A project with the aim to ascertain safety of the Sri Vakkira Kaliamman temple from the quarry blasts has been carried out at Thiruvakarai granite quarry in Tamilnadu. The work also involved design of a safe blasting method to prevent potential damage to the temple from future quarry blasts. The temple is about 2000 years old and has significant historical importance.

The exploding population and rapid growth of civilization is not only a concern for living but also challenges the exploitation of natural resources due to mushrooming of dwellings and habitats near the mining/quarry operations. The large scale application of explosives for breaking or splitting rock at the quarries have become a serious threat to the environment. The ill effects arising out of explosive application includes blast induced vibrations, air-overpressure, fly rock, noise, dust etc. The most common complaints from the people living near quarries were damage to the structures from ground vibrations and fly rock. This situation warrants meticulous planning and controlled blasting techniques to be employed for quarrying near critical structures. Needless to mention that many quarries with good deposit have been abandoned due to lack of prudent blasting practices. In view of this it is imperative that scientific methods should be adopted in all quarrying operations including monitoring and assessment of blast associated environmental hazards.

A case study has been presented here with a view to enumerate the importance of scientific studies in quarries. The Tamilnadu Minerals Limited (TAMIN) is operating a black granite quarry at the Thiruvakarai village, Vanur Taluk, Villupuram district located about 30 km from Tindivanam town, and 160 km from Chennai city in Tamilnadu. The quarry is said to be operating since last 15 years with an average monthly production of about 20 to 30 m³  per month.

Considering the proximity of the Sri Vakkira Kaliamman temple located near the quarry site the management of TAMIN approached the National Institute of Rock Mechanics (NIRM), Kolar Gold Fields to take up studies to monitor the ground vibration  produced by quarry blasts and  to ascertain the safety of the temple.  The Granite Mining Cell (presently Department of Dimensional Stone Technology) of NIRM carried out detailed field studies and monitored the blast vibrations. Predictor equations were derived and safe vibration limits were established. Recommendation were made for controlled blasting for future blasts.

Apart from the quarry office the only important structure near the quarry is the Sri Vakkira Kaliamman  temple located  at a distance of 100 m from the present quarry operations.  The temple is said to be about 2000 years old and has a few statues carved out of stones. A village called Thiruvakarai is also located 400 m away from the quarry consisting of few hutments with tile roof and a few single storey RCC roof buildings. The main concern of this study is the safety  of the Sri Vakkira Kaliamman  temple.

BLAST MONITORING

Blast vibrations were recorded with a seismograph (S-6, US make) and vibration meter (SMU 31, German make). The seismograph S-6 is a microprocessor-based instrument, which receives analyses and displays/prints peak particle velocities, peak sound pressure levels and frequency summary tables. Each record furnishes peak values in three orthogonal directions and their vector sum. It also gives relative energy content for different frequency bands. Some other additional information like blast number, date and time of the blast are also given in the printouts The vibration meter is an indicating type and peak values are to be noted.

The size of the blast was small compared to the ore mining production blast. The nature of blasts include block splitting, toe blasting, boulder blasting, ramp making, overburden removal, other developmental work etc. Drilling of holes was done by jack hammers of 32 mm diameter with hole depth varying from 0.75 to 1.5 m. In total 16 blasts were monitored. The number of holes per blast varied from 3 to 60. The maximum charge per delay varied from 0.25 to 13.75 kg. The burden and spacing varied widely depending upon the nature of blast. Holes were charged with Special gelatin 90% and Gunpowder. The powder factor varied from 0.06 to 0.277 kg/m³. The details of blast parameters are given in Table1.  

For all the blasts, vibration levels were monitored and measurements were taken at 3 different locations simultaneously. For every blast monitored, one instrument was kept near the temple and the rest in the predetermined positions established in consultation with the TAMIN quarry officials.

Table. 1 Details of trial blasts at Thiruvakarai Granite Quarry of TAMIN

 

B = Burden   S = Spacing   St = Stemming length          MCD = Maximum Charge per delay

TC = Total Charge             PF = Powder Factor

Particle Velocity

Particle velocity is the velocity at which the ground vibrates. It is measured in millimeters per second. Peak particle velocity has been accepted as an important indicator of structural damage. It depends mainly on the maximum charge per delay, the distance from the blast to the measuring point and the characteristics of the intervening medium. For a new area in which the seismic transmission characteristics are unknown site constants are determined by monitoring the ground vibration at different distances for known blasting parameters. The details of blast monitored with reference to the sensor locations at critical structures are given in Table 2.

Table 2. Measured values of peak particle velocity at different distances and maximum charge per delay.

 

MCD  Maximum Charge per Delay              PPV  Peak Particle Velocity

Estimation of Peak Particle Velocity

The US Bureau of Mines has developed a mathematical model that relates peak particle velocity, charge weight and distance. The formulation is

V= K (D/ÖQ)^b 

Where V = peak particle velocity

        D = radial distance from blast to monitoring station (m)

        Q = maximum charge per delay (kg)

        K and b are site constants.

