Production Mining Based on the Results of Seismic Measurements at the "Zve č aj" Quarry Site - A Case Study

: During mass blasting, seismic effects occur that can significantly damage existing buildings near the blasting site, depending on the amount of explosives and the blasting distance itself. An additional problem during blasting is the facilities located in the critical zone of mining works - the problem of unpredictable behavior of the soil or rock in relation to the propagation of seismic waves. Based on the measurements performed on the example presented in this paper at the site of the quarry EP "Zve č aj" (minefield at the level of the second floor K + 163 to K + 179), the method is explained, how quality and well-conducted research can determine optimal amounts of explosives per ignition level. Based on the performed measurement procedures and processed data, measured and obtained results of values of soil oscillation velocities in the direction of populated areas caused by production blasting in the quarry EP "Zve č aj", it follows that the conducted mining activity, consequently, had no damage. Blasting significantly affects the environment, due to the emission of noise and seismic action, which is manifested in the form of oscillations and elastic deformations of the rock, which is an artificial earthquake in which the place of explosion is its epicenter.


INTRODUCTION
In order to find out about the existence of mineral deposits, it is necessary to conduct research that boils down to the selection of a geographical area based on empirical data and geological knowledge. The main goal of such research is to determine the quality and quantity of raw material in the deposit. Total reserves of mineral resources are classified into certain categories according to the level of exploration of the deposit, the level of knowledge of the quality of the raw material and the parameters for determining the reserves [1,2]. Exploration of solid mineral deposits according to Croatian regulations is determined by geological, geophysical, geochemical, hydrogeological and engineering-geological methods, all types of surface and underground exploration and surface and pit exploratory drilling [3,4]. When a mineral deposit is detected, seismic waves propagate through the surrounding area during mass blasting at quarries. As a result, certain seismic effects also occur, which can significantly damage existing buildings near the site of blasting [4,5]. Experiences over many years of measurements have shown that the velocity, acceleration and displacement of particles in soil oscillations are the best parameters for assessing the damage caused to nearby buildings. The extent of the damage depends on the distance from the blasting site and the amount of explosives used [6]. At the same time, as the amount of explosives increases, the propagation space as well as the duration of the seismic disturbance increases. Therefore, it is necessary to perform preliminary research works and precise measurements of the seismic effects of test blasting in order to be able to determine the allowed quantities of explosives as reliably as possible. Often in practice, an additional problem when blasting are objects that are within the critical zone of mining operations, especially if they are objects that are under a certain degree of protection. The application of blasting as a technique in mining and construction is generally inseparable from the seismic action on both rock and soil, as well as on structures based on them. Oscillations initiated by blasting have a significant impact on the environment, and the effects vary from disturbing people to damage to buildings [7,8]. Damage to buildings is difficult to distinguish according to their occurrence, so even those caused by blasting cannot be clearly separated from damage caused in any other way. The degree of damage caused by this directly depends on the speed and frequency of vibrations of the building particles and the foundation soil, on the amount of explosives, the distance from the blasting site and the properties of the geological environment. Increasing the amount of explosives increases the duration and area of seismic disturbance, so the problem of determining the allowable amount of explosives is extremely sensitive and important, and therefore special attention should be paid to it. Considering demands of modern societies and primarily for technical and economic advantages, explosives for rock mass excavation have been widely used. Nevertheless, safety and particularly environmental concerns may arise in the use of these techniques; therefore, complex tools are needed to control these environmental effects, namely dust, noise, fly rock, and ground vibrations [9,21]. In order for the drilling and blasting works at the "Zvečaj" quarry to take place without harmful consequences, it was necessary to perform field measurements of the soil oscillation speed. Measurements were performed during production (trial) blasting at the level of the second floor, i.e. K + 163 to K + 197 m of the quarry, in order to determine the effects of seismic earthquakes on the surrounding settlements (buildings), or to determine the amount of explosive charge by ignition in order to avoid damage to nearby buildings near the quarry, [10].

SEISMIC EFFECTS OF BLASTING
The magnitude of seismic effects on the rock mass is determined by structural-textural features of rocks (stratification, cleavage, and joints), composition (mineral) and the bonding nature between individual grains [11]. The seismic effects of blasting include all forms of seismic action caused by mass blasting. The key criterion on the basis of which the endangerment of an individual object from seismic action is assessed is the speed of oscillations of soil particles or rocks caused by detonations of explosives in a mine well [6,12]. It is generally accepted that the speed of soil oscillations caused by blasting is related to the amount of explosive that detonates in each ignition interval, the distance of the observation site from the minefield and the characteristics of the basic tectonic structure of the rock mass. Defining the characteristics of rock masses in both geotechnical environments was carried out by classifying rock masses up to the level of determining the "Geological Strength Index (GSI)" as defined by Hoek [13], according to the "RMR" classification of Bieniawski [14] and part "Q" classification of Bartons [15]. Classification parameters were selected according to the direction in which, when measuring seismic effects, geophones were placed.

