INVESTIGATION OF THE PROCESS OF DESTRUCTION OF A ROAIN MASSIF BY BOREHOLE CHARGES WITH AN AXIAL AIR CAVITY BY THE METHOD OF PHYSICAL SIMULATION

: the design of downhole explosive charges with an axial air cavity was developed, which ensures uniform crushing of the rock mass and reduces the specific consumption of explosives. A technique has been developed for modeling the process of destruction of a rock mass by borehole charges with an axial air cavity. It has been established that with an increase in the diameter of the axial cavity, the mass of the charge and the specific consumption of explosives for breaking decrease. However, this decrease occurs only up to a certain limit, after which the volume of the chipped mass decreases and the specific consumption of explosives increases. The most efficient way is to reduce the specific consumption of explosives for breaking when the ratio of cavity diameters and charge diameters is 1.3:5–2.2:5. For other values of the ratio, the specific consumption of explosives increases dramatically.


1.INTRODUCTION
In the open mining of mineral deposits, one of the most important components is the problem of controlling the action of an explosion, which requires a correct understanding of the physical mechanism of its impact on the rock mass being destroyed [1,2].At present, thanks to scientists and practitioners of explosives, many complex issues are being successfully solved, including the creation of engineering methods for controlling the energy of an explosion, the improvement of technology, and the study of the physical foundations of the action of industrial explosions in rocks [11].Among the well-known modern methods of blasting, which affect the mechanical effect and allow you to control the degree of crushing of rocks, there are three main areas [10]: 1) development and improvement of a rational design of an explosive charge for efficient crushing of a rock mass; 2) observance of the principle of energy correspondence between the energy that is concentrated in a unit charge of explosives and the energy that is spent on crushing rocks; 3) development of various technological methods of blasting.

2.RESULTS AND DISCUSSION
In order to develop and improve the rational design of the explosive charge for effective crushing of the rock mass, the design of borehole explosive charges with an axial air cavity, which is formed as follows (fig.1).Vertical wells are drilled 1 according to the drilling and blasting passport.A polyvinyl chloride (PVC) pipe 2, previously sealed at one end, is lowered into the wells with a diameter of 1/6dc (dc is the diameter of the well) and a length of 2/3Lz (Lz is the length of the charge) to create an air cavity.An explosive 3 is laid, an intermediate detonator 4 is installed above the axial air cavity, mounted to a detonating cord 5, a stemming 6 is produced and blown up.

Fig. 1. Construction of a borehole explosive charge with an axial air cavity: 1 -bore; 2explosive; 3 -axial air cavity; 4 -intermediate detonator; 5 -stemming; 6 -detonating cord.
When choosing the diameter of the axial air cavity, it was taken into account that the limiting value of the shock wave velocity depends on the ratio of the charge and axial air cavity diameters [11].
Studies have established that the highest shock wave velocities are observed when the diameter of the axial air cavity is 3-6 times smaller than the diameter of the explosive charge.Depending on the ratio of the outer and inner diameters of the charge with the axial air cavity, the velocity of the shock wave moving in the channel varies from 8 to 12 km/s.The detonation of charges with an axial air cavity has specific features that are associated with the appearance of a channel wave moving ahead of the detonation front.The channel wave is a strong shock wave in the air filling the channel, which is excited by the expansion of the explosion products in the channel cavity.
In some cases, the flow of the explosion products themselves also outstrips the detonation front and, mixing with the shock-compressed gas, is organically included in the channel wave.Additional feeding of the channel wave with decomposition products occurs from the channel walls.Acting on the charge ahead of the detonation front, the channel wave can change the initial physical state (density, structure, etc.) and thus affect the propagation conditions and detonation structure of the detonation wave.In particular, the detonation can accelerate, slow down, lose instability and decay.This is the so-called "channel effect".The mechanism of the formation of shock waves in a cylindrical channel is associated with the phenomenon of collapse of detonation products (gas cumulation) inside the channel, resulting in the formation of a gas jet that is ahead of the detonation front.The speed of the jet increases with increasing distance traveled by the detonation wave, and the speed of the shock wave also increases accordingly (fig.2).The physical nature of the processes occurring during the explosive destruction of rocks can be most fully established on models using special equipment that registers fast processes.In order for the phenomena occurring during breaking on models to be comparable with those occurring in industrial explosions, it is necessary to ensure that the conditions for destruction of the material of the model and nature are identical.The modeling method, which was used in the study of the process of destruction of solid media by explosive charges of various designs, is based on the provisions set forth in the works [12][13][14][15][16][17][18].

