Vibration control in an opencast mine based on improved blast vibration predictors

Mining Science and Technology, 12 (1991) 157-165

157

Elsevier Science Publishers B.V., Amsterdam

Vibration control in an opencast mine based on improved blast vibration predictors P. Pal Roy Blasting Department, Central Mining Research Station, Barwa Road, Dhanbad 826001, Bihar, India (Received October 3, 1989; accepted May 14, 1990)

ABSTRACT Pal Roy, P., 1991. Vibration control in an opencast mine based on improved blast vibration predictors. Min. Sci. Technol., 12: 157-167. Vibrations due to rock blasting are studied based on the data measured on several test structures at the West Mudidih Opencast Project of BCCL, India. A single-, as well as a two-storey brick structure, a m u d house and concrete walls were constructed on the edge of the working area. All the existing ground vibration predictors including the two modified predictors of G h o s h - D a e m e n were tested against the data recorded. A new model is proposed as a blast vibration predictor. This model is simple and an improvement on existing vibration predictors, in that it gives a better index of determination and consistent charge/delay.

Introduction

Ground vibration from blasting is an undesirable side-effect of the use of explosive to fragment rock for mining, quarrying, and excavations. It is a continual problem for the mining industry, the public residing nearby, and the regulatory agencies responsible for setting environmental standards. The main reason for the existence of this problem is the fact that more rock excavation is being carried out near populated areas, and also in part to an increased sensitivity to environmental disturbance. The U.S. Bureau of Mines (USBM) has studied extensively various aspects of ground vibration, air blast and seismic instrumentation related to opencast and underground blasting. Nicholls et al. [1] have made a comprehensive review of blast design effects on 0167-9031/91/$03.50

the generation of ground vibration, air blast propagation and seismic instrumentation. They established the significance and use of peak particle velocity in place of displacement and acceleration. They suggested a m i n i m u m delay interval of 9 ms for scaled distance calculations, and a safe scaled distance of 23 m / k g 1/2 for quarry blasting in the absence of vibration monitoring. Their work also included a damage summary analysis originally published in 1962 by Duvall and Fogelson [2]. USBM recommended 25 m m / s as the limiting vibration level to assess the damage potential of the ground vibrations. Siskind et al. [3] noticed that, even below the 25 m m / s level, severe house rattling caused fear of property damage. In 1974, USBM started to reanalyse the blast damage criteria and structural response to ground vibrations. They

© 1991 - Elsevier Science Publishers B.V.

158

modified and expanded the work of Duvall and Fogelson [2]. Their study included: (1) Direct measurements of structural response, and damage observation in residences from actual surface mine production blasting. (2) A probabilistic approach using various data sets, as well as the conventional statistical derivation of regression analysis and standard deviation for the various damage thresholds. (3) The analysis of frequency dependence through frequency spectral analysis. (4) Human tolerance to vibrations through steady-state sources. Siskind et al. [3] developed the damage criteria to quantify the response of and damage to residential-type structures from the small to intermediate sized blasts used in mining, quarrying, and excavation. Their study was beneficial for the coexistence of blasting and an environment-conscious society. Walker et al. [4] pointed out that a more rational method for predicting damage is based on response spectra, an engineering technique originally developed for earthquake analysis. Adaptation of these techniques for opencast mining and the consequent results of their research showed that the conventional methods, based on peak particle velocity measurements, can result in over-restrictive site blasting practice. Huh Ginn et al. [5] studied in a recent paper the effect of blasting pattern, rock strength and different explosives on blast-induced ground vibrations. From this they determined the maximum charge weight/delay within a given vibration level. The model proposed by Ghosh and Daemen [6] separated the descriptors of geometrical and of inelastic attenuation. They modified the USBM [1], Ambraseys and Hendron [7] and general form of empirical equations [8,9,10] by incorporating inelastic attenuation, represented by an exponential decay function e -~D, where a is the inelastic attenuation factor. These equations improved the values of the index of determination marginally

