Blast Induced Ground Vibration

OPEN PIT MEGA BLASTING WITH IN-HOLE DELAYS AND / OR PRE-SPLITTING OF PRODUCTION BLAST – MEASURES TO CONTROL ADVERSE IMPACT OF COMPLEX VIBRATION ARISING DUE TO PRESENCE OF UNDERGROUND WORKINGS IN THE VICINITY OR IN OTHERWISE SENSITIVE AREAS. By: Partha Das Sharma ABSTRACT One of the most troublesome and controversial issues facing the mining and excavation industries is that of blast induced ground vibration. With the general trend towards larger blasting in mines, increased population and spread of urbanization near to the mining sites ground vibration problems and complaints have risen manifold. All blasting operation in mines has to be associated with some form of vibration. Even the best designed and executed blasts generate a certain amount of unwanted energy in the form of ground vibration waves, which radiate away from the blast site. However, the impact of ground vibration wave can be minimized by proper blast design. Another strange aspect in ground vibration, now a days, coming to fore is increased problem due to complex vibration wave arising out of presence of underground workings or cavity in the vicinity of open pit coal mines. As more and more larger open pit mines are expanding its periphery for their enhanced requirement of production, the chances of coming them to the vicinity of either abandoned or working underground mines becoming more. A new complex ground vibration problems are encountered when dealing with safety of dwellings and structures created above such underground cavities. It has become a challenge to the mining engineers to understand the peculiar pattern of vibration wave generated because of the presence of underground cavities and also to design blast to cope up such problems. The types of waves generated, reflection and refraction occurred when these waves encountered the Solid-Air interfaces in the underground and implication of their change in behaviour on ground vibration has been discussed. The modification of blast design in order to keep the allowable Peak Particle Velocity (PPV) within the limit with less frequency generated due to presence of Solid-Air interfaces in the underground beneath the surface structures/ area in question has been also been discussed. Introduction: One of the most troublesome and controversial issues facing the mining and excavation industries is that of blast induced ground vibration. With the general trend towards larger blasting in mines, increased population and spread of urbanization near to the mining sites ground vibration problems and complaints have arisen manifold. All blasting operation in mines has to be associated with some form of vibration. Even the best designed and executed blasts generate a certain amount of unwanted energy in the form of ground vibration waves, which radiate away from the blast site. However, the impact of ground vibration wave can be minimized by proper blast design. A strange aspect in ground vibration, now a days, coming to fore is increased problem due to complex vibration wave arising out of presence of underground workings or cavity in the vicinity of open pit coal mines. As more and more larger open pit mines are expanding its periphery for their enhanced requirement of production, the chances of coming them to the vicinity of either

1

abandoned or working underground mines becoming more. A new complex ground vibration problems are encountered when dealing with safety of dwellings and structures created above such underground workings/ cavities. It has become a challenge to the mining engineers to understand the peculiar pattern of vibration wave generated because of the presence of underground cavities and also to design blast to cope up such problems. Another aspect of peculiar phenomena of vibration waveform arises in Mega Blast carried out in Opencast Mines, when Ground Vibration waves interfere with each other and the resultant waveform is very much complex in nature. Even by the use of NONEL or Electronic Detonators the occurrence of this complex nature of waveform can not be avoided. Thus, it becomes all the more difficult task for blasting engineers to restrict such waves to propagate. Concept of ground vibration: When an explosives charge is detonated in a blast hole, the rock immediately surrounding the charge is fractured, split apart and is displaced. At a certain distance from the blast holes, the explosives energy decreases to a level, which causes no further shattering or displacement and continues to travel through the rock as an elastic ground vibration. The ground motion is literally a wave motion spreading outwards from the blast, much as ripples spread outwards from the impact of a stone dropped into a pool of water. The ground / rock through which the wave travels is considered to be elastic medium, composed of innumerable individual particles. As a disturbance, of these particles are set into a random oscillatory motion, a ground motion wave being generated. Each particle transmits energy successively to the next. The total energy of ground motion wave generated in the rock around a blast varies directly with the quantity of explosives detonated. As the ground motion wave propagates outwards from a blast, the volume of the rock subject to the compression wave increases. Since the energy in a ground shot is distributed over successively greater volume of rock, the ground motion must decrease. Thus energy losses occur with each successive transmission, so that as the ground wave spread outwards, it diminishes in intensity and the particles gradually return to the rest position. In a well-designed blast, most of the explosion energy is spent in breaking of ground and throw of the blasted rock. A small amount of energy is converted into ground vibration. When blast holes are under or over charged and absence of proper free face a great deal of liberated energy is wasted and converted into ground vibration, as explosion energy is not utilized in fragmenting / breaking of rock and throw. Ground motion wave produced by blasting is generally of two types: a) Body wave: Body wave travels through earth material. It may be reflected or refracted to the surface to become surface wave. b) Surface wave : The surface waves travel along surfaces and interfaces of earth materials. The most important Surface wave is the Rayleigh wave (as shown in Fig-1), denoted as “R”. Rayleigh wave cause the ground to shake in an elliptical motion, with no transverse or perpendicular motion. Another form of surface wave is Love wave, having a horizontal motion that is transverse (perpendicular) to the direction the wave is traveling; and is of very less significance.

