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Achieving Effective Fragmentation

A case study on the role of the stemming column on fragmentation in an overburden sandstone formation

By Dr Piyush Rai, A.K. Ranjan and Bhanwar Singh Choudhary

The confinement provided by the stemming column in a blasthole is an important controllable parameter that greatly influences both the release of energy and its proper utilization. This research paper investigates the influence of stemming confinement on fragmentation by blasting in a moderately strong (12.5–20MPa) sandstone formation at an opencast mine. The study highlights the influence of stemming length on better fragmentation results, and also proposes the concept of optimum stemming length under the given geo-mining conditions.

Stemming of the collar region of a blasthole with inert material confines and retains the gases produced by the explosion within the blasthole. With inadequate stemming, improper explosive confinement occurs, which can lead to the loss of up to 50% of the explosive energy due to premature venting of gases through the collar region of the blasthole1. Furthermore, inadequate stemming can generate oversize at the face and perimeter zones of the blast round2.

Adequate stemming, on the other hand, provides proper confinement and retention of explosive gases within the blasthole and promotes rock fracture by transmitting a major proportion of shock, as well as gas pressure, through the broken rock mass prior to the release of the stemming column.

Although the length of a stemming column is a function of many factors, an excessively long column results in excessive confinement, which can lead to numerous problems. An excessive amount of boulders in the blasted muckpile have been reported, especially from the collar zone, due to excessive stemming column lengths3. For optimum rock breakage in bench blasting, many researchers have expressed the length of the stemming column as a function of hole diameter, bench height and the burden dimension.

The use of coarse angular material for stemming, in contrast to finely powdered drill cuttings, has also been advocated3&4. Coarse, angular material, such as crushed rock, offers increased resistance to the premature ejection of blasthole pressure due to the interlocking properties of the material.

It is obvious, therefore, that stemming confinement plays a vital role in blast performance. Bearing this in mind, this paper presents a case study to quantitatively ascertain the influence of stemming column length on the fragment size results in the overburden sandstone formation at an opencast mine.

Case study description

To meet the stated research objective, field-scale blasts were conducted on the sandstone overburden formation at an opencast coal mine belonging to Northern Coalfields Ltd (NCL), a subsidiary of Coal India Ltd (CIL). A bench with an average height of 18–18.5m was chosen for the purpose of study. Comprising a fine-grained sandstone with an average compressive strength of 12.5–20 MPa and an average tensile strength of 1.0–2.5Mpa, this particular bench was chosen primarily due to the fact that it appeared to contain no major or significant geological discontinuities. The bench was underlain by a coal seam (of inferior grade) measuring almost 18m in thickness.

Research methodology

Four full-scale blasts were conducted with incremental variations in the column length. These incremental changes in the stemming column length were, however, kept within the prescribed norms of stemming length in relation to the burden dimension (0.5–0.7B; where B is the burden dimension in metres). As the four blasts were conducted on the same sandstone bench, and as the explosive used in all four blasts was a similar doped emulsion supplied by a single manufacturer, for the purposes of the study, the rock and explosives parameters were assumed to be reasonably uniform for all four blasts. This is an important and realistic field-based assumption in order to avoid complicating the interpretation as a result of changing too many parameters in the study blasts. Moreover, close perusal of table 1 clearly indicates that the important blast design parameters, with the exception of the stemming column length, for all four blasts were also kept virtually the same, such that any changes in fragment size could be largely attributed to the changes in the stemming column length alone. It is also appropriate to mention here that the blast design parameters were monitored through strict field surveillance and control.

For the quantification of fragmentation in the blasted muckpiles, a widely acclaimed and state-of-the-art digital image-analysis technique was deployed5,6,7&8. Using a digital camera, a series of high-resolution photographs were captured on the blasted muckpiles to cover the entire excavation history of each blast. The field-captured photographs were processed and analysed by Fragalyst, a commercially proven image-analysis software system based on the principles of granulometry. The output, in terms of mean fragment size (K50), coarse fragment size (K95) and maximum fragment size (K100), along with the entire fragment size distribution in the muckpile, was provided by the software. As already mentioned, table 1 gives the salient blast design parameters for the four study blasts conducted at field scale, while table 2 represents the corresponding fragment size results for these blasts.