P. PAL ROY

compared to the conventional predictors. In addition, the authors did not focus on the calculated charge/delay as obtained from their equations for specific distance and vibration level, in order to validate their proposed model. From a practical point of view, although some predictor equations may happen to give slightly higher values for the index of determination, they lead to charge weights/delay which are not feasible for production blasting. In some cases, in place of attenuation, it was observed that their equations led to amplification of particle velocity with distance. It was not possible to determine the physical cause of this. From these several existing methods for predicting ground motion induced by blasting, one must select a predictor which is relatively simple and is both suitable and gives a reasonable calculated charge weight/ delay for field use. It must be recognised that ground vibrations induced by blasting are the consequence of a complex series of events and it is most unlikely that all of these can be accounted for, in detail, by means of simple closed form equations. It has been proved that better predictability can be achieved by taking into account the dominant aspects of pulse initiation and propagation. The model proposed here as a blast ground vibration predictor is an improvement on the seven existing predictors as it fits better and gives a very consistent and feasible charge weight/ delay.

General description of the mine West Mudidih opencast mine is situated near a populated area of Katras, Bihar. Due to urban sprawl encroaching on quarry and mine operations, and in part to an increased sensitivity tO environmental disturbance, prediction and standardization of ground vibration caused by blasting to different types of structures was most essential for the mine.

VIBRATION

CONTROL

IN AN OPENCAST

159

MINE

Z.sin

MUGwails with smoothrapping boths,des

I m

--

[-

z, Srn - - - - - - - ~

--Z.Sm

[.

O

Fig. 3. Diagram to show the construction o[ a normal

village mud house.

The overburden was removed by drilling and blasting.

Fig. 1. The test structures constructed at the site.

Field investigation The coal seams of the area belong to the Barakar Measures. The general strike of the seams was N76 o W - $ 7 6 ° E and they dip at 1 in 5.6 towards S14 o W. There were five seams in the area. Out of these, only numbers IV and V were quarried out. The area was almost free from any geological disturbances, except for (the down throw side of) one strike fault in the western area and one oblique fault, with a 4.5 m throw in the middle portion of the area. The overburden mainly consisted of sand and shale and friable shale. The thickness of the overburden varied from 3 to 23 m.

Brick wall with mortar jolnting

The investigation was carried out in two phases. In the first phase, an attempt was m a d e to determine the damage threshold of various types of structures (Fig. 1) normally found in and a r o u n d the mining areas. In the second phase, the object was as before but with a difference in structure. This time a two-storey building with a circular window on the upper floor and a rectangular window on the ground floor was constructed. The purpose of the two types of windows was to allow a comparison to be m a d e of the devel-

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Roof for p r o t e c t i o n (concrete with girder)

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END E L E V A T I O N

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Fig. 2. Diagram to show the construction of single-storey structure.

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160

v. PALROY Concrct¢ with /- girder roof

+1

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(d

= -~7S ~i_j_ mm FRONT ELEVATION t 2m Zm I~

2m

Normal foundation 11"

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1.5rn SIDE VIEW

CENTRAL WALL

PLAN (FROM TOP)

Fig. 4. Diagram to show the construction of a two-storey structure.

opment of fractures. To provide more specific detail, a m u d house and two concrete walls were also constructed adjacent to the twostorey building. The specifications of the masonry buildings were similar to the " l o w cost house" of colliery areas, as were the concrete walls, which were constructed using a 1 : 2 : 4 ratio. The m u d house with khapra was similar to the m u d houses of the villages. Figures 2 - 4 show the specifications used in making the constructions. A series of experimental blasts were carried

Fig. 5. Type of fragmentation in an experimental blast.

out (Fig. 5) and vibrations measured using the equipment shown in Fig. 6. In all, forty vibration readings were taken on the ground and first floor of the structures. As can be seen from the photographs (Figs. 7-9), the effect of repeated blasting on the m u d house and its susceptibility to blast vibrations was much less than for the brick and concrete structures.

Fig. 6. The main instruments used for monitoring; vibration measuring unit SMU31, vibration monitors SINCO-3 and VS-1600, and Bruel and Kjaer strain indicator.

161

VIBRATION C O N T R O L IN AN OPENCAST M I N E

Blast vibration predictors The generation and prediction of ground vibration from blasting has gained the attention of researchers for a long time. Early U.S. Bureau of Mines work [12] tried to correlate

Fig. 9. Crack on the lower portion of the concrete wall.

