INFORMATION REQUIRED DURING THE BLAST

INFORMATION REQUIRED DURING THE BALST.

The performance of explosives can only be properly assessed by monitoring during blasts. Equipment and techniques are available to record blast dynamics and collect information on complex events which occur very rapidly during the explosive rock interaction process.

There are a number of tools available for blast analysis which are used on a regular basis in mines to obtain:
  1. general qualitative  information:
  2. firing times of detonators;
  3. VOD of explosives charges;
  4. burden moment data;
  5. information on energy losses; and
  6. values of ground vibration and air blast.

General qualitative information - Qualitative information (simply obtained with the use of a video) that can be used to assist with blast design includes the flowing:

  • observation of the initiation sequence;
  • potential misfired blast holes;
  • effectiveness of type and length of stemming;
  • face moment – degree and location;
  • sources of fly rock and air blast;
  • origin of oversize rock fragments; and
  • exposition gas fumes, indicating possible reduction in explosives performance (e.g., as a result of water contamination)

High speed filming (500 frames per second) can be used in special cases but it is expensive and requires interpretation. Even photographs taken using a standard hand-held camera, can give useful information and can form part of a blast record. High-speed photography is often used to assess the reliability, accuracy and precision of the initiating system. In the hole VOD is one the most commonly measured characteristics of explosives. Several techniques are available for VOD measurement, including continuous and point-to point systems.

VODs Measurement by the Electrical Contact System - In-hole VOD is one of the most commonly measured characteristics of explosives. Several techniques are available for VOD measurement including continuous and point-to point systems. VOD should be used essentially as a quality control measure. The ectrical contact system also measures point to point in hole VODs, using light weight electric cable inside blast holes. Each probe consists of a pair of insulated wires which are short circulated when the explosive’s detonation front consumes them. A data recorder measures the time at which successive in hole probes are shorted together. Data is analyzed after the blast, to calculate the electrical contact system is inexpensive and is simple to use in dry blast holes. However, the system requires expensive recording and analysis equipment, does not produce an immediate result, and is not reliable in wet conditions because of current leakage problems inside the blast hole.

Video Recording - Video recording cameras provide a simple and inexpensive method of recording the general appearance of production blasts. Video recording enables a blast to be reviewed immediately after it has been fired and, as stated previously, can provide a range of useful information. The video camera should have a high speed electronic shutter, to ensure clear images. A fully charged battery and a long tape a helps to capture the blast, even if it takes longer to fire than expected. A tripod is need to keep the camera steady and for remote recording of the blast.

Burden Moment - High-speed photography enables an accurate assessment of the time delay between blast hole detonation and initial movement of the free face, plus the velocity at which the face moves burden velocity measurements are used to calibrate the Sabrox computer blasting model and can be used to compare the heave energy of different explosives under similar conditions. The time to first movement is used to determine the optimum delay required between dependent blast holes.

To measure front-row burden movement, large markers are placed in vertical rows on the free face of the blast. The most effective markers are empty 200 liters drums, which are usually placed adjacent to front-row blast holes. The high-speed camera is located at a safe distance in front of and above the blast, where burden movement and the surface initiating system can be viewed clearly. The location of the camera, each marker, and the collar of each front-row blast hole must be accurately established b surveying.

After the high speed film has been developed, it is analyzed using computer software to determine burden. Specific relationships between burden distance, burden velocity and time to first movement can be determined for any explosive in a particular rock type.

In general, velocity is inversely proportional to burden, whereas the time to first movement is directly proportional to the burden. The time to first movement can be used to determine the minimum delay required between depend  on blast holes to ensure that the face is moving before the adjacent blast hole fires. The recommended minimum delay time is twice the time to first movement.

Power waver Radar - Orica Explosives had developed the Power wave radar as a new method of measuring face velocity and time to first movement. This patented system is a unique tool for blast performance assessment and works similar to poico radars. It is capable of measuring numerous objects all moving at different velocities. Traditional techniques for measuring face velocity, such as high-speed cameras, have invaluable in analyzing face motion. But they have drawbacks, results are not available for several days or weeks, and actual face velocities can be measured only where scale control exists on the face. Power wave radar can be used to monitor any face and the results can be analyzed immediately.

Face velocities from power wave radar are presented as a velocity distribution graph. This reflects the fact that, the blast, the blasted face does not travel at a single velocity, but different areas of the face travel at different velocities. 

 A Crying Boy. 1955 PhotoN G Nair Kerala travel 2010.
  

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