Investigation Of Firing Patterns On Fragmentation In An Indian Opencast Limestone Mine

For efficient blasting practice the parameters incorporated in a blast design can be categorized as: rock parameters, explosive parameters and blast-design parameters. Blasting results are sensitive to the complex interactions between these three parameters¹. At the field scale, there is no scope for the blasting engineer to alter the rock parameters, however the explosive and the blast-design parameters can be modified so that the blast energy is used more efficiently. This in turn helps to improve the fracturing, piling and fragmentation characteristics and thus the overall economy of the blasting operation.

One of the most fundamental and important controllable parameters is the firing pattern in multi-row blast rounds². Nominal drilling patterns with a burden ‘B’ and spacing ‘S’ can be subject to radical modification depending on the firing sequence, which brings into focus the role of effective burden ‘Be’ and effective spacing ‘Se’. The effective burden and spacing affect the fragmentation, displacement and swelling of the rocks³, and thus the role of the firing pattern is absolutely crucial for the success of the blasting operation^4,5.

It is in this context that the following paper investigates the influence of four different firing patterns on fragmentation and fragment size distribution at an Indian opencast limestone mine.

OBJECTIVES OF THE STUDY

The main objectives of the study were:

• To conduct field-scale blasts using four different but prevalent firing patterns.
• To ascertain the influence of the four different firing patterns and two different blasthole diameters (150mm and 100mm) on fragmentation.
• To analyze quantitatively the fragmentation and fragment size distribution in the blasted rock piles as a result of the different firing patterns.

BRIEF DESCRIPTION OF THE MINE

Full-scale blasts were conducted at an opencast limestone mine operated by Century Cements Ltd in Raipur, India. The total leasehold area of the mine is 237ha, of which the mineralized area represents some 126ha. The annual output of the mine is
1.5 million tons.

The limestone deposit belongs to the Chattisgarh basin of the Precambrian Vindhyan Group, which is largely made up of horizontal, thickly bedded stromatolitic limestones. Other associated rocks include dolomitic limestones and shales. The overburden comprises hard murrum (loose overburden) and clay with an average thickness of 0.5–3m. Figure 1 shows a representative borehole section through the deposit. The 22–24m thick limestone is worked in 6–9m high benches.

Drilling was carried out by down-the-hole drill rigs to provide both 100mm and 150mm diameter blastholes. Prilled ANFO explosive with a booster charge was used for primary blasting, with detonation carried out by means of an Exel shock tube bottom-initiation system. Representative diagrams of the charged blasthole sections for the 100mm and 150mm diameter holes are illustrated in figures 2 and 3 respectively.

The salient physio-mechanical properties and chemical composition of the limestone are shown in table 1.

METHODOLOGY OF STUDY

In order to fulfil the stated research objectives, an imaging technique was used whereby views of the blasted rock pile were captured by high-resolution digital camera. These images were then analysed using suitable computer software to provide a measure of the fragment size distribution in the rock pile. With the widespread use of computer hardware and software, the cost of such imaging techniques is relatively low and the characterization of fragment size can be carried out quickly, precisely and with greater ease^6,7,8.

In this study images were captured during the excavation of the entire rock pile from front to back. Depending on the size of each blast, some 20–30 good-quality high-resolution photographs were captured for analysis. For calibration purposes, a red-coloured square scale marker measuring 20cm by 20 cm was placed at the centre of each image frame.

A wide range of computer software is commercially available for the analysis of such images, but Fragalyst Version 2.0, developed by the Central Mining Research Institute, Nagpur, in collaboration with the Wavelet Group, Pune, was used in this study as it represents an indigenous system which is both cheaper and proven under conditions in India.

After enhancement and calibration of the captured images, the software performed an edge-detection function to demarcate the boundaries of fragmented rocks as they appear in the rock pile. The edges detected by the software were observed on a computer screen and corrected where necessary by means of edit network functions. On completion of the edge-detection function the images were subjected to further analysis to generate a typical Rosin-Rammler distribution. This provides the entire range of fragment sizes (with percentages) present in a rock pile, from which the mean fragment size (MFS), coarse fragment size (K95), maximum fragment size (K100) etc, and the uniformity index, of the rock pile can be obtained. In this way, fragment size characterization was conducted in a quantitative manner for all the rock piles generated during the study.

RESULTS AND DISCUSSION

As already stated, four different types of firing pattern were implemented in the field-scale blasts to investigate their influence on fragmentation. The firing patterns used were: ‘skewed V’, ‘in line’, ‘diagonal’ and ‘extended V’.

Results for blast designs using a 150mm blasthole diameter

The field observations and the results of the four blasts, all conducted on the same limestone bench, are tabulated in table 2. The drilling and firing patterns for the four blasts are shown in figures 4–7.

Perusal of table 2 reveals that the blast-design parameters for all four blasts were almost identical. Also, since all the blasts were conducted on same limestone bench with similar explosive, the differences in the fragmentation results can clearly be attributed to changes in the firing pattern alone.

The results in terms of MFS, K95 and K100 were most favourable for the ‘extended V’ firing pattern. The ‘in-line’ pattern yielded poor results, whereas the ‘skewed V’ firing pattern appeared to be better than the ‘diagonal’ pattern. Figure 8 presents the cumulative passing percentages at different screen sizes for the four firing patterns under investigation.

