Overbreak: Result of poor Blasting

B.O. Taiwo*

Giant Miner

*Graduate of Federal University of Technology Akure, Nigeria 

author's mail: taiwoblessing199@gmail.com

 Check other articles on Mining by Taiwo Blessing Olamide

Abstract

The presence of boulders in the blasted muck causes not only loss in production, but also increases the cost of hauling. In another word, the cost and efficiency of mucking, loading, and Comminution operations (crushing and grinding) will be highly altered by the outcome of the blasting operation. This article gives a detailed explanation of the causes and effects of overbreak during blasting. It points out the ways to control and minimize blast overbreak and give recommendation on the ways to improve blasting operation and safety. The safety of the mine is important to ensure a guarantee of life, therefore generation of overbreak on the mine face affects both profitability and safety status of the mine. All possible means and designs must be put in place to mitigate and control the generation of overbreak during the blasting operation. Rock mass with existing fractures and joint set likely become detrimental in the presence of overbreak fracture, such become unstable and can undergo plane or wedge failure if all required conditions are fulfilled. The ore requires high-grade status to guarantee high economic value, external dilution resulting from overbreak may increase dilution resulting in an increase in haulage cost and processing cost. Overbreak floor also supports the outflow of groundwater into the mine and demands for de-watering operation if not result in Acid Mine Drainage or total closure of the mine. Overbreak can therefore be controlled from the Pre-blasting stage which involves a proper understanding of the ore deposit geological condition, groundwater condition, explosive material knowledge among others. Special blasting techniques like pre-splitting, post splitting, line hole, cushion blasting, air decking among others can also be used to control overbreak in mine wall, toe and face. 

 Keywords: Blast, overbreak, fragmentation, safety, Mining

1. INTRODUCTION

The profitability of blasting operation depends upon the ability of the Mining Engineer to produce fragment size distribution as close as possible to the optimal range for downstream operations. In addition, from a practical standpoint, oversize is defined as size ranges greater than the crusher’s gape and therefore requires secondary breakage before further handling. In most hard rock mining methods, drilling and blasting are the most widely used method of fragmenting the rock for handling (transportation and stock-piling) (sang and Katsuhiko, 2004). Sang and Katsuhiko, (2004) noted that, an optimal blast is that which yields the specified fragmentation size distribution in a safe, economic, and environmentally friendly manner. On the other hand, a poorly conducted blast would typically end in poor fragmentation and may generate adverse effects like fly rocks, ground vibration, air blast, and back break (Nassib et al, 2016). Nassib et al, (2016) also discovered that blasting operation is capital intensive due to the following; the need for rock mass reduction to smaller size ranges, efficient use of explosive energy at a high safety level; and control of blasting to avoid oversize materials. Therefore, there is a need for proper blasting optimization using the controllable factors which can be predicted using machine learning packages and another empirical formula (Tiile, 2016).   

The primary requisites for any blast design are to ensure optimum results for existing operating conditions, possess adequate flexibility, and be comparatively simple to use (Muhammad, 2009). Several factors affect the output of a blast, these factors can be generally classified into two; uncontrollable factors and controllable factors.  Uncontrollable factors are those that the blast designer has no direct control over. They are controlled by the intrinsic (DIANE) properties of the in-situ rock formation or ore deposit to be blasted. According to Konya and Walter (1990), these factors include geology, rock characteristics, regulations as well as the distance to the nearest structures. These limitations usually require that the blaster makes correct modifications to a standard design to fit the conditions of the in-situ rock conditions.

2. Blast OVerbreak    

The chance of breaking rock beyond the design reference is highly possible during the Blasting operation. This is known as overbreak when the Explosive energy fractures the rock beyond the bench width. This is highly dangerous and even disastrous to both safety and profitability. Deep deposits are mined in benches to ensure easy access to the pit and support of multiple operations within the mine. The pit benches are designed such that the height is higher than the loader boom height and selected based on geological conditions and grade Control constraints. Each bench width is designed such that it accommodates truck turning radius and its overall width, this is done with high consideration of grade Control and geological condition also to ensure discontinuity daylight is not supported and likewise, external dilution is not supported. With all this in mind, the access road to the pit called ramp is also designed, each with consideration width (Be in expectant of my lecture on pit design and ramp design). Over breaking the bench width beyond this design width will compromise the safety in place of bench wall stability and also affect production in place of dilution and poor fragmentation. 

2.1 How overbreak occurred during Blasting 

Muhammad, (2009) establishes that blast design is usually aimed at providing adequate fragmentation and ensuring that loading, haulage, and subsequent processing is accomplished at the lowest possible cost. However, for optimum and efficient blasting performance, it is essential to consider critically the rock mass intrinsic properties. Rocks are usually characterized by several properties. The nature and properties of the rock mass vary sharply over a short distance. It is therefore important that the influence of the rock mass parameters must be understood during the blast design process (Bhandari, 1997).

Overbreak begins from the blast design point, selection of Minimum burden distance must be done with a good decision and rock condition justification. One important parameter that supports overbreak is the wall burden distance. If this is small then the implication is to produce overbreak beyond the bench wall. Such occur because the Explosive energy (shock wave) is underuse within the wall and has sufficient strength to proceed slabbing into the wall. Since slabbing is extended into the wall explosion will definitely fall rock beyond the wall Konya and Walter (1990). This follows the law of the conservation of energy directly.

Another factor is the power factor, this is the quantity of explosives designated to fracture a ton of the rock, this depends strictly on the charge weight per hole.

