Short-delay Blasting Research Articles

Characteristics of blasting vibration induced by short-delay cylindrical charges based on Heelan model

As the electronic detonators can control the detonation error within 1 ms, short-delay blasting was nowadays proposed to replace multi-hole simultaneous blasting to reduce the vibration hazards. In this study, the Heelan model was used to study blast-induced vibration of short-delay cylindrical charges. The influence of delay time on the vector peak particle velocity ( VPPV) of superimposed vibration waveforms was analyzed and the calculating formula of the shortest delay time between adjacent holes based on vibration reduction was established. It is found that as the delay time increases, the overlapping area of VPPV becomes further away from the explosion source. The main influencing factors for the shortest delay time between holes include the distance between the monitoring point and the explosion source, the difference between P wave and S wave velocities, and the duration of S wave in the first and second explosion holes. As the distance from the explosion source increases, the shortest inter hole delay time based on vibration reduction also increases. The research results have guiding significance for the control of blasting vibration in underground mines.

Numerical modelling of rock fragmentation under high in-situ stresses and short-delay blast loading

Since the invention of electronic detonators with a delay accuracy within 1 ms, short-delay blasting technology has been used in mining and tunnelling engineering to improve rock fracturing and control blast-caused vibration. However, the fragmentation characteristics of pre-stressed rock under short-delay blast loading have not been studied in detail. In this study, the interaction between the short-delay blast loading and static in-situ stress is investigated using a FEM-based modelling method and image-processing approach. Firstly, single-hole blasting tests were conducted in an underground roadway with a burial depth of 620 m. Subsequently, a single-hole blasting model was developed in LS-DYNA and validated against one of the experimental results, a two-hole delay blasting model was established to simulate rock fracture and fragmentation under different delay times and in-situ stresses. Finally, the underlying mechanism of in-situ stress affecting the optimum delay time was theoretically analyzed and an analytical solution for optimum delay was proposed. The tested results show that under a borehole length and charge weight of 1 m and 1.5 kg, respectively, the average fragment size is approximately 10.5 cm, and the crater radius and depth are 0.67 m and 0.96 m, respectively. The numerical results indicate that both of average fragment size and fragment size distribution range significantly increase with the increase of in-situ stress, while they first decrease and then increase with the increase of short delay times. Moreover, there is a specific delay window ranging from 0.5 to 3 ms where short-delay blasting improves fragmentation performance, and the optimum delay time increases with the increase of in-situ stress. The findings of this study can aid in improving the efficiency and safety of blasting operations in deep hard rock mining.

Substantiating parameters of short-delay blasting and seismic safety while constructing the inclined tunnel

Implementation of blasting works in mining engineering and underground construction requires new effective blasting technologies in conducting an explosion. Short-delay blasting is a technology that requires scientific substantiation of its parameters. The application of explosion technology with increased blast intervals between charges is characterized as successful. However, the degree of reduction of seismic action and, accordingly, an increase in seismic safety needs clarification of the parameters of short-delay blasting. The scientific paper analyzes the delay time of the blast of charges and the classification of the degree of damage to buildings and structures. Seismic safety parameters for the inclined tunnel “Skhidnyi” while tunneling at the quarry of the Inhulets Ore Mining and Processing Plant (in the city of Kryvyi Rih) have been substantiated. The functional dependence of the maximum vibration velocity on the seismically safe distance has been obtained, as well as the dependence of the coefficient, the value of which characterizes the mass of the explosive. This allows for a quick evaluation of seismic safety parameters when calculating the parameters of blasting operations in the case of short-delay blasting.

Influence of detonator delay scatter on rock fragmentation by bunch-holes blasting

Bunch-holes blasting (several blastholes arranged in a bunch with short spacing and detonated simultaneously to form a large common blasting crater) is typically used in underground mines to increase blasting efficiency under a single free surface. Considering the low accuracy of pyrotechnic detonators, which are commonly used in Chinese mines, bunch-holes blasting with the same nominal times is actually short-delay blasting, and the actual delay intervals induced by the delay scatter of detonators can lead to unsuccessful bunch-hole blasting, which seriously affects rock fragmentation. In this study, theoretical analysis is firstly conducted to explore the mechanism of short-delay bunch-holes blasting, and the formation time of a new free surface is defined as the critical delay interval of bunch holes. Then, the formula of the new free surface forming time is deduced to determine the critical delay interval of the short-delay bunch-holes blasting. Considering the delay scatter of the detonator, the probability method is used to quantitatively analyse the probability of successful bunch-holes blasting with different delay nominal time detonators. Field experiments are performed to verify the results. This research demonstrates that as the detonator delay scatter increases, the probability of successful bunch-holes blasting generally decreases. In addition, the charge length also has an impact on the probability, which is positively correlated but not as dramatic as the detonator delay scatter. Hence, it is recommended to use high-precision detonators or low-segment detonators for bunch-holes blasting to ensure the quality of the cutting blasting.