Fig. 7. Fine cracks on mud walls.

the amplitude of the seismic pulse with the amount of charge and with the distance from the source. Their research showed that scaling of distance was necessary to predict peak particle velocity when both the charge weight/delay, and the distance of the measuring transducer vary. Two of the most popular approaches are square root scaling and cube root scaling. If long cylindrical charges are used, the explosive geometry can be assumed to be cylindrical. From dimensional analysis, any linear dimension should then scale with the square root of the charge weight. In the case of spherical symmetry, any linear dimension should be scaled to the cube root of the charge size [15]. Some authors, namely Davies et al. [8], Birch and Chaffer [91 and Attewell [10], did not consider any particular charge symmetry but used a general type of predictor equation. The present study has considered the following seven conventional and modified predictors: (1) USBM (Duvall and Petkof [12], Duvall and Fogelson [2], Siskind et al. [3], Nicholls et al. [11):

V= K( D/~-Q)"

(1)

(2) L F K H (Langefors-Kihlstrom [131): Fig. 8. Cracks on the top of the single-storeystructure.

V= K ( ~ ) "

(2)

162

P. PAL

ROY

TABLE 1 Site constants a n d index of d e t e r m i n a t i o n as o b t a i n e d for g r o u n d floor observations Equation

n

U S B M

K

241.009

-1.38140

LFKH

1.60846

AMHEN

-1.35602

IS GEN

1.20635 - 1.10559

66.4648 716.441 66.4649 39.0183

m

a

I n d e x of determination

Remarks

-

-

0.754

-

-

0.823

-

-

0.554

w

-

0.823 0.829

Lower value of i n d e x of d e t e r m i n a t i o n c o m p a r e d to eqn. (8). Shows less c h a r g e / d e l a y c o m p a r e d to eqn. (8) after a distance of 70 m. I n c o n s i s t e n t increase in c h a r g e / d e l a y with distance. Same as eqn. (2). Gives very low charge at greater distances (over 50 m). A t t e n u a t i o n factor is positive, showing n o physical meaning. A t t e n u a t i o n factor is positive. Very consistent a n d feasible c h a r g e / d e l a y obtained.

0.827835

GHOS-DAE

1

- 1.70373

224.672

-

0.008755

0.383

GHOS-DAE Authors' eqn. (8)

2

-2.59065 -18.0266

3499.92 235.229

-

0.021289

0.837 0.851

-

(3) AMHEN (Ambraseys-Hendron [7]): (3)

V=K(D/Q1/3) "

(4) IS (Indian Standard [14]): V = K(Q/D2/3)

(5) GEN (Davies et al. [8], Birch and Chaffer [9], Attewell [10]): V= KDnO m

(5)

(6) G H O S - D A E 1 (Ghosh-Daemen [6]):

n

(4)

V =. g ( o / o ° 5 )

n e -aD

(6)

TABLE 2 Site constants a n d i n d e x of d e t e r m i n a t i o n as o b t a i n e d for first floor observations Equation

U S B M

n

IS GEN GHOSDAE 1 GHOSDAE 2 Authors' eqn. (9)

m

a

I n d e x of determination

Remarks

C o m p a r a t i v e l y lower charges t h a n t h a t of eqn. (9). Charges are lower as c o m p a r e d to eqn. (9). Very low for lower distance a n d m o d e r a t e l y low for greater distances. Same as eqn. (2). Very low c h a r g e / d e l a y obtained. Very low c h a r g e / d e l a y c o m p a r e d to eqn. (9). A t t e n u a t i o n factor is positive so amplification in place of a t t e n u a t i o n occurs. H i g h e r a n d feasible c h a r g e / d e l a y c o m p a r e d to other equations.

1.38858

591.950

-

-

0.805

1.57704

155.873

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-

0.746

2879.10

-

-

0.861

1.18278 - 1.54555 - 1.11460

155.873 2900.~2~ 61/3.866

0.513999 -

- 0.0066338

0.746 0.860 0.845

-1.67975

3310.25

-

0.0026567

0.862

- 65..2219

1593.64

-

-

LFKH AMHEN

K

-

1.54508

-

0.871

VIBRATION

CONTROL

IN AN OPENCAST

163

MINE

250-

(7) G H O S - D A E 2 [6]: V = K(D/Q'/3)

225

(7)

n e -~D

Where K, n, m and a are empirical constants which can be determined by regression analysis of one or two independent variables.

