The decrease of the blasting seismic effects on open pits in the Coal Mine “Banoviæi”

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Tom 24 2008 Zeszyt 4/3

ADMIR SOFTIC*, ZAHID BASIC**, AMIR BRIGIC*, ENIZ LACIC*

The decrease of the blasting seismic effects on open pits in the Coal Mine “Banoviæi”

Introduction

On the coal open pit “Cubric”, overburden is composed of clayey and marl layers which are dug out by shovel excavators of various technical characteristics, with preceding shaking by deep borehole mining.

At detonation of explosives in deep blasting boreholes, a sudden energy release occurs, which by its larger part is consumed for crushing of the rock body (productive mechanical work), while the rest of explosives’ potential energy is wasted for unproductive forms of work, such as seismic and air waves creation.

The specified events are destructive and unwanted when blasting in the proximity of residential objects or other structural objects, because damage can be generated on such objects.

Since coal has been exploited for 25 years on the open pit “Cubric”, the works on overburden are approaching the surrounding residential objects to distance of 200 meters, which means that useless energy forms of explosives come into significance, especially the seismic waves.

From the specified above, by apart mining of etaza on the base terrain, the quake intensity becomes very emphasized and therefore it is necessary, by permanent measuring, to bring the explosives quantity at one ignition degree into allowable limits.

** RMU “Banovici”d.d. Banovici, Banovici, Bosna i Hercegovina.

** GPP Banovici, Banovici, Bosna i Hercegovina.

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1. Technical description of blasting

Execution of mass blasting on the open pit “Cubric” is done by deep blasting boreholes with diameter of 115 mm. The mine fields contain more series of boreholes, and blasting field geometry depends on technical characteristics of the loading machinery, type of blasting material, and it is in range of 3×3 m (mine fields for coal) to 6×5 m (mine fields for overburden).

In order to reduce the blasting seismic effect on surrounding objects, it is applied the mili-second explosive charge ignition with different retard intervals, depending on need.

Blasting fields’ data is shown in the Table 1 and 2.

TABLE 1 Boreholes’ length and explosive charges’ quantity per borehole for mining field MF-1

TABELA 1 D³ugoœæ otworów wiertniczych i iloœæ ³adunków wybuchowych na otwór dla terenu wydobywczego MF-1

Ordinal number of

borehole serie

Berehole number

Length of clean berehole

Explosive quatity

Ordinal number of

borehole serie

Berehole number

Explosive quatity

I

1 13.0 50.00

III

20 13.0 50.00

2 13.0 50.00 21 13.0 50.00

3 13.0 50.00 22 13.0 50.00

4 13.0 50.00 23 13.0 50.00

5 13.0 50.00 24 13.0 50.00

6 13.0 50.00 25 13.0 50.00

7 13.0 50.00 26 13.0 50.00

8 13.0 50.00 27 13.0 50.00

9 13.0 50.00 28 13.0 50.00

10 13.0 50.00 29 13.0 50.00

II

10 130.0 500.00

IV

10 130.0 500.0

11 13.0 50.00 30 13.0 50.00

12 13.0 50.00 31 13.0 50.00

13 13.0 50.00 32 13.0 50.00

14 13.0 50.00 33 13.0 50.00

15 13.0 50.00 34 13.0 50.00

16 13.0 50.00 35 13.0 50.00

17 13.0 50.00 36 13.0 50.00

18 13.0 50.00 37 13.0 47.5

19 13.0 50.00 38 13.0 47.5

9 117.0 450.00 9 117.0 445.00

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2. Seismic effect

In modern blasting, it is unthinkable to get minerals in solid environment without usage of explosives, especially at big proportion exploiting as it is a case in open pits coal exploiting.

Drilling-blasting works are one factor of influence to economical exploiting of minerals.