Controlling blast energy parameters to ensure intensive open-pit rock fragmentation

Controlling blast action, in order to increase its energy efficiency in a production blasthole is quite an important issue. This is because it enables the formation of broken rock mass with preset coarseness parameters. Increasing the blast pressure and the time of the blast impact on a rock mass is traditionally recommended as one of the ways to improve the blast action on the rock mass, thus reducing the oversize yield in open pits. One device which enables this approach to a certain extent is a turbulator. The turbulator is fabricated of aluminum plate twisted in a helical fashion around its longitudinal axis. It is mounted in a production blasthole according to a specially designed scheme. The methodology developed to study the stress and strain state of a rock mass when using a turbulator in a blasthole explosive charge allows the size of radial fracture zone and the radius of rock fragmentation to be defined. A method was developed to initiate blasthole charges in a pit blasting block. It includes drilling blastholes, filling them with explosive, installing downhole blasting caps, and blasting using non-electric initiation system. A blasting block is divided into two equal parts (sections), which in turn contain three series of blastholes for short-delay blasting. Blasthole charges are initiated simultaneously in the two parts of the block based on a trapezoidal blasting pattern, thus ensuring meeting detonation waves. In the first series, instantaneous blasting of blastholes located on both ends of the blasting block and forming a trapezoid (in plan view) is carried out. Then after 42 ms, the second series of blastholes (also forming a trapezoid) is detonated. After another 42 ms, the remaining blastholes are detanoated along the perimeter of the blast block in the third series. Implementation of this design with the effect of turbo-blasting for rock fragmentations by blasthole charges at the Kalmakyr deposit of JSC “Almalyk Mining and Metallurgical Complex” has led to the reduction of consumption of explosives, volume of drilling, secondary fragmentation costs, and increased productivity of excavators and mining safety.

Fragment size distribution and its probability distribution in blasted rock mass in a natural stone quarry

The key objectives in the drilling and blasting design is the determination of a powder factor, explosive weight and distribution in rock mass, as well as some other parameters required to ensure the wanted quality of rock fragmentation by blasting. These objectives have different complexities but their common feature is the probabilistic nature of fragmentation results. An integral distribution function can provide an insight into the probability of yield of certain fragment sizes. This article describes fragment size estimation of blasted rock mass at Mikashevichi granite quarry. Mikashevichi quarry has 10 producing benches mostly 12 m high and uses the blasting method of quarrying. The quarry rock mass is watered, has a hardness factor of 15–20 on Protodyakonov’s scale and belongs to jointing categories III and IV. The multiple-row and short-delay blasting uses nonelectric detonators Iskra manufactured in Russia. The explosive is nitronite E-70. The continuous explosive charges are installed in polypropylene hoses placed in jointed rock mass. The workability of different models of fragment size distribution in rock mass after blasting in evaluation of blasting results in a natural stone quarry is analyzed. The theoretical and applied research and estimation of fragment sizes after blasting in different geological conditions uses various theoretical models, including the most popular gamma-distribution (Pearson type III distribution), Weibull distribution and log-normal distribution. The comparison of the theoretical distributions and in-situ estimates of fragment sizes in blasted rock mass using Pearson’s chi-squared test shows that the best model of fragment size distribution in blasted rock mass for a natural stone quarry is the Weibull distribution.