~ ~

I 200i

.... : Author's equation . : Ambraseys- Hccndron equation

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c 150 >

•

~125 Proposed ground vibration predictors

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100-

I\\ \\

oj

Based on the analysis of field data, two simple predictor equations are proposed here for the prediction of peak particle velocity on the ground and first floor of structures in the vicinity of mining operations. The equations assume that only geometrical spreading causes a decrease in the amplitude of ground vibrations. They are: Ground

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nO

-~ 50Z

~x

e•

25"

0001

o'5 cube

o',

root

scale

;3

1;

2'1

25

distance (D/-~)

Fig. 11. Peak particle velocity versus scale distance for the first floor.

floor

V = n + K(D/~)

(8)

-1

First floor V = n q- K ( D / Q 1 / 3 )

(9)

-1

The empirical constant n is related to the category of parameters which are influenced

by rock properties and geological discontinuties. In contrast the empirical constant K is related to the category of parameters which are influenced by design parameters, including charge weight, distance from the explosion source, charge diameter, delay interval, burden, spacing, subdrilling and stemming 1000

250 225

900 ....

aI 200

USBM equation

: Author's equation : U S B M . equation

o---o LANGEFORS-KIHLSTRDM equation H AMBRASEY-HENDRON equatlon GENERAL TYPE equatlon x----x GHOSH-DAEMEN equation & - ~ A U T H O R ' 5 equation

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t I

E .5 150

700

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o

100

500

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300

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50

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00 01

100'

015 square

0t9 root

scale

113

1=7

2~1

25

d i s t a n c e (DIJ~)

Fig. 10. Peak particle velocity versus scale distance for the ground floor.

00

.

30

50

70 Distance

90 (m} . . . .

110 >

130

150

Fig. 12. Variation in charge/delay with distance for the ground floor. Vibration level = 15 mm/s.

164

v. PAL ROY 1000

900

and first floor. For the ground floor, the suggested equation is compared with the USBM equation, while for the first floor it is compared with the Ambraseys-Hendron equation. The calculated charge/delay from all the predictors are compared in Figs. 12-14 for distances varying from 20 m to 150 m, keeping vibration levels to 15 m m / s and 25 m m / s , respectively.

USBM equa6on LANGEFORS- KIHLSTROM equat;on ~o AMBRASEYS-HENDRON e q u a t i o n =---o GENERAL TYPE equot;on x~ GHOSH-DAEMEN e q u o t l o n & ~ A AUTHOR'S e q u a t i o n --

800

,A

i

7O0

--

600

-~

500

o. 4 0 0

.c o

300

Summary and conclusion

200

100 00

1

30

50

70

90

110

Distance Iml .....

130

150

>

Fig. 13. Variation in c h a r g e / d e l a y with distance for the first floor. Vibration level = 25 m m / s .

length. Tables 1 and 2 list the values of empirical constants as well as the index of determination for all the equations, which were obtained by using the data monitored on the ground and first floor of the structures constructed at the site. Figures 10 and 11 show the peak particle velocity versus scaled distance plot for ground 1000

USBM e q u a t i o n o----o L A N G E F O R S - K I H L S T R O M

900

--

800

H

A N B R A S E Y S - HENORON equation

700

~ ~ ~

GENERAL TYPE e q u a t i o n GHOSH - DAEM EN equation AUTHOR'S e q u a t i o n

equation

Ii i -

600-

500"

/,00'

=o 300.

A simple, new and improved blast vibration predictor is introduced for analysing blast induced vibration data at the West Mudidih opencast project. The proposed equation has one important factor, that is, it gives very consistent, as well as improved, charge/delay values compared to other conventional predictors. The study shows some inconsistency in the G h o s h - D a e m e n model for charge/ delay calculations. The G h o s h - D a e m e n model is not suitable for building structures where relative amplification occurs in place of attenuation. The attenuation relation is not entirely site-specific. Although it depends on geology, it is also heavily dependent upon the blast geometry. The general form of the predictor equation--GEN, eqn. (5)--requires regression analysis of two independent variables and thus is not easily applicable in engineering practice. Moreover it gives lower charges compared to other equations. The Indian standard and Langefors-Kihlstrom equations also give low charges. The equations that we suggest lead to a very simple calculation for charge/delay at any specific distance and vibration level. They are:

Ground floor:

o 200'

100"

061'0

• 30

50

70

90

D i s t a n c e (m} - -

110

J 130

150

->

Fig. 14. Variation in c h a r g e / d e l a y with distance for the ground floor. Vibration level = 25 m m / s .