2

Body waves can be further subdivided into compressive (compression/tension) or sound like waves (‘P’ wave as shown in Fig-2) and distortional or shear waves (‘S’ wave as shown in Fig3). Explosions produce predominantly body waves at small distances. These body waves propagate outward in a spherical manner until they intersect a boundary such as another rock layer, soil, cavity or the ground surface. At this intersection, shear and surface waves are produced. Rayleigh surface waves become important at larger transmission distances. Rayleigh wave relatively has larger “R” amplitude compared to the “P” or “S” amplitude. Thus, S and R waves pose problems as vibration pollution. The effect of R waves is particularly big. When exciting force is applied vertical to the propagation direction, R waves account for 67% of total energy, S waves for 26% and P waves for 7%.

Fig-1

3

Fig-2

Fig-3

4

The propagation velocity of a vibration differs slightly between R waves and S waves. Nevertheless, it is fair to think that, when the vibration source and the receiving point are a few metre to a few hundred metre apart, as in the case of blasting vibration, and a shock wave is applied to one point on the surface of a vast homogeneous land, a P wave reaches the receiving point first followed by a mixed wave of R and S waves (as shown in Fig- 4).

Fig – 4 Reflected and Refracted Wave Path: Whenever an elastic wave (Body wave) encounters a medium with different elasticity and density reflection and refraction of the wave occurs. P – waves travel through all type of media, i.e., solid, liquid and gas; in contrast, S – waves have its inability to propagate through a fluid or gas because fluids and gasses cannot transmit shear stress. In a discontinuous ground, wave amplitude reduced due to mismatch of impedance (Impedance ‘Z’ is product of density ‘ρ’ and seismic velocity ‘V’; Z= V. ρ) (as in Fig – 5).

5

Fig - 5 Parameter of ground vibration: Typical blast vibrations, no matter the wave type, can be approximated as sinusoidal varying in either time or distance along all the three directions. There are interrelated parameters that may be used in order to define magnitude of ground vibration at any location. These are a)

Particle Displacement – The distance that a particle moves before returning to its original position (measured in mm).

b)

Particle Velocity – The rate at which the displacement changes (measured in mm/s).

c)

Particle Acceleration – The rate at which the particle velocity changes (measured in mm/s²).

d)

Frequency – The number of oscillation per second that a particle undergoes (measured in Hz).

The most preferred parameter of measurement of ground vibration is Peak Particle Velocity (PPV). The peak particle velocity is a function of borehole pressure, confinement, charge weight, distance from the blast site, the manner in which the compressive wave decays through rock mass and supposition of stress created by firing sequence of adjacent holes. The measurement of particles by vibration wave is usually measured in three mutually perpendicular directions, they are: a)

Longitudinal (sometimes termed Radial) – Back and forth particle movement in the same direction that vibration wave is traveling.

b) Vertical – Up and down movement perpendicular to the direction of vibration wave is traveling.

6

c)

Transverse – Left and Right particle movement perpendicular to the direction of vibration wave is traveling.