Fig. 2. Scheme of the flow of a channel wave in a charge with an axial air cavity: Vd.w.the detonation wave velocity; Vc.wcanal wave velocity 1.
When modeling the action of an explosion, the correspondence between the model and nature can only be achieved if the scaling conditions are met.First of all, the geometric similarity must be observed N=L/l, (1) where L and l -linear dimensions of nature and model.2. The detonation velocity of explosives used to load wells in the model and in real life should be the same.3. The destruction of the material of nature and the model occurs when the limiting value of the stress in the material is reached.The similarity of the destruction of the materials of nature and the model is observed when using the material of the model with the same properties as in nature.The use of materials with the same properties provides an analogy in the propagation of the shock wave and the similarity of areas in which the stresses caused by the explosion will exceed the allowable values and cause the destruction of the material.Such a property is the acoustic stiffness, which is the product of the density of the material (ρ, g/cm 3 ) and the velocity of propagation of a longitudinal wave in it (Cp, cm/s).In materials with the same acoustic stiffness, the condition is maintained (ηу)m=(ηу)n, (2) where (ηу)mis the coefficient of energy transfer into the shock wave during the explosion of materials of the explosive charge in the model; (ηу)nis the coefficient of energy transfer into a shock wave during the explosion of an explosive charge in nature.
η=Еу/Еww, (3) where Еуis the energy transferred into the shock wave; Eww is the total energy of explosives.Condition (3) is met if the blasting takes place in media with similar acoustic hardness.Thus, the main criteria of similarity in modeling the processes of explosive destruction of rocks is compliance with the scale of modeling, the use of an explosive model and nature with the same detonation velocities, the use of a material with acoustic hardness of nature for the manufacture of a model.For the manufacture of models, an ore-cement mixture was used.The ratio of the diameters of the cavity and the charge during the explosion of charges with longitudinal channels on the models changed within (0…1):2.The explosive charge was initiated by a micro electric detonator, which was installed in the upper part of the charge.During the destruction of models from the ore-cement mixture, one or two rows of holes were blown up with charges of various designs.The blasting of wells in a row is instantaneous, between the rows it is short-delayed.Since short-delayed blasting on models with the help of industrial short-delayed electric detonators is impossible, a scheme of automatic short-delayed blasting of charges using shunts was used [19][20][21][22][23][24].The designs of the charges are shown in fig. 3. A mixture of PETN and ammonite №6JV was used as an explosive.The detonation velocity of such a mixture during an explosion in charges of small diameter is equal to the detonation velocity of a granulated explosive in boreholes with a diameter of 105 mm.The parameters of the location of the wells during the blasting of charges of various designs were the same.During the production of experimental explosions, the specific consumption of explosives for breaking was recorded on the models; granulometric composition of the exploded model; speed of movement of the broken layer; the sequence of the destruction of the array; stresses that arise in the array during the explosion; the time from the beginning of the charge

Fig. 3. Explosion charge designs on models: а -solid column explosive; bcharge explosive substance with axial cavity; 1 -stalk; 2 -microelectrodetonator; 3 -explosive charge; 4 -axial cavity.
The results of an explosion of a continuous explosive charge are significantly different from the explosion of an explosive charge with an axial cavity.In the first case, the height of the broken layer corresponds to the depth of the well.The lower part of the model, which had no wells, was left undamaged; during the explosion of charges with an axial cavity, the lower part of the model, in which there were no wells and whose height was 20% of the well depth, was destroyed along with the overlying layer.This is of particular importance when breaking rock in quarries, where the over drill reaches 20% of the ledge height, and in underground mines with chamber mining systems or when forming vertical compensation chambers.Block breaking with explosive charges with an axial cavity can make it possible to reduce the volume of drilling operations by reducing the amount of over drill and ensure the completeness of breaking in underground mines [25][26][27][28][29]. Figure 4 shows graphs of changes in the granulometric composition of the part of the model destroyed by the explosion during the explosion of charges with an axial cavity of various diameters.

