April 2016

Evolution of sublevel caving – safety improvement through technology

  • By Mike Lovitt FAusIMM, Global Underground Technical Specialist, Orica Limited

An overview of a wireless blasting system for use in sublevel caving operations that significantly reduces the risk posed to mine workers

Sublevel cave (SLC) mining is breaking ore-bearing rock through blasting and drawing it out under the waste rock that is caving into the void created. SLC mines have the most intensive blasting process of all the mining methods, as generally one or two rings are fired at a time at each drawpoint in order to maximise recovery and reduce dilution. The charging process can be made more efficient by precharging, which is charging rings well ahead of those to be fired, such that the charging operation is kept away from the choked brow. In these choked conditions there is no open void to blast more than 1 or 2 rings without poor recovery and high dilution. It is therefore the brow area that has major hazards that are a safety risk to the operation.

Hook-up of the initiation system must presently be carried out in this hazardous area just before firing. This also limits productivity of the drawpoints to be fired and adjacent drawpoints. If this hook-up process could be eliminated, the risk profile of the SLC operation could be significantly reduced, while increasing the drawpoint availability and productivity.

A Wireless Electronic Blasting System that receives communications through the ground from a safe location can minimise the risk profile of SLC operations. All data and power is with the initiator, and it only requires the correct special signal to ‘wake up’ and then the correct signal to fire. This system has no lead wires to connect and the sequence is ‘stored’ during the charging of the ring.

This technology has now been established, with Orica having completed 20 000 lab tests, 1000 blast range tests and many full scale blasts in a long process to prove the concept and develop the system. The safety-critical elements of the wireless system have been designed and tested to IEC 61508 (SIL 3). This is the highest level that can be practically achieved for a commercial blasting product and reviewed by an external authority. A SIL 3 component running continuously should have a safety-critical failure less than one in 1000 years.

This article discusses the system details and the impact this system can have on the SLC method that is used in many underground mines throughout the world.

Safety issues in sublevel caving

The brow, the edge of the intact rock and the broken rock mass, is the area where many safety hazards are present and is the area where hook-up operations for blasting rings are carried out. The major hazards are:

  • unstable rill of material moving causing impact and slip trip injury
  • unstable brow causing rock fall, leading to impact injuries and damage
  • wear of the brow causing back break, thereby causing limited or no access to the next ring
  • unstable body of fill causing ejection into the drawpoint
  • blast damage cutting off the holes in the next blast ring
  • rework processes to mitigate safety concerns at blast hook-up time (bunds, shotcrete, and extra ground support) is a cost and reduces drawpoint availability.

The rill of material at a sublevel cave drawpoint is inherently unstable as it is the flow of material after broken ore has been withdrawn. It can move due to subsidence of compacting material over time or through destabilisation due to external movement. Rills are unstable, which is why most mines ban their use as a work place due to their rough nature and the fact that the material can re-mobilise at any stage. Where brow break back exists, and where a basket does not afford enough work room, workers must work off the rill to recover initiation leads out of the blast hole. Some mines build up the floor of the drive to get workers closer to the collars of the hole and to protect the instability of the rill by buttressing it.

The Wireless Electronic Blasting System eliminates these concerns by removing the need to carry out any hook-up of initiation system. The selection of correct data to ‘load’ into the firing system is carried out on the surface with all personnel out of the mine. This elevates the need for best practice data integrity systems to ensure that the correct rings are fired.

The brow of a drawpoint is an intersection between ground support systems and blasted ground. The draw of material in the extraction process can destabilise rock or ground support mechanisms close to the brow, thereby making it more prevalent to unexpected rock falls. Any system or process which removes personnel from this area will reduce the exposure to these falls of ground.

OLYMPUS DIGITAL CAMERA
Draw point open (major hazard for falling rock) showing next ring to fire and rill underneath.

Wear to the brow is common, and in some cases the collars of the next ring are consumed by the retreating brow. In this case if the collars are not able to be accessed, these holes or even the whole ring could be lost. This could mean that there is significant burden left to be blasted on that ring, but as the collar cannot be accessed, the hole or ring has to be abandoned. This causes significant dilution and recovery issues. There may be an instance where the remaining holes or the next ring does not have the energy to recover the extra ground. This can result in bridging which will require extra drilling, or even re-slotting.