Broadly speaking, the key factor that controls the amount and type of blast vibration produced is explosives energy confinement and the distance of the structure from the blasting site. On this basis the concept of Scaled Distance equation has been developed, which is given below: Distance of Structure from blast site Scaled Distance = √ Max. Charge wt. Per (8 ms) delay interval In some of the country following scaled distance restrictions have been imposed in mines when seismograph is not used to monitor the blast: 0 – 300 ft away – minimum allowable scaled distance is 50 301 – 5000 ft away - minimum allowable scaled distance is 55 Over 5000 ft away - - minimum allowable scaled distance is 65 Vibration propagation Equation: The most accepted index (USBM’s PREDICTOR EQUATION) of Ground vibration generated by blasting is the Peak Particle Velocity (PPV). It has been well established that, PPV depends on maximum charge per delay; distance from blast-site to measurement point and Ground geology, and the relationship is as follows: V = K (D/√Q)-B Where, V is the Peak Particle Velocity (PPV), D is the Distance of the measuring transducer Q is the Maximum charge weight per delay. K and B are Site constants, to be determined by regression analysis. D/√Q is called as Scaled Distance. Taking Logarithm of both sides of the Predictor equation, we get Log V = Log K – B Log (D/√Q) If, Y = Log V; X = Log (D/√Q) and C = Log K; then the above equation represents a straight line of the form Y = C - BX (Straight Line plotted on Log V as Y axis and Log (D/√Q) as X axis below); where B is the negative slope of the straight line and C is the point of interception on Y axis. Relationship of Scaled Distance with Peak Particle Velocity (PPV) on Log scale shown in Fig.- 6.

7

Fig- 6 Criteria for limiting safe vibration level (Regression Analysis): Regression Analysis is the process of estimating PPV statistically from the independent variables of explosive weight / delay and the distance to the structure of concern. Now the prime objective is to determine the maximum charge to be fired per (at least 8 ms) delay interval, in order to keep PPV within the safe limit. Following are the procedures involved: a) Measurement of PPV with different scaled distances. PPV tend to decrease with the increase of Scaled Distance. b) Plotting these values on a Log – Log scale as described above in Fig-6. c) The value of site constants K and B are to be determined by extrapolation of straight line plotted as described above. d) Using propagation straight line (plotted as per point ‘b’ above) safe scaled distance to be determined to keep PPV below safe limit (on the basis of threshold limit prescribed by DGMS, India). e) From the determined safe scaled distance above, the maximum charge per delay can be found out for various distances for a particular site. Normally, in blasting, a 95% confidence line is calculated. To develop a good analysis, a minimum of 30 – 35 data sets need to be analysed. If done properly and enough seismographs are used, this can be accomplished and began in the test blast programme. Regression analysis should be continued throughout the project and blast design should be adjusted accordingly. Besides peak particle velocity, the Frequency is one of the most important factors controlling the response of structures. As stated earlier, frequency is the number of times the particles move back and forth in one second. This back and forth motion can also be referred to as

8

oscillations per second or cycle per second that a particle makes under influence from the vibration wave and is measured in Hz (Hertz). Frequency is dependent on site geology, distance of the blast, delay sequencing and condition of available free face of the blast. It has been observed that, presence of buffer in front of face holes develop low frequency waves. The effect of frequency generated during blasting relates to the condition of structural response and also can allow higher peak particle velocities with higher frequency. For example, within very short distance of blast the frequency can be very high; although they attenuate or decay quickly. It has also been observed that, the ground motion frequencies are relatively high when solid or tough rock is present; frequency is relatively low when transmission medium is medium-hard / softer strata and frequency is considerably low when there is presence of void beneath. Thus, allowable peak particle velocity reduces considerably when there is void or underground workings beneath the structures in question. The typical range of ground vibration for surface open pit predominantly from 20 to 30 Hz in case of hard rock quarries; 8 to 20 Hz in case of open pit site having medium hard to soft strata and less than 8 Hz in case of ground has cavities/voids in the form of underground workings. Effect of blast induced ground vibration: Ground vibration can cause physical damages to mine plant structures and to neighbouring residences. The most type of damage associated with ground vibration is the aggravation of existing minor cracks in the structure. The damages to the structure depend on the intensity of the vibration; which mostly depend on the distance of the structure from the blast site, Explosives charge weight per Delay, Frequency of vibration, Blast geometry and confinement. Therefore, the following primary variables are responsible for damages: • • •

Distance of structure from blast site (Peak particle velocity reduces as the distance increases) Explosive charge weight per delay Frequency of vibration (Low frequency wave below 10 Hz cause more damage to structure)

