HANDLING OF MISFIRES IN MINES

HANDLING OF MISFIRES IN MINES: Dealing with it is potentially most dangerous activity.

1. INTRODUCTION – Misfire means the complete or partial failure of a blasting charge to explode as planned. The explosive or pyrotechnical products that remain in the ground or in the muckpile might be triggered by any mechanical effect during the digging, milling or crushing stages of the mining process, causing injuries or fatalities to blasters or operators.

The potential consequences of a misfire are such that every reasonably practicable means available to site managers should be taken to avoid misfires. The emphasis should be made on prevention rather than cure.

Dealing with a misfire is potentially the most dangerous activity that site managers and Shotfirers will be involved in during blasting operations. In the event of a misfire, it is likely that unexploded charges and detonators will be left in the face or in the muckpile. These charges could be detonated if drilled into, if struck by an excavator bucket, wheels or tracks, or if inadvertently fed through a crushing plant. Unexploded charges may also be loaded out accidentally and taken either off site in road vehicles or to site tips. In any of these circumstances there may be a risk of danger to the operator or to the public, particularly from fly-rock in the event of a detonation.

Unexploded charges may need to be recovered by hand. All those likely to be involved must realise that this is a potentially dangerous operation. Great care and attention to detail is required to ensure that this is carried out safely.

2. RECOGNITION OF MISFIRES – After firing, a proper examination must be carried out to check the state of the face, that all the charges have fired and that there is no indication of a misfire. However, explosives can still be discovered at the face, in the muckpile or at the processing plant.

Any discovery of undetonated explosives or detonating cord must be reported since their presence constitutes a misfire. Indications of a misfire can include noxious fumes, inadequate ground movement, poor fragmentation, unusual blast sound or vibration trace, flyrock or evidence of undetonated explosives

3. POST BLAST INSPECTION – Post blast inspection is a hazardous task and in all circumstances must be carried out in accordance with the site rules.

Hazards exist not only from the existence of undetonated explosives but also from the post blast environment.

There is the possibility of a misfire remaining undetected even after inspection. It is therefore essential that adequately trained personnel regularly check the muckpile and face throughout the loading operation.

All personnel but especially those operating loading equipment, hauling equipment and crushers should be aware of this possibility and must be instructed to report abnormalities.

The extent and nature of the misfire must be determined as soon as possible after the misfire has been detected.

An exclusion zone must be established and secured until such time as any readily retrievable explosive has been collected and removed.

4. IN THE EVENT OF A MISFIRE – If a misfire is discovered during the post blast inspection then the “all clear” signal should not be given until a new exclusion zone has been established and secured.

The exclusion zone must be established by the responsible person who could be either the site manager or the blasting engineer or the shotfirer.

The immediate priority must be to ensure that arrangements to safeguard personnel in the event of a misfire are adequate and complied with.

Only those personnel directly concerned with the misfire should be within the exclusion zone.

5. DEALING WITH MISFIRES – The following procedures should be considered as possible courses of action in dealing with misfires.

* Removing stemming and re-priming

There may be circumstances in which it is possible to remove stemming in order to gain access and to reprime the charge. This is a potentially hazardous operation, which requires great care. It should only be attempted after detailed consideration.

When a hole contains detonators and it is anticipated that excessive force will be required to remove the stemming then the operation must not be attempted. This could result in premature initiation of the charge, particularly if the detonator is close to the top of the main charge and is immediately below the stemming.

If the hole contains an electric detonator the use of high velocity air to remove the stemming should not be attempted. Static charges sufficient to initiate electric detonators can be created.

Bulk explosive can be washed out of misfired shotholes but the utmost care must be taken in removing cartridges, particularly where detonators are involved. Under no circumstances must explosives or detonators be removed from a borehole by pulling on the detonator leads. Suitable extraction tools are available to enable cartridges to be removed. These usually take the form of a corkscrew or barb of nonferrous material which can be connected to stemming rods.