Pf = charge per hole ÷ Tonnage of rock fragment per hole 

From my short study of jackhammer drilling and Blasting operation, a single hole of jackhammer blasted is expected to produce nothing less than one (1) ton of the rock. Meaning if 5 holes are blasted, more than 5Ton capacity trucks will be produced. In a situation where at the last hole row, the Explosive weight is more than that to fracture within the holes area o influence then, the unused Energy will definitely be used to overbreak the wall and create a problem.

It's important to know that during the Blasting of a single hole the burden is divided into two and distributed at the four cardinal directions of the hole to form the area of influence around the hole. Each blast-generated energy from an Explosive exothermic reaction is expected to be active within this area of influence. Placing our blast holes at the last bench edge should be done with respect to this understanding. This area of influence is defined by 1ft/ms of shock energy traveling and it corresponds to the gas Energy explosion time and Rick casing time according to scholars' research work in the 1970s. Explosive shock and gas energy will always be actively provided the area of influence is still yet completed. Over breaking in the rock mass is easy to control when the place of understanding the blast parameters balancing is fulfilled. This can be prevented by increasing the burden distance.

Nevertheless, if the design burden distance is much, the implication is that the blast charge initiated will have more of the energy contained within the designated region thereby producing back-break or under-break as shown in Fig 1 which generates more oversize. Oversize or Large Particle size materials require secondary Blasting or hydraulic breaker to Re-break the rock. This form a source of additional cost and waste of time which any company will never want.

Another factor that causes overbreak is rock mass's Anisotropic and heterogeneous nature. Rock consist of various Mineral composition, the response of each mineral component to Explosive energy is different due to their Strength and hardness variation (Bhandari, 1997). This heterogeneity nature makes rock and ore deposit Anisotropic to explosive energy. As a result of this, there will be a change in fracturing along the rock wall. For instance, a formation containing dyke or sill which are both geological intrusion, this intrusion according to the principle of intrusion which states that "the rock been intruded is older than the intrusion itself". With the understanding of this principle, if all things been equal in absence of weathering and leaching, the intrusion (dyke, batholith, laccolith among others) are proved to have higher strength than the old rock been intruded especially in the case of metamorphosed carbonate rocks (marble and dolomite). Blasting such rock, understanding the geology of the area and deposit, with proper strata test, geotechnical mapping work, and observation during drilling will help to put good charge design in place to handle any variation in rock stratigraphy and lithology.

Other factors include discontinuity, Explosive property, and conditions, misfire among others which I will explain in my full article. 

2.2 Effect of Overbreak 

Some of overbreak effects are:

1. Produce rough bench wall

2. Support external dilution

3. Support slope and wall failure

4. Poor fragmentation and non-uniform size distribution among other 

2.3 Ways to control and prevent overbreak

1. Proper Blasting design: making balance in blast parameters (burden, spacing, stemming length e.t.c.) Design

2. Proper study of formation geology and lithology during Drilling: This requires the implementation of a drilling log in the mine, the drilling team should provide information of each hole clearly as observed during Drilling and from the drill cutting. Such include interception depth at which a hole lithology changes, the strength variation with depth as indicated by Penetration rate During drilling, collar condition, and others.

3. Discontinuity information and joint set mapping to understand the in-situ rock stability and competency status.

4. The place of powder factor control is also important and the place of drill hole row alignment with design.

5. Preventive measures against misfires such as Explosive checking, proper loading, and connection, hole water status checking e t.c should be intact.

Conclusion

The safety of the mine is important to ensure a guarantee of life, therefore generation of overbreak on the mine face affects both profitability and safety status of the mine. All possible means and design must be put in place to mitigate and control the generation of overbreak during the blasting operations. Rock mass with existing fractures and joint set likely become detrimental in the presence of overbreak fracture, such become unstable and can undergo plane or wedge failure if all required conditions are fulfilled. The ore requires high-grade status to guarantee high economic value, external dilution resulting from overbreak may increase dilution resulting in an increase in haulage cost and processing cost. Overbreak floor also supports the out flow of groundwater into the mine and demands for de-watering operation if not result in Acid Mine Drainage or total closure of the mine. Overbreak can therefore be control from the pre-blasting stage which involves the proper understanding of the ore deposit geological condition, groundwater condition, explosive material knowledge among others. Special blasting techniques like pre-splitting, post splitting, line hole, cushion blasting, air decking among others can also be use to control overbreak in mine wall, toe and face.  

References

[1]        Sang Ho, C. and Katsuhiko K. (2004). Rock Fragmentation Control in Blasting. The Mining and Materials Processing Institute of Japan , pp. 1722 to 1730.

[2]         Tiile, R. N. (2016). Artificial neural network approach to predict blast-induced ground vibration, Airblast and rock Fragmentation . Masters Theses. 7571.

       https://scholarsmine.mst.edu/masters_theses/7571

[3]          Muhammed, A. R. ( 2009). The Effect of Fragmentation Specification on Blasting Cost. Unpublished MSc Thesis Report, Queen’s University, Kingston, Ontario, Canada , pp.192-  200.

[4]         Konya, C. J. and Walter, E. J.  ( 1990). Surface Blast Design. Prentice-Hall Inc., New Jersey, USA , pp 125.

[5]         Bhandari, S. (1997). Engineering Rock Blasting Operations. A.A. Balkema, Rotterdam, Brookfield , pp 375