Effect of ground vibration when encountered underground workings: It has already been discussed the nature of various types of waves generated and their nature when they get reflected or refracted in the interface of strata in the ground. A strange and complex phenomenon perhaps takes place, when waves encounter Solid-Air interfaces in the underground (similar to a condition when vibration waves passes through underground voids/cavities created due to underground workings). An amplified form of wave (relatively higher PPV) with low frequency has been experienced at the vibration monitoring points near structures located above such underground voids/cavities. The probable justifications of such anomaly, effects and control could be explained as below: a) The ordinary reflection/refraction takes place by some of vibration waves in the conventional interfaces (solid-solid interface) just above the underground cavities and reaches the receiving monitor in the usual manner. b) Portion of vibration waves which otherwise had lost without making any impact on receiving monitor as they would pass to a greater depth, get reflected back from wider Solid-Air interfaces in underground (created due to underground workings) towards surface with much lower frequency, larger wave lengths and larger duration (perhaps follow Rayleigh’s principle).

9

The percentage of such reflected wave would be about 60 to 80% (depend on the nature, width and extent of Solid-Air interfaces) of the waves that would have lost through the zone. c) Thus, when this complex vibration waveform reaches receiving monitor at the surface, we get relatively enlarged peak particle velocity accompanied with low frequency waves; which endanger the stability of surface structures and inhabitants located above such underground workings. The rate of attenuation of such low frequency wave is much lesser. d) In practice also, it has been observed that, keeping surface distance same the intensity of ground vibration is enormously more with low frequency vibration wave when there is presence of underground workings beneath a structure than there is no underground cavities. e) Thus, in order to minimize the adverse effects of such enhanced intensity of ground vibration at low frequency, the allowable PPV reduced drastically and a great deal of monitoring/ recording of ground vibrations are done in order to design a proper blast by keeping every relevant aspect in mind. f) To keep PPV within allowable limit (as per threshold limit prescribed by DGMS, India, given in Annexure-II), one of the most important aspects in designing the blast is reduction of charge per delay; which we intend to discuss here broadly. g) One of most dependable practice to limit charge per delay is by using in-hole delays in deck charges. With amplified situation of complex waveform, it is very difficult to decide the quantity of explosives to be kept in a deck and the number of decks to be used. This becomes more complicated when bench height is large (for a mines where Dragline is being worked and bench height is kept more than 30 metre). Some time as many as four to five decks depending on the situation may have to use in order to reduce charge per delay. Each deck of explosives charge has to be given separate delay timing having interval between each delay more than 8 ms. h) Regular regression analysis is to be carried out in the mines in order to establish the charge per delay by calculating scaled distance. Effect of Ground Vibration on surface structures: Ground vibration may cause damage to structures and annoyance to inhabitants in the vicinity of mines. Damage to structures depends on the type of structure, its resonant frequency, construction material used, dimension of structure etc., and the characteristics of vibrations. Attempts have been made to relate structural damage to more than one parameter, i.e., frequency, amplitude, velocity and acceleration. The types of damage caused to the structures in order of increasing intensity are (a) dust falling from old plaster cracks, (b) extension of old plaster cracks, (c) new plaster cracks formation, (d) plaster flaking, (e) plaster drops from large areas, (f) masonry crack formation, (g) partition separating from exterior walls, (h) further severe damages and building collapses. Potential for new approach of addressing the issue of structural damage due to Ground Vibration has opened up; which include vibration resistant structural designs, ways to minimize the Ground Vibration effects on structures by proper blast design by monitoring the vibration intensity of the blast and keeping proper record of the blast vibration data collected from site. Use of in-hole delays for limiting charge per delay – (A case study): The blast detail and of some of the design applied in an open pit coal mines in the dragline bench blasting and record

10

of blast induced ground vibration measured at a place where human habitants/ villages are located over an abandoned underground coal mines at a distance of about 700 to 800 metres from the blast site are given in Table - 1. The blast design with quantity of charge per delay, number of decks, burden & spacing etc., have been decided after no. of trials conducted with varying patterns. The necessity of using such a large number of decks (some time as many as four to five decks depending on the situation, Fig-7 and Fig - 8), many a times, has arisen to reduce weight of explosives per delay drastically to keep the PPV within allowable limit. With this system of decking the total lapsed time of the entire blast in a round becomes more, but PPV in most of the blast kept within reasonable limit as frequency obtained is considerably low. This is mainly due to presence of underground workings/ cavities beneath the area in question.

