Outbursts are a failure of coal and associated material that is accompanied by significant energy and gas release. They are dangerous in the underground environment because they are unexpected events that are sudden, and may kill or injure by mechanical force or asphyxiation.

Sigra’s Managing Director has been involved with outbursts in underground coal mines since 1979 when he commenced working with the problem in Queensland at Bowen No 2 Mine and the notorious Leichhardt Colliery, which had some 300 outbursts. Sigra’s Principal Geologist has experience which extends back to the same period. In this early period some useful lessons were also learned from the Japanese coal mining industry, which had some significant outburst problems.

Since the incorporation of the Company in 1994, work has continued in determining outburst proneness and in determining methods to reduce the likelihood of its occurrence. This has not only included work within Australia but has involved mines in Russia, China and Kazakhstan.

Sigra has developed its own approach to determining the risk of outbursts. This has drawn on the experience of others internationally, and has been somewhat broadened to deal with outbursts ranging from the purely gas driven, to those that are substantially rock bursts with a small amount of gas drive. The key to this is the determination of the energy that may be released in an outburst.

Sigra’s Approach Takes Into Account the Energy Release From:

  • Strain energy of the breaking coal
  • Strain energy contributed from the surrounding strata

    • Pore space
    • Desorbing coal particles and the energy that is absorbed in breaking the coal into particles
    • Adiabatic gas expansion from free gas

The Factors That Sigra Uses to Assess Outburst Propensity Therefore Includes:

  • Stress Strain and Strength

    • Stress within the coal and surrounding strata
    • Young’s modulus of the coal and surrounding strata
    • Tensile strength
    • Toughness
  • Gas Related Factors

    • Diffusion coefficient
    • Gas content
    • Sorption pressure
    • Sorption Isotherm
    • Porosity
    • Characteristic particle size on failure
  • Structural Features

    • Faulting and the nature of the fault gouge material
    • Cleat structure and infill

Sigra has a suite of techniques which it can use to determine the factors that affect outbursting. These include field work from surface, underground and in the laboratory.

Based on this work Sigra will determine what needs to be undertaken to prevent outbursts. This may involve gas drainage, if that is possible, or de-stressing of the seam by adjacent mining, or by sluicing to remove large quantities of material. The latter would always be undertaken using adequate safeguards so that no risk to mine personnel would exist.