TABLE 2 Boreholes’ length and explosive charges’ quantity per borehole for mining field MP-2

TABELA 2 D³ugoœæ otworów wiertniczych i iloœæ ³adunków wybuchowych na otwór dla terenu wydobywczego MP-2

Ordinal number of borehole serie

Berehole number

Explosive quatity

Ordinal number of borehole serie

Berehole number

Explosive quatity

I

1 13.0 52.00

III

19 13.0 52.00

2 13.0 52.00 20 13.0 52.00

3 13.0 52.00 21 13.0 52.00

4 13.0 52.00 22 13.0 52.00

II

4 52.0 208.00 23 13.0 52.00

24 13.0 52.00

5 13.0 52.00 25 13.0 52.00

6 13.0 52.00 26 13.0 52.00

7 13.0 52.00 27 13.0 52.00

8 13.0 52.00 28 13.0 52.00

9 13.0 52.00 29 130.0 52.00

10 13.0 52.00 30 52.00

11 52.00 31 13.0 52.00

12 13.0 52.00

IV

13 169.0 676.00

14 13.0 52.00 32 13.0 52.00

15 13.0 52.00 33 13.0 52.00

16 13.0 52.00 34 13.0 52.00

17 13.0 52.00 35 13.0 52.00

18 13.0 52.00 36 13.0 52.00

14 182.0 728.00 37 13.0 52.00

38 13.0 52.00

39 13.0 52.00

40 13.0 60.00

41 13.0 60.00

10 130.0 536.00

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TABLE3 Geo-mechanicalparametersofworkingenvironment TABELA3 Parametrygeomechaniczneœrodowiskapracy Working environmentLitološki CompositionVolumemass [t/m3]Wetness.[%], Porosity.[%]

Presure resistance [MPa]

Tension resistance [MPa]

Cohesion c[MPa]

Innerfriction angle [j°]

ModulEla [MPa]ModulDefor [MPa] Krovina

Gray-clayeymarl2.05–2.38W=10.5 n=11.46 e=0.126.60.943.12–6.524–58380–1490290 Gray-macularmarl2.05–2.47W=4.3–17.518.31.659.0–33.021–59940–2950800–2000 Gray-whitelime.marl2.33–2.49W=7.4–21.040.05.720–2722–502390–54001490–4000 Graylime.marl2.34–2.38W=10.3 n=8.48 e=0.16–0.2618.1–51.92.61–4.1626.04031901786 DirectGraymarl2.33–2.45W=9.5 n=13.0 e=0.152.5–20.70.3–1.7513.431.51800–56401160–4390 KrovinaGraywhitelime.marl3.22–2.51W=3.312.0–44.11.56–4.7316–4844–572060–45601610–3000 CoalCoal1.28–1.43W=9.2–18.4 n=16.8 e=0.207.3–10.80.49–1.226.1–7.522–391460–22001030–1400 Shelf Graymarl2.34–2.64W=3.8–120.8–14.00.39–1.662.5–4422–511940–17802260–3830 2.34–2.64W=3.8–12 n=13.1 Coalmarl1.65–1.86e=0.151.1–1.60.1–0.20.6–0.833.32500–50002700–4200

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TABLE4 Acousticparametersofworkingenvironment TABELA4 Parametryakustyczneœrodowiskapracy Environment markerLitološkiCompositionDepth h[m]Vp [m/s]Vs [m/s]Y [t/m3]Puesson’s coeff.[m]Edyn [MPa]Gdyn [MPa]Kdyn [MPa] 1Laxcallow:marls, damagedbyblastingor overburden2.0–7.0760–1160250–4501.80–2.000.439–0.411330–1165115–415900–2180 2Claymarls10.0–40.01500–2300515–8252.05–2.200.433–0.4261590–4350550–15253950–9800 3Marlinthefaultingzone>5–20.01930–2780650–10002.15–2.300.436–0.4252660–6680925–23455910–14850 3Marl>5.0–40.02600–3170820–12002.20–2.370.444–0.4164350–98501510–348012960–19550 4Coal,damagedbymining oroverburden2.5–8.0625–805255–2701.05–1.080.400–0.436195–23070–80325–600 4Coal10.0–15.01035–1370375–5901.15–1.250.424–0.386470–12301030–1800 5Shelfstratum: marl.coalmarl.clay>10.0312010252.300.4397090246519370 Vp–Speedoflongitudinalelasticwaves,Vs–Speedoflateralelasticwaves,Y–Volumemass,Edyn–Dynamicelasticitymodulus,Gdyn–Dynamicshearmodulus, Kdyn–Dynamiccohesion

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Open pit blasting releases the energy of explosive charges which is converted into kinetic energy of seismic waves which are radially spread in all directions, from the point of blasting.

The seismic waves generate ground oscillations, in other words, an artificial earth quake occurs. This is very unfavorable event in case when blasting is performed in proximity of residential objects, because certain quake intensity may damage such objects.

