Damage threat to sensitive structures of a thermal power plant from hard rock blasting operations in track hopper area: A case study

Abstract

The article deals with the development of controlled blast design patterns and methodologies for excavation of hard rock for foundation work of a track hopper in a running thermal power plant in close proximity of many sensitive structures. Test blasts were conducted with different blast geometries and charge loading patterns at selected locations. The results of the test blasts were analysed to evolve safe controlled blast design patterns and methodologies for completing the total excavation work. The threshold value of vibration for the safety of various structures/foundations present has been taken as 25 mm/s based on the dominant frequency content as well as guidelines from the regulatory authority. The dominant frequency content of the ground vibration waves was in the range of 10–40 Hz. Controlled blast design parameters, namely maximum explosive per delay, size of the blasts and blast geometry were formulated based on established ground vibration predictor equation and nearness of structures concerned from the blasting point. Two blasting zones were classified for safe excavation work. For the first zone within 20 m from the structures, controlled blasting using 32-mm diameter blast holes with small blast geometry was suggested, whereas for the second zone beyond 20 m, controlled blasting with 100-mm diameter holes with lesser hole depth and limited number of holes was suggested. The whole excavation work in the track hopper area was completed safely without causing any damage to the nearby sensitive structures of the running thermal power plant.

Keywords

Controlled blasting thermal power plant blast vibration dominant frequency hard rock excavation

Introduction

The Damodar Valley Corporation (DVC) is a publicly owned agency of the Indian government with responsibility for flood control, irrigation, generation, transmission and distribution of electricity, afforestation, as well as economic and social development in the areas affected by DVC projects. While DVC’s activities are broad in scope, power station development plans have become the dominant focus of the corporation. However, over the past few decades, power generation has gained priority. DVC has more than 7000 MW of power generation capacity (DVC website Available at: www.dvc.gov.in). To cater the growing need of power, DVC has recently commissioned the expansion of its Bokaro Thermal Power Plant having a 500 MW capacity nearby the existing thermal power plant in the Bokaro District of Jharkhand State in India.

At the new expansion plant site, for construction of new track hopper for handling of coal, excavation work of 245 m length, 30 m width (top) and 18 m (bottom) with 14–25 m depth was required to be carried out. During the initial assessment of the strata being excavated for construction of new track hopper, it was observed that hard sandstone strata of 1.2–1.5 m thickness is present below the soil cover all along the eastern side of the required excavation area (Plate 1). Below this hard sandstone bed, about 2.5 m thick shale strata of softer formation is present. Again, below these shale strata, hard rock strata of shaley-sandstone are present. Hence, the excavation of rock layers required drilling and blasting work with commercial explosives available in the market. Near the excavation site, several sensitive structures, namely ash pond, weighbridge, underground tunnel for existing reclaimer hopper, chimney, foundations of transfer point structures, fire-station building, under-construction electric substation and transfer point buildings were also present. The ash pond/ash lagoon was situated at merely 5 m from the boundary of the excavation area in the northern direction, while the foundation pillars for transfer point structures were at 5 m in the southern direction. The other important structure was the underground tunnel for existing reclaimer hopper, situated at 8.5 m at the top and 14.5 m at the bottom of excavation level. The other structures like under-construction transfer point building, fire-station building, electric substation building and weighbridge were situated within 50 m from the excavation boundary. A 275-m-high chimney was situated at 100 m distance from the excavation boundary in the south-eastern direction. A schematic layout plan showing the different sensitive structures nearby the excavation area is shown in Figure 1. Therefore, it was apprehended that the impact of blasting for excavation purpose may damage the existing or under-construction structures and concrete foundations near the site for construction purposes.

In this context, a detailed scientific investigation supported with a number of experimental blasts was carried out and based on the results of the blasts, safe controlled blast design patterns for different blasting zones and methodology for excavation of hard rock were recommended, keeping in view the safety of the nearby sensitive structures and foundation works (CSIR-CIMFR Report of Investigation, 2018).

