A Technical overview of Overbreak
B.O. Taiwo*
Giant Miner
*Graduate of Federal University of Technology Akure, Nigeria
author's mail: taiwoblessing199@gmail.com
Abstract: The presence of boulders in the blasted muck
causes not only loss in production, but also increases the cost of hauling. In
another words, the cost and efficiency of mucking, loading, and Comminution
operations (crushing and grinding) will be highly altered by the outcome of
blasting operation. This
article give detail explanation about the causes and effect of
overbreak during blasting. It point 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 guarantee of life,
therefore generation of overbreak on the mine face affect both
profitability and safety status of the mine. All possible means and
design must be put in place to mitigated and control the generation of
overbreak during blasting operation. Rock mass with existing fractures
and joint set likely become detrimental in the present of overbreak
fracture, such become unstable and can undergo plane or wedge failure if
all require conditions are fulfilled. The ore require high grade status
to guarantee high economic value, external dilution resulting from
overbreak may increase dilution resulting into increase in haulage cost
and processing cost. Overbreak floor also support the out flow of ground
water into the mine and demanding for de-watering operation if not
result into Acid Mine Drainage or total closure of mine. Overbreak can
therefore be control from the Pre-blasting stage which involve proper
understanding of the ore deposit geological condition, ground water
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.
Keywords: Blast, overbreak, fragmentation
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 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 at safe, economic and environmental
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; need for rock mass reduction to smaller size ranges, efficient
use of explosive energy at high safety level; and control of blasting to avoid
oversize materials. Therefore, there is need for proper blasting optimization
using the controllable factors which can be predicted using machine learning
packages and other empirical formula (Tiile, 2016).
The primary requisites for any blast design is to ensure
optimum results for existing operating conditions, possesses adequate flexibility
and 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 modification
to a standard design to fit the conditions of the in-situ rock conditions.
2. Blast OVerbreak
Fig 1. Theory of blast Overbreak
The chance of breaking rock beyond the design reference is highly possible during 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 safely 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 design such that the height is higher than the loader boom height and selected base on geological condition and grade Control constraints. Each bench width is designed such that it accommodate truck turning radius and it 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 design, 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 safely in place of bench wall stability and also affect production in place of dilution and poor fragmentation.
Fig 2. Site view of Overbreak effect on mine toe (After ismge.org)
2.1 How overbreak occurred during Blasting
Muhammad, (2009) establish that blast design
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 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 good decision and rock condition justification. One important parameters that support 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 under use within the wall and have sufficient strength to proceed slabbing into the wall. Since slabbing is extend into the wall explosion will definitely fall rock beyond the wall Konya and Walter (1990). This follows the law of conservation of energy directly.
Other factor is the powder factor, this is the quantity of explosive 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 jack hammer drilling and Blasting operation, a single hole of jack hammer blasted is expected to produce nothing less than one (1) ton of the rock. Meaning if 5 holes are blasted,ore than 5Ton capacity truck will be produced. In 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 use to overbreak the wall and create problem.
It's important to know that during Blasting of a single hole the burden is divided into two and distributed at the four cardinal direction of the hole to form the area of influence around the hole. Each blast generated energy from 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 define by 1ft/ms of shock energy traveling and it correspondence to the gas Energy explosion time and Rick casing time according to scholars research work in 1970s. Explosive shock and gas energy will always be active provided the area of influence is still yet completed. Over breaking in rock mass is easy to control when the place of understanding the blast parameters balancing is fulfilled. This can be prevent by increasing the burden distance.
Nevertheless, if design burden distance is much, the implication is that,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 generate more oversize. Oversize or Large Particle size materials requires 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 Anisotropic and heterogenous nature. Rock consist of various Mineral composition, the response of each mineral components 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 the this there will be change in fracturing along the rock wall. For instance, 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 absent 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 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 effect 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 require the implementation of drilling log in mine, the drilling team should provide information of each holes clearly as observes 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 insitu rock stability and competency status.
4. The place of powder factor control is also important and place of drill hole row alignment with design.
5. Preventive measure against misfire such has 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 guarantee of life, therefore generation of overbreak on the mine face affect both profitability and safety status of the mine. All possible means and design must be put in place to mitigated and control the generation of overbreak during blasting operation. Rock mass with existing fractures and joint set likely become detrimental in the present of overbreak fracture, such become unstable and can undergo plane or wedge failure if all require conditions are fulfilled. The ore require high grade status to guarantee high economic value, external dilution resulting from overbreak may increase dilution resulting into increase in haulage cost and processing cost. Overbreak floor also support the out flow of ground water into the mine and demanding for de-watering operation if not result into Acid Mine Drainage or total closure of mine. Overbreak can therefore be control from the pre-blasting stage which involve proper understanding of the ore deposit geological condition, ground water 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
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