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Figure 1. A blast-induced “back break”. All holes were loaded the same but the blast ratio on the ends is double in the middle. It can be avoided by reorienting the faces and altering the design and timings.
Figure 1. A blast-induced “back break”. All holes were loaded the same but the blast ratio on the ends is double in the middle. It can be avoided by reorienting the faces and altering the design and timings.
 











Reducing piles through better blasting

All too often blasting is undertaken with little regard for the geological conditions and the wrong type of explosives are used, resulting in blast piles higher than the actual face. Ian Thomas explains why a more scientific – and efficient - approach is needed in the blasting process.
Blasting has four objectives: to loosen in-situ rock from a quarry face, reduce block size for processing by crushers, to excavate and load the blast pile and to leave a stable face afterwards. Efficient blasting also reduces fines and oversize and the fragmentation of the blast pile is designed to match the loading equipment and crushers.


The cheapest rock breakage uses explosives in correct quantities at the correct locations. This requires known geological conditions, a stable rock face with minimal blast damage and consistent blasting parameters. With a fragmentation model like Kuz-Ram, it is possible to predict fragmentation and this improves production planning.

The current situation in blast engineering is summarised by Evert Hoek, the modern father of rock engineering (1999):
… a handful of highly skilled and dedicated researchers ... in association with explosives manufacturers, have developed techniques for producing optimum fragmentation and minimising damage in blasts. At the other end of the spectrum are miners who have learned their blasting skills by traditional apprenticeship methods, and who are ... are not convinced that the results ... from ... these techniques justify the effort and expense.

At fault ... are owners and managers who are more concerned with cost than with safety and design ... The need to change the present system is not widely recognised because the impact of blasting damage upon the stability of structures in rock is not widely recognised or understood.

This was a criticism of mining engineers but equally applies to quarry managers. As an industry, we have been preoccupied by issues such as health and safety and environmental and planning concerns. Many experienced managers and engineers have also retired or left the industry and the younger generation is less questioning of what they perceive as established blasting practices.


Today we use more powerful explosives in the same unscientific way that we did in the 1970s, and local and senior management focus on primary blasting costs. Drilling fewer holes may be cheaper but is not ideal for geotechnical stability or total production cost reductions.

The firing of shots with more than two rows should also be monitored; it may be cheaper and require less manpower but can cause extreme damage to the rock face. The geotechnical specialists give optimum and maximum face heights but a four-row blast can give a blast pile higher than the face which is problematic for the excavator operator.

My research indicates that blasting practices have not progressed in 20 years. Blast designers often do not know the properties of the explosives (velocity of detonation [VOD], gas volume produced, etc) and can use the wrong explosives for the geological conditions. In some cases, shot hole sizes are too large for pumped bulk explosives and the only option is to charge or stem which is inefficient.

INCONSISTENT SHOT HOLE GEOMETRY

The main problem is our shot hole geometry is often inconsistent and does not follow published guidance; the spacing to burden ratio should be one (1) and up to 1.4, depending on rock strength. Most rock breakage is caused by the reflection of compressive shockwaves, thereby breaking the rock in tension. If the burdens are too great the shock waves travel too far and attenuate before optimum breakage occurs. If the spacing is too small, the effective free face, which is often not the face itself in a multi-row blast, does not allow the blast pile to be effectively displaced.

Figure 4. How sedimentary rock with closely space joints appears in a blast pile.
Figure 4. How sedimentary rock with closely space joints appears in a blast pile.
In turn, this can cause excessive back break and “tight” blast piles which increase face dangers and slow down loading. In Figure 1, all holes were loaded the same but the blast ratio on the ends is almost double that in the middle. Each hole requires careful positioning if the ability to reduce charge weights is not available.

The UK Health and Safety Executive (HSE) have highlighted the lack of geological input into blast designs, and realise that in many cases the design cannot be correlated to any of the geotechnical or geological features in the quarry. In reality, the case is even more complicated if we consider the material we are blasting; we should consider the rock mass, not just the rock.

Terzaghi in 1946 pointed out that knowledge of the “type and intensity of the rock defects (discontinuities) will be much more important than the type of rock which will be encountered’’. A discontinuity is defined by Edelbro (2004) as a ‘’significant mechanical fracture that has low shear strength, negligible tensile strength, and high fluid conductivity compared with the rest of the rock material’’.

Rock mass is rarely continuous, homogenous or isotropic and is composed of intact blocks of rock separated by discontinuities like faults, cleavages, fissures, fractures, joints, bedding planes, shear zones, etc. The behaviour of the rock mass is dependent on the nature and frequency of these discontinuities, the shape of the intact rock defined by the discontinuities and the properties of the intact rock. From a geotechnical engineering view point, it is the discontinuities that control the engineering performance of a rock mass, not the intact rock.

Therefore, the more angular shaped the rock pieces are with clean, rough discontinuity surfaces, the stronger the rock mass. The behaviour of a rock mass is also influenced by external conditions, generally the in situ stress state and groundwater. Hoek and Karzulovic (2000) studied the effects of blasting on discontinuities (see Figure 2).

From the results, it can be seen that this blast-induced damaged area can extend to twice the face height. It is the main reason why pre-split blasting fails if it is undertaken too close to an existing face. Often this damage is seen in extreme cases as “back break” and can be avoided by reorientating the faces and altering the design and timings.