Damage to structures and concrete foundations due to blasting

Drilling and blasting with commercial explosives is used widely as an economical tool for excavation of rock/ore in construction, quarry and mining projects. However, a part of the explosive energy is always used in the form of elastic waves during rock excavation by blasting. These waves travelling in all directions from the blasting site give rise to ground vibrations, which in excess may cause damage to nearby structures (Ak et al., 2009; Elevli and Arpaz, 2010; Nateghi, 2011; Tripathy et al., 2016). Blast-induced ground vibrations are characterized by two important parameters: the peak particle velocity (PPV) and frequency. The damage potential of ground vibrations is largely quantified either in terms of only PPV (Chae, 1978; Duvall and Fogelson, 1962; Edwards and Northwood, 1960; Esteves, 1978; Langefors and Kihlstrom, 1978) or PPV and its associated frequency (BS 7385-2, 1993; Dowding, 1985; Khandelwal and Singh, 2006; Ozer, 2008; Siskind et al., 1980).

The extent of structural damage produced from blast vibration depends largely on the quantity of explosive charge used, the distance from the blasting site, the properties of the media through which vibrations are transmitted and the various blast design parameters adopted in addition to the characteristic properties of the concerned structure (Dowding, 1985; Siskind et al., 1980). From analysis of a large number of data on blast damage, investigators from various countries have established that the damage produced in a structure could be related to the PPV of ground motion (Duvall and Fogelson, 1962; Edwards and Northwood, 1960). In addition to PPV, the associated frequency also plays a significant role in causing blast-induced damages in structures (Dowding, 1985; Khandelwal and Singh, 2006; Ozer, 2008; Siskind et al., 1980).

According to Oriard (2002), there are some important points to keep in mind to avoid damage to a concrete foundation. If blasting is taking place adjacent to concrete, it means that the concrete is resting on the rock and it is important to limit the rock rupture in order to avoid damage to the concrete. Therefore, in such situations when blasting is to be carried out very near to the foundations of structures or near the structures, the parameters of blasting should be formulated very cautiously after carrying out a series of test blasts supported with precise vibration monitoring at concerned locations and assessing the results of test blasts under different strata loading conditions.

Investigational works

Initially, test blasts were conducted with different hole diameters to assess the impact of charge loading parameters on blast performance as well as on various sensitive structures nearby the excavation site. The ground vibration magnitudes were recorded at different locations and analysed for further designing of the controlled blasting patterns. Based on the results of the test blasts, a sufficient number of blasts were conducted at different locations of the excavation area with different blast design patterns and charge loading parameters. Blasts were conducted using small hole diameter, that is, 32 mm with Jack Hammer drilling machine as well as using large hole diameter, that is, 100 mm with Crawler-mounted wagon drilling machine.

Test blasts with small hole diameter

Three test blasts were conducted using 32-mm diameter holes drilled with Jack Hammer drilling machine. The details of blasting parameters, explosives used and type of initiation systems are given in Table 1.

Table 1.

Details of test blast patterns with small hole diameter of 32 mm.

Sl. no.	Hole depth (m)	No. of holes	Burden × spacing (m × m)	Avg. explosive per hole (kg)	Total explosive in the blast (kg)	Max^m explosive charge/delay (kg)	Initiation system
1.	0.9	21	0.6 × 0.6	0.3125	6.56	2.19	DF^* with cord relays of 25 ms
2.	0.6–0.8	43	0.6 × 0.6	0.1875–0.25	7.50	4.00	DF with cord relays of 25 ms
3.	1.2–1.3	23	0.6 × 0.6	0.375	8.625	3.00	DF with cord relays of 25 ms

Detonating fuse.

Depending on the thickness of the rock bed, the depth of holes varied between 0.6 and 1.3 m. Burden and spacing were 0.6 m each in all the blasts. Small diameter cartridge explosive, 25 mm dia. and 125 g weight per cartridge was used in all the holes. The explosive charge per hole varied between 0.1875 and 0.375 kg whereas the total explosive charge varied from 6.56 to 8.625 kg. The maximum charge per delay also varied between 2.19 and 4.00 kg. Detonating fuse (DF) was used for initiation of explosives in the holes. Cord relays of 25 ms delay were used between rows to provide sufficient delay timing between the row of holes. A view of the blasting face after final connection of holes is given in Plate 2. The total blasting area was covered with conveyor belts and sandbags.