All the blastholes were drilled in a rectangular pattern and detonated using a V-type firing pattern with inter-row delays only. The blastholes were bottom initiated with a shock-tube system. A representative blasthole section (for study blast 1) is illustrated in figure 1, while a representative rectangular drilling and V-type firing pattern with designated inter-row delay timing for study blast 1 is represented in fig.2.

Results and discussion

The results shown in table 2 are suggestive of the significant influence of the length of stemming column on the fragment sizes in the blasted muckpiles. The plot of various fragment sizes (K50, K95 and K100) versus the stemming length (see fig. 3) clearly reveals this influence. From this plot it may be inferred that mid values (almost 0.6B) of the prescribed stemming length range (of 0.5–0.7B, as prescribed in literature), in terms of burden, appear to provide the optimum fragment size for the moderately strong sandstone formation in the case study. Extreme values (approaching 0.5B and 0.7B) have yielded larger fragment sizes. Although these results are case specific and involved the use of finely powdered drill cuttings as stemming material, they offer a meaningful insight. In some recent studies9&10 in the sandstone formations and limestone quarries of India, and also in the Philippines, the mid values of stemming column length were implemented with fine drill cuttings as well with crushed, angular aggregates. The results were reasonably uniform and acceptable, although some modifications were also attempted in the firing sequence in the said studies.

Furthermore, looking at the size of the blast and the poor fragment size results from blast 4 (with a stemming column length of 0.66B), no further increase in stemming column length beyond 0.66B was attempted owing to the likelihood of excessive generation of oversize.

Conclusion

In this moderately hard sandstone with a compressive strength of 12.5–20Mpa, the mid-value stemming column length (of almost 0.6B) appeared to offer optimum results in terms of fragment size and distribution in the blasted muckpiles, in comparison with the more extreme values.

Acknowledgement

The authors would like to thank the management and staff of Northern Coalfields Ltd (NCL) for permission to conduct the study and for their support in implementing the field-scale blasts.

Dr Piyush Rai is a Reader in Mining Engineering, A.K. Ranjan is a Former Post Graduate Student and Bhanwar Singh Choudhary is a Senior Research Fellow with the Department of Mining Engineering, Institute of Technology, Banaras Hindu University, India.

References

  1. BRINKMANN, J.R.: ‘An experimental study of the effects of shock and gas penetration in blasting’, Procs 3rd Int. Symp. on Rock Fragmentation by Blasting, Brisbane, Australia, 1990, pp55-66.
  2. FLOYD, J.L.: ‘Explosive energy relief – the key to controlling overbreak’, Procs Int. Conf. Explo '99, Kalgoorlie, Western Australia, 1999, pp147-153.
  3. JIMENO, C.L., JIMENO, E.L. and F.J.A. CARCEDO: Drilling and Blasting of Rocks, Pub. by A.A. Balkema, Rotterdam, The Netherlands,1995.
  4. KONYA, C.J.: Blast Design, Pub. by Intercontinental Development, Montville, Ohio, USA, 1995.
  5. ATASOY, Y., BRUNTON, I., TAPIA-VERGARA, F. and S.S. KANCHIBOTLA: ‘Implementation of split to estimate the size distribution of rocks in mining and milling operations’, Procs Conf. Mine to Mill, 1998, pp227-234.
  6. RUSTAN, P.A.: ‘Automatic image processing and analysis of rock fragmentation – comparison of systems and new guidelines for testing the systems’, Fragblast, 1998, vol. 2, pp15-23.
  7. KANCHIBOTLA, S.S., VALERY, W. and S. MORELL: ‘Modelling fines in blast fragmentation and its impact on crushing and grinding’, Procs Explo ’99, Kalgoorlie, Western Australia, 1999, pp137-144.
  8. OUCHTERLONY, F., OLSSON, M., NYBERG, U., ANDERSSON, P. and L. GUSTAVSSON: ‘Constructing the fragment size distribution of a bench blasting round, using the new swebrec function’, Fragblast-8, May 2006, pp332-344.
  9. RAI, P. and R. NATH: ‘Firing pattern vis-à-vis fragmentation on sandstone benches of a surface coal mine’, Coal International, Sept-Oct 2003, pp207-212.
  10. RAI, P. and S.S. BAGHEL: ‘Investigation of patterns on fragmentation in an Indian opencast limestone mine’, Quarry Management, 2004, vol. 31, no. 2, pp21-30.

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