Influence of Seismic Effects of Blasting on Nearby Buildings
Seismic waves created by blasting cause mechanical movements of the soil, which are non-stationary periodic oscillations. When seismic waves reach a building, part of the ground oscillation energy is transferred to its foundations, so dynamic stresses occur in certain parts of the structure. To more accurately determine the allowable oscillation velocities, Karlheinz Arnold (Olofsson, 1990) takes into account the substrate on which the structure is based and the visible damage that occurs with stronger soil oscillations [16]. At a certain earthquake intensity, these stresses can exceed the ultimate strength of the material from which the building is built, which can cause permanent deformations [16,17]. Damage to buildings is very difficult to classify according to the method of its occurrence. Therefore, blasting is often attributed to damage to building structures that is actually caused by other causes, such as subsidence, uneven load, landslides, traffic, etc. Most of the vibrational energy appeared to be below 100 Hz and human perception of vibrations is usually examined in frequency ranges below 100 Hz [18]. Damage first occurs in the basement walls closest to the blasting site, and cracks can extend in all directions. The degree of damage to other buildings depends on the amount of explosive charge and the distance of the blasting site [19]. Damage will also occur at the points of weakening in the walls, such as openings, structural joints and old cracks due to subsidence, as well as contact joints with extended parts of the building [20].

Criteria for Assessing Seismic Safety
Many authors have dealt with the protection of buildings from seismic damage, and as a result of their research, a number of theoretical solutions and empirical formulas have emerged that define the mathematical relationship between soil vibration intensity and the amount of explosives [3,20,21]. These formulas have one or more correction factors whose values are determined by field measurements or based on statistical data. In recent years, there has been a growing awareness in Croatia of the need for more detailed research into the seismic effects of blasting in the vicinity of buildings [17]. On the other hand, the allowed speeds of rock mass and soil oscillations are not standardized by domestic legal regulations, so in practice verified standards from abroad are used. The International Society for Rock Mechanics (ISRM) recommends the combined use of the German standard DIN 4150 and the American OSM (U.S. Office for Surface Mining), which treat soils and foundations [3,12]. To calculate the peak ground oscillations at a certain distance on the terrain surface for normalized energy sources Wiss gave an expression similar to the USBM expression for the ratio of soil oscillation -distance -hammer impact energy, except that in the USBM expression the amount of explosive charge CW was replaced by energy W and the following expression is obtained [22]: The intensity of earthquakes caused by blasting is influenced by a number of factors, such as: physical and mechanical properties and geological structure of the rock through which seismic waves propagate, the amount and type of explosive charge, method and size of blasting and distance from blasting site. The intensity of oscillations is expressed in various measurands such as displacement, velocity, acceleration, frequency or energy of oscillations. Which of these quantities, and to what extent with the others, best represents the intensity of the seismic effect remains an open question, so different criteria are used to assess the seismic hazard. In Germany, the DIN 4150 standard classifies buildings by category and the corresponding permitted ground oscillation speeds depending on the frequency of oscillations. The display of the limit values of permissible ground oscillation speeds according to DIN standard 4150 is given in Tab. 1 [10]. The geological strength index is an assessment of the properties of rock masses, and is based on a visual assessment of the state of the rock mass structure and the state of rock mass discontinuities obtained by surface mines and borehole cores [23]. Therefore, we can conclude that the intensity of soil oscillation, the amount of explosive charge and the distance from the place of filling are expressions that have one or more correction factors whose values are determined by field measurements or statistics.