Test blasts with large hole diameter

With a large hole diameter of 100 mm, eight test blasts were conducted at different locations of the excavation site with varying hole depths and blast design parameters. The details of blast design parameters used with different hole depths and their charge loading patterns are given in Table 2.

Table 2.

Details of test blast patterns with large hole diameter of 100 mm.

Sl. no.	Hole depth (m)	No. of holes	Burden × spacing (m × m)	Avg. explosive per hole (kg)	Total explosive in the blast (kg)	Max^m explosive charge/delay (kg)	Initiation system
1.	4.0	18	2.0 × 2.0	11.12	200.00	22.24	Nonel* system, DTH – 250 ms, TLD – 25 and 42 ms
2.	3.5	18	2.0 × 2.0	8.34	150.12	16.68	Nonel system, DTH – 250 ms, TLD – 25 and 42 ms
3.	2.0	69	1.5 × 1.75	2.78	191.82	11.12	Nonel system, DTH – 250 ms, TLD – 25 and 42 ms
4.	3.0	24	1.6 × 2.0	8.34	200.16	16.68	Nonel system, DTH – 250 ms, TLD – 25 and 42 ms
5.	3.0	7	1.6 × 2.0	8.34	58.38	8.34	Nonel system, DTH – 250 ms, TLD – 25 and 42 ms
6.	3.0	7	1.6 × 2.0	8.34	58.38	8.34	Nonel system, DTH – 250 ms, TLD – 25 and 42 ms
7.	3.0	7	1.6 × 2.0	8.34	58.38	8.34	Nonel system, DTH – 250 ms, TLD – 25 and 42 ms
8.	3.0	18	1.6 × 2.0	8.34	150.12	16.68	Nonel system, DTH – 250 ms, TLD – 25 and 42 ms

Nonel: non-electric initiation; DTH: down-the-hole; TLD: trunk-line delay.

The depth of holes in the test blasts varied between 2.0 and 4.0 m with the number of holes varying between 7 and 69. The burden values varied from 1.5 to 2.0 m, whereas spacing varied between 1.75 and 2.0 m. Large diameter cartridge explosive, 83 mm dia. and 2.78 kg weight per cartridge was used in all the holes. Depending on the depth of holes, the average explosive charge per hole varied between 2.78 and 11.12 kg. The total explosive charge varied between 58.38 and 200.16 kg, whereas the maximum explosive charge delay also varied from 8.34 to 22.24 kg. Non-electric (Nonel) initiation system, (DTH – 250 ms and TLD – 25, 42 ms) was used to initiate the explosive charge in the holes as well as to connect one hole to another at the surface. The total blasting face was muffled with conveyor belts and sandbags whenever experimental blasts were conducted very near to the sensitive structures. The view of blasting face in one of the test blasts is given in Plate 3 and muffling arrangement in the same blast is shown in Plate 4.

Ground vibration monitoring near sensitive structures

The ground vibration waves generated due to blasting were monitored using digital seismographs. The sensors of the seismographs were firmly fixed on the ground surfaces. Ground vibrations were recorded in terms of PPV in millimetre per second (mm/s). Depending on the locations of the blasting point and its nearness to the particular sensitive structure, seismographs were placed near the sensitive structure, which was very close to the particular blasting point. Whenever the test blasts were conducted on the northern side of the excavation area, ground vibrations were monitored near the ash pond, weighbridge, electric substation building, fire-station building and in the direction of other structures. However, when the blasts were conducted on the southern side of the excavation area, vibrations were monitored near the foundations of transfer point structures, transfer point building and near chimney. The distances of vibration monitoring points from the blast sites varied from 21 to 249 m. The views of some of the vibration monitoring points near the different sensitive structures are shown in Plates 5 to 8.