Engineering formulas for dimensioning of deformation and damage zones in jointed rock mass under impact of blasting in quarries in Transbaikalia

Currently, there are five characteristic zones of blast-induced impact in quarries: crushed zone, radial cracking zone, severely fractured zone, blast-induced residual stress zone, and blast-induced load zone. Knowledge about the size of the zones is very important in the technological processes of mining. The size of the crushed zone governs the amount of ore over-grinding and loss. The size of the radial cracking zone is reflective of the spacing of blastholes. The size of the severe fracturing zone is characteristic of the geometry of an unstable pit wall area. The zone of blast-induced residual stresses in rock mass and its size affects the slope stability of pit wall. The purpose of this study is to develop engineering formulas for calculating the sizes of deformation and damage zones in jointed rock mass during short-delay blasting in quarries. The mathematical and numerical analyses of the complex theoretical expression provided engineering formulas for dimensioning of the blasting-induced impact zones in quarries. The reliability of the formulas is proved by the comparison with the full-scale testing data obtained in quarries in Transbaikalia.

MODELING AND PRACTICAL APPLIKCATION OF A POWER FIELD AT CONIC CHARGE EXPLOSION

An analysis over of character of the isodenses of conical charge explosive products, got by mathematical design, is brought. The features of distribution of the power field are certain depending on direction of initiation of the truncated conical charge. The complex character of intermediate initiator using in a downhole charge as the truncated cone at his location in the ground part of hole is discussed. It is established that the shape of the field of isolines of the density of PD is due primarily to the form of the charge, regardless of the place of its initiation; the prevailing field development occurs relative to the Z axis, that is, along the X axis of the conical charge by the action of its narrow part. This is due to the spread of the products of detonation normal to the sloping surface of the cone. If you associate the total area of the PD density field with the total explosive energy, it is possible to redistribute the flow of energy due primarily to the orientation of the bases and sides of the truncated cone charge, and then to its initiation location. Control of the dynamics and direction of development of the force field is possible by choosing the location of application of the initiating pulse, namely, the location of the initiator on the opposite charge basis for the development of the process in the desired direction of concentrating the energy of the explosion. Due to the specific "pear-shaped" form of the radiated energy field, the explosion of the inverted conical fighter at the stage of initiation of the main (downhole) charge directs the vector of the initial section of the stress wave front towards the upper free surface, ie towards the surface sideways. This area is not subject to the direct action of a system of compressive radial and tensile tangential loads from a charge explosion in a well. changing the direction of development of the force field toward the upper layers of the rock should contribute to better grinding of this part of the array. This method can be implemented using the mechanism of interaction of rock masses, moving towards and additionally destroyed similar to the conditions of short-delay blasting in a schematic diagram, ie, the mechanism of interaction at the level of a slower process - the collision (collision) of the mass directed to the upper side. Simultaneous lower initiation of prolonged charges in adjacent rows gives rise to the front edges inclined to the free surface of the ledge (Fig.). In fact, in this case, the scheme of a wedge or trapezoidal cut is implemented, but oriented instead of the lateral free surface towards the roof of the ledge, which because of its large area is the main source of oversized fractions. consists in the simultaneous simultaneous detonation of three adjacent series of downhole charges treated as one group, followed by the short-term detonation of the adjacent three-row group and the like. The implementation of such a scheme allows to take advantage of its advantages by combining mechanisms of movement of masses in the vertical direction and traditional movement with the coincidence of areas of the array in the horizontal direction. The advantages of using a mixed circuit switching scheme have been proven by industrial testing of the method.

Numerical study on the fracturing mechanism of shock wave interactions between two adjacent blast holes in deep rock blasting

With the application of electronic detonators, millisecond blasting is regarded as a significant promising approach to improve the rock fragmentation in deep rock blasting. Thus, it is necessary to investigate the fracturing mechanisms of short-delay blasting. In this work, a rectangle model with two circle boreholes is modeled as a particles assembly based on the discrete element method to simulate the shock wave interactions induced by millisecond blasting. The rectangle model has a size of 12 × 6 m (L × W) and two blast holes have the same diameter of 12 cm. The shock waves are simplified as time-varying forces applied at the particles of walls of the two boreholes. Among a series of numerical tests in this study, the spacing between two adjacent boreholes and delay time of millisecond blasting are considered as two primary variables, and the decoupling charge with a coefficient of 1.5 is taken into account in each case. The results show that stress superposition is not a key factor for improving rock fragmentation (tensile stress interactions rather than compressive stress superposition could affect the generation of cracks), whereas collision actions from isolated particles or particles with weakened constraints play a crucial role in creating the fracture network. The delay time has an influence on causing cracks in rock blasting, however, whether it works heavily depends on the distance between the two holes.

Numerical simulation of stress wave interaction in short-delay blasting with a single free surface.