MEASUREMENT OF SOIL OSCILLATION SPEED 3.1 Measuring Instruments and Equipment
Soil oscillation velocities resulting from blasting can be measured in soil and construction, depending on the purpose and intent of the test. Specially designed portable seismographs are used to measure the speed of soil oscillations caused by blasting, which can be placed at any place where it is necessary to measure the resulting oscillations [16]. As a result of measurements in the foundation soil, oscillation values should be expected that have a general character and do not take into account the interaction of the foundation soil-structures.The oscillation velocity of the material particle in three mutually perpendicular planes is registered at the measuring points. Vibrations are captured by three-component geophones connected to a seismograph, each of which registers all three components of the ground oscillation velocity at the measurement site. Three-component geophones were placed at the Zvečaj quarry in such a way that: -two geophones were placed in the horizontal plane, one in the direction of the detonation point (for registration of the longitudinal component of oscillations, marked with 1 in Fig. 1), the other perpendicular to the previous one (for registration of the transverse component of oscillations, marked with 2 in Fig. 1) -the third geophone was located perpendicular to the horizontal plane (for the registration of the vertical component of oscillations, denoted by 3 in Fig. 1) [8,16]. Instruments operating on the principle of a seismograph were constructed to measure the magnitude of soil oscillations. The speeds of soil oscillations resulting from blasting can be measured in the soil itself, the structure depending on the purpose of the test. The oscillation velocity of the material particle is registered at the measuring points. After the detonation, each geophone registers one irregular curve that is obtained on the recording (seismogram) of the resulting seismic disturbance (earthquake). After detonation, each geophone registers one irregular curve obtained on the recording (seismogram) of the resulting seismic disturbance (Tab. 2 and Fig. 2 and Fig. 2a) [23].
For the needs of measurements during the development of the blasting project at the quarry "Zvečaj", digital seismic devices Instantel (MiniMate Plus) were used, which measure the speed, displacement, acceleration and the corresponding frequencies of soil oscillations and air shock wave.

Measurement of Oscillations in the Field
During production blasting at the "Zvečaj" quarry, soil oscillation velocities and an air shock wave were measured at a total of six observation points. A total of one minefield was fired at the level of the second floor of the terrain (K + 163 to K + 179). The distance of the observation site from the minefield was measured using GPS (Global Positioning System). By knowing the coordinates for two points, for the minefield (MP) and observation points (MO) the values of coordinates Y, X and Z are determined with an accuracy of up to 3.0 m, and based on them the shortest distance in space between these two points (R = D xyz ) according to the expression [10]:

DETERMINATION OF THE RADIUS OF THE VULNERABLE ZONE IN MINING
The magnitude of the seismic effects depends on the amount of explosives, the distance from the blasting site, the method of blasting and the properties of the rock mass and soil.
Many researchers have published empirical expressions to determine the speed of oscillations and the maximum amount of explosive charge by degree of ignition.
One of the most commonly used terms for calculating the speed of oscillations is the expression Langefors [24]. Long-term observations confirmed the interdependence of the measured oscillation velocity and the stated parameters [25]. Langefors expressed the interrelationship with the expression: that is, the stated dependence of the ground oscillation rate and other parameters according to the United States Bureau of Mines (USBM) is expressed by the following formula: where: V R -resultant ground oscillation speed, mm / s; Rdistance of observation point from minefield, m; Qamount of explosive charge detonating instantaneously, (kg); K -transmission coefficient and blasting method. Based on the measured resultant oscillation velocities at two observation points, their distances from the minefield and the adopted permissible limit oscillation velocities according to the appropriate standard, the permissible explosive charge per ignition rate for certain distances from the minefield is calculated and graphically represented by QR diagram [1] (Fig. 3).
Knowing the maximum value of the resultant V R velocity obtained by measuring at the observation point MO, the amount of explosive charge Q and the distance of the observation point R from the minefield (MP) from the above expressions we calculate the transmission coefficient K. while the distances and markings of the measuring positions are shown in Tab. 3, [10].  Based on the measurements and measured values of soil oscillation velocities in the direction of the nearby settlements of Donji Zvečaj, Novo Brdo and Grganjica, or family houses (Fudurić, Mataković, Benić), caused by production blasting at the quarry EP "Zvečaj", minefield at the level of the second floor K + 163 to K + 179), it was determined that there was no damage from blasting. 0.5 cm/s, which corresponds to the speed of oscillations required to turn on the seismograph. The frequency spectrum and the measured transmission coefficients differ significantly for the direction of Zvečaj, or for Novo Brdo -quarry plant. The highest amount of transmission coefficient is calculated in the direction of MO 4 (Donji Zvečaj), K = 125, which represents the direction of maximum energy transfer. Maximum explosion quantity of explosive charge in one interval on a minefield in level II eta was 80 kg (Eg + Ep) [23]. Langefors was chosen as the reference expression for the permissible value of the oscillation velocity of soil particles of 0.8 cm/s (direction Novo Brdo -plant), and 1.5 cm/s (direction Gornji Zvečaj) (DIN 4150). The analysis was performed in three groups with respect to the direction of propagation of seismic waves. The direction of Donji Zvečaj, especially MO 4, proved to be a critical direction. Drilling at the "Zvečaj" quarry was performed with self-propelled, hydraulic, percussion-rotary drills. As drilling was performed on a route that passes through terrain of different configurations, the drilling depth as well as the drilling geometry differed for each abutment or column foundation. Based on the "Study of deep drilling and mass blasting", blasting at the quarry "Zvečaj" was successfully performed. The obtained granulation of the mined rock mass meets the set requirements, except in the part of the minefield where 4 mine wells could not be filled due to their collapse. A sketch of a minefield with boreholes and a binding scheme in the Zvečaj quarry is shown in Fig. 4, while Fig. 5 shows a characteristic profile 1-1 through a well with a filling structure [10].