Fig. - 7

11

Fig. - 8 (Table – 1) S.N.

PARTICULARS

BLAST NO. 1

BLAST NO. 2

BLAST NO. 3

1

Strata condition / geology

Medium hard stand stone with minor faults. Mostly watery strata.

Medium hard stand stone with minor faults. Mostly watery strata.

2 3 4 5 6 7 8 9 10 11 12

No. of holes in the round Dia. Of hole (mm) Av. Bench Height (m) Depth of holes (m) Av. Depth of holes (m) Av. Spacing (m) Av. Burden (m) Av. Stemming (m) Sub-grade drilling No. of Decks used for explosives DTH delay sequence from bottom deck

13

Surface STL delay sequence

14 15 16

Max. charge per delay (KG) Av. Charge per hole (KG) Total explosives used in the round (MT) Distance of measurement point from the blast site (m) Frequency (Hz) Resultant PPV obtained (mm/sec)

72 260 32 From 28 to 35 32 7.5 7.5 5.3 Nil 5 450ms, 475ms, 500ms, 525ms, 550ms. Hole to hole -42ms and Row to row – 336ms. 250 1100 79.780

Medium hard stand stone with minor faults. Mostly watery strata. 39 260 32 From 28 to 35 32 7.5 7.5 5.3 Nil 5 450ms, 475ms, 500ms, 525ms, 550ms. Hole to hole -42ms and Row to row – 336ms. 250 1100 42.630

750

780

775

5.0 7.73

4.0 5.19

5.5 5.03

17 18 19

71 260 32 From 28 to 35 32 7.5 7.5 5.3 Nil 5 450ms, 475ms, 500ms, 525ms, 550ms. Hole to hole -42ms and Row to row – 336ms. 250 1100 81.380

12

(The explosives used for above blasts were Bulk emulsion explosives, Solar BE-101 of M/s Solar Capitals Ltd. and Shock Tube NONEL (DTH & STL) Supremedet of M/s Economic Explosives Ltd.). The blasts with In-hole delay system described above can also be effectively done by using Electronic Detonators. Direct delay timings can be assigned to the various decks. The Surface Trunk Line Detonators are not needed in case of blasting with Electronic Detonators. Pre-splitting of Production Blast or Main Blast: Another effective way of restricting Ground vibration is by Pre-splitting of the production blast. Pre-splitting helps in isolating blasting area from remaining rock mass by creating an artificial discontinuity along the final designed excavation line / plane against which subsequent main blast breaks. A row of holes are drilled at the periphery (three sides) of the main blasting block at a closer spacing, charged preferably with lesser quantity of explosives than the production blast and blasted prior to the main blast in an effort to create a fracture line and a reflective plane at the excavation limit or plane. Some of the shock waves from subsequent main blast are reflected at the pre-split plane which results in arresting a considerable portion of blast induced ground vibration generated in the main blast to propagate. The theory of pre-splitting is that when two charges are shot simultaneously in adjoining holes, collision of shock waves between the holes places the web in tension and causes cracking that give a sheared zone between the holes to open a narrow crack / separation along the three sides of the production or main block before the main blast goes-off (Fig – 9). This results in a smooth wall with little or no over break. The pre-sheared plane reflect some of the shockwaves generated from the primary blasts that follow, which prevent them from being transmitted into the finished wall and minimizes shattering and over break.

Fig - 9 The separation of timing between blasting of pre-splitting holes and production blast are kept with the help of delays. The delay gap of 200ms to 250ms between pre-split and main blast is considered to be enough.

13

The quantity of explosives to be used in pre-split holes, burden and spacing are estimated keeping in view the insitu tensile strength of rock mass. The borehole spacing of pre-split holes is normally kept at 8 to 12 times the blast hole diameter and the burden may be kept as of the burden of the Main Blast. Depth may be kept as of last row of main blast. Mostly, light distributed decoupled charges are used in pre-splitting holes. Air-deck in between deck charges improve the quality of pre-split fracture and avoid extension of radial cracks around the holes (Fig – 10). Generally, the quantity of explosives kept in pre-splitting holes is 8 to 12 % of the explosives charged in one hole in the Main Blast.