Ground vibration results and analyses

The magnitudes of ground vibration recorded during the three test blasts conducted with 32 mm diameter holes varied between 0.684 and 5.94 mm/s with the monitoring distances varying from 42 to 93 m. However, the magnitudes of vibration recorded during the test blasts with 100 mm diameter holes were greater compared with that of 32 mm dia. holes. The magnitudes of vibrations with 100 mm dia. holes varied between 0.540 and 27.1 mm/s. The distance of monitoring points also varied between 21 and 249 m. The maximum magnitude of ground vibration recorded was 27.1 mm/s at a distance of 22 m from the blasting point with associated dominant peak frequency of 83.5 Hz. This was recorded on the floor of the underground tunnel when the blast was conducted with seven holes of 3.0 m depth. The total explosive fired in the blasting round was 58.38 kg, whereas the maximum explosive charge per delay was only 8.34 kg. The details of ground vibration recorded near the different sensitive structures are given in Table 3.

Table 3.

Ground vibrations recorded from test blasts near different sensitive structures.

SL. no.	Structure	Range of monitoring distance (m)	Range of PPV recorded (mm/s)	Maximum PPV (mm/s)
SL. no.	Structure	Range of monitoring distance (m)	Range of PPV recorded (mm/s)	Magnitude (mm/s)	Distance (m)	Frequency (Hz)
1.	Ash pond	25–127	2.13–24.6	24.6	45	33.5
2.	Weighbridge	50–134	2.41–16.0	16.0	50	34.1
3.	Near chimney	103–210	0.696–1.67	1.67	184	11.3
4.	Near fire-station building	41–138	0.540–12.1	12.1	41	238
5.	Near electric substation building	66–139	0.684–3.23	3.23	139	16.4
6.	Floor of the tunnel	21–77	1.55–27.1	27.1	22	83.5
7.	Near transfer point structure	50–249	0.603–2.72	2.72	65	28.4

The underground tunnel and transfer point structures were the nearest sensitive structures situated very close to the excavation boundary. The test blasts near the underground tunnel were conducted with very less number of holes with both 32 and 100 mm diameter holes. However, the test blasts towards the southern side of the excavation boundary, near the transfer point structures, were conducted using 32 mm diameter holes only. Hence, the magnitudes of vibration recorded near transfer point structure are less.

It has long been recognized that there is a relationship between PPV, distance from blast to monitoring location (d) and the maximum instantaneous charge (MIC; Devine et al., 1967). The distance and MIC have been combined to form the parameter known as scaled distance (SD) which is defined in equation 1 as

S D = \frac{d}{\sqrt M I C}

(1)

The actual relationship between PPV and SD varies from site to site and is given in general form as

P P V = a {(S D)}^{b}

(2)

where a and b are site factors. The ‘a’ factor measures the intensity of the seismic energy that is transferred to the ground and is propagated away. It is a function of confinement, explosive density and energy. The factor ‘b’ is related to the geology through which the seismic wave passes and is a measure of the decay of the velocity intensity with distance (Dowding, 1985). These site factors can be determined empirically from a number of blast monitoring records for a particular site. This is usually carried out by transforming equation 2 to equation 3 by taking the log of both sides

\log [P P V] = b \times \log [S D] + \log [a]

(3)

This has the form of the equation for a straight line $(y = m x + c)$ , where m is the gradient relating to site factor b, and c is the intercept relating to the log of the intercept, log[a]. If the regression is reasonable (high correlation coefficient and low standard error), it is possible to predict the likely PPV value given a certain charge weight and distance (Birch and White, 2013).

Based on the above globally accepted empirical correlation for predicting safe quantity of explosive charge per delay for different distances of structures, a site-specific attenuation relation for the site has been developed using the vibration data recorded in the test blasts at different distances. The empirical equation is established correlating the maximum explosive charge per delay (Q_max in kg), distance from blast to monitoring location (D in m) and recorded vibration (V in mm/s) from the test blast data. The regression plot thus obtained is depicted in Figure 2.