The following factors must be considered:

The use of high pressure water is unlikely to overcome the mechanical lock of stemming comprising chippings; The use of large quantities of water could desensitise any non waterproof explosive and dissolve any explosive with a high concentration of water soluble ingredients;

In situations where multiple decks of explosives are employed, all the above considerations magnify the difficulty of gaining access to the lower decks of explosives. Irrespective of the number of explosive decks, removal of the stemming in order to gain access to the charge as to re-prime is a technique, which ranges from unattractive at best to extremely difficult.

Any tools used inside the borehole to remove stemming must be non-ferrous.

If all the stemming can be removed and access to the top of the charge is achieved, the charge may be reprimed and refired. However it should be noted that in the event of a partial misfire the burden on the misfired shothole can often be reduced or fractured and a careful assessment of the situation must be made before any decision is reached.

* Drilling and firing relieving holes

The hazards in drilling relieving holes are:-

a) intersecting an explosives column, with a high risk of detonation

b) operating a drill in unstable rock conditions

The object of such holes when fired is:

  •  To disturb and displace the adjacent explosive column so that any primers and detonators remaining unfired are not located within an undisturbed explosive column after this blasting;
  •  To break up the rock mass in the region of the misfired hole in order to facilitate the search for and retrieval of any unexploded charges, primers and detonators.

One or more relieving holes may be drilled behind the misfired hole. The separation distance between the holes depends on the diameter, the inclination and the type of drilling equipment and the sensitiveness of the explosives. Any relieving hole must be drilled parallel to the misfired hole and to the same depth. To ensure that the holes are parallel it is essential that the information relating to the inclination and azimuth of the misfired hole is accurate. Care must be taken to drill the relieving hole at the same angle. The precise location of the relieving hole can only be established after careful assessment of the local conditions. Consideration should also be given to operating the drill rig remotely. It may be necessary to seek expert advice.

A further option is to drill small diameter relieving holes around the collar of the misfired hole. These are systematically fired to work off the rock and expose the charge. There may be adjacent charged holes, which must be considered and their location confirmed before any action is taken.

* Discovery and retrieval of explosives

It may be necessary to move rock from the immediate vicinity of the misfire before access to the charge can be gained. The remaining rock next to any misfired charge is likely to be solid and any attempt to remove this can be fraught with danger. Remedial action can only be decided after careful inspection and appraisal of the misfire site.

It may be possible to remove part of a misfired charge by hand from the socket of a hole but this should only be attempted by experienced personnel after due consultation with, and the approval of site management.

Removal of some charge from a hole will allow the introduction of a primer and detonators. Some stemming may then be added to the hole to create additional confinement, before firing.

If a misfired hole contains more than one deck of explosive, it may be necessary to deal with each deck in turn as a separate misfire, with either full retrieval of charges from each deck or re- priming. Sufficient confinement must be provided before refiring each deck.

Explosive which is recovered should be placed in containers for storage or disposal. Detonators should be separated from the explosives and primers carefully and stored separately from explosives. Explosives should be placed in plastic bags and placed in clearly labelled boxes.

The process of searching for explosive material in the heap may involve the use of loading equipment. Note that it is possible to utilise specially protective devices in order to protect the operators during this process.

Material picked up by the bucket should be taken to a leve1 area, carefully deposited on the floor and searched thoroughly. The minimum number of people should be exposed during this process.

Before excavation commences precise instructions should be given to the machine operator as to how to proceed.

This procedure should help minimise the risk of the impact of the bucket or falling rocks detonating unexploded charges. This work must only be done under direct supervision. From the location of misfired material and information from the blast design it may be possible to determine the quantities and types of explosive involved. A search should continue until, as far as possible, all explosive material has been accounted for. Be aware that explosive material may be concealed below the floor where sub-drilling is used.

A more serious situation occurs when explosive material is found when loading out or processing. It must be assumed that some has made its way into the product, stocking area or tip. It may even have been taken off site. An assessment must be made of the dangers and risks likely to be involved should the explosive be inadvertently detonated. Steps must be taken to arrange for the search and inspection of any location where undetonated explosives have found their way. All personnel must be instructed to report the finding of any explosive material to the shotfirer, the face foreman or the manager as soon as possible.

6. MISFIRE INVESTIGATION – After a misfire has occurred it is important that the “lessons learned” are recorded in order to attempt to prevent a repetition of the event.