Fig - 10 Advantages of Pre-splitting: (1) Field observation reveals that with the introduction of presplitting the back breaks are eliminated, improving the stability of high-wall slopes and to provide uniform burden to the front row of holes for next blasting round. (2) As back breaks are eliminated, formations of pre-formed boulders are reduced resulting better fragmentation in the subsequent blasts. (3) Field observation reveals that, there is substantial reduction of ground vibration level to the tune of nearly 1/3rd of normal production blast due to pre-splitting. (4) Presplitting is most suitable for controlling ground vibration level in the case of Overburden Side Cast Blasts. (5) Mega blasts conducted in opencast mines, the interference of ground waves result. A very complex phenomenon of resultant waveform occurs, which is very difficult to control only by NONEL or Electronic Detonators. Pre-splitting of production blast is the best method of controlling or restricting the intensity of the waveform to propagate outside the Mega blasting zone and thereby, protecting of Surface Structures. Modern Technology & Equipment used: Some of the modern equipments available which can be employed for the purpose of control of Ground Vibration are: a) Seismograph or Ground vibration recorder is essential in documenting the un-wanted side effects of blasting such as Ground vibration and Air blast. b) It has been observed by use of sophisticated, precise and perfect timing Electronic Detonators in Mega blast in opencast mines reduces intensity of Ground Vibration, as the

14

scattering (deviation of actual timing from nominal delay timing) of delay timing is almost negligible or nil. c) Modern surveying equipment and its Computer software provided wealth valuable information on a blast and post-blast scenario. These include overburden – optimum front hole location, custom loading information for front row hole burden, pre-blast mapping, post-blast mapping & determination of cast / throw, volumetric calculation for swell factor and cast percentage for basis of evaluation of economic advantages. d) High Speed Photography recording is very efficient and extremely useful instrument for analyzing of blasts and to assess delay sequence used for the blast. Now a days, high speed photography to the tune of about 1000 frames per second is in vogue, which is very useful for the purpose. e) Velocity of Detonation Recorder is another useful instrument which can be used to determine explosive performance in the hole. f) Use of Precise Drilling Equipment to prevent any deviation of holes. Conclusion: As a complex ground vibration phenomena are observed when dealing with safety of dwellings and structures created above such underground workings/ cavities or in the sensitive areas, it has become a challenge to the mining engineers to understand the peculiar pattern of vibration wave generated. Extensive field study is required in order to understand the wave pattern generated, change of characteristic of various waves when pass through Solid-Air interfaces in the ground and to establish the Scaled Distance and safe charge per delay for a area / structures in question. Pre-splitting of production blast is another important way to control adverse impact of ground vibration, which blasting engineers should adopt. References 1. Atlas Powder Company, Dallas, Texas, USA - “Explosive and Rock Blasting” 1987. 2. Siskind.,D.E., Stachura., V.J., Nutting, M.J.; (1987): “Low frequency produced by surface mine blasting over abandoned underground mines”; USBM RJ9078. 3. Sharma., P.D : “Control of adverse effects of Explosives Blasting in mines by using Shock tubes(Nonelectric) Initiation system and its Future challenges”; Advances in drilling and blasting techniques- Procc. of DRILL BLAST’99 – National Seminar on drilling and blasting, Bhubaneswar, India, January 2000. 4. DGMS (Tech)/S&T Circular No. 7 of 1997; “Damage of structures to blast induced ground vibrations in the mining area”; Dhanbad, India. 5. Siskind.,D.E. (2000) : “Ground vibration from blasting”; Int. society of Explosives Engineers. 6. Siskind.,D.E. and Stachura., V.J.; “Vibrations from blasting over abandoned underground mines”; USBM report - IC 9135h (1988). 7. Lucca., F.J.; “Tight construction blasting, Ground vibration basics, monitoring and prediction”; Terra Dinamica L.L.C (2003). 8. Konya., C.J. & Watler., C. J.; “ Surface blast design”; Prentice Hall (1990). 9. Siskind., D.E., Stagg., M.s., etal ‘Structural response and damage produced by ground vibration from Surface mine blasting’, USBM RI 8507, (1980). 10. Dowding,.C.H., ‘Blast Vibration Monitoring and Control’, Prentice Hall Inc., 1985. 11. Blasters’ Handbook – E.I.du Pont de Nemours & Co. (Inc.), Wilmington, Delaware 19898.