Spread speed of waves generated by blasting, that is the quake intensity due to blasting depends on distance from the blasting location, the quantity of explosives that are being activated in one time interval, as well as on physical-mechanical characteristics of the working environment.

Geo-mechanical parameters of working environment on open pits as well as acoustic parameters are shown in the following tables: (Table 3 and 4).

Earth vibrations always emit, even at the best planned and executed blasting and they spread radial from the blasting location, fading with distance.

There are four mutually dependable parameters which can be used for definition of earth vibration magnitude, at any spot. For sinusoidal movement are:

— Drift on which the earth particle moves to, before it returns to starting position of inaction

S = A Sin2pfT [mm] (1)

Where is:

A – Maximum amplitude [mm], f – Oscillation frequency [Hz], T – Vibration duration [s].

— Speed of earth’s particles displacement around the position of inaction

V = 2pfA [mm/s] (2)

— Acceleration or speed change of earth’s particles

A = 4p²f²A [mm/s²] (3)

Frequency, that is, number of oscillations per second, which an earth’s particle does under influence of seismic waves.

3. Measuring mode for seismic effect

Registering of elastic seismic waves which occur due to blasting can be performed on ground, structure or object, depending on target and assignment of testing. For this purpose, specially designed seismograph is used, which is able to register oscillatory appearances.

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Up to date digital seismographs are constructed so they can register the maximum oscillation speed, maximum acceleration of particles, particles’ drift and dominant frequencies in any plain. Resulting speed of earth’s particles is also registered as well as value of the air overpressure in decibels (dB).

Dependence between oscillation intensity (oscillation speed), distance from the blasting location, quantity of explosives and physical-mechanical characteristics of rock is defined by Sadovsky formula:

V K q R

= (³ )n

[Cm/s] (4)

Where:

V – Oscillation speed [cm/s],

K – Coefficient of seismic activity in rock (250–350), N – Coefficient of seismic waves’ reduction (1.40–1.60), Q – Quantity of explosives simultaneously activated [kg], R – Measuring spot distance from the blasting field [m].

For evaluation of seismic effect, we use seismic scale so called IFZ (Table 5), which has been established at the Institute of Earth’s Physics of Soviet Science Academy.

TABLE 5 Seismic scale

TABELA 5 Skala sejsmiczna

Quake

degree Quake’s characteristics

Oscillating speed [cm/s]

At earth quake At explosion

I Only instrument can register oscillations 0.125 0.2

II Some people in calm and at higher floors are able to feel

oscillations 0.125–0.25 0.2–0.4

III Dangling of hanging object can be noticed 0.25–0.5 0.4–0.8

IV Many people feel oscillations, glass and plate rattle 0.5–1 0.8–1.5 V All people feel oscillations, damage appears on older buildings 1–2 1.5–3 VI Little cracks occur and buildings damages are easily noticeable 2.1–4 3–6 VII Damages occur even on solid buildings, chimney damage 4.1–8 6–12

VIII Significant damages occur, ruptures of bearing walls, collapse

of concrete forms and chimneys 8–16 12–24

IX Collapse of walls, ceilings and roofs – 24–48

X–XII Bigger destructions, appearance of ground cracks, collapse of

many buildings – >48

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The scale is descriptive and it contains 12 seismic degrees, and it is compatible with the European scale (Mercal’s scale) used for quake evaluation due to earth quakes.

By this scale it is possible to compare the values of oscillation speeds generated by blasting and oscillation speeds originated due to earth quakes and to identify interrelation of the oscillations due to blasting and earth quakes.

4. Measurement of ground oscillation speeds

Ground oscillation speeds provoked by blasting on etazama of the open pit “Cubric”, were measured by seismographs of type MiniMate Plus/8 and Blast Mate III. These instruments detect speeds of three types of waves: longitudinal, vertical and transversal.

On the occasion of blasting on the open pit “Cubric”, ground oscillation speeds were measured after blasting of two experimental blast fields of which, each had four observation stations with different distances.

Based on the measured components of oscillation speeds, the speed resultants of all observation stations were calculated, and the results are given in the Table 6.