These quantitative results can be qualitatively ascertained by some of the images captured in the field, as shown in figures 9–12. Figure 9 shows a fine, uniformly fragmented rock pile created using the ‘extended V’ firing pattern. Figure 10 shows generally good fragmentation with some large sized boulders in the rock pile produced using the ‘skewed V’ pattern. On the other hand, figure 11 clearly reveals the occurrence of large to very large fragments in the rock pile created using the ‘in-line’ firing pattern, whereas figure 12 shows the occurrence of a few coarse fragments in the rock pile produced using the ‘diagonal’ firing pattern.

These quantitative and qualitative results highlight the influence of the firing pattern on the fragmentation. A change in the firing pattern also results in a change to the Se (effective spacing) to Be (effective burden) ratio. For blasts one and three this was the same at 2.16, for blast two it was around 1.5 and for blast four it was 4.0. With an increasing Se to Be ratio the blast energy and the stress developed by the blast is distributed more evenly and uniformly, which improves the degree of fragmentation⁹.

Another noteworthy result — even though the Se to Be ratio was the same for blasts one and three — is that blast number one, fired using the ‘skewed V’ pattern, yielded better results in comparison to blast number three which was fired using the ‘diagonal’ pattern. This can be explained on the basis that in-flight collisions among the broken rock fragments are increased in a ‘V’ firing pattern in comparison to ‘diagonal’ firing. These in-flight collisions were seen to be of paramount significance in enhancing the fragmentation results in the hard and strong limestone formation being studied.

Results for blast designs using a 100mm blasthole diameter

In this part of the investigation four more blasts were conducted on the same bench with a similar explosive. Table 3 shows the salient design parameters for these blasts, while the four types of firing pattern used are illustrated in figures 13–16.

Perusal of table 3 reveals that, with the smaller 100mm blasthole diameter, the blast-design parameters have changed for all four blasts. The fragmentation results indicate that, for smaller-diameter blastholes with reduced spacing and burden, fragment sizes are also reduced for all the blasts when compared to larger-diameter (ie 150mm) blastholes with corresponding firing patterns. Figure17 shows the fragment size distribution curves for the four blasts.

The fragment size and size distribution follows a similar trend to the larger 150mm diameter hole size results. Once again, the ‘extended V’ pattern yielded the best results while the ‘in-line’ pattern yielded poor results. Also, the ‘V’ type firing yielded better results than the ‘diagonal’ pattern. These results may again be attributed to the Se to Be ratio, which was almost 4.0 for blast eight and about 2.11 for blasts five and seven. For blast six it was 1.4. Good and uniform fragmentation from blasts eight and five can clearly be seen in figures 18 and 19 respectively. On the other hand, very large sized fragments are quite distinct in the rock pile from blast six
(fig. 20).

CONCLUSIONS

The following main conclusions can be drawn from the study:

• Imaging techniques coupled with computer software analysis provide a powerful tool for quantitatively characterizing blast fragmentation at the field scale.
• Firing pattern greatly influences the degree of fragmentation as it affects the effective spacing (Se) to effective burden (Be) ratio, which in turn plays a significant role in altering the fragmentation results.
• Fragmentation improves with an increase in the Se to Be ratio because of the even and uniform distribution of the explosive energy.
• In hard, strong formations the in-flight collisions between broken rock fragments are of great significance in improving the degree of fragmentation.

ACKNOWLEDGEMENT

The authors would like to express their thanks to the management and staff of Century Cements Ltd for permission to carry out the field study and for their full co-operation in conducting the various blasts.

REFERENCES

1. MAMUREKLI, D.: ‘A user-driven computer model for open-pit blast design’, Journal of Min., Met. and Fuels, Expl. & Blasting special issue, 1990, pp 17–23.
2. RUSTAN, P.A.: ‘Automatic image processing and analysis of rock fragmentation-comparison of systems and new guidelines for testing the systems’, Fragblast, V.2, 1998, pp 15–23.
3. JIMENO, C.L., JIMENO, E.L., and F.J.A. CARCEDO: Drilling and blasting of rocks, A.A.Balkema, Rotterdam, The Netherlands, 1995.
4. HAGAN, T.N.: ‘The influence of some controllable blast parameters upon muckpile characteristics and open-pit mining costs’, Procs. Conf. Large open-pit mining, Aust. Inst. of Min. Met., 1986, pp 123–132.
5. BHANDARI, S.: Engineering rock blasting operations, A.A. Balkema, Rotterdam, The Netherlands, 1997.
6. MAERZ, N.H., FRANKLIN, J.A., ROTHENBURG, L., and D.L. COURSEN: ‘Measurement of rock fragmentation by digital photo analysis’, 5th Int. Congr. Int. Soc. Rock Mech., 1987, pp 687–692.
7. MAERZ, N.H.: Photo analysis of rock fabric, Ph.D. thesis, Dept. of Earth Sciences, Univ. of Waterloo, 1990.
8. WANG, W., BERGHOLM, F., and O. STEPHANSSON: ‘Image analysis of fragment size and shape’, Procs. Fragblast-5, Montreal, 1996, pp 233–243.
9. DOJCAR, O.: ‘Investigation of blast parameters to optimize fragmentation’, Trans. Inst. Min. Metall., 1991, 100, pp A31–A41.

The authors, Dr Piyush Rai and Satyendra Singh Baghel, are senior lecturer and post-graduate students, respectively, at the Department of Mining Engineering, Institute of Technology, Banaras Hindu University, India