15

12. Nicholls., H.R., Jhonson., C. F. and Duvall., W.I., ‘Blasting Vibrations and their effects on Structures’, USBM Bull. 656. (1971). 13. Gupta., R.N., Ghose., A.K., Mozumdar., B.K., Nabibullah., Md. - ‘Design of Blasting rounds with Airdeck pre splitting for Dragline and Shovel benches near populated Areas – A case study’, Int. Symposium on Explosives and Blasting techniques, N.Delhi, Nov. – 1990. 14. Sharma., P.D. – ‘Overburden Side casting by Blasting – Operating large opencast Mines in a cost effective way’ , Proc., of 1st Asian Mining Congress, 16-18 January 2006, Kolkata, India, (Page No. 307 – 315), MGMI Centenary Vol.-1. 15. Watson., John. T. – ‘Developments with Electronic Detonators’, Proc., of Int. Conf. On Expl. & Blasting Tech, ISEE (2002). 16. Sharma., P.D. - Open pit blasting with in-hole delays and / or pre-splitting of production blast – Measures to control adverse impact of complex vibration arising due to presence of underground workings in the vicinity or in otherwise sensitive areas; Mining Engineers’ Journal, August 2006.

Annexure - I Known methods and techniques to reduce Ground Vibrations: The following methods and Techniques have been successful in reducing ground vibration and resulting annoyance complaint: 1. Reduce weight of Explosives per delay. 2. Reduce explosives confinement by : a. Reducing burden and spacing. b. Reducing buffers in front of face holes. c. Reducing sub-drilling. d. Reducing Hole depth. e. Using a blast design that produces maximum relief: this means using large delays between holes or rows of holes. Optimum delay intervals can be determined and substantiated with the use of high-speed motion picture photography. f. Allowing maximum number of free face to blast. 3.

Whenever possible, the progression of detonating holes or a row of holes through millisecond delay intervals should progress away from the structure.

4.

Use larger delays, where geological conditions in conjunction with initiation system permit.

5.

Where possible, keep the total lapsed time of the entire blast below 1-second duration.

6.

Use electric millisecond detonators with sequential blasting machines or an initiating system with an adequate number of delay intervals preferably, with down-the-hole delays causing bottom charge and deck charge blast separated by delays.

7.

Use of accurate delay timing detonators, i.e., Electronic Detonators, wherever possible minimizes the intensity of ground vibration.

8.

It has been observed that, using pre-splitting the production blast and by using air decking the ground vibration is reduced considerably.

16

Annexure - II THRESHOLD VALUE OF GROUND VIBRATION AS PER DGMS (INDIA)

TYPE OF STRUCTURE

Building structure not belonging to owner 1.Demostic House structure kucha brick &cement 2. Industrial building RCC& Framed Structure 3.Objects of historical importance and sensitive structure Building belonging to owners with limited span to life 1.Domastic houses structure Kucha bricks & cement 2.Industrial Building RCC & framed structure

PPV IN MM/SEC AT A FOUNDATION LEVEL OF STRUCTURE AT A FREQUENCY. 25Hz

5

10

15

10

25

25

2

5

10

10

15

25

15

25

50

Author’s Bio-data: Partha Das Sharma is Graduate (B.Tech – Hons.) in Mining Engineering from IIT, Kharagpur (1979) and was associated with number of mining and explosives organizations, namely MOIL, BALCO, Century Cement, Anil Chemicals, VBC Industries, Mah. Explosives etc., before joining the present organization, M/s Solar Explosives Ltd., few years ago. Author has presented number of technical papers in many of the seminars and journals on varied topics like Overburden side casting by blasting, Blast induced Ground Vibration and its control, Tunnel blasting, Drilling & blasting in metalliferous underground mines, Controlled blasting techniques, Development of Non-primary explosive detonators (NPED) etc. Currently, author has following useful blogs on Web: http://www.environmentengineering.blogspot.com • http://saferenvironment.wordpress.com • www.coalandfuel.blogspot.com •

Author can be contacted at E-mail: [email protected] ***

17