First floor:

V I B R A T I O N C O N T R O L IN A N O P E N C A S T M I N E

They are simpler than any of the other conventional predictor mentioned in eqns. (1)(7). Vibration readings on the walls and floor of the m u d house showed some interesting features. The value for particle velocity was much less there in comparison to other nearby structures. Significant damping was observed in the m u d house. To study this effect more closely, the distance from the source was decreased and the c h a r g e / d e l a y increased stepwise to see the comparative stability of the structures. At a distance of 10 m from the source and with a m a x i m u m c h a r g e / d e l a y of 483 kg the concrete as well as the brick structures completely collapsed, whereas the m u d house was unchanged, except for some extended crack fines a few feet long. The general applicability of the proposed equation is under review and will appear in a forthcoming publication.

Acknowledgements The present work relates to the part of the investigation sponsored by the D e p a r t m e n t of Science and Technology, Government of India. The author is grateful to Dr. R.N. Gupta, Professor, Indian School of Mines, D h a n b a d for some helpful suggestions during the period of investigation. He would also like to thank his colleagues for their co-operation and suggestions. The author expresses his gratitude to the Director, Central Mining Research Station for his permission to publish this paper. Opinions expressed here are the author's and do not necessarily reflect the views of the Institution to which the author belongs.

References 1 Nicholls, H.R., Johnson, C.F. and Duvall, W.I., Blasting Vibrations and their effects on structures. US Bur. Mines, Bull. 656 (1971), 105 pp.

165

2 Duvall, W.I. and Fogelson, D.E., Review of criteria for estimating damage to residences from blasting vibrations. US Bur. Mines R15968 (1962), 19 pp. 3 Siskind, D.E., Stagg, M.S., Kopp, J.W. and Dowding, C.H., Structure response and damage produced by ground vibration from surface mine blasting. US Bur. Mines R.I. 8507 (1980), 74 pp. 4 Walker, S., Young, P.A. and Davey, P.M., Development of response spectra techniques for prediction of structural damage from open-pit blasting vibrations. Trans. Inst. Min. Metall. (Sect. A, Min. Ind.), 91 (1982), A55-A62. 5 Huh, Ginn, Lee, Kyung Won and Lim, Han Uk, A determination of vibration equation by empirical methods. Proc. Int. Soc. Rock Mech. (1987), 629632. 6 Ghosh, A. and Daemen, J.K., A simple new blast vibration predictor (based on wave propagation laws). U.S. Symp. Rock Mechanics, 24th (1983), pp. 151-161 7 Ambraseys, N.R. and Hendron, A.J., Dynamic behaviour of rock masses. In: K.G. Stagg and O.C. Zeinkiewicz (Editors), Rock Mechanics in Engineering Practice. Wiley, London (1968), pp. 203-207. 8 Davies, B., Farmer, I.W. and Attewell, P.B., Ground vibrations from shallow subsurface blasts. Engineer, 217 (1964), 553-559. 9 Birch, W.J. and Chaffer, R., Prediction of ground vibrations from blasting on opencast sites. Trans. Inst. Min. Metall. (Sect. A, Min. Ind.) (1983), A102A107. 10 Attewell, P.B., Recording and interpretation of shock effects in rock. Min. Miner. Eng. (1964), 21-28. 11 Blair, B.E. and Duvall, W.I., Evaluation of gages for measuring displacements velocity and acceleration of seismic pulses. US Bur. Mines RI 5073 (1954), 21 PP. 12 Duvall, W.I. and Petkof, B., Spherical propagation of explosion--generated strain pulses in rock. US Bur. Mines RI 5483 (1959), 21 pp. 13 Langefors, U. and Kihlstrom, B., Rock Blasting. Wiley, New York (1973), 405 pp. 14 Indian Standards Institute, Criteria for safety and design of structures subjected to underground blast. ISI Bull. IS-6922 (1973). 15 Dowding, C.H., Blast Vibration Monitoring and Control. Prentice-Hall, Englewood Cliffs, NJ. (1985), 297 pp.