It is generally believed that stress wave superposition does occur and plays an important role in cutting blasting with a single free surface, in which explosive columns of several blast holes with short spacing are simultaneously initiated. However, considering the large scatter of pyrotechnic delay detonators that are used in most underground metal mines in China, the existence of stress wave superposition and the influence of this effect on rock fragmentation are questionable. In the present study, the stress wave interaction in short-delay blasting with a single free surface was studied through the use of the LS-DYNA code. Stress waves induced by two blast holes blasting with different delays were compared with the single blast hole case, and the effects of delay time, detonating location and spacing on stress wave superposition were investigated. The numerical results showed that for blast holes with a 1 m spacing, stress wave interaction only occurs when the delay time is 0 ms and does not occur for blasting with delays of more than 1 ms. An increase in the duration of a stress wave via optimizing the detonation location does not improve the stress wave interaction. For a 1 ms delay, stress wave superposition only occurs when the spacing is more than 4 m, which is a rare case in practice. The results indicated that the occurrence of stress wave superposition for blasting with a single free surface is strictly limited to conditions that would be difficult to achieve under the existing delay accuracy of detonators. Therefore, it is unrealistic to improve fragmentation via the stress wave interaction in field blasting. Furthermore, the numerical results of the stress wave interaction also show that there would be a great potential to reduce the hazardous vibrations induced by short-delay blasting by using electronic detonators with better control of delays in an order of several milliseconds.

Short-delay blasting with single free surface: Results of experimental tests

Delay blasting with relatively long-delay intervals is widely used in mining engineering since the former detonated blast-holes can produce new free surfaces for the later detonated blast-holes. With the application of electronic detonators, which have a minimum delay and a delay accuracy of 1 ms, a new blasting pattern using short-delay intervals is proposed in the present study in order to improve rock breaking and control blast-induced vibrations in cutting blasting with single free surface in underground mines. Theoretical analyses are firstly conducted to investigate the mechanisms of blasting crater formation and vibration reduction of short-delay blasting. Then a series of blasting crater tests with different delay intervals are performed to compare the characteristics of blasting craters and blast-induced vibrations produced by short-delay and simultaneous blastings. The results of crater sizes show that it is possible to form a common blasting crater only when the delay intervals are shorter than the formation time of a new free surface. It is also found that the short-delay blasting can effectively reduce PPV compared with the simultaneous blasting, particularly in the near-field. Spectral analysis indicates that there is less energy in the low-frequency content in short-delay blasting than simultaneous blasting. The possibility and feasibility of reducing vibration via short-delay blasting in underground mines are also discussed in this study.

Internal forces analysis of gas permeable shelter with emphasis on short-delay blasting

A mathematical model of transformable gas permeable blasting shelter made from worn out tires of heavy trucks bound together with chains, ropes or cables is proposed. Large displacements and unilateral constraints are specific features of this model. An algorithm was developed to calculate the displacements of tires and internal forces in connecting elements of blasting shelter subjected to dynamic loading. The results of the numerical simulation of simple shelter are presented for assessment of the adequacy of the mathematical model. This algorithm can be used to determine the parameters of shelter such as mass, number of tires, diameter of connecting elements which ensure its safe operation under any sequence of explosions.

Standards for seismic security during blasting in quarries

Based on the analysis of experimental data, pattern of seismic waves propagation in real mines and the various existing methods of calculations of seismic safe and secure mass distance charges in these specific circumstances blasting were examined. Using National Standards (Codes) of blasting seismic safety of USA, Australia, Germany, Switzerland and others. The degree of compliance with the law of similarity between the seismic effect of explosions at different mass of explosives was analyzed. The results of statistical and graphical analysis of processing and presentation methods of experimental data of seismic effect as instant and short-delay blasting of the explosive charges. Established that as the degree of compliance with the law of similarity, and the accuracy of calculation parameters seismic waves with formula M.A. Sadovsky, which is the basis formula for the Code 4704:2008 Ukrainian Standard, gives the best results compared with similar dependencies in foreign countries It is shown that seismic wave parameters which are calculated by the formula Mining Bureau in the US epicenter distances exceeding 800 meters are half times bigger compare with data from the actual values in terms of Dobriansky limestone quarry. It is proved that the using of the law of similarity, in terms Dobriansky limestone quarry, can more accurately determine the level of seismic hazard and more reasonably predict the outcome seismic effect from blasting and the law is efficient for simulation of charge explosions in research. The efficiency of blasting operations at the mining enterprises is largely determined by the state of the legal and procedural documentation. It is known that the main regulatory documents, governing the conduct of blasting operations are «safety regulations for handling with explosive materials for industrial use» and state standards of Ukraine. The implementation of these regulations in our state is mandatory for all ministries, departments, enterprises and organizations that are engaged in blasting operations. In other countries, such as USA, Australia, Germany, Switzerland, there are also seismic safety standards in the performance of man-made explosions. The uniting of these rules, including in Ukraine it is that the seismic explosion of the evaluation criteria for the protection of buildings are oscillation speed and frequency.