Reporting is an important part of this procedure and records must be maintained. This is particularly important if it is suspected that all of the misfired material has not been recovered.

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Tunnel Construction Methods and their comparison

Tunnel Construction Methods and their comparison

1. Introduction – This paper gives a general description of the tunnelling techniques such as cut and cover, drill and blast, bored tunnelling and sequential mining construction, reviewed for possible use in various projects.

A summary of environmental merits and demerits associated with these methods are also given.

Above tunnelling techniques are mostly used to construct small tunnels and find their applications in utility projects to a great extent.

2. Construction Methods:

a. Cut and Cover Tunnelling – Cut and cover tunnelling is a common and well-proven technique for constructing shallow tunnels. The method can accommodate changes in tunnel width and non-uniform shapes and is often adopted in construction of underground stations. Several overlapping works are required to be carried out in using this tunnelling method. Trench excavation, tunnel construction and soil covering of excavated tunnels are three major integral parts of the tunnelling method.

Most of these works are similar to other road construction except that the excavation levels involved are deeper. Bulk excavation is often undertaken under a road deck to minimise traffic disruption as well as environmental impacts in terms of dust and noise emissions and visual impact.

b. Drill and Blast – This tunnelling method involves the use of explosives. Drilling rigs are used to drill blast holes on the proposed tunnel surface to a designated depth for blasting. Explosives and timed detonators (Delay detonators)are then placed in the blast holes. Once blasting is carried out, waste rocks and soils are transported out of the tunnel before further blasting. Most tunnelling construction in rock involves ground that is somewhere between two extreme conditions of hard rock and soft ground. Hence adequate structural support measures are required when adopting this method for tunnelling.

Compared with bored tunnelling by Tunnel Boring Machine (see below), blasting generally results in higher but lesser duration of vibration levels. A temporary magazine site is often needed for overnight storage of explosives.

c. Bored Tunnelling by Tunnel Boring Machines (TBM) – Bored tunnelling by using a Tunnel Boring Machine (TBM) is often used for excavating long tunnels. An effective TMB method requires the selection of appropriate equipment for different rock mass and geological conditions. The TBM may be suitable for excavating tunnels which contain competent rocks that can provide adequate geological stability for boring a long section tunnel without structural support. However, extremely hard rock can cause significant wear of the TBM rock cutter and may slow down the progress of the tunnelling works to the point where TBM becomes inefficient and uneconomical and may take longer time than the drill-and-blast tunnelling method.

d. Sequential Excavation Method – This method is also known as the New Austrian Tunnelling Method (NATM). The excavation location of a proposed tunnel is divided into segments first. The segments are then mined sequentially with supports. Some mining equipments such as road-headers and backhoes are commonly used for the tunnel excavation. The ground for excavation must be fully dry for applying the NATM and ground dewatering is also an essential process before the excavation. Another process relates to the ground modifications such as grouting and ground freezing is also common with this method in order to stabilize the soil for tunnelling. This method is relatively slow but is found useful in areas where existing structures such as sewer or subway could not be relocated.

3. Environmental Merits and Demerits – Selection of the techniques to be adopted for construction of a tunnel section shall take into account the nature of the substrata and the levels of the tunnel involved. A summary of the environmental advantages and disadvantages associated with the construction methods is tabulated below:

Tunnel Construction Methods Environmental advantages and disadvantages

(on relative terms)

Cut and cover tunnelling Disadvantages:

  • More dust and noise impact may arise, though these can be mitigated through implementation of sufficient control measures;
  • Temporary decks are often installed before bulk excavation to minimise the associated environment impacts;
  • Larger quantity of C&D materials would be generated from the excavation works, requiring proper handling and disposal.
Drill and blast Advantages:

  • Potential environmental impacts in terms of noise, dust and visual on sensitive receives are significantly reduced and are restricted to those located near the tunnel portal;
  • Compared with the cut-and-cover approach, quantity of C&D materials generated would be much reduced;
  • Compared with the cut-and-cover approach, disturbance to local traffic and associated environmental impacts would be much reduced;
  • Blasting would significantly reduce the duration of vibration, though the vibration level would be higher compared with bored tunnelling (with proper blast design & techniques vibration can be reduced);

 

Disadvantages:

  • Potential hazard associated with establishment of a temporary magazine site for overnight storage of explosives shall be addressed through avoiding populated areas in the site selection process.
Bored tunnelling by TBM Advantages:

  • Potential environmental impacts in terms of noise, dust and visual on sensitive receives are significantly reduced and are restricted to those located near the launching and retrieval shafts;
  • Compared with the cut-and-cover approach, disturbance to local traffic and associated environmental impacts would be much reduced;
  • Compared with the cut-and-cover approach, quantity of C&D materials generated would be much reduced;
Sequential Excavation Method Advantages:

  • Similar to the drill-and-blast and bored tunnelling methods, only localised potential environmental impacts would be generated;

Disadvantages:

  • As the method is relatively slow, duration of potential environmental impacts would be longer than that of the other methods.

References:

* Partha Das Sharma; ‘Tunnel Excavation’: https://miningandblasting.wordpress.com/2009/12/16/tunnel-excavation/

*Partha Das Sharma; ‘Techniques of Controlled Blasting’: https://miningandblasting.wordpress.com/2009/09/02/techniques-of-controlled-blasting/

* Partha Das Sharma Blast design for Drifting and Tunnelling with Wedge and Burn Cut

Factors in Controlling Drilling and Blasting Costs in Mines – Discussion

Factors in Controlling Drilling and Blasting Costs in Mines – Discussion

The recent increase in ammonium nitrate  and diesel oil prices have invited many operations in mine to re-examine their drilling and blasting methods to find ways of reducing costs. Under the changed scenario, drilling and blasting companies require to optimize their blasting and costs through a blasting optimization review. Some of the questions faced during blast optimization are discussed here.

Many operations only look for less costly explosives, which company feel would do the job. What can be done to reduce costs if we are only looking for less expensive methods for firing the same blasting pattern? Let’s examine the blasting pattern and supplies used and consider the following questions and how they relate to the specification prevailing in the field.

Do you really know what is your exact drilling and blasting cost actually should be? Countless times when we work with operations we find that their actual costs are much higher than their calculated ideal cost. Some times the reason for this is as simple as, for example, they are assuming a 4 by 4 m drill pattern while in actual fact the drill pattern on average is 3 by 3.5 m. This small difference in average pattern dimensions would already increase cost per ton by over 11%. There is Blasting Cost software available, which will accurately determine calculated ideal costs.

What types of initiators are you using in the blast? Are you using a redundant path system for a shock tube initiation system? Is the redundant path really needed or are you paying double for the cost of the initiators to protect the manufacturer for product defects? Are you paying double for initiators to cover mistakes made by the blasting crew? A single path shock tube system is used by many operations worldwide. Why would it not work in your operation?

One of the lowest cost methods to produce better fragmentation is to use the proper delay times in the blastholes. How do you know if the delay times used produce the best fragmentation, back wall control, and lowest vibration in your particular operation. Selection of proper delay times are site specific and depend on the local geology. Do your blasters know how to select the proper delay times? Using the wrong delay times can greatly increase your production costs.

How many cast primers are used routinely in each blasthole? Is more then one really needed in all holes? What size cast primer are you using: 500 gm, 400 gm, 250 gm, 100 gm? What size and how many primers are really needed from a technical standpoint?

What explosive are you using as the main explosive charge? Many operations are using more expensive, more energetic explosives than needed with the same drill pattern they would use for ANFO. All that results is additional throw of the broken for the additional cost.

Are you using cartridged explosives rather than bulk explosives? The additional explosive that you can place into the blasthole because you are filling the annular space that you would not fill with cartridged explosives will allow you to reduce the drilling cost by expanding the drill pattern.

What do you do with the used motor oil that you generate from your equipment? Some companies use the old motor oil as part of the fuel for their ANFO or in the manufacture of their emulsions rather than paying to have the used oil taken away. Research has shown that used oil diluted with diesel oil produces as much or more energy than pure diesel oil in ANFO.

Deck loading is used in many operations to reduce the quantity of explosives per delay in order to reduce vibration. Deck loading increases the time it takes to load a blast, increases the number of initiators and primers needed, and often produces less efficient fragmentation than when using a full column of explosives. In my experience most blasts are not efficient when it comes to minimizing vibration. If the blast efficiency is improved then deck loading may not be necessary.  By considering these factors and others, many operations have saved as much as 40% of their explosive costs without effecting their blast performance.

