Analysis of shock wave parameters at the explosive cavity wall during refraction of detonation waves through the air and water
Authors:   St. Petersburg Mining University, AFANASEV Pavel 
Published: December 2023

Introduction. Drilling and blasting operations are carried out in various mining and geological conditions. The pressure transmitted from the detonation wave into the rock is an important factor affecting the parameters of the borehole network. At the same time, the calculation of the parameters of shock waves in engineering practice is not accompanied by the analysis of changes in the detonation wave parameters when passing to the rock. At decomposition of the explosive inside the borehole, the movement of the detonation wave along its surface is characterized by normal and sliding incidence. At the moment of initiation, at the initial stage, the incidence of the detonation wave on the borehole surface is normal; later, when the detonation wave propagates along the surface, a sliding interaction occurs. This sliding motion is the main one, since the detonation wave front proper, which moves along the borehole, does not have a strong surface curvature. Refraction of the detonation wave is not always possible directly into the rock, since the medium near the explosive can be surrounded by an air or aquatic medium. Therefore, it is necessary to take into account possible options for the performance of drilling and blasting operations: a radial air gap and the presence of water between the explosive and the rock. As a rule, the occurrence of an air gap shows up in case of intense fracturing of the massif, the use of hoses of a smaller diameter than the drilled borehole. The presence of water between the explosive and the rock depends on the rate of water inflow into the borehole and can be also due to breaching the charging technology, when charging of the borehole under the water column is neglected. The presence of inert media, such as water and air, change the detonation pressure and the velocity of detonation products acting on the rock; therefore, it is necessary to analyze the parameters of the explosive cavity wall for various types of refraction of the detonation waves and on the basis of such analysis, substantiate your choice of the characteristics of the explosive. Research methods and materials. Assessment of the detonation wave parameters is based on the hydrodynamic theory of detonation with the calculation of the indicators of gaseous explosion products using the method proposed by L.V. Landau and K.P. Stanyukovich. Estimation of stress wave parameters in the massif is based on the solution of the Riemann problem of the breakdown of an arbitrary discontinuity when the detonation products (DP) refract into different media (Fig. 1). In the first case, the “DP – rock” is considered; in the second case, the “DP – air medium – rock” and in the third one, the “DP – aquatic medium – rock”. Discussion of research results. It was revealed: when a detonation wave is refracted through an aquatic medium into siltstone, the pressure values on the explosive cavity wall will be higher than without an iner t medium. Thus, for acoustically softer rocks, the presence of an inert medium (water) leads to an increase in pressure, and hence to an increase in the brisance of the explosion. In acoustically harder rocks, this result is not recorded. The analysis of the data obtained showed that the parameters of the detonation waves refracted into different inert media differ significantly both in pressures and in par ticle velocities in such media. Conclusive statements. 1. When a detonation wave passes into an acoustically hard medium, the values of the pressure transmission ratios increase in the presence or absence of an intermediate inert medium, and the mass velocity transmission ratios decrease. Thus, there is a redistribution of the destruction character in favour of the wave action in relation to the quasi-static one. 2. When a detonation wave is refracted through an aquatic medium, the pressure in acoustically soft rocks increases. For acoustically hard rocks, no pressure increase is recorded. 3. The highest values of the ratio of pressure transmission into the rock are achieved at the detonation velocity of EEM about 4,000-4,500 m/s. 4. The choice of an explosive for assessing the impact on the massif should be based on the estimation of the initial parameters recorded on the explosive cavity wall, since you can choose the following characteristics of impact on the massif: “hard” – wave or “soft” – quasi-static. The initial values of mass velocities can be used for assessing the collapse of the rock, and the pressure values for assessing the shear (regrinding) zones. Conclusions. 1. The article presents the results of research on estimating the parameters of shock waves on the explosive cavity wall when the sliding detonation wave is refracted into the rock. 2. It is shown that the choice of an explosive for assessing the impact on the rock mass should be based on the calculation of the initial parameters recorded on the explosive cavity wall taking into account the character of the detonation wave refraction through inert media. 3. It is ascertained that when a detonation wave is refracted through the aquatic medium in an acoustically “softer” rock (siltstone), the stresses arising on the explosive cavity wall will be higher than in the absence of an aquatic medium. Thus, in acoustically “soft” rocks, an increase in stresses is recorded due to an inert medium; for acoustically “hard” rocks, there is no increase in stress. The air medium always reduces the stress on the explosive cavity wall. Results. The results of the research can be useful for design and scientific organizations that validate the explosives used by the companies of the mineral resource complex and also for those engaged in mining operations and improvement of the blasted rock mass quality.