PROPOSING OPTIMUM BOREHOLE SPACING FOR A NON-VIBRATIONAL ROCK SPLITTING METHOD USING DISCRETE ELEMENT MODELING

Abstract

Various rock excavation methods are used in the field of tunneling and underground space. Most of the mechanized and non-mechanized tunnel construction methods involve the use of explosion and vibration techniques. These methods are not feasible to be used for excavation in urban areas mainly due to the reason that the production of noise, vibration, and dust due to these methods is unavoidable. As tunnels construction is often considered an important part of the development projects in urban areas especially big metropolitans, the environmental effects of such projects require the minimum production of vibration, noise, and dust so that these projects can be made as bearable for the local residents as possible. In the light of these considerations, it is highly required that such a tunnel excavation method is chosen for urban areas in which minimum noise, vibration, and dust is produced. In this study, a non-vibrational rock splitting method is introduced that involves the fragmentation of rock blocks by inserting a split-wedge system mounted on an excavator into already drilled boreholes and splitting of rock segments with their subsequent excavation. Lack of research work about various parameters and fracture propagation phenomenon involved in this method requires detailed investigation of non-vibrational rock splitting method. For this purpose, numerical analysis of this method was performed using PFC2D code based on particle mechanics approach because of the inherent function of micro crack formation and coalescence in the rock material made in PFC2D that leads to the macro-scale fracture propagation in rocks. Based on the uniaxial compressive strength, the rock material in this study was categorized in to three types namely; soft rock, normal rock, and hard rock. Bonded particle model (BPM) was used to generate the rock splitting model in PFC2D so that the mechanical behavior of the material could be represented realistically. In addition to that, the concept of Representative Elementary Volume (REV) was used to investigate the effects of specimen size, particle size, and particle heterogeneity in the PFC material by performing a number of numerical simulations of Brazilian tensile strength (BTS) tests. Afterwards, the REV for rock splitting model was estimated so that the specimen size for obtaining the microscopic parameters of rock splitting model could be obtained for all three types of rocks. In this study, various values of borehole spacing according to the strength parameters of rocks were mainly investigated using numerical simulation. Fracture propagation was achieved by inserting the full length (1 m) of the split-wedge system in the borehole and the results were obtained according to two different splitting widths of the system at the head of borehole. The splitting width of 20 mm is normally used in practice but the borehole spacing results using the splitting width of 25 mm were also included in this study. Additionally, three different borehole depths of 1.4 m, 1.3 m, and 1.2 m were also analyzed for all types of rocks using splitting width of 20 mm. The numerical analysis results show that when the splitting action is performed in rock, a breakout zone is formed vertically from the rock surface to the tip of fractured plane approaching the free space. Based on the results, it is suggested that the borehole spacing values greater than those in practice can be used in soft, normal, and hard rocks. The results also show that decreasing the borehole depth generally results in an increase of borehole spacing. The application of the results suggested in this study can help undergo an economical and efficient excavation process as they shall not only reduce the noise production during borehole drilling as well as the time and power usually consumed for the excavation.