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Slotting While Drilling for Reduced Blast Damage to the Coal Seam

Open Cut » Drilling & Blasting

Published: April 05Project Number: C13033

Get ReportAuthor: Mehmet Doktan, Dihon Tadic | CRCMining

This project has developed an innovative technique and tool to reduce blast damage to the coal seam. Blast damage that leads to coal loss in open cut mines is generally seen in two forms:

  • Damage to the top of the coal seam and loss of coal during mining
  • Coal edge movement or block movement and subsequent burial of coal under overburden

Research to date indicates that the confinement and excess energy at the toe is responsible for these mechanisms. The standard preventive measures include strict control of stand off distances, adherence to the plans, baby decking and seam buffering. These are complicated, difficult to achieve, expensive and in most cases ineffective while being only a partial solution to the problem.

CRCMining has proposed and developed a novel technique to overcome the problem. The technique is based on creating hollow slots at the toe of the blast hole that will effectively reduce confinement and divert excess blast energy into the rock rather than damage the coal.

The project is intended to be completed in two phases. The objectives of the first phase of the research included:

  • Develop tools and systems for blasthole slotting
  • Conduct tests to prove the concept, the system and quantify the benefits of slotting
  • Develop guidelines for retrofitting a drill rig and site trials
  • Transfer project outcomes back to the industry for next stage funding

Various options for a suitable slotting tool were evaluated and a self-extending arm concept was selected as the most suitable system. A slotter that can be mounted on a drill rig or forklift was designed, constructed, and tested on various samples.

The tests conducted showed that coal measure rocks such as sandstone, were efficiently slotted (approximately 300 mm in three minutes) while hard rock, such as granite took significantly longer (50 mm in 8 minutes).

To demonstrate the effectiveness of slots in controlling fracturing, a number of specimens were drilled, slotted and blasted at a quarry. The results showed that slotting the hole resulted in a reduction of the Peak Particle Velocity (PPV) which is a measurable indication of blast damage. The photos below illustrate the difference in the fragmentation of the toe section of the slotted and non-slotted samples after blasting.

Upon successful completion of the first stage, a set of guidelines were developed to aid in the implementation of the next stage of work. The overall recommendation for the next stage of work is to develop, test (full-scale field test), and validate a slotting system independent of the drill rig (Figure A6).

Anticipated benefits of blasthole slotting include the following:

  • Reduced blast damage to the coal seam. Slots created at the toe of the blasthole will reduce confinement and provide a route for the gases to enter and break the rock. This will effectively reduce stresses otherwise acting on the coal seam.
  • Improved presplit operation and quality. Vertical slots created around the blasthole will encourage the gas entry and promote fracturing along the plane defined by slots.
  • Improved fragmentation. Cavities (large slots) and slots created at harder segments of the overlying rocks will significantly improve the fragmentation in those sections.
  • Improved cast blast throw. Cavities created and filled with additional explosives will promote better cast and throw.
  • Reduced need for sub drill. The need for sub drill may be reduced when the fracturing is controlled at the toe of the blasthole.
  • Increased safety. Due to better fracture control and reduced blast damage to the highwall, highwalls will have smother surface and therefore will be safer.
  • Reduced blast vibration. In areas where blasting is conducted close to residential areas, reduction of blast vibration is important. This can be achieved by multiple slotting along the blast hole.

The first stage of the blasthole slotting project was completed on time and budget with all project objectives achieved.

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