Fig. 4. Graph of the change in particle size distribution during the blasting of explosive charges with different diameters of the axial cavity: а -one-row; б -two-row
As can be seen from Fig. 4, a, the yield of well-crushed material (fractions less than 20 mm) is always higher in the case of a charge explosion with an axial cavity.The output of large fractions (50-60 mm and more) is higher when a solid charge is blasted.
Changing the cavity diameter and the specific consumption of explosives for breaking affects the yield of the 0-5-and 40-60-mm fractions (fig.5).Fluctuations in the output of the 5-40 mm fraction do not exceed 4%.This pattern takes place both in the explosion of continuous explosive charges and in the explosion of explosive charges with longitudinal channels [3][4][5][6][7][8].
With double-row blasting, the same patterns are observed as with single-row blasting.With an increase in the diameter of the axial cavity, the mass of the charge and the specific consumption of explosives for breaking decrease.However, this decrease occurs only up to a certain limit (optimum), after which the volume of the chipped mass decreases and the specific consumption of explosives increases (Fig. 6).The most efficient way is to reduce the specific consumption of explosives for breaking with a charge diameter of 5 mm and a ratio of the cavity diameters and the charge diameter as 1.3:5-2.2:5.For other values of the ratio, the specific consumption of explosives increases dramatically.

Fig. 5. Graph of the output of fractions crushed by the explosion of material at different values of the specific consumption of explosives: 1 -fractions 0-5 mm; 2 -fractions 5-40 mm; 3 -fractions 40-60 mm
The destruction of the broken layer occurs almost simultaneously both from the side of the exposed surface and from the side of the line of wells.In this case, along the line of wells, cracking occurs under the action of shock waves from the explosion of charges, and from the side of the exposed surface -under the action of tensile stresses and spall phenomena due to the process of reflection of waves from free surfaces.As measurements have shown, the departure of detonation products from wells during the explosion of charges with an axial cavity occurs later than during the explosion of solid charges.Consequently, the time of their impact on the array is longer.The products of the explosion have a piston effect on the destroyed mass up to the complete outflow of gases from the well or into the newly formed cracks.Therefore, an increase in the time of exposure to detonation products has a significant impact on improving the crushing of the chipped mass [8,9].
During the explosion of charges with longitudinal channels, the speed of movement of the broken layer increases significantly (fig.7).As can be seen, the speed of movement of the broken layer during uniform blasting is maximum when the ratio of the diameters of the volost and the charge is 1:5.With a double-row short-delayed blasting, the speed of movement of the broken layer is maximum when the ratio of the diameters of the cavity and the charge is 2:5.The speed of movement of the broken rock mass is of particular importance when additional fragmentation of the broken pieces occurs during their collision or in systems with explosive delivery.In these cases, the increased speed of the rock mass will contribute to the growth of the efficiency of blasting.It should be noted that the speed of movement of broken rock mass during short-delayed blasting of several layers is always lower than during simultaneous instantaneous blasting under similar conditions [30][31][32][33][34][35][36][37][38][39][40][41][42].

Fig. 7. Dependence of the speed of movement of the broken layer on the ratio of the diameters of the axial cavity and the charge during blasting:
1 -one-row; 2 -two-row short-delay.

3.CONCLUSIONS
Thus, analyzing the above study, we came to the following conclusions: -The design of a downhole explosive charge with an axial air cavity was studied, which ensures uniform crushing of the rock mass and reduces the specific consumption of explosives.
-It has been established that the highest shock wave velocities are observed when the diameter of the axial air cavity is 6-8 times smaller than the diameter of the explosive charge.Depending on the ratio of the outer and inner diameters of the charge with the axial air cavity, the velocity of the shock wave moving in the channel varies from 8 to 12 km/s.-A technique has been developed for modeling the process of destruction of a rock mass by borehole charges with an axial air cavity.
-It has been established that with an increase in the diameter of the axial cavity, the mass of the charge and the specific consumption of explosives for breaking are reduced.However, this decrease occurs only up to a certain limit (optimum), after which the volume of the chipped mass decreases and the specific consumption of explosives increases.The most efficient way is to reduce the specific consumption of explosives for breaking when the ratio of cavity diameters and charge diameters is 1.3:5-2.2:5.For other values of the ratio, the specific consumption of explosives increases dramatically.

24 Fig. 6 .
Fig. 6.Dependence of the specific consumption of explosives onthe diameter of the axial cavity.