The noise is now being recognised as a major health hazard; resulting in annoyance, partial hearing loss and even permanent damage to the inner ear after prolonged exposure. The problem underground is of special importance because of the acoustics of the confined space.
The ambient noise level of the underground mining area is affected by the operation of the cutting machines, tub/conveyor movement and blasting of the coal. The movement of coaling machines and transport units-conveyor, tubs and transfer points caused audible noise which becomes disturbing underground because of the poor absorption by the walls.
Noise Pollution due to Mining Activities:
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The most noise generating equipment underground are the haulage, ventilators-main, auxiliary and forcing fans, conveyor transfer points, cutting and drilling machines. The ambient noise level due to different operations in underground mines varies within 80-104 dBA. In a mine of Singareni Collieries Company Ltd.,—Bihar, the noise level near fan house, conveyor system shearer and road headers was reported to be within 92-93 dBA.
The values increased in many Indian mines because of poor maintenance of the machines and exceeded the permissible limit of 90 dBA for 8 hours per day exposure. The transfer points of the coal underground were the main point of the noise menace. The result of a noise survey for a coal mine conducted by Director General Mines and Safety (DGMS) is summarised in Table 13.1, which indicates noise over 90 dB by the drills, breaking and crushing units and transport system underground.
The mechanised mines have lower noise problem in comparison to the old conventional mines operating with haulage and coal cutting machines. The results (Table 13.2) covering wholly manual, partly mechanised with cola cutting machines and partly mechanised with SDL loading, showed reduction in the noise level underground.
Noise Pollution due to Blasting:
The blasting underground cause’s high frequency subaudible noise measured in terms of air over pressure. The magnitude of air over pressure was found to the 164 dB (1) at 30 m distance reduced to 144 dB (1) at a distance of 70 m.
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The total noise menace due to blasting underground is the result of the audible and subaudible noise. The subaudible noise responsible for vibration causes vibration of the surface features and in case of thin overburden cracks in surface structures. The societal reaction of Jharia town development forum over blasting forced the pick mining in some of the situations. The failure of a large number of houses in October 1998 was reported to be because of the blasting underground.
The reaction of blasting is reported in the following forms:
1. Damage of old structures due to vibrations.
2. Public nuisance vis-a-vis disturbance of sleep.
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3. Disturbance of sewerage and water supply line.
The amplitude of vibration due to blast wave was observed to be reduced with increase in the height of the building and hence drop in the level of nuisance in the upper floors. The investigation in some of the mines revealed that in case of machine cut, the blasting in the lower section generated more vibration than that of the upper portion. The restriction of total charge was essential to minimise the vibration due to blasting underground. The P5 of explosives generated low vibration in comparison to P3 grade of explosives.
The noise control measures in general are categorised in three groups’ viz., personal protective measures, engineering control measures and administrative measures. The engineering control measures are the most effective as they are based on sophisticated techniques like Retrofit approach for installation of noise control treatment on mining equipment.
Designing of inherently quite mining equipment is also included in this technique which aims to control and reduce the noise emission successfully. The preferred cost effective system for the underground mining has been the personal protective system ear muffs for the operator of the noise producing units.
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Air Blast and Impulsive Noise from Blasting:
Air blast is a measure of explosive energy inefficiency in series of blasting round. Both these problems are associated with each other and jointly or singly can cause structural damage such as the breaking of windows of the houses or of the equipments.
Noise, which is originated by airblast, causes discomfort and gives rise to complaints from those living close to the mines. In Indian context, as the opencast mines are closely situated near the human habitations, the ‘noise’ from blasting has become a social issue of greater concern.
Factors resulting into an air-blast and noise are:
1. Overcharging of blastholes.
2. Poor stemming.
3. Uncovered detonating cord.
4. Improper blast geometry.
5. Variation of burden on vertical front row blasthole alongside high and/or shallow-dipping face.
Because of premature bursting of explosion gases high levels of noise, airblast and/or flyrock occur.
6. Atmospheric temperature, wind velocity, direction and pressure—altitude relationship e.g., clouds/fogs can also cause reflection of detonating pressure wave back to the ground level at some distance.
7. Stemming material, by using coarse, angular material for stemming can reduce air blast.
8. Orientation of benches right at the inception of the mine taking general wind direction. This is because velocity of wind influences to a greater extent on the propagation of the sound.
9. Temperature inversions can have a disturbing effect on airblast-sites close to the blast epicenter. These experience little response whereas those at greater distance may be severely disturbed, when the air temperature increases with altitude; the speed of sound is also increased. This effect deflects the propagating wave front in the direction of the ground and the sound waves are ‘bent’ downwards.