Figure 1 shows a plot of peak particle velocity against square-root scaled distance on a log-log graph. The square root scaled distance is the distance divided by square-root of the maximum charge per delay. A best fit curve gives a straight line while the value of K is given by the intercept at a scaled distance of unity and b by the slope of the line. Thirty three sets of data were used for regression analysis. The following predictor equation was derived with a correlation coefficient of 0.8

V= 93 (D/ÖQ)^-0.972     50% Confidence level        (1)

V= 241.71 (D/ÖQ)^-0.972  95% Confidence level        (2)

When a suitable instrument is not available, vibration levels can be estimated from this empirical equation by substituting the value of the maximum charge per delay (Q) and the distance (D). Alternatively, squared-root scaled distance, is calculated and the peak particle velocity may be estimated from Figure 1.

Figure 1 Peak particle velocity Vs Scaled distance

Frequency of Ground Motion

The rock/soil cover at Thiruvakarai has a dominant effect on the frequency, which is ranging from 19 to 107 Hz (Figure 2a and 2b). Considering the fact that all single and doubled storey residential buildings have their natural frequencies in the range of 19 Hz to 151 Hz, the influence of blast induced frequency that may cause damage was also examined for arriving at the safe limits of blast vibration.

Figure 2a. Energy content in different frequency bands for sensor A

Figure 2b Energy content in different frequency bands for sensor B

SAFE LIMITS  AND CONTROL OF GROUND VIBRATION

Various codes and standards have been prescribed for ground vibration limits in different countries. The safe limits of blast vibrations are prescribed taking into consideration the vibration characteristics, the type and condition of the structures coupled with the previous experiences on monitoring, analysis and control of ground vibration in different conditions. The most practical method of containing ground vibration is to limit the maximum charge per delay. The use of delays in blasting permits to divide the total charge into smaller charges, which are detonated in a predetermined sequence at specified intervals.

In order to arrive at the safe limits of vibration for the Sri Vakkira Kaliamman temple, the recommended permissible peak particle velocity (mm/s) at the foundation level of structures as per Directorate General of Mines Safety (DGMS) was used (Table 3). Since the Sri Vakkira Kaliamman temple is very old and sensitive historical structure, a peak particle of 2.0 mm/s was recommended as permissible limit of vibration level.

Considering 2 mm/s as safe limit of vibration, the maximum charge per delay was computed using the equation 2 for different distances (Table 4) which may provide adequate protection against blast vibration damage to the temple. 

Table 4 Recommended maximum charge per delay for different distances from Sri Vakkira Kaliamman temple to the blast site at 95% confidence level.

 

Radial distance from the blast site to temple (m)	Recommended safe maximum charge per delay (kg)
55	0.2
65	0.2
75	0.3
85	0.4
95	0.5
105	0.6
115	0.7
125	0.8
135	1.0
145	1.1
155	1.3
165	1.4
175	1.6
185	1.8
195	2.0

 

CONCLUSIONS

Considering the fast depletion of dimensional stone deposits due to rapid production and technological advancements, quarrying in the vicinity of habitats cannot be averted in all. In order to combat the adverse environmental hazards that may arise from explosive application in quarries, meticulous planning and prudent designs of extraction shall be employed. Monitoring and assessment of the extraction process using appropriate scientific equipments/instruments shall pave way for optimum and safe exploitation in the vicinity of critical structures.  In the case study discussed, a peak particle velocity of 2 mm/s was established as permissible limits for the safety of Sri Vakkira Kaliamman temple. Predictor equation was derived for the estimation of peak particle velocity and maximum charges per delay for corresponding distances were suggested for safe quarrying.

The work was carried under a sponsored project at Thiruvakarai granite quarry, Tamilnadu Minerals Limited. The authors are thankful to the Director, Dr. M.K. Lakshminarayanan AGM (Production), and Mr. Kannadhasan, Divisional Manager for their interest and co-operation in conducting this study. We are also thankful to the staff of  Thiruvakarai quarry and others who directly or indirectly helped for the successful completion of the project. Last but not the least we sincerely thank  Mr. S. Dominic Xavier who equally toiled with us during field investigations and helped in preparation of this report.

1. C.J. Konya and E.J. Walter, ‘Surface Blast Design’ Prentice Hall, 1990.

2. ISRM Suggested Method for Blast Vibration Monitoring, Int. J. Rock Mech. Min. Sci. & Geomech. Abstr. Vol. 29, No. 2, pp. 143-156, 1992.

3. D.E. Siskind, V.J. Stachura, M.S. Stagg and J.W. Kopp, ‘Structure Response and Damage Produced by Airblast from Surface Mining’, US Bureau of Mines, RI 8485/1980.

4. A. Rajan Babu, D.S. Subrahmanyam & R. N. Gupta, 'Final Report on Ground Vibration monitoring at Thiruvakarai Granite quarry, Tamilnadu Minerals Limited, Tamilnadu.