The established equation for 95% confidence level is given as

V = 1856 {[\frac{D}{\sqrt{Q_{m a x}}}]}^{- 1.68}

(4)

Coefficient of determination = 0.829

Standard deviation = 0.242

The established equation with good correlation coefficient is in agreement with the form of predictor equations used by various researchers throughout the globe for calculating the safe explosive charge per delay to be used in designing controlled blasting parameters. Based on the established equation (4), the safe values of explosive charge per delay have been calculated for the blasting operations near various sensitive structures.

The potential for damage from blast vibration is strongly dependent on the frequencies present in the vibration along with its magnitude. The vibration standards have set different allowable vibration intensities for different frequencies of vibration. Thus, finding out what wave frequencies and intensities contribute to the overall vibration is extremely important in gaining an understanding of its damage potential. The time histories of a few of the recorded ground vibration data are given in Figures 3 and 4 and their frequency spectra in Figures 5 and 6 respectively.

The dominant peak frequency content of all the recorded vibration waves were calculated using Fast Fourier Transform (FFT) analysis. An FFT is an algorithm that samples a signal over a period of time (or space) and divides it into its frequency components. The FFT analysis was performed over the complete waveform. In simple waveform, not composed of many different frequencies, the dominant frequency may be at the PPV of the waveform. However, in more complex waveforms, the dominant frequency is not necessarily the frequency at the PPV, but at the frequency with the greatest amplitude. The dominant frequency bands for recorded vibration waves in test blasts varied widely between 11.3 and 238 Hz. The peak dominant frequency content calculated from the FFT analysis, in most of the cases, varied between 10 and 40 Hz. The distribution pattern of frequency content of the ground vibration waves recorded from the test blasts is shown in Figure 7.

It is clear from the histogram shown in Figure 7 that in 14 instances, the frequency of recorded ground vibrations varied between 10 and 20 Hz and in 28 instances, the dominant frequency content ranged between 10 and 40 Hz.

A plot of dominant peak frequency component against the magnitude of the recorded ground vibrations is also shown in Figure 8.

Assessment of safe vibration level for nearby sensitive structures

The ground vibration standards prescribed by the Directorate General of Mines Safety (DGMS) and Bureau of Indian Standards are given in Tables 4 to 6. DGMS, Dhanbad, authorized regulatory body in India for mines safety has stipulated the damage threshold values for different types of structures in India based on PPV and dominant peak frequency of vibration waves arising due to blasting activities in mines and quarries. The permissible standards for different types of structures have been arrived by considering the importance of buildings and structures. The buildings of historical importance and multistoried structures are likely to be damaged with low levels of vibration and therefore, permissible standards for them are the lowest. The permissible levels of vibrations for different types of structures are given in Table 4. However, in the case of Indian Standards (IS:6922-1973), the safe values of ground vibrations have been assigned based on the nature of rock strata for the foundation of different structures.

Table 4.

DGMS ground vibration standards (Technical circular number 7 of 1997).

Type of structure	Dominant excitation frequency
	<8 Hz	8–25 Hz	>25 Hz
(A) Buildings/structures not belonging to the owner
1. Domestic houses/structures (Kuchcha, brick and cement)	5 mm/s	10 mm/s	15 mm/s
2. Industrial buildings	10 mm/s	20 mm/s	25 mm/s
3. Objects of historical importance and sensitive structures	2 mm/s	5 mm/s	10 mm/s
(B) Buildings with limited span of life and belonging to owner
1. Domestic houses/structures	10 mm/s	15 mm/s	25 mm/s
2. Industrial buildings	15 mm/s	25 mm/s	50 mm/s

Table 5.

Indian Standard (IS:6922-1973: 4.1.1.1) for safety of structure from threshold damage, the ground particle velocity shall not exceed.

Soil, weathered or soft rock	50 mm/s
Hard rock	70 mm/s

Table 6.

Indian Standard (IS:6922-1973: 4.1.1.2) where monitoring of ground particle velocity by means of suitable instrument is adopted as a means of vibration control.

Soil, weathered or soft rock	70 mm/s
Hard rock	100 mm/s

Although the DGMS-prescribed threshold values of vibrations are more stringent and suggest much conservative values compared with IS:6922-1973, yet, for better safety of the different sensitive structures nearby the excavation site of track hopper area, the DGMS vibration standards have been considered for calculating the safe quantity of explosive charge per delay in designing the controlled blast design patterns.