Figure 4 Sketch of a minefield with boreholes and a mooring scheme
Seismic waves created by blasting at the "Zvečaj" quarry caused mechanical ground movements that represent non-stationary periodic oscillations. When seismic waves reach a building, part of the energy of the soil oscillation is transferred to its foundations, so in some parts dynamic stresses occur. At a certain earthquake intensity, these stresses can exceed the ultimate strength of the material from which the building is built, which can cause permanent deformation. The blasting sounds that cause the greatest reactions of discomfort are the rattling of window panes, doors, unstable objects or sounds caused by blows from outside the building or from the roof. Sounds like this cause even more discomfort if they occur suddenly and unexpectedly. Based on research during the blasting at the "Zvečaj" quarry, the concerns of locals living nearby arose at times when they were exposed to the sound shock because it was related to potential damage to their homes rather than their bodies, and reactions diminished when they were not in own homes. When the sound shock occurred after a warning sign, aspects of discomfort were minimized, but some observed individuals erroneously concluded that the sound reaction of the apartment building indicated some kind of potential damage [5]. The localization of microseismic sources/acoustic emission in the rockmass structure can provide the basis for determining the potential areas of rockmass instability and rockburst in the underground mining [26]. Destress blasting is a rockburst control technique where highly stressed rock is blasted to reduce the local stress and stiffness of the rock, thereby reducing its burst proneness [27,28]. Such sounds are much better accepted by the public, if people see the importance of such activities, either for them personally or for the general public.

CONCLUSION
Based on the measurements, a selected example from this paper at the location of the quarry EP "Zvečaj" shows the way (procedure) how quality and well-performed previous research can clearly determine the optimal quantities of explosives by ignition level (explosive consumption norm: 2265 kg, 8.022 m 3 = 0.280 kg/m 3 ), and whose detonation of seismic oscillations along the protected buildings remains within the permitted limits. In addition to the use of permissible amounts of explosives, it is always useful to apply known methods and techniques to reduce soil vibration. The allowable amount of explosive charge per ignition stage was performed according to Langefors, and according to the recommendation of the United States Bureau of Mines (USBM). Based on the performed measurements, measured and obtained results of the values of soil oscillation velocities in the direction of Donji Zvečaj, Novo Brdo and Grganjica, caused by production blasting at the location of the quarry EP "Zvečaj", minefield at the level of the second floor (K + 163 to K + 179), it can be concluded that the blasting activity did not result in any damage. The measured values at the measuring points meet the standards of permitted ground oscillation speeds and are within the permitted criteria for residential buildings.
According to DIN 4150 the obtained value of permissible soil oscillations for residential buildings is 0.8 cm/s and 1.5 cm/s, and for the dominant frequencies of oscillations of particles measured at the location. Measured values indicate complex geological conditions and dispersion of seismic waves in relation to the direction of propagation. Finally, the measured values undoubtedly show that the frequency spectrum and measured coefficients transmissions differ for the direction of the settlement Zvečaj, in relation to the settlement Novo Brdo -quarry plant. It should be noted that the largest amount of the transmission coefficient was calculated in the direction MO 4 (Donji Zvečaj), K = 125, (shown in Fig. 2 and Fig.  2a.), which is also the largest direction of energy transfer. It is important to conclude that the total duration of the detonation at the "Zvečaj" quarry should not be more than one second, due to the adverse impact of the seismic effect on residential buildings and the psychophysical reactions of the surrounding population. When the total oscillation time exceeds one second, the frequency of complaints and thus "reported" damages increases.
That is why instrumental observations of seismic oscillations in each blasting are an excellent way of control. However, if the project itself is undesirable, any project operation can be seen as undesirable, and the number of damage reports will be much higher than for a project that is well accepted or a project that is considered significant for the benefit of society.