10. Firing pattern may also cause an increase in airblast. Experimental work conducted have shown that airblast levels were atleast 6 dB higher perpendicular to the firing pattern rather than parallel due to blasthole reinforcement. Furthermore, if the direction of initiation was towards the affected location, then increases of 10, 15 dB were possible.
These can be minimised by:
(i) Maximum of two holes in the front row to be fired on the first and subsequent delays:
(ii) Selection of burden and spacing and blastholes, delay intervals and firing sequence to avoid reinforcement in critical directions.
Noise Study Conducted in Some of Coalfields:
The noise study sponsored by the Department of Coal in some of the coalfields has revealed that-
(i) Blasting;
(ii) Operation of heavy duty earth moving machines;
(iii) Crushing;
(iv) Screening; and
(v) Loading operations pushed up the noise level as high as 150 dB in place of the permissible limit of 90 dB in the day time and 40—45 dB for night time in and around residential areas.
The noise pollution caused hearing loss and affected the mental and physical health in many ways. It has also physiological effect such as increase in blood circulation rate, heart beat rate, elevated blood cholesterol and gastric secretion. The noise interfered sleep and forced the use of sleeping pills.
Because of the problems associated with the noise, efficiency of the individuals dropped, time required to perform a task extended and the quality of the work lowered. The noise was likely to reduce the accuracy of work rather than the total quantity.
The impact of noise is dependent on the following factors:
1. Frequency and intensity of noise.
2. Band width of noise.
3. Duration of exposure to noise during a day.
4. Number of years of working day exposures.
The audiogram showing progressive loss of hearing amongst the miners and others is shown in Fig. 13 2.
The auditory effects were of three types:
(i) Temporary threshold shift (TTS);
(ii) Noise induced permanent threshold shift (NIPTS); and
(iii) Acoustic trauma.
The operators of the heavy earth moving machinery and the inhabitants in the close vicinity of dumper roads or crushing plants developed noise induced permanent threshold shift (NIPTS) due to prolonged exposure while damage occurred even with single intense exposure to blasting noise. The intensity of the noise defined as dB was dependent upon the pressure on the ear membrane over and above the atmospheric pressure.
It is defined as follows –
dB = 20 Log10 (P/Po)
Where,
P = over pressure in pounds/sq., inch
Po = Reference pressure (2.9 × 10-9 Psi)
The impact of air blast to individuals was based on age, state of health posture, position and frequency of event counts. The information generated in USBM indicated that a well mounted glass window could stand 168 dB while the safe limit for a poorly mounted window was only 15 dB.
The recommended safe limit for different conditions was defined as follows:
1. 134 dB with 1 Hz high pass system
2. 133 dB with 2 Hz high pass system
3. 129 dB with 5-6 Hz high pass system
4. 105 dB when the event did not exceed 2 second duration.
The level of noise generated in different working places of the opencast mining in some of the Indian mines is shown in Fig. 13.3. The observation revealed noise level with most of the machinery above threshold limit over the total operation cycle. The maximum permissible level of noise for 8 hours exposure per day for different countries is summarised in Table 13.4.
The noise menace is increasing every day with the use of heavier earth moving machines. The roaring noise by thundering 170 tonne dumpers and large diameter drills disturbed large area around their zone of operation. The noise emission with different operation in opencast mines is summarised in Table 13.5. The disturbance level in general exceeded the threshold limit of 90 dB.
The noise level with the deep hole blasting and the pressure wave at wide range of frequency was transmitted in the atmosphere. Loss of hearing and damage to the surface structures were the result of the impulse noise and air over pressure. The noise level in the draft proposal for coal mining area was recommended below 75 dBA during 6 AM to 9 PM and 70 dBA for 9 PM to 6 AM.
The Ministry of Environment and Forest in its notification No. GSR 1063 (E) of December 1989 specified standards for the noise level in industrial area, commercial area, residential area and silence zone separately for day and night time, highest figure being 75 dBA in the industrial area during day time. For the coal mining area, the permitted noise level by the DGMS is 90 dBA.
As the noise pollution with different mining operation was much above the International threshold limit and higher than even the suggested Indian standard, the following corrective measures were suggested:
1. Suitable selection of equipment with inbuilt effective silencer.
2. Proper mounting of the system to reduce shock waves.
3. Use of sound insulated enclosures or enclosures padding.
4. Creation of boundary walls or green tree belt.
5. Provision of ear plugs to the operators.
6. Use of suitable delay and burden for heavy blasting to reduce impulse noise.
Less than 25 per cent of the explosive energy was utilised in rock breaking during opencast mining while the rest was converted into shock waves traveling through air or ground, causing noise, flying of rocks or vibration of ground structures. Heavy blasting in overburden and thick coal bed often caused severe ground vibration and affected the safety of the surface structures.