MO-1 from MP-1 – 1.838 m MO-2 from MP-1 – 2.357 m MO-3 from MP-1 – 807 m MO-4 from MP-1 – 795 m MO-1 from MP-2 – 1.482 m MO-2 from MP-2 – 2.145 m MO-3 from MP-2 – 1.116 m MO-4 from MP-2 – 1.373 m

TABLE 6 Measured results of the ground oscillation speeds

TABELA 6 Zmierzone wyniki szybkoœci oscylacji gruntu

Blasting spot

Observation post

Components of ground oscillation speeds [cm/s] Resultant oscillation speed [cm/s]

Vt Vv VI

MP-1

MO-1 0.01 0.008 0.01 0.02

MO-2 – – – –

MO-3 0.02 0.03 0.02 0.04

MO-4 0.02 0.01 0.035 0.041

MP-2

MO-1 0.02 0.005 0.035 0.041

MO-2 – – – –

MO-3 0.06 0.08 0.06 0.12

MO-4 0.03 0.02 0.06 0.07

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Based on the values of resultant ground oscillation speeds in observation posts, we can calculate allowable quantities of explosive charge which may be momentary activated for different distances of residential objects from the inaction place.

By this, the following conditions were set:

— Allowable speed of ground oscillations – 1 cm/s.

— Allowable degree of quake – 4 degree.

Analyzing the overall results of these experimental blasting, emphasizing the seismic effects, that is, their influence on surrounding objects, we come to dependency of maximum allowable explosives’ quantity per one degree of ignition in one hand, and the distance of structural objects from the blasting field in the other hand (Fig. 1).

Fig. 1. Diagram for determining of maximum allowable quantity of explosives Rys. 1. Diagram okreœlenia maksymalnej dopuszczalnej iloœci materia³ów wybuchowych

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Conclusion

Considering all relevant factors which affect or may have influence on reduction, that is, decrease of seismic waves, we come to the following conclusions:

— Because the craters on open pits of Coal Mine “Banovici” will grow in the future, their size will increase, and for the reason that etaze which are being blasted will get closer to residential objects, the drilling and blasting process must be updated (modernized).

— Update means, initiation of emulsion explosives into blasting process because they produce seismic waves with substantially less speed, therefore contribute in decrease of damage on surrounding residential objects and other structures around crater of the pit.

— Moreover, the update also means initiation of information technologies into the drilling and blasting process.

Therefore, it is necessary to make new software model, especially for blasting process, which will treat all factors that are essential for better usage of explosives’ energy. In other words, this means that there must be a model which will decrease negative effects of the explosive charge and seismic waves in particular, because their work is manifested on surrounding structural objects in the proximity of open pits.

REFERENCES

[1] G u s t a f s s o n R., 1973 – Swedish blasting technique. Nora Boktryckeri AB, Gothenburg

[2] L a n g e f o r s U., K i h l s t r ö m B., 1967 – The modern technique of rock blasting. Almquist u. Wiksell, Stockholm.

[3] T h u m W., 1978 – Sprengtechnik im Steinbruch und Baubetrieb. Bauverlag GMBH, Wiesbaden und Berlin.

ZMNIEJSZENIE SEJSMICZNYCH SKUTKÓW ROBÓT STRZELNICZYCH NA ODKRYWKI W KOPALNI WÊGLA „BANOVIÆI”

S ³ o w a k l u c z o w e

Materia³y wybuchowe, roboty strzelnicze, kopalnia odkrywkowa, wibracje gruntu

S t r e s z c z e n i e

Roboty strzelnicze prowadzone w ska³ach p³onnych zwiêkszaj¹ intensywnoœæ wstrz¹sów ziemi. Dobrano najbardziej odpowiednie materia³y wybuchowe i optymalne parametry robót strzelniczych dla odkrywki. Bardzo ma³a czêœæ energii powsta³ej w czasie robót strzelniczych zostaje zamieniona na energiê kinetyczn¹ fal sejsmicznych o ma³ej intensywnoœci. W przysz³oœci konieczne bêdzie narzucenie stosowania emulsyjnych materia³ów wybuchowych dla uzyskania lepszego efektu robót strzelniczych, to znaczy lepszego wykorzystania energii materia³ów wybuchowych.

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THE DECREASE OF THE BLASTING SEISMIC EFFECTS ON OPEN PITS IN THE COAL MINE “BANOVIÆI”

K e y w o r d s Explosives, blasting, open pit, round vibration

A b s t r a c t

Due to blasting of waste-rock, a higher intensity quakes happen. It has been chosen the most suitable explosives and optimal blasting parameters on open pit. A very small part of blasting energy is turned into kinetic energy of lower intensity seismic waves. In the future, it is necessary to impose the emulsion explosives for better blasting effects, that is, better utilization of explosives’ energy.

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