A bigger picture must be considered if we are truly concerned about reducing production costs. Blasting is only the first step in the production process for mines and quarries and the costs of this first step is normally only 8% to 12% of the total costs. The total product costs are composed of: drilling, blasting, secondary breakage of oversize, digging, haulage, crushing. Blasting affects every step in the production cycle. What is important is to reduce total costs. If you try to reduce explosive costs alone you may raise drilling cost per ton, secondary breakage costs, digging costs, hauling costs, and crushing costs. If explosives cost would increase but produce better breakage and cost reductions could made in the other production costs then total production cost may radically decrease.

If savings on drilling and blasting costs are desired it is important to determine the actual costs of the production process. What are the drilling and blasting costs per ton? What percent of the blast is oversized and what does it cost to break up the oversized boulders? What is the average cost per ton to dig and load out the blast? What does it cost to haul the material to the crusher? How many trucks dump at the crusher per hour? With good blasting you may be able to greatly increase the truck count per hour. What are the average crushing costs. How many tons per hour go through the primary crusher? Most operations either have the raw data needed or can easily get this data to be able to determine a total production cost.

The major problem we find is that many operations do not have a good understanding and easily obtainable data on of the costs associated with secondary breakage, digging and haulage costs and how this would be affected by better drilling and blasting procedures. It is much easier to look at explosive costs. To totally optimize costs and to make informed decisions, all production costs must be considered when selecting drilling and blasting methods.

Drilling and blasting is only the first phase of the production cycle but influences all costs for all the other activities.

ReferL Cost effective new formulation of ANFO by using Biomass Briquette as Additive

 

Characteristics of Rock and Geology influence Surface, UG and Tunnel Rock Blasting Results

Characteristics of Rock and Geology influence Surface, UG and Tunnel Rock Blasting Results

1. GENERAL CHARACTERISTICS OF ROCK – The result of any blast is more dependent on the characteristics of the rock than on the explosives being used to break it. The more important characteristics of the rock influencing the blasting result include:

  • Tensile and compressive strength,
  • Density and
  • Seismic velocity (acoustic velocity).

a. Tensile and Compressive Strength – Most types of rock have a compressive strength which is 8 to 10 times greater than the tensile strength. These properties are important factors in rock blasting.

Type of rock Compressive strength (kg/cm2) Tensile strength (kg/cm2)
Granites 2000-3600 100- 300
Diabase 2900-4000 190- 300
Marble 1500-1900 150- 250
Limestone 1300-2000 170- 300
Shale 300-1300
Sandstone, hard 3000 300

b. Density – Rock of high density is normally harder to blast than rock of lower density. One reason for this is that high density rock is heavier to move during detonation.

c. Seismic Velocity – The seismic velocity (acoustic velocity) of the various types of rock varies from 1500-6000 m/s. Hard rock of high seismic velocity will shoot more easily, spe­cially when explosives with high velocity of detonation are being used.

Type of rock Density (kg/dm3) Seismic velocity (m/s)
Granite 2,7-2,8 4500-6000
Gneiss 2,5-2,6 4000-6000
Limestone 2,4-2,7 3000-4500
Dolomite 2,5-2,6 4500-5000
Sandstone 1,8-2,0 1500-2000
Clay mudstone 2,5-2,7 4000-5000
Marble 2,8-3,0 6000-7000
Diabase 2,8-3,1 4000-5000

2. ROCK STRUCTURE – The planning process should include a survey of the rock structure and other rock characteristics so that the drilling and loading pattern and direction of advance can be optimized as far as possible.

The rock structure included cracks and fissures and other zones of weakness.

Two expressions commonly used to describe the rock structure are ‘strike’ and ‘dip’. Strike is the horizontal direction of the structure on the rock surface. Dip is the angle of the structure relative to the horizontal rock surface.

 

For further reading refer following paper:

https://miningandblasting.wordpress.com/2010/12/21/modification-of-blasting-techniques-as-per-geological-conditions-of-strata/