The analyses of vibration wave signatures clearly revealed that in a majority of blasts, the dominant peak frequency of vibration waves are greater than 10 Hz. The FFT analyses also confirmed the maximum concentration of vibration energy ranging between 10 and 40 Hz. Hence, the safe limit of vibration (PPV) for the safety of various structures/foundations nearby the excavation site has been taken as 25 mm/s as per the DGMS standard given in Table 4.

Safe blasting zones and development of blast design patterns

On the basis of vibration data collected and their subsequent analyses, an empirical equation has been developed for the site. Considering the PPV level of 25 mm/s as the safe and threshold of ground vibration for the safety of nearby sensitive structures, the maximum explosive charge weight per delay to be fired in a blasting round has been calculated and are given in Figure 9.

For example, if we have to calculate the safe maximum explosive charge per delay (Q_max) when our structures are situated at 5 m from the blasting site, we can calculate it using equation 4.

In equation 4, if V = 25 mm/s and D = 5 m, then Q_max = ???

Log (25) = (- 1.68) x Log (D / Q_{m a x}) + Log (1856)

(5)

Equation 5 may be simplified to the form

Q_{m a x} = (D / 12.9867)^{2}

(6)

By putting the value of D for different distances, the safe explosive charge to be fired in the blast can be calculated for any distance of sensitive structure. Here, for D = 5 m, the value of Q_max becomes 0.15 kg.

The safe value of maximum explosive charge per delay when blast is to be conducted at 5 m from the structure is only 0.15 kg, whereas for 10–20 m distance, it is varying from 0.59 to 2.37 kg only. It is also evident that in the excavation area, various sensitive structures, such as ash pond, transfer point structures and underground tunnel are lying within 5–10 m of the blasting point, which need total safety from the blasting. In actual blasting operations, the safe charge weight per delay evaluated from the established equation (4) is distributed in a number of holes drilled to the required depth in a specific pattern and fired at small time intervals. The linear charge concentration, that is, quantity of explosive charge per unit length of blast hole, in the case of different diameters of holes varies largely with the type of explosives in use. Therefore, based on the allowable maximum explosive charge per delay at 5–20 m from the structures, only smaller blast hole diameter can be used for safe blasting operations. However, beyond 20 m from the different structures present at the excavation site, bigger blast hole diameters can be used as the safe values of the maximum charge per delay increase with the distance. The rate of excavation can also be accelerated using a bigger blast hole diameter. Hence, two different blasting zones can be classified for design of controlled blasting at the site as follows:

Blasting zone within 20 m from structures – 32 mm hole diameter;

Blasting zone beyond 20 m from structures – 100 mm hole diameter.

The above blasting zones have been classified based on the hole diameter of the drilling machines. It is evident that the linear charge concentration in larger diameter holes would be very high compared with small diameter holes. This will in turn, enhance the damage probability of the structures located very near to the blasting point. Therefore, within the blasting zone of 20 m, controlled blasting using 32 mm diameter blast holes is suggested, whereas beyond 20 m distance from the structures, controlled blasting using 100 mm diameter blast holes is suggested. The controlled blast design parameters, namely number of holes, depth of holes, charge per hole, and maximum charge per delay to be used for different blasting zones are also suggested.

Blasting zone within 20 m from structures

Some sensitive structures were present within 5–10 m at the two extreme ends, both on the northern and southern sides of the track hopper area. On the northern side, the ash pond was located within 5 m from the required excavation line, whereas on the extreme southern side, transfer point structures were located within 5 m. At both the extreme ends, proper attention was needed to control ground vibration, particularly when blasting is to be conducted within 5–10 m from the structures. Hence, line drilling of holes using 100 mm diameter was suggested to reduce the propagation of ground vibration towards those structures to some extent. In all the blasts, complete muffling of holes using conveyor belts and sandbags was suggested to restrict the flying fragments on the blasting face only.