Improper heavy blast design resulted in the following damages:
1. Failure of natural slopes of the working site.
2. Damage to nearby building or structures.
3. Severe over break causing damage to pit wall.
4. Damage to men and materials due to fly rock.
The ground vibration has resumed one of the severe menaces, major source of structural damage and public complaints. The effect of vibration on human responsive system was indicated in terms of peak particle velocity with the following level of response (Table 13.6).
In subsequent studies, frequency spectrum analysis was found to be equally important for tall structures. The sensitive and delicate instrumented panels were found to be more sensitive to amplitude then peak particle velocity. For example – the peak particle velocity was found to amplify with increase in the height of the structures.
It increased by 10 times that of ground level at 52 m height while the frequency dropped from 35 Hz to just 1 Hz for the same condition. The safe limit for different types of structures against frequency and peak particle velocity for different countries is summarised in Table 13.7.
The ground vibration may be reduced to the permissible limit by improved blasting pattern, proper selection of delay detonators, proper control over the charge weight, spacing of holes and burden. The vibration reduction with the delay control in blasting is shown in Fig. 13.4a according to which the level of vibration with 25 Ms decreased to 20 per cent that of 50 Ms at 50 m distance.
Similarly low frequency waves caused more severe damage to the houses in comparison to that of a long wave (Fig. 13.4b). The walls develop x cracks with vibration of the foundation (Fig. 13.4c) leading to their instability. The other dimension of the blast vibration—lift, acceleration, shear and settlement cause different types of damages (Fig. 13.4d) to the buildings.
Noise Monitoring:
The surface mines generate noise from blasting, excavation and transport while the major noise sources from underground mines are the ventilation fan, haulage, conveyor, drilling and blasting and coal cutting machines. The noise is measured on a logarithmic scale and expressed in decibels dB. In view of its response to the human ear, the scale is adjusted to a weighted average dBA.
In case of blasting, even the low peak particle velocity with higher frequency caused perceptible to unbearable response to the people around. For the prediction of structural response of a building to the vibration. Fast Fourier Transform (FFT) method is a simple approach. It gives an indication of the vibration limit likely to damage or disturb a building for any blast design.
The dimension of blasting are the ground vibration manifested in form of peak particle velocity, acceleration, dynamic strain and dynamic pressure. The noise menace associated with blasting is the noise and air over pressure; more damaging in case of underground mining because of the dimensional constraints. The conventional sciesmographs used for monitoring of the vibration were heavy in weight and practically non portable. The system has been improved to monitor the following parameters with better precision.
The land disturbance is characterised by the type of sciesmic waves, wave parameters, geometric attenuation and nature of the ground mass and the building material. The data logger development is an effort in the direction to monitor all the damaging parameters associated with blasting underground or on the surface. The data trap model of the same has 12 channels with the provision for monitoring the peak particle velocity dynamic strain and pressure in addition to velocity of detonation.
The ground movement is monitored by a sciesmography computerised to analyse the recorded signals. The acceleration sciesmographs are based on the difference of potential generated by piezoelectric crystals under force. The recorders are used for the visualisation and amplification of the signals coming from the sensors. The sciesmographic system is an analog or digital instrument to reproduce and visualise the signals.
Noise known as a major health hazard is spot specific in case of underground mining where the operators are exposed to continuous background noise while high level short duration noise peaks are common in case of surface mining. The human ear is most sensitive in 2 to 5 KHz range and less sensitive in high and low frequencies.
The sensitivity is more pronounced at lower sound pressure level in comparison to that of the high level. A convenient and practical way of assessing the risk of hearing damage is with the aid of the following instruments.
The instrument with microphone, input amplifier, weighting network, RMS rectifier and a display meter responds to sound like that of a human ear.
The light weight assembly with LCD indicator and time weighting dynamic characteristic weighting has the following peripherals:
1/3 octane band meter
Calibrator
Frequency analyser
Data Analyser
The frequency weighting in the latest system is specified as ± 2 dB in the range of 63 Hz, ± 1.5 Hz in the range of 250 Hz and 1 khz, ± 3 dB in the range of 4 kHz and ± 5 dB for 8 kHz. The instrument has setting for 30-80 dB, 50-100 dB, 80-130 dB, and 50 dB on each step with over and under range indication.
The noise dose meter is used to measure short and long duration accumulated noise dose as percentage of maximum permitted dose, assessment of hearing loss risk, indication of allowable noise dose, exposure time and warning of impact and continuous noise hazard.