There are similarities between Figures 2 and 3. Figure 3 is a blast damaged face excavated with a 360 degree excavator where the digging has halted when it has become harder. This situation takes many blasts to rectify because the subsequent hole may have a large burden and little explosive in the upper regions and the damage can extend many metres back into the face.

To improve blasting, it is time to go back to basics and to concentrate on the blasting parameters used. The UK Government report Reclamation of limestone quarries by landform simulation (Cripps et al, 1999) calculated that only 15 per cent of the blast energy is utilised in breaking the rock and four per cent displacing it; the rest is wasted as noise, vibration and fines production. Thus, a small decrease in the blast ratio will only produce more unwanted vibration.

Figure 4 shows how a sedimentary rock with closely spaced joints can appear in a blast pile. A close examination shows iron staining on the joints and indicates very little breakage of the rock; it has only been displaced. This shows how accurately explosives should be used; too much only increases fines because once there is sufficient explosive to break the rock, the rest of the energy is wasted.

EMPLOYING KUZ-RAM, GSI

It is easy to reduce blasting costs with fewer explosives; the goal is to use the optimum amount of explosives to produce effective fragmentation for the crushing plant. A simple method to predict blast fragmentation from the drilling and explosives used is the Kuz-Ram model. It has never been fully taken up by the quarrying industry but is widely used by larger mining companies.


An additional benefit to efficient blasting is the reduction in power required to crush rock. There has been a major drive in the extractives industry to reduce power consumption and improve efficiency for the whole quarry operation. The European Union funded research project Less fines in the aggregate and industrial minerals industry has been working since 2000, and in the UK, sustainability research was funded by the Minerals Industry Sustainable Technology research programme until 2006.

From the latter research came Lowndes and Jeffrey’s paper (2007) which highlighted the importance of a holistic mine to mill approach to optimise primary aggregates production. They stated that the project aimed to “utilise a combined blast fragmentation and comminution optimisation to maximise desired product, minimise fines and energy consumption”.

My modification to the Kuz-Ram model (Figure 5) is to include an efficiency factor and relate the rock factors to the geological strength index (GSI), a system for estimating the reduction in rock mass strength for different geological conditions and tables found in Rock Slope Engineering (Wyllie and Mah, 2001). The GSI indicates the frequency of the joints and their surface conditions which dictate the breakage that will be achieved.

The efficiency factors include the burden to spacing ratio, optimum stemming lengths, timings, row patterns and number of rows. For instance, the fragmentation for a well designed staggered row with correct digital timings can be up to 30 per cent. The measurement of fragmentation is often difficult to achieve. Ultimately, I used the power consumption from the primary crusher. Bond’s 1952 theory of comminution shows that the energy requirement to reduce fragments decreases as the feed size reduces.

Thus fragmentation can be estimated over a period of months (excluding variances in moisture, ambient temperature and crusher maintenance). A more accurate method is to digitally scan blast piles with a LiDAR (Light Detection and Ranging) laser scanner (Figure 6).

The block size distribution in the blast pile was analysed by Adrian Wilkinson of Quarry Design Ltd, using SplitFX software which triangulates LiDAR point clouds and then creates polygons around groups of triangles within a user defined orientation. A series of “polygons” were produced upon a scan of just the blast pile, with the areas of each polygon exported as a CSV file into Excel, and the average polygon area was determined.

Based upon the polygon size (ie one side of a cubic block), the block volumes were calculated.

In conclusion, efficient use of explosives can reduce the instabilities in quarry faces. The more stable a face, the more consistent the fragmentation is and the more predictable it is with models such as Kuz-Ram. Blasting costs, along with wear and power consumption downstream in the plant, are reduced while production increases.

Figure 6. A LiDAR scan of a blast pile.
Figure 6. A LiDAR scan of a blast pile.


This story originally appeared in the March 2012 issue of Quarry Management (UK) and is reprinted with kind permission.

REFERENCES

Bond FC. The third theory of comminution. Transactions of the American Institute of Mechanical Engineers 193: 484-494.
Cripps JC, Czerewko MA, Johnson D, Wilton T. Reclamation of limestone quarries by landform simulation. In Bradley P, Brashaw P, editors. Office of the [UK] Deputy Prime Minister (ODPM), now transferred to the Department for Communities and Local Government.
Edelbro C. Evaluation of rock mass strength criteria. Technical report. Luleå University of Technology, 2004. ISSN 1402-1757.
Hoek E, 1999. Putting numbers to geology – An engineer’s viewpoint. Quarterly Journal of Engineering Geology 1999; 32: 1-19.
Hoek E, Karzulovic A. Rock mass properties for surface mines. In: Hustralid WA, McCarter MK, van Zyl DJA, editors. Slope stability in surface mining, Society for Mining, Metallurgical and Exploration. 2000. p59-70.
Lowndes IS, Jeffrey K. Sustainable provision of aggregates: Optimising the efficiency of primary aggregate production. DEFRA. Aggregate Levy Sustainable Fund.
Terzaghi K. Rock defects and loads on tunnel supports. In: Proctor RV, White T. Rock tunnelling with steel supports. Published by Commercial Shearing and Stamping Co. Youngstown, 1946. p15-99.
Wyllie DC, Mah CW. Rock Slope Engineering. 4th edition. Spon Press, 2007. ISBN 0-415 28001-X.










ABOUT THE AUTHOR
Ian Thomas
is the geotechnical manager for Aggregate Industries (UK).








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