Controlled blast design pattern near the ash pond (northern side)

The required excavation depth in this area near the ash pond was about 2.5 m. Hence, it was recommended to initially conduct controlled blasting at the location nearest to the ash pond side with ‘Cautious Blasting’ for trench cutting as shown in Figure 10. After the trench cutting, hard rock excavation using controlled blasting were suggested in an opposite direction to the ash pond.

Controlled blast design pattern at southern end

On the southern side, transfer point structures were located very close to the excavation line. Therefore, initial trench cutting having 4–5 m width using cautious blasting were suggested along the end portions. The suggested process of excavation and the controlled blast design parameters for cautious blasting are shown in Figure 11. Line drilling of 6–8 m hole depth using 100 mm blast hole diameter was suggested all along the southern portion of the excavation boundary to reduce ground vibration.

Blasting zone beyond 20 m from structures

Beyond 20 m from the sensitive structures of the plant, use of 100-mm diameter blast holes was recommended, but only after obtaining clear-cut free faces from the cautious blasting. Within the blasting zone of 20–50 m, the blast hole depth used was 2.5–4.0 m with burden and spacing value of 1.0–1.25 m and 1.25–1.75 m, respectively. It was observed that by reducing the burden and spacing values, explosive charge per hole can be reduced significantly to contain ground vibration within safe limits. However, within the blasting zone of 50–100 m, blast hole depth up to 5.0 m have been used with burden and spacing values of 1.5 and 2.0 m, respectively. The explosive type used for 100-mm diameter holes was 83-mm diameter cartridge explosive, 2.78 kg weight per cartridge. In order to obtain safe values of explosive charge as given in Figure 7, the cartridge explosives were cut into smaller pieces. Nonel (shock tube) initiation system was used in all the blasts with muffling arrangements using conveyor belts and sandbags to prevent flyrock.

Conclusion

The extent of damage produced in a structure due to blast-induced ground vibrations depends in a complicated way on the ground motion parameters, blast design parameters and the type of geological strata in addition to its inherent strength and dynamic properties. The characteristics of ground vibrations, namely magnitude and frequency, need to be considered carefully in determining the safe PPV level for nearby sensitive structures. For safe excavation work near various sensitive structures in a running thermal power plant, a number of test blasts were conducted with different patterns and explosive charges. Based on the results of test blasts and analyses of vibration data, specific design patterns were suggested for different distances. The controlled blasting patterns and methodology helped in successful excavation of rock in the track hopper area in the vicinity of various sensitive structures of the running thermal power plant. A few conclusions are drawn as follows:

There is a direct impact of the quantity of maximum explosive charge per delay over the magnitude of ground vibrations. The more the maximum charge per delay, the more the magnitude of vibration. The magnitudes of ground vibrations are less with 32 mm diameter blast holes compared to blasts with 100 mm diameter.

The dominant peak frequency content of the recorded vibrations predominantly ranged between 10 and 40 Hz. Hence, the safe limit of vibration (PPV) for the safety of various structures/foundations in the excavation area has been taken as 25 mm/s.

A site-specific empirical equation has been developed with good correlation coefficient and accordingly, the values of maximum explosive charge per delay to be fired in the blasts were estimated for designing of controlled blasting parameters.

Two blasting zones were classified, one within 20 m from the structures while the other beyond 20 m. For different blasting zones, blasting patterns and methodologies to excavate the hard rock were formulated.

Within 20 m from the structures, controlled blasting with only 32-mm diameter blast holes was suggested. However, beyond 20 m, use of 100-mm diameter holes with lesser depth and limited number of holes was suggested.

The empirical equation developed from this study is expected to be useful for estimation of preliminary safe maximum explosive charges for rock excavation in similar geological conditions in the absence of site-specific blasting studies. The equation can be further improved to accommodate the actual site conditions by collecting the ground vibration data during actual blasting operations.

Footnotes

Acknowledgements

The authors are thankful to M/s S K Samanta & Co. Ltd for their kind support to the study. They are also thankful to Dr P. K. Singh, Director, CSIR-CIMFR for his kind permission to publish this research work. The ideas and findings mentioned in the paper are those of the authors and not necessarily of the organizations to which they belong.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship and/or publication of this article.

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