Wireless systems remove this issue. All parts of holes that have a primer in contact with good explosives will be initiated to carry out rock breakage work and stop isolated live boosters in the muck pile.

There are a number of deep mining SLC mines which have experienced rushes of material out of drawpoints that have filled the extraction drift by up to 20 m from the brow. These events can bury mining equipment and result in injuries or fatalities. Operations with this exposure have looked at alternative drilling and blasting processes and have installed large bunds to avoid this exposure. The simplest solution now is to use the wireless system of initiation along with precharging.

SLC mines blast intensively as they blast against a rill of broken material in a choked manner. This causes extra damage back to the next firing ring (slept ring) which is not fired in this blast. This damage can be in the form of hole dislocation and in damage to the initiation systems. With conventional systems it is believed that this is one major reason for the misfire rate in present initiation systems. The wireless system will solve the dislocation issues as there will be no wires to be cut. The biggest exposure to the new system will be dynamic shock, and initial trialling of the system in this application will highlight how much of a problem this will be.

The hazards outlined above have caused mines to resort to many controls to mitigate them. This results in extra cost and time to perform them. Bunds, shotcrete, and extra ground support all take time and introduce complexity to the process that reduces drawpoint availability and therefore productivity.
A solution to this problem is to eliminate the high risk process of ‘hook-up’ occurring underground by moving the hook-up process to the office with a data integrity system.

Wireless communication
through rock

Low frequency electromagnetic waves have been used for communication through liquid and/or solid media for decades (Mallette, 2015). The frequency ranges are variously called very low, ultra low or extremely low frequency, hence VLF, ULF and ELF. Military navies use ELF for submarine communication. VLF communications are widely used in underground mining for PED (personal emergency device) systems. Such communication is one-way from a large surface transmitter to receivers carried (usually in a belt pack) by each miner underground.

Low frequency communication for mining applications requires a range from the transmitter to the in-hole receivers in the order of hundreds to thousands of meters. The transmitting antenna required to achieve this range is a loop of one or more copper wires of tens to hundreds of square meters in area and carrying several hundred amps of current. There is a trade-off between current and area. The system has to be highly reliable.

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Long section showing the brow of a ring being lost.

Alternating currents generate oscillating fields which can be made to carry data by modulating the frequency or amplitude. Lower frequencies generate lower bandwidth and slower communications. On the other hand lower frequencies have much greater penetration range in a given medium. The choice of frequency is thus a trade-off between the desired range and the amount and rate of data transmission to safely initiate a blast. The system described has a frequency of less than 5000 Hz.

Electromagnetic signals generated in this way are dominated by their magnetic component. As a result, communication in the VLF or ULF range is called magneto-inductive or magneto-resistive. In Orica’s magneto-inductive system the receiving antenna is a coil of copper wire with thousands of turns. Such coils can detect tiny changes in their ambient magnetic field and reflect such changes in sophisticated electronic circuitry that amplifies and interprets the incoming signals.

The physical embodiment of these principles is shown in Figure 1 where it can be seen that data originally generated in the controller is conveyed via an oscillating magnetic field to the in-hole receiver.

The critical attribute of the signal detected in the borehole is its signal to noise ratio. Magnetic noise in a mining environment is generated mainly by electric motors and AC power lines. When power lines convey power at 50 or 60 Hz, magnetic fields are created at harmonics of these frequencies. Very sensitive magneto detectors can easily detect such harmonics of 60 Hz (ie 120, 180, 240 Hz etc). The exact communication frequency for blasting must therefore be chosen carefully and controlled precisely to avoid likely noise sources.

Designs for a safe system

In a fully wireless blast there is no connection from the in-hole device to the collar (Mallette, 2015). This brings two new and potentially hazardous situations into play. Firstly, the in-hole device of necessity contains its own power source. Secondly, with wired initiation systems, only initiators actually connected to the wire could possibly respond to a fire command. That is not the case here as any wireless initiator that is energised and within range of the transmitter will ‘hear’ the fire command. These new circumstances required many hazard studies and the invention or adoption of new hardware, software and procedures to ensure the safety of the system.