ESTABLISHMENT OF THE REGULARITIES OF THE DESTROY OF ROCKS BY EXPLOSION OF BOREHOLE CHARGES WITH THE SLOWDOWN

Proposed for studies of the interaction of energy flows in short-delay blasting of borehole charges system to determine the stress state of the massif based on the principle of superposition of stress waves based on the solution to the problem of destruction of rocks by explosion of a single charge. It is established that the maximum value of the amount of destruction of rocks in the presence of a single plane of exposure is explicit dependence on the interval of delay between the explosions of related charges.

Enhancement of seismic safety of short-delay blasting in mines

Enhancement of seismic safety of short-delay blasting in mines

Study on Mining Techniques for Improving Lump Coal Rate of Anthracite

Based on the decisive impact of mining techniques on lump coal formation, according to the specific mining conditions of 2206# coalface in Xinqiao Mine, the turning direction of rollers in coal winning machine had been changed, the cutting coal ways of coal winning machine had been transformed, the supporting strength of hydraulic supports had been reduced and the presplit short-delay blasting had been implemented for improving the lump coal rate of anthracite. The application results show that those mining techniques could greatly improve the quantity and rate of lump coal and make better technical and economic benefits.

A field testing study on negative skin friction along piles induced by seismic subsidence of loess

At present piles foundation is widely used in the earthquake-prone loess area of China. Unfortunately, negative skin friction (NSF) along piles induced by an abrupt settlement of loess under strong earthquakes, so called seismic subsidence, has not been taken into account in design of piles foundation due to the lack of NSF data from field test. In order to investigate NSF along piles caused by seismic subsidence of loess and develop a method to estimate this earthquake-induced NSF, the authors perform a field test at a loess site by means of a series of explosions, i.e. a short delay blasting, in which two reinforced concrete piles was grouted. The expected ground motion generated by the short delay blasting is strong enough to induce an obvious seismic subsidence in the loess site. There are 40 stress gages to be averagely disposed into the two piles with a certain interval to collect the data of NSF during this field test. Meanwhile, ground motion and seismic subsidence of loess were, respectively, observed by accelerographes and a level gauge. The obtained data particularly disclose the developing characteristics and magnitude of loess seismic subsidence and NSF at the condition of strong ground motion. The loess seismic subsidence and NSF both have a rapid development in the explosion process and then keep a slower increase for some days after the blasting. At the end of observation, the maximum seismic subsidence of loess in the field reaches 33 mm; the average NSF along piles attains to 54 kPa, which is much greater than previous cases of NSF caused by collapse of loess. This field test shows the NSF along piles induced by seismic subsidence of loess is significant and should not be ignored in design of piles foundation in seismic loess regions.

Geomechanical Substantiation of Ore-Block Breaking Schemes Taking into Account Static Stress Field Redistribution under the Short-Delay Blasting

The features of the static stress field redistribution in a rock mass are established for schemes of blasting that form a sinusoid-shaped ore block without a free surface on the contact with caving zone. The influence exerted by caved rocks on the dimensions and location of tension zones in a broken block is estimated.

Stress Redistribution in Ore Block During Breaking

The stress fields are calculated for the pattern of short-delay blasting in ore block with the formation of curved free surfaces. It is established that in all breaking stages, the tension areas favoring the reduction in the energy intensity of failure arise in the massif.

Use of short-delay blasting during well-shooting for water output

This article examines physical, technical, and technological aspects of the shooting of water wells with the use of short-delay blasting, which makes it possible to increase the productivity of well operations by a factor of 2–12. The experimental use of short-delay blasting is described and the efficiency of the new technology is compared to other blasting techniques.