model very much resemble the positions where falloutswere observed in the field close to the footwall drift in the unsupported crosscut. (PDF) Dynamic modelling of rock bolts at Kiirunavaara mine. 

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.

Sublevel caving (SLC) is a mass mining method based upon the utilization of gravity flow of blasted ore and caved waste rock. Production blasting has significant impact on the efficiency and productivity of SLC output, affecting material fragmentation, flow and recovery. Misfires could occur during blasting in SLC, which results in the loss of explosives and poor fragmentation. Usually, the blastholes are top-initiated with redundant lower primers set at + 25 ms as a precaution. If the top primers fail, the lower primers may initiate the explosive, which will induce out-of-sequence delays that can influence fragmentation and gravity flow. This work investigated the impact of misfires and out-of-sequence delays on fragmentation and gravity flow through numerical simulations using a coupled finite element and discrete element method. Different scenarios of misfires and out-of-sequence delays were studied, and the fragment size distribution was obtained to evaluate the blasting performance. The material flow behaviour was investigated to assess the gravity flow process. The results indicated that misfires significantly affect the fragmentation and the gravity flow process while the effects of out-of-sequence delays are not so significant for the investigated cases. 

The gravity flow behavior of blasted ore and caved waste in sublevel caving (SLC) mines is complex. The shape of fragmented ore and caved waste is identified as one of the principal factors influencing the gravity flow of ore. To investigate the effect of the particle shapes on the gravity flow, a code was developed to generate polyhedral fragments in different shapes and divide them into internal elements. Then these fragments were imported in the LS-DYNA code to generate SLC models containing blasted ore and caved waste and model the extraction process. To model the non-continuous loading process, the gravity flow was considered to be an intermittent process by setting a switcher at the extraction point. The flow behavior of ore from the numerical modeling is in agreement with the experimental results. The cumulative dilution of ore by waste is up to around 30%, which agrees with the results of the field survey.

Blasting is widely used in tunneling when mechanical excavation methods cannot be applied due to rock conditions or cost constraints. The blast design of the contour holes defines the damage to the remaining rock, which might change the rock support requirements. This study investigates the crack behavior in sequential boreholes through small-scale experiments on rock-like specimens. Cylindrical samples, prepared with speckles for Digital Image Correlation (DIC), varied in decoupling ratio, and the detonation cord was detonated simultaneously in the blast holes. The data was collected with an ultra-high speed camera (UHSC) for DIC. The results indicated the development of the cracks between the boreholes and their behavior towards the boundary of the samples. The results showed that in this experimental configuration, there is no significant difference between the different decoupling ratios. This study shows the importance of an optimum blast design to minimize the damage to the remaining rock.

Coalbed methane (CBM) is a significant factor in triggering coal and gas outburst disaster, while also serving as a clean fuel. With the increasing depth of mining operations, coal seams that exhibit high levels of gas content and low permeability have become increasingly prevalent. While controllable shockwave (CSW) technology has proven effective in enhancing CBM in laboratory settings, there is a lack of reports on its field applications in soft and low-permeability coal seams. This study establishes the governing equations for stress waves induced by CSW. Laplace numerical inversion was employed to analyse the dynamic response of the coal seam during CSW antireflection. Additionally, quantitative calculations were performed for the crushed zone, fracture zone, and effective CSW influence range, which guided the selection of field test parameters. The results of the field test unveiled a substantial improvement in the gas permeability coefficient, the average rate of pure methane flowrate, and the mean gas flowrate within a 10 m radius of the antireflection borehole. These enhancements were notable, showing increases of 3 times, 13.72 times, and 11.48 times, respectively. Furthermore, the field test performed on the CSW antireflection gas extraction hole cluster demonstrated a noticeable improvement in CBM extraction. After antireflection, the maximum peak gas concentration and maximum peak pure methane flow reached 71.2% and 2.59 m3/min, respectively. These findings will offer valuable guidance for the application of CSW antireflection technology in soft and low-permeability coal seams.