An early decision for the wireless electronic blasting system was to base its safety and reliability on the excellent platform that has been established with electronic detonators. These specific detonators are inherently safe, ie the programming device to establish timing has neither sufficient voltage nor sufficient current to initiate the detonator. Further, this can be tested during manufacture such that any detonator components, especially the fusehead and capacitor that fall outside the inherent safety limits, are culled during a simulated test-fire procedure.

Since this detonator has an excellent track record in the field, it was incorporated into the wireless system with only minimal changes.

Despite the confidence in being able to design and build safe electronic initiation systems for blasting, it was decided this wireless system should also comply with the guidelines of an external standard. IEC61508 was chosen as the standard to meet the rigorous safety requirements needed for the blasting and mining industries.. Level 3 is the highest level that can practically be achieved for a commercial blasting product. A SIL 3 component running continuously, for example in a chemical plant, should have a safety-critical failure less than one in 1000 years. The safety-critical elements of the wireless system have been designed and certified to IEC 61508 (SIL3).

The DRX is a sophisticated communications device that also controls the safety-critical generation of the firing voltage and fire command to initiate countdown in the attached detonator. During programming or encoding on the blast pattern, the DRX communicates with the detonator only at programming voltage. Communications between the encoder and DRX include the passing to the DRX of the delay time for its detonator, and its group identity number (GID). This group number allows pre-selected groups of primers to be fired as a subset of all the primers within communication range of the transmitter. As an example (explained in more detail below) pre-loaded rings of holes in a sublevel cave blast could be selectively fired in groups of one or two rings at a time. Ensuring that all selected DRXs, and only those selected, respond at firing time is also a safety-critical function.

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Figure 1. Magneto-Inductive communication in blasting.

After deployment of the wireless primer into the borehole, the primer actively listens to the ambient magnetic field. If the wake-up signal (from the transmitter) is detected the DRX enters the active state and listens for further commands. The next command tells the DRX which GIDs have been selected for that firing event. When a DRX receives a code that matches its stored GID it enters the armed state. All other DRXs revert after a timeout to the listening state. Great care has been taken in the system design to ensure that this DRX selection and arming step is reliable.

During all of these actions the DRX is operating at or below the inherently safe voltage of the detonator. Only at the last moment before sending the fire command to the detonator does each armed DRX generate the firing voltage. Control of this step is safety critical, and failure of any single component is the entire system will not compromise this control. The well-established safety practice of requiring at least two independent faults for failure of a safety-critical function is used within the DRX for control of the detonator firing voltage.

As was mentioned above, ambient magnetic noise is a concern for reliable magneto-inductive communications. The set of digital commands and data used in this wireless system has been carefully chosen to minimise the chances of any command or GID number being mistaken for any other command or number, even in a noisy environment.

In summary, the safety design is based on:

  • thorough hazard studies and the management of each identified hazard
  • use of the detonator with its inherently safe communications mode
  • control of the generation of the firing voltage by the DRX via established dual-fault methodology
  • careful selection of a command and data set to minimise the chance of misinterpretation.

Conclusion

The Wireless Electronic Blasting System has been designed to be safe, reliable and is specifically designed for underground mining. The system is IEC61508 qualified (SIL3 rating) and aimed at six sigma reliability.

This system of initiation has no wires and so there is no need for a hook-up process in a SLC ring blasting application (or any other) – thereby removing a high safety risk operation from these mines. The initiation system is programmed at charging and with unique codes that limit initiation to that mine only. These initiators are rendered remote firing inert if they are taken offsite and safety systems render them inert if they are not placed in a hole within 30 minutes. With no wires to damage, there are fewer problems that could cause misfires, there are no problems with lack of access to the holes, there is no bogging down time for a hook-up stage and there are no safety risks with hook-up. Wireless initiation makes most sense to SLC mines where multiple small blasts can be controlled better with less safety risk and greater productivity. Access to holes is the main reason SLC mines move away from advanced drilling and precharging, the wireless system eliminates this access issue – enabling advance drilling and charging, which significantly reduces cost and time in standing up rills, rework and redrilling.

Safety is every company’s first priority and when safety risks can be eliminated with an engineering solution there is a need to test the value of it. Initial trials will test this wireless system in SLC mines and determine if it can perform in a most challenging environment.  

References

Mallette D, (2015). Wireless Electronic Blasting. New Orleans: ISEE.


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